The Lipids.
Some people fear fat. We may have friends
who have so-called "fat attacks." These attacks
are reported to be both the craving for tasty, fatty foods and the worrying about the way fat appears on the body. However,
fat is not always an enemy. Fat is
valuable and necessary to health. It is important to learn about fat in food,
what the fat we eat does in our bodies, and how
it can be both helpful and harmful to our
health. Individual preference for fat is developed either in infancy or early childhood; innate preferences for sweet
taste are observed at birth.' Thus children learn
to prefer tastes, flavors, and textures that are associated with foods that are rich in fat, sweet, or both. Aging may be
associated with increasing acceptance of bitter
tastes and consumption of more fruits, vegetables, and whole grains.
Nonetheless, decreasing fat consumption
takes time and effort, perhaps because of food
selection habits, symbolic meaning associated with certain foods, and sensory values of fats in foods.
The five dimensions of health provide ways
to think about the effects of changing dietary
fat consumption. Physical health is maintained by consuming dietary fats that are necessary for essential
fatty acids, for energy, and for fat-soluble vitamins.
Excessive intake of fats, though, may
increase the risk of obesity and dietrelated diseases.
The intellectual health dimension encompasses the skills necessary to assess the type of dietary fat
modification most appropriate for our clients' and our own health needs. How we emotionally
approach nutritional lifestyle changes for
our clients and ourselves affects success, which reflects the emotional health
dimension.
Can these emotions be expressed, or are
changes simply disregarded because they
make us feel uncomfortable? The social dimension is tested as change is initiated. Are relationships of family and
friends based on sharing high-fat meals?
Can you or your clients refuse to take
part in social situations without jeopardizing relationships
or making others feel defensive?
Can food preparation suggestions to lower the fat content be made without
seeming overly critical?
Some religions maintain that taking care of one's body is
necessary to achieve spiritual goals. Adopting a
healthier fat intake supports these spiritual health dimension goals.
Fat actually
refers to the chemical group called lipids.
Lipids
are divided into three
classifications: fats (or triglycerides) and the fat-related substances of phospholipids and sterols.
Triglycerides
are the largest class of lipids and may be in the form
of fats (somewhat solid) or oils (liquids).
About 95% of the lipids in foods and in our bodies are in the triglyceride
form of fat.
The
other two lipid classifications are
the fat-related substances of phospholipids and sterols. Lecithin is the best-known phospholipid; cholesterol is
the best-known sterol. All are organic, composed
of carbon, hydrogen, and oxygen; and cannot dissolve in water.
Lipids are water-insoluble organic biomolecules that can be extracted from cells and tissues by nonpolar solvents,
e.g., chloroform, ether, or benzene.
Lipids are an amphiphilic class of hydrocarbon-containing organic compounds. Lipids are categorized by the fact that they have complicated solvation properties,
giving rise to lipid polymorphism. Lipid molecules have these properties because they consist largely of
long hydrocarbon tails which are lipophilic in nature as well as polar headgroups (e.g. phosphate-based functionality,
and/or inositol based functionality). In living organisms, lipids are used for energy
storage, serve as the structural components of cell membranes, and constitute important signalling molecules. Although the term lipid is often used as a synonym for fat, the latter is in fact a subgroup of lipids called triglycerides.
There are
several different families or classes of lipids but all derive their
distinctive properties from the hydrocarbon nature of a major portion of their structure.
Biological functions of lipids
Biological molecules that are
insoluble in aqueous solutions and soluble in organic solvents are classified
as lipids. The lipids of physiological importance for humans have four major
functions:
Lipids have several important biological functions, serving
(1) as structural
components of membranes,
(2) as storage
and transport forms of metabolic fuel,
(3) as a protective
coating on the surface of many organisms, and
4) as cell-surface
components concerned in cell recognition, species specificity, and tissue
immunity. Some substances classified among the lipids have intense biological
activity; they include some of the vitamins and hormones.
Although lipids are a
distinct class of biomolecules, we shall see that they often occur combined,
either covalently or through weak bonds, with members of other classes of biomolecules
to yield hybrid molecules such as glycolipids,
which contain both carbohydrate and lipid groups, "and lipoproteins, which contain both lipids
and proteins. In such biomolecules the distinctive chemical and physical properties
of their components are blended to fill specialized biological functions.
Classification of lipids
Lipids have been classified in
several different ways. The most satisfactory classification is based on their
backbone structures:
1. Simple lipids:
1) acylglycerols;
2) steroids;
3) waxes.
2. Complex lipids:
1) phospholipids
a) glycerophospholipids;
b) sphingophospholipids.
2) glycolipids
a) glycosylglycerols;
b) glycosphingolipids.
Lipids
usually contain fatty acids as components. Such lipids are called saponifiable lipids since they yield soaps
(salts of fatty acids) on alkaline hydrolysis. The other
great group of lipids which do not contain fatty acids and hence are
nonsapomfiable.
Let us first consider the structure
and properties of fatty acids, characteristic components of all the complex
lipids.
Fatty Acids
Fatty acids and
glycerides
Fatty acids fill two major roles in the body:
· 1. as the components of more complex
membrane lipids.
· 2. as the major components of stored fat
in the form of triacylglycerols.
Fatty acids are
long-chain hydrocarbon molecules containing a carboxylic acid moiety at one
end. The numbering of carbons in fatty acids begins with the carbon of the
carboxylate group. At physiological pH, the carboxyl group is readily ionized,
rendering a negative charge onto fatty acids in bodily fluids.
Fatty acids that contain no carbon-carbon double bonds are termed saturated
fatty acids; those that contain double bonds are unsaturated fatty acids. The
numeric designations used for fatty acids come from the number of carbon atoms,
followed by the number of sites of unsaturation (eg, palmitic acid is a
16-carbon fatty acid with no unsaturation and is designated by 16:0). The site of
unsaturation in a fatty acid is indicated by the symbol and the number of the first carbon of the
double bond (e.g. palmitoleic acid is a 16-carbon fatty acid with one site of
unsaturation between carbons 9 and 10, and is designated by 16:1?9).
Saturated fatty
acids of less than eight carbon atoms are liquid at physiological temperature,
whereas those containing more than ten are solid. The presence of double bonds
in fatty acids significantly lowers the melting point relative to a saturated
fatty acid.
The majority of body fatty acids are acquired in
the diet. However, the lipid biosynthetic capacity of the body (fatty acid
synthase and other fatty acid modifying enzymes) can supply the body with all
the various fatty acid structures needed. Two key exceptions to this are the
highly unsaturated fatty acids know as linoleic acid and linolenic acid,
containing unsaturation sites beyond carbons 9 and 10.
These two fatty acids cannot be synthesized from
precursors in the body, and are thus considered the essential fatty acids; essential in the sense that they must be
provided in the diet. Since plants are capable of synthesizing linoleic and
linolenic acid humans can aquire these fats by consuming a variety of plants or
else by eating the meat of animals that have consumed these plant fats
Chemically, fatty acids can be described as long-chain monocarboxylic acids
and have a general structure of CH3(CH2)nCOOH.
The length of the chain usually ranges from 12 to 24, always with an even
number of carbons. When the carbon chain contains no double bonds, it is a saturated
chain. If it contains one or more such bonds, it is unsaturated. The presence
of double bonds generally reduces the melting point of fatty acids.
Furthermore, unsaturated fatty acids can occur either in cis or trans geometric isomers. In naturally
occurring fatty acids, the double bonds are in the cis-configuration.
Glycerides are lipids
possessing a glycerol (propan-1, 2, 3-triol) core structure with one
or more fatty acyl groups, which are fatty acid-derived chains attached to the
glycerol backbone by ester linkages. Glycerides with three acyl groups (triglycerides or neutral fats)
are the main storage form of fat in animals and plants.
An important type of glyceride-based molecule
found in biological membranes, such as the cell's plasma membrane and the intracellular membranes of organelles, are the phosphoglycerides or glycerophospholipids. These are phospholipids that contain a glycerol core linked to two fatty acid-derived
"tails" by ester or, more rarely, ether linkages and to one "head" group by a phosphate ester linkage. The head groups of the phospholipids found in biological membranes are phosphatidylcholine (also known as PC, and lecithin), phosphatidylethanolamine (PE), phosphatidylserine and phosphatidylinositol (PI). These phospholipids are subject to a variety of functions in the cell: for
instance, the lipophilic and polar ends can be released from specific
phospholipids through enzyme-catalysed hydrolysis to generate secondary messengers involved in signal transduction. In the case of phosphatidylinositol, the head group
can be enzymatically modified by the addition of one, two or three phosphate
groups, this constituting another mechanism of cell signalling. While
phospholipids are the major component of biological membranes, other
non-glyceride lipid components like sphingolipids and sterols (such as cholesterol in animal cell
membranes) are also found in biological membranes.
A biological
membrane is a form of lipid bilayer, as is a liposome. Formation of lipid bilayers is an
energetically-favoured process when the glycerophospholipids described above
are in an aqueous environment. In an aqueous system, the polar heads of lipids
orientate towards the polar, aqueous environment, while the hydrophobic tails
minimise their contact with water.
The lipophilic tails of lipids (U) tend to cluster together, forming a lipid bilayer (1) or a micelle (2). Other aggregations are also observed and form part of the polymorphism of amphiphile (lipid) behaviour. The polar heads (P) face the aqueous environment,
curving away from the water. Phase behaviour is a complicated area within biophysics and is the subject of current
academic research.
Micelles and bilayers
form in the polar medium by a process known as the lipophilic effect.
When dissolving a lipophilic or amphiphilic substance in a polar
environment, the polar molecules (i.e. water in an aqueous solution) become
more ordered around the dissolved lipophilic substance, since the polar
molecules cannot form hydrogen bonds to the lipophilic areas of the amiphphile. So, in an aqueous environment the water molecules form an ordered "clathrate" cage around the dissolved lipophilic molecule.
The
self-organisation depends on the concentration of the lipid present in
solution. Below the critical micelle concentration, the lipids form
a single layer on the liquid surface and are (sparingly) dispersed in the
solution. At the first critical micelle concentration (CMC-I), the lipids
organise in spherical micelles, at given points above this concentration, other
phases are observed.
Self-organization
of phospholipids. A lipid bilayer is shown on the
left and a micelle on the right.
Although
fatty acids occur in very large amounts as building-block components of the
saponifiable lipids, only traces occur in free (unesterified) form in cells and
tissues. Well over 100 different kinds of fatty acids have been isolated from
various lipids of animals, plants, and microorganisms. All possess a long
hydrocarbon chain and a terminal carboxyl group. The hydrocarbon chain may be
saturated, as in palmitic acid, or it
may have one or more double bonds, as in oleic
acid; a few fatty acids contain triple bonds.
Fatty acids differ from each
other primarily in chain length and in the number and position of their
unsaturated bonds. They are often symbolized by a shorthand notation that
designates the length of the carbon chain and the number, position, and
configuration of the double bonds. Thus palmitic acid (16 carbons, saturated)
is symbolized 16:0 and oleic acid [18 carbons and one double bond (cis) at
carbons 9 and 10] is symbolized 18:1. It is understood that the double bonds
are cis (see below) unless indicated otherwise.
Some
generalizations can be made on the different fatty acids of higher plants and
animals. The most abundant have an even number of carbon atoms with chains
between 14 and 22 carbon atoms long, but those with 16 or 18 carbons
predominate. The most common among the saturated fatty acids are palmitic acid
(Cis) and stearic acid (Cis) and among the unsaturated fatty acids oleic acid
(Cis). Unsaturated fatty acids predominate over the saturated ones,
particularly in higher
plants and in animals living at low
temperatures. Unsaturated fatty acids have lower
melting points than saturated fatty acids of the same chain length. In most
monounsaturated (monoenoic) fatty acids of higher organisms there is a double
bond between carbon atoms 9 and
There are two kinds of fats, saturated and unsaturated.
Unsaturated fats have at least one double bond in one of the fatty acids. A double
bond happens when two electrons are shared or exchanged in a bond. They are
much stronger than single bonds.
Saturated fats have no double bonds.
Fats have a lot of energy stored up in their molecular bonds. That's why
the human body stores fat as an energy source. When it needs extra fuel, your
body breaks down the fat and uses the energy. Where one molecule of sugar only
gives a small amount of energy, a fat molecule gives off many times more.
Some naturally occuring fatty
acids
Symbol |
Structure |
Systemic
name |
Common
name |
Saturated fatty acid |
|||
Ñ12:0 |
ÑÍ3(ÑÍ2)10ÑÎÎÍ |
n-Dodecanoic |
Lauric |
Ñ14:0 |
ÑÍ3(ÑÍ2)12ÑÎÎÍ |
n-Tetradecanoic |
Myristic |
Ñ16:0 |
ÑÍ3(ÑÍ2)14ÑÎÎÍ |
n-Hexadecanoic |
Palmitic |
Ñ18:0 |
ÑÍ3(ÑÍ2)16ÑÎÎÍ |
n-Octadecanoic
|
Stearic |
Ñ20:0 |
ÑÍ3(ÑÍ2)18ÑÎÎÍ |
n-Eicosanoic |
Arachidic |
Ñ22:0 |
ÑÍ3(ÑÍ2)20ÑÎÎÍ |
n-Docosanoic |
Begenic |
Ñ24:0 |
ÑÍ3(ÑÍ2)22ÑÎÎÍ |
n-Tetracosanoic |
Lignoceric |
Unsaturated monoenic fatty acid |
|||
Ñ16:1 |
ÑÍ3(ÑÍ2)5ÑÍ=ÑÍ(ÑÍ2)7ÑÎÎÍ |
|
Palmitooleic |
Ñ18:1 |
ÑÍ3(ÑÍ2)7ÑÍ=ÑÍ(ÑÍ2)7ÑÎÎÍ |
|
Oleic |
Unsaturated polienic fatty acid |
|||
Ñ18:2 |
ÑÍ3(ÑÍ2)4(ÑÍ=ÑÍÑÍ2)2(ÑÍ2)6ÑÎÎÍ |
|
Linoleic |
Ñ18:3 |
ÑÍ3ÑÍ2(ÑÍ=ÑÍÑÍ2)3(ÑÍ2)6ÑÎÎÍ |
|
Linolenic |
Ñ20:4 |
ÑÍ3(ÑÍ2)4(ÑÍ=ÑÍÑÍ2)4(ÑÍ2)2ÑÎÎÍ |
Arachidonic |
All Lipids are hydrophobic: that’s the one property they have in
common. This group of molecules includes fats and oils, waxes, phospholipids,
steroids (like cholesterol), and some other related compounds.
Structure of Fatty Acids
Fats and oils are made from two kinds
of molecules: glycerol (a type of
alcohol with a hydroxyl group on each of its three carbons) and three fatty acids joined by dehydration
synthesis. Since there are three fatty acids attached, these are known as triglycerides. “Bread” and pastries from a “bread factory” often contain mono- and
diglycerides as “dough conditioners.”
Can you figure out
what these molecules would look like? The main distinction between fats and
oils is whether they’re solid or liquid at room temperature, and this, as we’ll
soon see, is based on differences in the structures of the fatty acids they
contain
Essential fatty acids
When weanling or immature rats are placed on a fat-free diet, they grow
poorly, develop a scaly skin, lose hair, and ultimately die with many
pathological signs. When linoleic
acid is present in the diet, these conditions do not develop. Linolenic acid and arachidonic acid also prevent these symptoms. Saturated and monounsaturated
fatty acids are inactive. It has been concluded that mammals can synthesize
saturated and monounsaturated fatty acids from other precursors but are unable
to make linoleic and linolenic acids. Fatty acids required in the diet of
mammals are called essential fatty acids.
The most abundant essential fatty acid in mammals is linoleic acid, which makes up from 10 to 20 percent of the total
fatty acids of their triacylglycerols and phosphoglycerides.
Linoleic and linolenic acids cannot be synthesized by mammals but must be
obtained from plant sources, in which they are very abundant. Linoleic acid is
a necessary precursor in mammals for the biosynthesis of arachidonic acid, which is not found in plants.
The terms saturated,
mono-unsaturated, and poly-unsaturated refer to the number of
hydrogens attached to the hydrocarbon tails of the fatty acids as compared to
the number of double bonds between carbon atoms in the tail. Fats, which are
mostly from animal sources, have all single bonds between the carbons in their
fatty acid tails, thus all the carbons are also bonded to the maximum number of
hydrogens possible. Since the fatty acids in these triglycerides contain the
maximum possible amouunt of hydrogens, these would be called saturated fats. The hydrocarbon
chains in these fatty acids are, thus, fairly straight and can pack closely
together, making these fats solid at room temperature. Oils, mostly from plant
sources, have some double bonds between some of the carbons in the hydrocarbon
tail, causing bends or “kinks” in the shape of the molecules. Because some of
the carbons share double bonds, they’re not bonded to as many hydrogens as they could if they weren’t double bonded to
each other. Therefore these oils are called unsaturated fats. Because of the kinks in the hydrocarbon tails,
unsaturated fats can’t pack as closely together, making them liquid at room
temperature. Many people have heard that the unsaturated fats are “healthier”
than the saturated ones.
Hydrogenated
vegetable oil (as in shortening and commercial peanut butters where
a solid consistency is sought) started out as “good” unsaturated oil. However,
this commercial product has had all the double bonds artificially broken and
hydrogens artificially added (in a chemistry lab-type setting) to turn it into
saturated fat that bears no resemblance to the original oil from which it came
(so it will be solid at room temperature).
Although the specific
functions of essential fatty acids in mammals were a mystery for many years,
one function has been discovered. Essential fatty acids are necessary precursors
in the biosynthesis of a group of fatty acid derivatives called prostaglandins, hormonelike compounds
which in trace amounts have profound effects on a number of important physiological
activities.
Physical and chemical
properties of fatty acids
Saturated and unsaturated fatty acids have quite different conformations.
In saturated fatty acids, the hydrocarbon tails are flexible and can exist in a
very large number of conformations because each single bond in the backbone
has complete freedom of rotation. Unsaturated fatty acids, on the other hand,
show one or more rigid kinks contributed by the nonrotating double bond(s).
Unsaturated fatty acids undergo addition reactions at their double bonds.
Quantitative titration with halogens, e.g., iodine or bromine, can yield
information on the relative number of double bonds in a given sample of fatty
acids or lipid.
Triacylglycerols (Triglycerides)
Fat is also known as a triglyceride. It is made up of a molecule known as glycerol
that is connected to one, two, or three fatty acids. Glycerol is the basis of
all fats and is made up of a three-carbon chain. It connects the fatty acids
together. A fatty acid is a long chain of carbon atoms connected to each other.
Fatty acid esters of the alcohol glycerol are called acylglycerols or glycerides;
they are sometimes referred to as "neutral
fats," a term that has become archaic. When all three hydroxyl groups
of glycerol are esterified with fatty acids, the structure is called a triacylglycerol:
Although the name "triglyceride" has been traditionally used to
designate these compounds, an international nomenclature commission has recommended
that this chemically inaccurate term no longer be used.
Triacylglycerols are the most abundant family of lipids and the major
components of depot or storage lipids in plant and animal cells.
Triacylglycerols that are solid at room temperature are often referred to as
"fats" and those which are liquid as "oils."
Diacylgiycerols (also called diglycerides)
and monoacylgiycerols (or monoglycerides) are also found in nature, but in much
smaller amounts.
Triacylglycerols occur in many different types, according to the identity
and position of the three fatty acid components esterified to glycerol. Those
with a single kind of fatty acid in all three positions, called simple
triacylglycerols, are named after the fatty acids they contain.
Examples are tristearoylglycerol,
tripalmitoylglycerol, and trioleoylglycerol; the trivial and more
commonly used names are tristearin, tripalmitin, and trioiein, respectively. Mixed triacylglycerols contain two or more
different fatty acids. The naming of mixed triacylglycerols can be illustrated
by the example of 1-palmitoyldi-stearoylglycerol
(trivial name, 1-palmitodistearin).
Most natural fats are extremely complex mixtures of simple and mixed
triacylglycerols.
Properties of triacylglycerols
The melting point of triacylglycerols is determined by
their fatty acid components. In general, the melting point increases with the
number and length of the saturated fatty acid components.
For example,
tripalmitin and tristearin are solids at body temperature, whereas triolein and
trilinolein are liquids.
All triacylglycerols are insoluble in water and do not
tend by themselves to form highly dispersed micelles. However, diacylglycerols and monoacylglycerols
have appreciable polarity because of their free hydroxyl groups and thus can
form micelles. Diacyl- and monoacylglycerols find wide use in the food industry
in the production of more homogeneous and more easily processed foods; they are
completely digestible and utilized biologically. Acylglycerols are soluble in
ether, chloroform, benzene, and hot ethanol. Their specific gravity is lower
than that of water. Acylglycerols undergo hydrolysis when boiled with acids or
bases or by the action of lipases, e.g., those present in pancreatic juice.
Hydrolysis with alkali, called saponification, yields a mixture of soaps and
glycerol.
Steroids
Steroids occur in animals in something called hormones.
The basis of a steroid molecule is a four-ring structure, one with five carbons
and three with six carbons in the rings. You may have heard of steroids in the
news. Many body builders and athletes use anabolic steroids to build muscle
mass. The steroids make their body want to add more muscle than they normally
would be able to.
The body builders wind up stronger and bulkier (but not faster).
Never take drugs to enhance your body. Those body builders are actually
hurting their bodies. They can't see it because it is slowly destroying their
internal organs and not the muscles. When they get older, they can have kidney
and liver problems. Some even die.
The important class of lipids called
steroids are actually
metabolic derivatives of terpenes, but they are customarily treated as a
separate group. Steroids may be recognized by their tetracyclic skeleton,
consisting of three fused six-membered and one five-membered ring, as shown in
the diagram to the right. The four rings are designated A, B, C & D as
noted, and the peculiar numbering of the ring carbon atoms (shown in red) is
the result of an earlier misassignment of the structure. The substituents designated by
R are often alkyl groups, but may also have functionality. The R group at the A:B ring fusion is most commonly methyl or hydrogen, that at
the C:D fusion is usually methyl. The substituent at C-17 varies considerably,
and is usually larger than methyl if it is not a functional group. The most
common locations of functional groups are C-3, C-4, C-7, C-11, C-12 & C-17. Ring A is sometimes aromatic. Since a
number of tetracyclic triterpenes also have this tetracyclic structure, it
cannot be considered a unique identifier.
Steroids
are widely distributed in animals, where they are associated with a number of
physiological processes. Examples of some important steroids are shown in the
following diagram. Different kinds of steroids will be displayed by clicking
the "Toggle Structures" button under the diagram. Norethindrone is a
synthetic steroid, all the other examples occur naturally. A common strategy in
pharmaceutical chemistry is to take a natural compound, having certain desired
biological properties together with undesired side effects, and to modify its
structure to enhance the desired characteristics and diminish the undesired.
This is sometimes accomplished by trial and error.
The generic steroid structure
drawn above has seven chiral stereocenters (carbons 5, 8, 9, 10, 13, 14 &
17), which means that it may have as many as 128 stereoisomers. With the
exception of C-5, natural steroids generally have a single common
configuration. This is shown in the last of the toggled displays, along with
the preferred conformations of the rings.
Chemical studies of the steroids were very important to our present
understanding of the configurations and conformations of six-membered rings.
Substituent groups at different sites on the tetracyclic skeleton will have
axial or equatorial orientations that are fixed because of the rigid structure
of the trans-fused rings. This fixed orientation influences chemical reactivity,
largely due to the greater steric hindrance of axial groups versus their
equatorial isomers. Thus an equatorial hydroxyl group is esterified more
rapidly than its axial isomer.
Steroids are complex ethers of cyclic spirits sterols and fatty acids. Sterols are derivatives of the saturated
tetracylic hydrocarbon cyclopentanoperhydrophenanthrene
:
Cyclopentanoperhydrophenanthrene
Cholesterol
The general structure of cholesterol
consists of two six-membered rings side-by-side and sharing one side in common,
a third six-membered ring off the top corner of the right ring, and a
five-membered ring attached to the right side of that.
The central core of this molecule, consisting of four fused rings, is
shared by all steroids, including
estrogen (estradiol), progesterone, corticosteroids such as cortisol
(cortisone), aldosterone, testosterone, and Vitamin D. In the various types of
steroids, various other groups/molecules are attached around the edges. Know
how to draw the four rings that make up the central structure.
Cholesterol is not a “bad guy!” Our bodies make about
Many people have
hear the claims that egg yolk contains too much cholesterol, thus should not be
eaten.
An interesting
study was done at
A great many different steroids, each with a
distinctive function or activity, have been isolated from natural sources.
Steroids differ in the number and position of double bonds, in the type,
location, and number of substituent functional groups, in the configuration of
the bonds between the substituent groups and the nucleus, and in the
configuration of the rings in relation to each other.
Cholesterol is the most abundant steroid in animal
tissues. Cholesterol and lanosterol are members of a large subgroup of steroids
called the sterols. They are steroid alcohols containing a hydroxyl group
at carbon 3 of ring A and a branched aliphatic chain of eight or more carbon
atoms at carbon 17. They occur either as free alcohols or as long-chain fatty
acid esters of the hydroxyl group at carbon 3; all are solids at room
temperature. Cholesterol melts at
Lanosterol was first found in the waxy
coating of wool in esterified form before it was established as an important
intermediate in the biosynthesis of cholesterol in animal tissues.
Cholesterol is the precursor of many other steroids in animal tissues,
including the bile acids,
detergentlike compounds that aid in emulsification and absorption of lipids in
the intestine; the androgens, or male
sex hormones; the estrogens,
or female sex hormones; the progestational hormone progesterone; and the adrenocortical hormones. Among the most
important steroids are a group of compounds having vitamin D activity.
Waxes
Waxes are water-insoluble, solid esters of higher fatty acids with long-chain
monohydroxylic fatty alcohols or with sterols. They are soft and pliable when
warm but hard when cold. Waxes are found as protective coatings on skin, fur,
and feathers, on leaves and fruits of higher plants, and on the exoskeleton of
many insects. The major components of beeswax are palmitic acid esters of
long-chain fatty alcohols with 26 to 34 carbon atoms.
Lanolin, or wool fat, is a mixture of
fatty acid esters of the sterols lanosterol and agnosterol.
Waxes are used to coat and protect things in nature. Bees make wax. Your
ears make wax. Plant leaves even have wax on the outside of their leaves. It
can be used for structures such as the bees' honeycombs. Waxes can also be used
for protection. Plants use wax to stop evaporation of water from their leaves.
Prostaglandins Thromboxanes & Leukotrienes
The members of this group of structurally related natural hormones have an
extraordinary range of biological effects. They can lower gastric secretions,
stimulate uterine contractions, lower blood pressure, influence blood clotting
and induce asthma-like allergic responses. Because their genesis in body
tissues is tied to the metabolism of the essential fatty acid arachadonic acid
(5,8,11,14-eicosatetraenoic acid) they are classified
as eicosanoids. Many properties of
the common drug asprin result from its effect on the cascade of reactions
associated with these hormones.
The metabolic
pathways by which arachidonic acid is converted to the various eicosanoids are
complex and will not be discussed here. A rough outline of some of the
transformations that take place is provided below. It is helpful to view
arachadonic acid in the coiled conformation shown in the shaded box.
Phospholipids
Glycerophospholipids
(phosphoglycerides)
The basic structure of phospolipids is very similar to that of the
triacylglycerides except that C-3 (sn3)
of the glycerol backbone is esterified to phosphoric acid. The building block
of the phospholipids is phosphatidic acid which results when the X substitution
in the basic structure shown in the Figure below is a hydrogen atom.
Substitutions include ethanolamine (phosphatidylethanolamine), choline
(phosphatidylcholine, also called lecithins), serine (phosphatidylserine),
glycerol (phosphatidylglycerol), myo-inositol
(phosphatidylinositol, these compounds can have a variety in the numbers of
inositol alcohols that are phosphorylated generating
polyphosphatidylinositols), and phosphatidylglycerol.
Phosphoglycerides are
characteristic major components of cell membranes; only very small amounts of
phosphoglycerides occur elsewhere in cells.
Phospholipids are
made from glycerol, two fatty acids, and (in place of the third fatty acid) a phosphate group with some other
molecule attached to its other end. The hydrocarbon tails of the fatty acids
are still hydrophobic, but the phosphate group end of the molecule is
hydrophilic because of the oxygens with all of their pairs of unshared
electrons. This means that phospholipids are soluble in both water and oil.
An emulsifying agent is a
substance which is soluble in both oil and water, thus enabling the two to mix.
A “famous” phospholipid is lecithin which is found in
egg yolk and soybeans. Egg yolk is mostly water but has a lot of lipids,
especially cholesterol, which are needed by the developing chick. Lecithin is
used to emulsify the lipids and hold
them in the water as an emulsion.
Lecithin is the basis of the classic emulsion known as mayonnaise.
Our cell membranes are made
mostly of phospholipids arranged in a double layer with the tails from both
layers “inside” (facing toward each other) and the heads facing “out” (toward
the watery environment) on both surfaces.
In phosphoglycerides one of
the primary hydroxyl groups of glycerol is esterified to phosphoric acid; the
other hydroxyl groups are esterified to fatty acids. The parent compound of
the series is thus the phosphoric ester of glycerol.
Because
phosphoglycerides possess a polar head in addition to their nonpolar
hydrocarbon tails, they are called amphipathic
or polar lipids. The different types
of phosphoglycerides differ in the size, shape, and electric charge of their
polar head groups.
The parent compound of the
phosphoglycerides is phosphatidic acid,
which contains no polar alcohol head group. It occurs in only very small
amounts in cells, but it is an important intermediate in the biosynthesis of
the phosphoglycerides.
phosphatidic acid
The most abundant phosphoglycerides in higher plants and animals are phosphatidylethanoamme and phosphatidylchohne, which contain as
head groups the amino alcohols ethanoiamine
and choline, respectively. (The new
names recommended for these phosphoglycerides are ethanolamine phosphoglyceride
and choline phosphoglyceride, but they have not yet gained wide use. The old
trivial names are cephalin and lecithin, respectively.) These two
phosphoglycerides are major components of most animal cell membranes.
In phosphqtidylserine, the
hydroxyl group of the amino acid L-serine is esterified to the phosphoric acid.
Closely related to phosphatidylglycerol is the more complex lipid cardiolipin, also called diphosphatidylglycerol, which consists
of a molecule of phosphatidylglycerol in which the 3'-hydroxyl group of the
second glycerol moiety is esterified to the phosphate group of a molecule of
phosphatidic acid. The backbone of cardiolipin
thus consists of three molecules of glycerol joined by two phosphodiester
bridges; the two hydroxyl groups of both external glycerol molecules are
esterified with fatty acids. Cardiolipin
is present in large amounts in the inner membrane of mitochondria; it was first
isolated from heart muscle, in which mitochondria are abundant.
Lipid Soluble
Vitamins
The essential dietary substances called vitamins are commonly classified as "water soluble" or
"fat soluble". Water soluble vitamins, such as vitamin C, are rapidly
eliminated from the body and their dietary levels need to be relatively high.
The recommended daily allotment (RDA) of vitamin C is 100 mg, and amounts as
large as 2 to
Vitamin A 800 μg ( upper limit ca. 3000 μg)
Vitamin D 5 to 10 μg ( upper limit ca. 2000 μg)
Vitamin E 15 mg ( upper limit ca.
Vitamin K 110 μg ( upper limit not
specified)
From this data it is clear
that vitamins A and D, while essential to good health in proper amounts, can be
very toxic. Vitamin D, for example, is used as a rat poison, and in equal weight
is more than 100 times as poisonous as sodium cyanide. From the structures
shown here, it should be clear that these compounds have more than a solubility
connection with lipids. Vitamins A is a terpene, and
vitamins E and K have long terpene chains attached to an aromatic moiety. The
structure of vitamin D can be described as a steroid in which ring B is cut
open and the remaining three rings remain unchanged. The precursors of vitamins
A and D have been identified as the tetraterpene beta-carotene and the steroid
ergosterol, respectively.
Properties of phosphoglycerides
Phosphoglycerides are soluble in most nonpolar solvents containing some
water and are best extracted from cells and tissues with chloroform-methanol mixtures.
When phosphoglycerides are placed in water, they appear to dissolve, but only
very minute amounts go into true solution; most of the "dissolved"
lipid is in the form of micelles.
Phosphoglycerides have
variations in the size, shape, polarity, and electric charge and it plays a significant role in the structure of various types
of cell membranes.
Phosphoglycerides can be hydrolyzed by specific phospholipases, which have become important tools in the
determination of phosphoglyceride structure. Phospholipase A1 specifically
removes the fatty acid from the 1 position and phospholipase A2 from the 2
position. Removal of one fatty acid molecule from a phosphoglyceride yields a lysophosphoglyceride, e.g., lysophosphatidyl-ethanolamine. Lysophosphoglycerides are
intermediates in phosphoglyceride metabolism but are found in cells or tissues
in only very small amounts; in high concentrations they are toxic and injurious
to membranes. Phospholipase B can bring about successive removal of the two
fatty acids of phosphoglycerides. Phospholipase C hydrolyzes the bond between
phosphoric acid and glycerol, while phospholipase D removes the polar head
group to leave a phosphatidic acid.
Sphingophospholipids
Sphingolipids are composed of a backbone of sphingosine which is derived
itself from glycerol. Sphingosine is N-acetylated by a variety of fatty acids
generating a family of molecules referred to as ceramides. Sphingolipids
predominate in the myelin sheath of nerve fibers. Sphingomyelin is an abundant
sphingolipid generated by transfer of the phosphocholine moiety of
phosphatidylcholine to a ceramide, thus sphingomyelin is a unique form of a
phospholipid.
The other major class of sphingolipids (besides the sphingomyelins) are the
glycosphingolipids generated by substitution of carbohydrates to the sn1 carbon of the glycerol backbone of a
ceramide. There are 4 major classes of
glycosphingolipids:
Cerebrosides:
contain a single moiety, principally galactose.
Sulfatides:
sulfuric acid esters of galactocerebrosides.
Globosides:
contain 2 or more sugars.
Gangliosides:
similar to globosides except also contain sialic acid.
Sphingolipids
Sphingophospholipids, complex lipids containing as
their backbone sphingosine, are important membrane components in both plant and
animal cells. They are present in especially large amounts in brain and nerve
tissue. Only trace amounts of sphingophospholipids are found in depot fats. All
sphingophospholipids contain four characteristic building-block components: one
molecule of a fatty acid, one molecule of sphingosine, phosphoric acid and a
polar head group, which in some sphingolipids is very large and complex.
The most abundant sphingophospholipids in the tissues of higher animals are
sphingomyelins, which contain phosphorylethanolamine or phosphorylcholine as
their polar head groups, esterified to the 1-hydroxyl group of ceramide.
Sphingomyelins have physical properties very similar to those of phosphatidylethanolamine
and phosphatidylcholine.
Glycolipids
Glycosylglycerols
Glycosyldiqcylglycerols contain a sugar in glycosidic linkage with the
unesterified 3-hydroxyl group of diacylglycerols. A common example is galactosyldiacylglycerol, found in higher
plants and also in neural tissue of vertebrates.
Glycosphingolipids
Neutral glycosphingolipids
This class of glycolipids contains one or more neutral sugar residues as
their polar head groups and thus has no electric charge; they are called neutral
glycosphingolipids. The simplest of these are the cerebrosides, which contain as their polar head group a
monosaccharide bound in beta-glycosidic linkage to the hydroxyl group of ceramide. The cerebrosides of the brain
and nervous system contain D-galactose and are therefore called galactocerebrosides. Cerebrosides
are also present in much smaller amounts in nonneural tissues of animals,
where, because they usually contain D-glucose instead of D-galactose, they are
called glucocerebrosides.
Sulfate esters of galactocerebrosides (at the 3 position of the
D-galactose) are also present in brain tissue; they are called sulfotides.
The neutral glycosphingolipids
are important cell-surface components in animal tissues. Their nonpolar tails
presumably penetrate into the lipid bilayer structure of cell membranes,
whereas the polar heads protrude outward from the surface. Some of the neutral
glycosphingolipids are found on the surface of red blood cells and give them
blood-group specificity.
Acidic glycosphingolipids
(gangliosides)
Gangliosides contain in
their oligosaccharide head groups one or more residues of a sialic acid, which
gives the polar head of the gangliosides a net negative charge at pH 7.0. The
sialic acid usually found in human gangliosides is N-acetylneuraminic acid. Gangliosides are most abundant in the gray
matter of the brain, where they constitute 6 percent of the total lipids, but
small amounts are also found in nonneural tissues.
Function of glycosphingolipids
Although glycosphingolipids
are only minor constituents of membranes, they appear to be extremely
important in a number of specialized functions. Because gangliosides are
especially abundant in nerve endings, it has been suggested that they function
in the transmission of nerve impulses across synapses. They are also believed
to be present at receptor sites for acetylcholine and other neurotransmitter
substances. Some of the cell-surface glycosphingolipids are concerned not only
in blood-group specificity but also in organ and tissue specificity. These
complex lipids are also involved in tissue immunity and in cell-cell
recognition sites fundamental to the development and structure of tissues.
Cancer cells, for example, have characteristic glycosphingolipids different
from those in normal cells.
The most important derivatives of lipids
The lipids discussed up to this point contain fatty acids as building
blocks, which can be released on alkaline hydrolysis. The simple lipids
contain no fatty acids. They occur in smaller amounts in cells and tissues than
the complex lipids, but they include many substances having profound biological
activity—vitamins, hormones, and other highly specialized
fat-soluble biomolecules.
Prostaglandins are a family of fatty acid derivatives which have a variety
of potent biological activities of a hormonal or regulatory nature. Prostaglandins
function as regulators of metabolism in a number of tissues and in a number of
ways.
All the natural prostaglandins
are biologically derived by cyclization of 20-carbon unsaturated fatty acids,
such as arachidonic acid, which is formed from the essential fatty acid
linoleic acid. The prostaglandins differ from each other with respect to their
biological activity, although all show at least some activity in lowering
blood pressure and inducing smooth muscle to contract. Some, like PGE2,
antagonize the action of certain hormones. PGE2 and PGE2a may find clinical use in
inducing labor and bringing about therapeutic abortion.
Digestion of fats
By far the most common of the diet are the neutral fats, also known as triglycerides, each molecule of which is
composed of a glycerol nucleus and three fatty acids, as illustrated. Neutral
fat is found in food of both animal and and plant origin. In the usual diet are
also small quantities of phospholipids, cholesterol, and cholesterol esters.
Digestion of fats in the
intestine. A small amount of short chain triglycerides is
digested in the stomach by gastric
lipase.
Emulsification of fat by bile
acids. The first in fat digestion is
to break the fat globules into s sizes so that the water-soluble digestive
enzymes act on the globule surfaces. This process is called emulsification of the fat, and it is
achieved under the presence of bile
acids.
Bile contain a large quantity of bile salts, mainly in the form of
ionized sodium salts.
The
carboxyl and other parts of the bile salt molecule are highly soluble in water,
whereas most of the sterol portion of the bile is highly soluble in fat.
Therefore, the fat-soluble portion of the bile salt dissolves in the surface
layer of the fat globule and polar portion of the bile salt is soluble in the
surrounding fluids. This effect decreases the interfacial tension of the fat.
When the interfacial tension of a globule is low, globule is broken up into
many minute particles. The total surface area of the particles in the
intestinal contents is inversely proportional to the diameters of the
particles. The lipases are water-soluble compounds and can act on the fat globules
only on their surfaces. Consequently, it can be readily understood how
important detergent function of bile salts is for the digestion of fats.
Digestion
of fats by pancreatic lipase.
The most
important enzyme for the digestion of fats is pancreatic lipase in the pancreatic juice. However, the cells of
the small intestine also contain a minute quantity of lipase known as enteric lipase.
Products of fat digestion.
Most of the
triglycerides of the diet are split into free fatty acids and monoglycerides.
Role of bile salts in accelerating fat digestion —
formation of micelles. The hydrolysis of triglycerides highly
reversible process; therefore, accumulation of monoglycerides and free fatty
acids very quickly blocks further digestion. The bile salts play an important
role in removing the monoglycerides and free fatty acids from the vicinity of
the digesting fat globules almost as rapidly as these end-products of digestion
are formed.
This occurs in the following
way: bile salts have the propensity to form micelles, which are small spherical
globules composed of 20 to 40 molecules of bile salt. These develop because
each bile salt molecule is composed of a sterol nucleus, most of which is
highly fat-soluble, and a polar group that is highly water-soluble. The sterol nuclei of the 20 to 40 bile salt molecules of the
micelle aggregate together to form a small fat globule in the middle of the
micelle. This aggregation causes the polar groups to project outward to
cover the surface of the micelle During triglyceride digestion, as rapidly as
the monoglycerides and free fatty acids are formed they become dissolved in
the fatty portion of the micelles, which immediately reduces these end-products
of digestion in the vicinity of the digesting fat globules. The bile salt
micelles also act as a transport medium to carry the monoglycerides and the
free fatty acids, both of which would otherwise be relatively insoluble, to the
brush borders of the epithelial cells. There the monoglycerides and free fatty acids
are absorbed. On delivery of these substances to the brush border, the bile
salts are again released back into the chyme to be used again and again for
this "ferrying" process.
Digestion of Cholesterol Esters and Phospholipids.
Most of the cholesterol in the diet is in the form of cholesterol esters,
which are combinations of free cholesterol and one molecule of fatty acid. And
phospholipids also contain fatty acid chains within their molecules. Both the
cholesterol esters and the phospholipids are hydrolyzed by lipases in the
pancreatic secretion that free the fatty acids — the enzyme cholesterol ester
hydrolase to hydrolyze the cholesterol ester and phospholipase A to hydrolyze the
phospholipid.
The bile salt micelles play identically the same role in
"ferrying" free cholesterol as they play in "ferrying"
monoglycerides and free fatty acids. Indeed, this role of the bile salt
micelles is absolutely essential to the absorption of cholesterol because
essentially no cholesterol is absorbed without the presence of bile salts. On
the other hand, as much as 60 per cent of the triglycerides can be digested and
absorbed even in the absence of bile salts.
Absorption of fats
Monoglycerides and fatty acids - both of digestive end-products - become dissolved
in the lipid portion of the micelles. Because of the molecular dimension of
these micelles, only 2.5 nanometers, and also because of their highly charged,
they are soluble in the chyme. Micelles contact with the surfaces of the brush
border even penetrating into the recesses , agitating
microvilli.
The micelles then diffuse back through the chyme and absorb still more
monoglycerides and fatty acids, and similarly transport these also to the
epithelial cells. Thus, the bile acids perform a "ferrying" function,
which is highly important for fat absorption. In the presence of an abundance
of bile acids, approximately 97 per cent of the fat is absorbed; in the absence
of bile acids, only 50 to 60 per cent is normally absorbed.
The mechanism for absorption of the monoglycerides and fatty acids through
the brush border is based entirely on the fact that both these substances are
highly lipid-soluble. Therefore, they become dissolved in the membrane and
simply diffuse to the interior of the cell. The undigested triglycerides and
the diglycerides are both also highly soluble in the lipid membrane of the
epithelial cell. However, only small quantities of these are normally absorbed
because the bile acid micelles will not dissolve either triglycerides or diglycerides
and therefore will not ferry them to the epithelial membrane.
After entering the epithelial cell, the fatty
acids and monoglycerides are taken up by the smooth endoplasmic reticulum, and
here they are mainly recombined to form new triglycerides. However, a few of
the monoglycerides are further digested into glycerol and fatty acids by an
epithelial cell lipase. Then, the free fatty acids are reconstituted by the
smooth endoplasmic reticulum into triglycerides. Most of the glycerol that is
utilized for this purpose is synthesized de novo from alpha-glycerophosphate,
this synthesis requiring both energy from ATP and a complex of enzymes to
catalyze the reactions. Once formed, the triglycerides aggregate within the
endoplasmic reticulum into globules along with absorbed cholesterol, absorbed
phospholipids, and small amounts of newly synthesized cholesterol and
phospholipids. The phospholipids arrange themselves in these globules with the
fatty portion of the phospholipid toward the center and the polar portions
located on the surface.
This
provides an electrically charged surface that makes these globules miscible
with the fluids of the cell. In addition, small amounts of lipoprotein, also
synthesized by the endoplasmic reticulum, coat part of the surface of each
globule. In this form the globule diffuses to the side of the epithelial cell
and is excreted by the process of cellular exocytosis into the space between
the cells; from there it passes into the lymph in the central lacteal of the
villus. These globules are then called chylomicrons.
Transport of the Chylomicrons in the Lymph.
From the
sides of the epithelial cells the chylomicrons wend their way into the central
lac-teals of the villi and from here are propelled, along with the lymph, by the
lymphatic pump upward through the thoracic duct to be emptied into the great
veins of the neck. Between 80 and 90 per cent of all fat absorbed from the gut
is absorbed in this manner and is transported to the blood by way of the
thoracic lymph in the form of chylomicrons.
Direct
Absorption of fatty acids into the portal blood. Small quantities of short
chain fatty acids, such as those from butterfat, are absorbed directly into
the portal blood rather than being converted into triglycerides and absorbed
into the lymphatics. The cause of this difference between short and long chain
fatty acid absorption is that the shorter chain fatty acids are more
water-soluble and are not reconverted into triglycerides by the endoplasmic
reticulum. This allows direct diffusion of these fatty acids from the epithelial
cells into the capillary blood of the dlood.
Transport forms of lipids
Certain lipids associate with
specific proteins to form lipoprotein systems in which the specific physical
properties of these two classes of biomolecules are blended. In these systems
the lipids and proteins are not covalently joined but are held together largely
by hydrophobic interactions between the nonpolar portions of the lipid and the
protein components.
Transport
lipoproteins of blood plasma.
The plasma
lipoproteins are complexes in which the lipids and proteins occur in a
relatively fixed ratio. They carry water-insoluble lipids between various
organs via the blood, in a form with a relatively small and constant particle
diameter and weight. Human plasma lipoproteins occur in four major classes that
differ in density as well as particle size. They are physically distinguished
by their relative rates of flotation in high gravitational fields in the
ultracentrifuge.
The blood lipoproteins serve to transport water-insoluble
triacylglycerols and cholesterol from one tissue to another. The major carriers
of triacylglyeerols are chylomicrons
and very low density lipoproteins (VLDL).
The triacylglycerols of the chylomicrons and VLDL are
digested in capillaries by lipoprotein lipase. The fatty acids that are
produced are utilized for energy or converted to triacylglycerols and stored.
The glycerol is used for triacylglycerol synthesis or converted to DHAP and
oxidized for energy, either directly or after conversion to glucose in the
liver. The remnants of the chylomicrons are taken up by liver cells by the
process of endocytosis and are degraded by lysosomal enzymes, and the products
are reused by the cell.
VLDL is converted to intermediate
density lipoproteins (IDL), which is
degraded by the liver or converted in blood capillaries to low density lipoproteins LDL by further digestion of
triacylglycerols.
LDL is taken up by various tissues and provides
cholesterol, which the tissue utilize.
High density
lipoproteins (HDL) which is synthesized by the liver,
transfers apoproteins to ehylomicrons and VLDL.
HDL picks up cholesterol from cell membranes or from other
lipoproteins. Cholesterol is converted to cholesterol esters by the
lecithin:cholesterol acyltransferase (LCAT) reaction. The cholesterol esters
may be transferred to other lipoproteins or carried by HDL to the liver, where
they are hydrolyzed to free cholesterol, which is used for synthesis of VLDL or
converted to bile salts.
Composition of the blood lipoproteins
The major components of
lipoproteins are triacylglycerols, cholesterol, cholesterol esters,
phospholipids, and proteins. Purified proteins (apoproteins) are designated A,
B, C, and E.
Chylomicrons are the least dense of the
blood lipoproteins because they have the most triacylglycerol and the least
protein.
VLDL is more
dense than chylomicrons but
still has a high content of triacylglycerol.
IDL, which is
derived from VLDL, is more dense than chylomicrons but still has a high content of
triacylglycerol.
LDL has less triacylglycerol and
more protein and, therefore, is more dense than the IDL
from which it is derived. LDL has the highest content of cholesterol and its
esters.
HDL is the most
dense lipoprotein. It has the lowest triacylglycerol and the highest
protein content.
Metabolism of Chylomicrons
Chylomicrons
are synthesized in intestinal
epithelial cells. Their triacylglycerols are derived from dietary lipid, and
their major apoprotein is apo B-48.Chylomicrons travel through the lymph into
the blood. In peripheral tissues,
particularly adipose and muscle, the triacylglyerols are digested by lipoprotein
lipase.The chylomicron remnants interact with receptors on liver cells and
are taken+ up by endocytosis. The contents are degraded by lysosomal
enzymes, and the products (amino acids, fatty acids, glycerol, and cholesterol)
are released into the cytosol and reutilized.
Metabolism of VLDL
VLDL is synthesized in the liver, particularly after a
high-carbohydrate meal. It is formed from triacylglycerols that are package
with cholesterol, apoproteins (particularly apo B-100), and phospholipids and
it is released into the blood.
In peripheral tissues,
particularly adipose and muscle, VLDL triacylglycerols are digested by
lipoprotein lipase, and VLDL is converted to IDL.
IDL returns to the liver, is
taken up by endocytosis, and is degraded by lysosomal enzymes.
IDL may also be further
degraded by lipoprotein lipase, forming LDL.
LDL reacts with receptors on various
cells, is taken up by endocytosis and is digested by lysosomal enzymes.
Cholesterol, released from cholesterol
esters by a lysosomal esterase, can be used for the synthesis of cell
memmbranes or bile salts in the liver or steroid hormones in endocrine tissue.
Metabolism of HDL.
HDL is synthesized by the liver and released into the blood as disk-shaped
particles. The major protein of HDL is apo A.
HDL cholesterol, obtained from cell membranes or from other lipoproteins,
is converted to cholesterol esters. As cholesterol esters accumulate in the
core of the lipoprotein, HDL particles become spheroids.
HDL particles are taken up by the liver by endocytosis and hydrolyzed by
lysosomal enzymes. Cholesterol, released from cholesterol esters may be
packaged by the liver in VLDL and released into the blood or converted to bile
salts and secreted into the bile.
Catabolism of triacylglycerols
Dietary
triacylglycerols undergo hydrolysis in the small intestine by the action of
lipases, e.g., those present in pancreatic juice. Lipase digests the
triacylglycerols to 2-monoglycerols, glycerol and free fatty acids. These
components are absorbed and metabolized in the enterocytes, blood and liver. In
the enterocytes and liver the specific for organism acylglycerols are
synthesized. Then these are accumulated in adipose tissue and in much smaller quantity in other organs.
Fermentative hydrolysis of in
adipocytes and other cells is implemented in several stages. Diacylglycerols,
monoacylglycerols, glycerol and free fatty acids are formed in this process:
OXIDATION OF FATTY ACIDS
Fatty acids
play an extremely important part as an energy-rich fuel in higher animals and
plants since large amounts can be stored in cells in the form of
triacylglycerols. Triacylglycerols are especially well adapted for this role
because they have a high energy content (about 9 kcal/g) and can be accumulated
in nearly anhydrous form as intracellular fat droplets. In contrast, glycogen
and starch can yield only about 4 kcal/g; moreover,
since they are highly hydrated, they cannot be stored in such concentrated form. Fatty
acids provide up to 40 percent of the total fuel requirement in man on a normal
diet.
Sources of Fatty Acids.
Mammalian tissues normally
contain only vanishingly small amounts of free fatty acids, which are in fact
somewhat toxic. By the action of hormonally
controlled lipases free fatty acids are formed from triacylglycerols in fat or
adipose tissue. The free fatty acids are then released from the tissue, become
tightly bound to serum albumin, and in this form are carried via the blood to
other tissues for oxidation. Fatty acids delivered in this manner are first
enzymatically "activated" in the cytoplasm and then enter the
mitochondria for oxidation.
Long-chain fatty acids are
oxidized to CO2 and H2O in nearly all tissues of
vertebrates except the brain. Some tissues, such as heart muscle, obtain most
of their energy from the oxidation of fatty acids. The mobilization,
distribution, and oxidation of fatty acids are integrated with the utilization
of carbohydrate fuels; both are under complex endocrine regulation.
The pathway of fatty acid
oxidation.
Knoop
postulated that fatty acids are oxidized by b-oxidation, i.e., oxidation at the b carbon to
yield a b-keto acid,
which was assumed to undergo cleavage to form acetic acid and a fatty acid shorter
by two carbon atoms.
Outline of the fatty acid oxidation cycle.
Before
oxidation, long-chain fatty acids from the cytosol must undergo a rather
complex enzymatic activation, followed by transport across the mitochondrial membranes
into the major compartment. There the fatty acyl group is transferred to
intramitochondrial coenzyme A, yielding a fatty acyl-CoA thioester. The
subsequent oxidation of the fatty acyl-CoA takes place entirely in the
mitochondrial matrix.
The fatty acyl-CoA is
dehydrogenated by removal of a pair of hydrogen atoms from the a and b carbon atoms (atoms 2 and 3)
to yield the a,b-unsaturated acyl-CoA. This is
then enzymatically hydrated to form a b-hydroxyacyl-CoA, which in
turn is dehydrogenated in the next step to yield the b-ketoacyl-CoA. It then
undergoes enzymatic cleavage by reaction with a second molecule of CoA. One
product is acetyl-CoA, derived from carbon atoms 1 and 2 of the original fatty
acid chain. The other product, a long-chain saturated fatty acyl-CoA having two
fewer carbon atoms than the original fatty acid, now becomes the substrate for
another round of reactions, beginning at the first dehydrogenation step and
ending with the removal of a second two-carbon fragment as acetyl-CoA. At each
passage through this spiral the fatty acid chain loses a two-carbon fragment as
acetyl-CoA. The 16-carbon palmitic acid thus undergoes a total of seven such
cycles, to yield altogether 8 molecules of acetyl-CoA and 14 pairs of hydrogen
atoms. The palmitate must be primed or activated only once, since at the end of
each round the shortened fatty acid appears as its CoA thioester.
The hydrogen atoms removed during the dehydrogenation of the fatty acid
enter the respiratory chain; as electrons pass to molecular oxygen via the
cytochrome system, oxidative phosphorylation of ADP to ATP occurs. The
acetyl-CoA formed as product of the fatty acid oxidation system enters the
tricarboxylic acid cycle.
Activation and entry of fatty acids
into mitochondria.
There are three stages in the entry of fatty acids into mitochondria from
the extramitochondrial cytoplasm: (1) the enzymatic ATP-driven esterification
of the free fatty acid with extramitochondrial CoA to yield fatty acyl-CoA, a step
often referred to as the activation of the fatty acid, (2) the transfer of the
acyl group from the fatty acyl-CoA to the carrier molecule carnitine, followed
by the transport of the acyl carnitine across the inner membrane, and (3) the
transfer of the acyl group from fatty acyl carnitine to intramitochondrial CoA.
Activation of fatty acids.
At least three different
enzymes catalyze formation of acyl-CoA thioesters, each being specific for a
given range of fatty acid chain length. These enzymes are called acyl-CoA
synthetases. Acetyl-CoA synthetase activates acetic, propionic, and acrylic
acids, medium-chain acyl-CoA synthetase activates fatty acids with 4 to
12 carbon atoms, and long-chain acyI-CoA synthetase activates fatty acids with
12 to 22 or more carbon atoms. The last two enzymes activate both saturated
and unsaturated fatty acids. Otherwise the properties and mechanisms of all
three synthetases, which have been isolated in highly purified form, are nearly
identical. The overall reaction catalyzed by the ATP-linked acyl-CoA
synthetases is:
RCOOH + ATP + CoA–SH Û RCO—S—CoA + AMP + PP
Fatty acids
acyl-CoA
In this reaction a thioester linkage is formed between the fatty acid
carboxyl group and the thiol group of CoA; the ATP undergoes pyrophosphate
cleavage to yield AMP and inorganic pyrophosphate.
The acyl-CoA synthetases are found in the outer mitochondrial membrane and
in the endoplasmic reticulum.
Transfer to carnitine.
Long-chain saturated fatty acids
have only a limited ability to cross the inner membrane as CoA thioesters, but
their entry is greatly stimulated by carnitine.
The
stimulation of fatty acid oxidation by carnitine is due to the action of an
enzyme carnitine acyltransferase,
which catalyzes transfer of the fatty acyl group from its thioester linkage
with CoA to an oxygen-ester linkage with the hydroxyl group of carnitine. The
acyl carnitine ester so formed then passes through the inner membrane into the
matrix, presumably via a specific transport system.
Transfer to intramitochondrial CoA.
In the last stage of the entry process the acyl group is transferred from
carnitine to intramitochondrial CoA by the action of a second type of carnitine acyltransferase located on the
inner surface of the inner membrane:
Acyl carnitine + CoA Û acyl-CoA + carnitine
This complex entry mechanism,
often called the fatty acid shuttle, has the effect of keeping the
extramitochondrial and intramitochondrial pools of CoA and of fatty acids separated.
The intramitochondrial fatty acyl-CoA now becomes the substrate of the fatty
acid oxidation system, which is situated in the inner matrix compartment.
The first dehydrogenation step
in fatty acid oxidation.
Following the formation of intramitochondrial acyl-CoA, all subsequent
reactions of the fatty acid oxidation cycle take place in the inner
compartment. In the first step the fatty acyl-CoA thioester undergoes enzymatic
dehydrogenation by acyl-CoA dehydrogenase
at the a and b carbon atoms (carbons 2 and
3) to form enoyl-CoA as product. The double bond formed in this reaction has
the trans geometrical configuration. Recall, however,
that the double bonds of the unsaturated fatty acids of natural fats nearly always
have the configuration.
There are four different acyl-CoA dehydrogenases, each specific for a given
range of fatty acid chain lengths. All contain tightly bound flavin adenine
dinucleotide (FAD) as prosthetic groups. The FAD becomes reduced at the expense
of the substrate, a process that probably occurs through distinct one-electron
steps.
The FADH2 of the reduced acyl-CoA dehydrogenase cannot react
directly with oxygen but donates its electrons to the respiratory chain via a second
flavoprotein, electron-transferring flavoprotein, which in turn passes the
electrons to some carrier of the respiratory chain.
The hydration step.
The double
bond of the enoyl-CoA ester is then hydrated to form 3-hydroxyacyl-CoA by the enzyme enoyl-CoA
hydratase.
The
addition of water across the trans double bond is
stereo-specific and results in the formation of the L-stereoisomer of the
3-hydroxyacyl-CoA.
The second
dehydrogenation step.
In the next
step of the fatty acid oxidation cycle, the 3-hydroxyacyl-CoA is dehydrogenated
to form 3-ketoacyl-CoA) by 3-hydroxyacyl-CoA dehydrogenase. NAD+
is the specific electron acceptor. The reaction is:
This enzyme is
relatively nonspecific with respect to the length of the fatty acid chain but
is absolutely specific for the l stereoisomer.
The NADH formed in the reaction donates its electron equivalents to the NADH
dehydrogenase of the mitochondrial respiratory chain.
The cleavage step.
In the last step of the fatty acid
oxidation cycle, which is catalyzed by acetyl-CoA
acetyltransferase, more commonly
known as thiolase, the 3-ketoacyl-CoA
undergoes cleavage by interaction with a molecule of free CoA to yield the
carboxyl-terminal two-carbon fragment of the fatty acid as acetyl-CoA. The
remaining fatty acid, now shorter by two carbon atoms, appears as its coenzyme
A thioester.
This cleavage reaction, also
called a thiolysis or a thiolytic cleavage, is analogous to hydrolysis. Since
the reaction is highly exergonic, cleavage is favored. There appear to be two
(perhaps three) forms of the enzyme, each specific for different fatty acid
chain lengths.
The balance sheet.
We have described one turn of
the fatty acid oxidation cycle, in which one molecule of acetyl-CoA and two
pairs of hydrogen atoms have been removed from the starting long-chain fatty
acyl-CoA. The overall equation for one turn of the cycle, starting from
palmitoyl-CoA, is
Palmitoyl-CoA
+ CoA + FAD+ + NAD+
+ H2O ®
myristoyl-CoA
+ acetyl-CoA + FADH2 + NADH2
We can now write the equation for the seven turns of the cycle required to
convert one molecule of palmitoyl-CoA into eight molecules of acetyl-CoA:
Palmitoyl-CoA
+ 7CoA + 7FAD+ + 7NAD+ + 7H2O ®
8
acetyl-CoA + 7FADH2 + 7NADH2 + 7H+
Each molecule of FADH2 donates a pair of electron equivalents
to the respiratory chain at the level of coenzyme Q; thus two molecules of ATP
are generated during the ensuing electron transport to oxygen. Similarly,
oxidation of each molecule of NADH2 by the respiratory chain results
in formation of three molecules of ATP. Hence,
a total of five molecules of ATP is formed by
oxidative phosphorylation per molecule of acetyl-CoA cleaved.
The seven turns of the cycle
required to convert one molecule of palmitoyl-CoA rsults in the formation of 5
x 7 = 35 ATP.
The eight molecules of
acetyl-CoA formed in the fatty acid cycle may now enter the tricarboxylic acid
cycle. The degradation of 1 molecule of acetyl-CoA in tricarboxylic acid cycle
results in the formation of 12 molecules of ATP. 8 molecules of acetyl-CoA give
96 molecules of ATP.
Thus, the total output of
energy in full cleavage of 1 molecule of palmitoyl-CoA is: 35 + 96 = 131
molecules of ATP.
Since one molecule of ATP is in effect utilized to form palmitoyl-CoA from
palmitate, the net yield of ATP per molecule of palmitate is 130.
Oxidation of unsaturated fatty
acids.
Unsaturated
fatty acids, such as oleic acid, are oxidized by the same general pathway as
saturated fatty acids, but two special problems arise. The double bonds of
naturally occurring unsaturated fatty acids are in the cis configuration,
whereas the unsaturated acyl-CoA intermediates in the oxidation of saturated
fatty acids are trans, as we have seen. Moreover, the
double bonds of most unsaturated fatty acids occur at such positions in the
carbon chain that successive removal of two-carbon fragments from the carboxyl
end yields a D3-unsaturated
fatty acyl-CoA rather than the D2
fatty acyl-CoA serving as the normal intermediate in the fatty acid cycle.
These problems have been
resolved with the discovery of an auxiliary enzyme, enoyl-CoA isomerase,
which catalyzes a reversible shift of the double bond from the D3-cis to the D2-trans configuration. The
resulting D2-trans-unsaturated fatty
acyl-CoA is the normal substrate for the next enzyme of the fatty acid
oxidation sequence, enoyl-CoA hydratase,
which hydrates it to form L-3-hydroxyacyl-CoA. The complete oxidation of
oleyl-CoA to nine acetyl-CoA units by the fatty acid oxidation cycle thus
requires an extra enzymatic step catalyzed by the enoyl-CoA isomerase, in addition to those steps required in the
oxidation of saturated fatty acids.
Polyunsaturated fatty acids,
such as linoleic acid, require a second auxiliary enzyme to complete their
oxidation, since they contain two or more cis
double bonds. When three successive acetyl-CoA units are removed from
linoleyl-CoA, a D3-cis double bond remains, as
in the case of oleyl-CoA. This is then transformed by the enoyl-CoA isomerase
described above to the D2-trans isomer. This undergoes
the usual reactions, with loss of two acetyl-CoA's, leaving an eight-carbon D2-unsaturated acid. Note,
however that the double bond of the latter is in the cis configuration.
Although the D2-cis double bond can be
hydrated by enoyl-CoA hydratase, the product is the D stereoisomer of a
3-hydroxyacyl-CoA, not the L stereoisomer normally formed during oxidation of
saturated fatty acids. Utilization of the d
stereoisomer requires a second auxiliary enzyme, 3-hydroxyacyl-CoA
epimerase, which catalyzes epi-merization at carbon atom 3 to yield the l isomer. The product of this
reversible reaction is then oxidized by the L-specific 3-hydroxyacyl-CoA
dehydrogenase and cleaved by thiolase to complete the oxidation cycle. The
remaining six-carbon saturated fatty acyl-CoA derived from linoleic acid can
now be oxidized to three molecules of acetyl-CoA. These two auxiliary enzymes
of the fatty acid oxidation cycle make possible the complete oxidation of all
the common unsaturated fatty acids found in naturally occurring lipids. The
number of ATP molecules yielded during the complete oxidation of an unsaturated
fatty acid is somewhat lower than for the corresponding saturated fatty acid
since unsaturated fatty acids have fewer hydrogen atoms and thus fewer
electrons to be transferred via the respiratory chain to oxygen.
Oxidation of odd-carbon fatty acids and the fate of
propionyl-CoA
Odd-carbon fatty acids, which are rare but do occur in some marine organisms,
can also be oxidized in the fatty acid oxidation cycle. Successive acetyl-CoA
residues are removed until the terminal three-carbon residue pro-pionyl-CoA is
reached. This compound is also formed in the oxidative degradation of the amino
acids valine and isoleucine. Propionyl-CoA undergoes enzymatic carboxylation in
an ATP-dependent process to form Ds-methylmaionyl-CoA, a reaction
catalyzed by propionyl-CoA corboxylase. This enzyme contains biotin as its
prosthetic group. In the next step Ds-methylmalonyl-CoA undergoes
enzymatic epimerization to LR-methylmalonyl-CoA, by action of
methyimaionyl-CoA racemase. In the next reaction step, catalyzed by methylmalonyl-CoA mutase, LR-methylmalonyl-CoA
is isomerized to succinyl-CoA, which may then undergo deacylation by reversal
of the succinyl-CoA synthetase reaction to yield free succinate, an
intermediate of the tricarboxylic acid cycle.
Methylmalonyl-CoA mutase
requires as cofactor coenzyme B12.
Study of this intramolecular reaction with isotope tracers has revealed that
it takes place by the migration of the entire —CO—S—CoA group from carbon atom
2 of methylmalonyl-CoA to the methyl carbon atom in exchange for a hydrogen
atom.
Patients suffering from
pernicious anemia, who are deficient in vitamin B12 because of
their lack of intrinsic factor, excrete large amounts of methylmalonic acid and
its precursor propionic acid in the urine, showing that in such patients the
coenzyme B12-dependent methylmalonyl-CoA mutase reaction is
defective.
OXIDATION OF GLYCEROL
Glycerol formed in cleavage of tryacylglycerols enter catabolism or use for
biosynthesis of glycerides again. Before including of glycerol in metabolism it
is activated by ATP to glycerol-3-phosphate by action of glycerol phosphokinase:
Glycerol Glycerol-3-phosphate
Glycerol-3-phosphate is
oxidized by glycerophosphate
dehydrogenase and glyceroaldehyde-3-phosphate is produced:
Glycerol-3-phosphate Glyceroaldehyde-3-phosphate
Glyceroaldehyde-3-phosphate is the central metabolite of glycolysis.
The biosynthesis of lipids is a prominent metabolic process in most
organisms. Because of the limited capacity of higher animals to store
polysaccharides, glucose ingested in excess of immediate energy needs and
storage capacity is converted by glycolysis into pyruvate and then acetyl-CoA,
from which fatty acids are synthesized. These in turn are converted into
triacylglycerols, which have a much higher energy
content than polysaccharides and may be stored in very large amounts in adipose
or fat tissues. Triacylglycerols are also stored in the seeds and fruits of
many plants.
The formation of the various phospholipids and sphingolipids of cell membranes
is also an important biosynthetic process. These complex lipids undergo continuous
metabolic turnover in most cells.
Biosynthesis of saturated fatty acids
The biosynthesis of saturated fatty acids from their ultimate precursor
acetyl-CoA occurs in all organisms but is particularly prominent in the liver,
adipose tissues, and mammary glands of higher animals. It is brought about by a
process that differs significantly from the opposed process of fatty acid
oxidation. In the first place total biosynthesis of fatty acids occurs in the
cytosol, whereas fatty acid oxidation occurs in the mitochondria. Second, the
presence of citrate is necessary for maximal rates of synthesis of fatty acids,
whereas it is not required in fatty acid oxidation. Perhaps the most
unexpected difference is that CO2 is essential for fatty acid
synthesis in cell extracts, although isotopic CO2 is not itself
incorporated into the newly synthesized fatty acids. These and many other
observations have revealed that fatty acid synthesis from acetyl-CoA takes
place with an entirely different set of enzymes from those employed in fatty
acid oxidation.
In the overall reaction of
fatty acid synthesis, which is catalyzed by a complex multienzyme system in the
cytosol, the fatty-acid synthetase
complex, acetyl-CoA derived from carbohydrate or amino acid sources
is the ultimate precursor of all the carbon atoms of the fatty acid chain.
However, of the eight acetyl units required for biosynthesis of palmitic acid, only
one is provided by acetyl-CoA; the other seven arrive in the form of
malonyl-CoA, formed from acetyl-CoA and HCO3- in a
carboxylation reaction. One acetyl residue and seven malonyl residues undergo
successive condensation steps, with release of seven molecules of CO2,
to form palmitic acid; the reducing power is furnished by NADPH:
Acetyl-CoA + 7 malonyl-CoA +
14NADPH + 14H+ ®
CH3(CH2)14COOH
+ 7CO2 + 8CoA + 14NADP+ + 6H2O
Palmitic acid
The single molecule of acetyl-CoA required in the process serves as a
primer, or starter; the two carbon atoms of its acetyl group become the two
terminal carbon atoms (15 and 16) of the palmitic acid formed. Chain growth
during fatty acid synthesis thus starts at the carboxyl group of acetyl-CoA and
proceeds by successive addition of acetyl residues at the carboxyl end of the
growing chain. Each successive acetyl residue is derived from two of the three
carbon atoms of a malonic acid residue entering the system in the form of
malonyl-CoA; the third carbon atom of malonic acid, i.e., that of the
unesterified carboxyl group, is lost as CO2. The final product is a
molecule of palmitic acid.
A distinctive feature of the
mechanism of fatty acid biosynthesis is that the acyl intermediates in the
process of chain lengthening are esters, not of CoA, as in fatty acid
oxidation, but of a low-molecular-weight conjugated protein called acyl carrier protein (ACP). This protein
can form a complex or complexes with the six other enzyme proteins required for
the complete synthesis of palmitic acid. In most eukariotic cells all seven
proteins of the fatty acid synthetase complex are associated in a multienzymes
cluster.
In most organisms the end
product of the fatty-acid synthetase system is palmitic acid, the precursor of
all other higher saturated fatty acids and of all unsaturated fatty acids.
The carbon source for fatty acid synthesis
The ultimate source of all the
carbon atoms of fatty acids is acetyl-CoA, formed in the mitochondria by the
oxidative decarboxylation of pyruvate, the oxidative degradation of some of
the amino acids, or by the b-oxidation of long-chain fatty
acids.
Acetyl-CoA itself cannot pass out of the mitochondria into the cytosol;
however, its acetyl group is transferred through the membrane in other chemical
forms. Citrate, formed in mitochondria from acetyl-CoA and oxaloacetate, may
pass through the mitochondrial membrane to the cytoplasm via the tricarboxylate
transport system. In the cytosol acetyl-CoA is regenerated from citrate by ATP-citrate lyase, also called citrate cleavage enzyme, which catalyzes
the reaction:
Citrate + ATP + CoA ® acetyl-CoA + ADP + P +
oxaloacetate
In a second pathway the acetyl
group of acetyl-CoA is enzymatically transferred to carnitine, which acts as a
carrier of fatty acids into mitochondria preparatory to their oxidation.
Acetylcarnitine passes from the mitochondrial matrix through the mitochondrial
membrane into the cytosol; acetyl-CoA is then regenerated by transfer of the
acetyl group from acetylcarnitine to cytosol CoA.
Formation of malonyl-CoA:
acetyl-CoA carboxylase
Before the acetyl groups of acetyl-CoA can be utilized by the fatty-acid
synthetase complex, an important preparatory reaction must take place to
convert acetyl-CoA into malonyl-CoA, the immediate precursor of 14 of the 16
carbon atoms of palmitic acid. Malonyl-CoA is formed from acetyl-CoA and
bicarbonate in the cytosol by the action of acetyl-CoA
carboxylase, a complex enzyme that catalyzes the reaction:
Acetyl-CoA
Malonyl-CoA
The carbon atom of the CO2
becomes the distal or free carboxyl carbon of malonyl-CoA. However, the above equation give only the overall reaction, the sum of at least
three intermediate reactions.
Acetyl-CoA carboxylase
contains biotin as its prosthetic
group. The carboxyl group of biotin is bound in amide linkage to the e-amino group of a specific
lysine residue of a subunit of the enzyme. The covalently bound biotin serves
as an intermediate carrier of a molecule of CO2.
The reaction catalyzed by
acetyl-CoA carboxylase, an allosteric enzyme, is the primary regulatory,
or rate-limiting, step in the biosynthesis of fatty acids. Acetyl-CoA
carboxylase is virtually inactive in the absence of its positive modulators
citrate or isocitrate. The striking allosteric stimulation of this enzyme by
citrate accounts for the fact that citrate is required for fatty acid synthesis
in cell extracts without being used as a precursor.
Acetyl-CoA carboxylase occurs in both an inactive monomeric form and an
active polymeric form. As it occurs in the avian liver, the inactive enzyme
monomer has a molecular weight of 410,000 and contains one binding site for CO2
(that is, one biotin prosthetic group), one binding site for acetyl-CoA,
and one for citrate. Citrate shifts the equilibrium between the inactive
monomer and the active polymer, to favor the latter.
Polymeric acetyl-CoA carboxylase of animal tissues consists of long
filaments of enzyme monomers; each monomer unit contains a molecule of bound
citrate. The length of the polymeric form varies, but on the average each
filament contains about 20 monomer units, has a particle weight of some 8
megadaltons, and is about 400 nm long. Such filaments have been studied in the
electron microscope and have actually been observed in the cytoplasm of adipose
cells.
The acetyl-CoA carboxylase
reaction is complex. In fact, the monomeric unit of the enzyme contains four
different subunits. The sequence of reactions in the formation of malonyl-CoA
has been deduced from study of the four subunits of the monomer. One of these
subunits, biotin carboxylase (BC),
catalyzes the first step of the overall reaction, namely, the carboxylation of
the biotin residue covalently bound to the second subunit, which is called biotin
carboxyl-carrier protein (BCCP). The second step in the overall reaction is
catalyzed by the third type of subunit, called carboxyl transferase (CT). In these reactions the biotin residue of
the carboxyl carrier protein serves as a swinging arm to transfer the
bicarbonate ion from the biotin carboxylase subunit to the acetyl-CoA bound to
the active site of the carboxyltransferase subunit. The change from the
inactive monomeric form of acetyl-CoA carboxylase to the polymeric, active form
of the enzyme occurs when citrate is bound to the fourth subunit of each
monomeric unit.
The reactions of the fatty-acid synthetase system
Once malonyl-CoA has been generated from acetyl-CoA by
the action of the rather complex acetyl-CoA carboxylase reaction, the ensuing
reactions of fatty acid synthesis occur in a sequence of six successive steps
catalyzed by the six enzymes of the fatty-acid synthetase system. The seventh
protein of this system, which has no enzymatic activity itself, is acyl
carrier protein, to which the growing fatty acid chain is covalently attached.
Acyl carrier protein (ACP)
Acyl carrier protein, universally symbolized as ACP, was first isolated in pure
form from E. coli and has since been studied from many other sources. The E.
coli ACP is a relatively small (mol wt 10,000), heat-stable protein containing
77 amino acid residues, whose sequence has been established, and a covalently
attached prosthetic group.
The single sulfhydryl group of ACP, to which the acyl intermediates are
esterified, is contributed by its prosthetic group, a molecule of 4'-phosphopantetheine, which is covalently
linked to the hydroxyl group of serine residue 36 of the protein. The
4'-phosphopantetheme moiety is identical with that of coenzyme A, from which it
is derived. The function of ACP in fatty acid synthesis is analogous to that of
CoA in fatty acid oxidation: it serves as an anchor to which the acyl
intermediates are esterified.
The priming reaction
To prime the fatty-acid
synthetase system, acetyl-CoA first reacts with the sulfhydryl group of ACP by
the action of one of the six enzymes of the synthetase system, ACP-acyltransferase, which catalyzes the
reaction:
The malonyl transfer step
In the next reaction,
catalyzed by ACP malonyltransferase,
malonyl-S-CoA formed in the acetyl-CoA carboxylase reaction reacts with the
—SH group of the 4'-phosphopantetheine arm of ACP, with loss of free CoA, to
form malonyl-S-ACP:
Malonyl—S—CoA + ACP—SH Û malonyl—S—ACP + CoA—SH
As a result of this step and
of the preceding priming reaction, a malonyl group is now esterified to ACP and
an acetyl group is esterified to an —SH group on the
ACP molecule.
The condensation reaction
In the next reaction of the sequence, catalyzed by b-ketoacyl-ACP synthase, the acetyl group esterified
to the cysteine residue is transferred to carbon atom 2 of the malonyl group
on ACP, with release of the free carboxyl group of the malonyl residue as CO2:
Study of the reaction
equilibrium has revealed the probable basis for the biological selection of
malonyl-CoA as the precursor of two-carbon residues for fatty acid synthesis.
If acetoacetyl-CoA were to be formed from two molecules of acetyl-CoA by the
action of acetyl-CoA acetyltransferase, the reaction would be endergonic, with
its equilibrium lying to the left.
Acetyl—S—CoA + acetyl—S—CoA Û acetoacetyl—S—CoA + CoA—SH
The first reduction reaction
The acetoacetyl-S-ACP now undergoes reduction by NADPH to form b-hydroxybutyryl-S-ACP. This reaction is catalyzed by
b-ketoacyl-ACP
reductase:
The dehydration step
b-Hydroxybutyryl-S-ACP is next
dehydrated to the corresponding unsaturated acyl-S-ACP, namely,
crotonyl-S-ACP, by b-hydroxyacyl—ACP-dehydratase:
The second
reduction step
Crotonyl-S-ACP is now reduced to butyryl-S-ACP by enoil-ACP reductase (NADPH);
the electron donor is NADPH in animal tissues:
Crotonyl-S-ACP Butyryl-S-ACP
This reaction also differs
from the corresponding reaction of fatty acid oxidation in mitochondria in that
a pyridine nucleotide rather than a flavoprotein is involved. Since the
NADPH-NADP+ couple has a more negative standard potential than the
fatty acid oxidizing flavoprotein, NADPH favors reductive formation of the saturated
fatty acid.
The
formation of butyryl-ACP completes
the first of seven cycles en route to palmitoyl-S-ACP.
To start the next cycle the butyryl group is transferred from —SH group of
phosphopantetheine to the —SH group of cysteine, thus allowing —SH group of ACP
phosphopantetheine to accept a malonyl group from another molecule of
malonyl-CoA.
Then the cycle repeats, the
next step being the condensation of malonyl-S-ACP with butyryl-S-ACP to yield b-ketohexanoyl-S-ACP and CO2.
After seven complete cycles, palmitoyl-ACP is the end product. The
palmitoyl group may be removed to yield free palmitic acid by the action of a thioesterase, or it may be transferred
from ACP to CoA, or it may be incorporated directly into phosphatidic acid in
the pathway to phospholipids and triacylglycerols.
It is remarkable that in most
organisms the fatty-acid synthetase system stops with the production of
palmitic acid and does not yield stearic acid, which has only two more carbon
atoms than palmitic acid and thus does not differ greatly in physical
properties.
Saturated fatty acids having
an odd number of carbon atoms, which are found in many marine organisms, are
also made by the fatty-acid synthetase complex. In this case the synthesis is
primed by a starter molecule of propionyl-S-ACP (instead of acetyl-S-ACP), to
which are added successive two-carbon units via condensations with
malonyl-S-ACP.
We can now write the overall
equation for palmitic acid biosynthesis starting from acetyl-S-CoA:
8 Acetyl—S—CoA + 14NADPH + 14H+
+ 7ATP + H2O ®
palmitic acid + 8CoA + 14NADP+ + 7ADP + 7P.
The 14 molecules of NADPH required
for the reductive steps in the synthesis of palmitic acid arise largely from
the NADP-dependent oxidation of glucose
6-phosphate via the phosphogluconate
pathway. Liver, mammary gland, and adipose tissue of vertebrates, which have a
rather high rate of fatty acid biosynthesis, also have a very active
6-phosphogluconate pathway.
The enzymatic steps leading to the biosynthesis of palmitic acid differ
from those involved in oxidation of palmitic acid in the following respects:
1. Their intracellular location.
2. The type of acyl-group carrier.
3. The form in which the two-carbon units are added or removed.
4. The pyridine nucleotide specificity of the b-ketoacyl-b-hydroxyacyl reaction.
5. The stereoisomeric configuration of the b-hydroxyacyl intermediate.
6. The electron donor-acceptor system for the crotonyl-butyryl step.
These differences illustrate how two opposing metabolic processes may
proceed independently of each other in the cell.
Elongation of saturated
fatty acids in mitochondria and microsomes
Palmitic acid, the normal end product of
the fatty-acid synthetase system, is the precursor of the other long-chain
saturated and unsaturated fatty acids in most organisms. Elongation of
palmitic acid to longer-chain saturated fatty acids, of which stearic acid is most abundant, occurs by
the action of two different types of enzyme systems, one in the mitochondria
and the other in the endoplasmic reticulum.
In mitochondria palmitic and
other saturated fatty acids are lengthened by successive additions to the
carboxyl-terminal end of acetyl units in the form of acetyl-CoA; malonyl-ACP
cannot replace acetyl-CoA. The mitochondrial elongation pathway occurs by
reactions similar to those in fatty acid oxidation. Condensation of
palmityl-CoA with acetyl-CoA yields b-ketostearyl-CoA, which is
reduced by NADPH to b-hydroxystearyl-CoA. The
latter is dehydrated to the unsaturated stearyl-CoA, which is then reduced to
yield stearyl-CoA at the expense of NADPH. This system will also elongate
unsaturated fatty acids.
Microsome preparations can
elongate both saturated and unsaturated fatty acyl-CoA esters, but in this case
malonyl-CoA rather than acetyl-CoA serves as source of the acetyl groups. The
reaction sequence is identical to that in the fatty-acid synthetase system
except that the microsomal system employs CoA and not ACP as acyl carrier.
Formation of monoenoic acids
Palmitic
and stearic acids serve as precursors of the two common monolenoic (monounsaturated) fatty acids of animal tissues, namely,
poimitoleic and oleic acids, both of which possess a cis double bond in the D9 position.
Although most organisms can form palmitoleic and oleic acids, the pathway and
enzymes employed differ between aerobic and anaerobic organisms. In vertebrates (and most
other aerobic organisms) the D9 double bond is introduced by
a specific monooxygenase system; it is located in the endoplasmic reticulum of
liver and adipose tissue. One molecule of molecular oxygen (O2) is
used as the acceptor for two pairs of electrons, one pair derived from the
palmitoyl-CoA or stearyl-CoA substrate and the other from NADPH, which is a
required coreductant in the reaction. The transfer of electrons in this
complex reaction involves a microsomal electron-transport chain which carries
electrons from NADPH (or NADH) to microsomal cytochrome b5 via cytochrome
b5 reductase, a
flavoprotein. A terminal cyanide-sensitive factor (CSF), a protein, is required
to activate the acyl-CoA and the oxygen.
The overall reaction for
palmitoyl-CoA is:
Palmitoyl—CoA + NADPH + H+
+ O2 ® palmitoleyl—CoA + NADP+ + 2H2O.
Formation of polyenoic acids
Bacteria do not contain polyenoic acids; however, these acids are abundant both
in higher plants and in animals. Mammals contain four distinct families of
polyenoic acids, which differ in the number of carbon atoms between the terminal
methyl group and the nearest double bond. These families are named from their
precursor fatty acids, namely, palmitoleic,
oleic, linoleic, and linolenic acids. All polyenoic acids found in mammals
are formed from these four precursors by further elongation and/or desaturation
reactions. Two of these precursor fatty acids, linoleic and linolenic acids,
cannot be synthesized by mammals and must be obtained from plant sources; they
are therefore called essential fatty
acids.
The elongation of chains of
polyenoic acids occurs at the carboxyl end by the mitochondrial or microsomal systems
described above. The desaturation steps occur by the action of the cytochrome b5-oxygenase
system with NADPH as coreductant of oxygen, like the steps in the formation of
palmitoleic and oleic acids, also described above.
Arachidonic acid is the most abundant
polyenoic acid. When young rats are placed on diets deficient in essential
fatty acids, they grow slowly and develop a scaly dermatitis and thickening of
the skin. This condition can be relieved by administration not only of linoleic
or linolenic acid but also of arachidonic acid. The essential fatty acids and
some of their derivatives serve as precursors of the prostaglandins.
In plants linoleic and
linolenic acids are synthesized from oleic acid via aerobic desaturation
reactions catalyzed by specific oxygenase systems requiring NADPH as
coreductant.
The double bonds of naturally
occurring fatty acids do not in general undergo hydrogenation to yield more
completely saturated fatty acids; only a few organisms appear to carry out this
process. Unsaturated fatty acids, however, are completely oxidized by the
fatty acid oxidation system.
In most organisms the
conversion of saturated to unsaturated fatty acids is promoted by low
environmental temperatures. This is an adaptation to maintain the melting point
of the total cell lipids below the ambient temperature; unsatu-rated fatty
acids have lower melting points than saturated. In some organisms the enzymes
involved in fatty acid desaturation increase in concentration in response to
low temperatures; in others the unsaturated fatty acids are inserted into
lipids at increased rates.
Biosynthesis of triacylglycerols
The triacylglycerols, which
function as depot, or storage, lipids, are actively synthesized in the cells of
vertebrates, particularly liver and fat cells, as well as those of higher
plants. Bacteria in general contain relatively small amounts of
triacyglycerols.
In higher animals and plants two major precursors are required for the
synthesis of triacylglycerols: L-glycerol
3-phosphate and fatty acyl-CoA.. L-Glycerol
3-phosphate is derived from two different sources. Its normal precursor is dihydroxyacetone phosphate, the product
of the aldolase reaction of
glycolysis. Dihydroxyacetone phosphate is reduced to L-glycerol 3-phosphate by
the NAD-linked glycerol- 3-phosphate
dehydrogenase of the cytosol:
Dihydroxyacetone phosphate +
NADH + H+ ® L-glycerol 3-phosphate + NAD+
It may also be formed from
free glycerol arising from degradation of triacylglycerols, through the action
of glycerol kinase:
ATP + glycerol ® L-glycerol 3-phosphate + ADP
The first stage in triacyglycerol formation is the acylation of the free
hydroxyl groups of glycerol phosphate by two molecules of fatty acyl-CoA to
yield first a lysophosphotidic acid
and then a phosphatidic acid:
Glycerol phosphate Lysophosphotidic acid
Lysophosphotidic acid Phosphatidic acid
Free glycerol is not acylated.
These reactions occur preferentially with 16- and 18-carbon saturated and
unsaturated acyl-CoA.
Phosphatidic acids occur only
in trace amounts in cells, but they are important intermediates in the
biosynthesis of triacylglycerols and phosphoglycerides.
In the pathway to triacylglycerols, phosphatidic acid undergoes hydrolysis
by phosphatidate phosphatase to form
a diacylglycerol:
Phosphatidic acid Diacylglycerol
The diacylglycerol then reacts
with a third molecule of a fatty acyl-CoA to yield a triacylglycerol by the action of diacylglycerol acyltransferase:
Diacylglycerol Triacylglycerol
In the intestinal mucosa of higher animals, which actively synthesizes
triacylglycerols during absorption of fatty acids from the intestine, another
type of acylation reaction comes into play. Monoacylglycerols formed during
intestinal digestion may be acylated directly by acylglycerol palmitoyltrans-ferase and thus phosphatidic acid is
not an intermediate:
Monoacylglycerol +
palmitoyl-CoA ® diacylglycerol + CoA
In storage fats of animal and
plant tissues the triacylglycerols are usually mixed, i.e., contain two or
more different fatty acids.
Nutrition and health
Lipids play diverse and important roles in nutrition and health. Many lipids are absolutely essential for life,
however, there is also considerable awareness that abnormal levels of certain
lipids, particularly cholesterol (in hypercholesterolemia) and, more
recently, trans fatty acids, are risk factors
for heart disease and other diseases.
We need fats in our bodies and in our diet. Animals in general use fat for
energy storage because fat stores 9 KCal/g of energy. Plants, which don’t move
around, can afford to store food for energy in a less compact but more easily
accessible form, so they use starch (a carbohydrate, NOT A LIPID) for energy
storage. Carbohydrates and proteins store only 4 KCal/g of energy, so fat
stores over twice as much energy/gram as other sources of energy.
We need fats in our bodies and in our diet. Animals
in general use fat for energy storage because fat stores 9 KCal/g of energy.
Plants, which don’t move around, can afford to store food for energy in a less
compact but more easily accessible form, so they use starch (a carbohydrate,
NOT A LIPID) for energy storage. Carbohydrates and proteins store only 4
KCal/g of energy, so fat stores over twice as much energy/gram as fat. By the
way, this is also related to the idea behind some of the high-carbohydrate weight
loss diets.
The human body burns carbohydrates and fats for fuel in a given proportion
to each other. The theory behind these diets is that if they supply
carbohydrates but not fats, then it is hoped that the fat needed to balance
with the sugar will be taken from the dieter’s
body stores. Fat is also is used in our bodies to a) cushion vital organs like
the kidneys and b) serve as insulation, especially just beneath the skin.