Biosynthesis and biotransformation of cholesterol





Cholesterol is an extremely important biological molecule that has roles in membrane structure as well as being a precursor for the synthesis of the steroid hormones and bile acids. Both dietary cholesterol and that synthesized de novo are transported through the circulation in lipoprotein particles. The same is true of cholesteryl esters, the form in which cholesterol is stored in cells.

The synthesis and utilization of cholesterol must be tightly regulated in order to prevent over-accumulation and abnormal deposition within the body. Of particular importance clinically is the abnormal deposition of cholesterol and cholesterol-rich lipoproteins in the coronary arteries. Such deposition, eventually leading to atherosclerosis, is the leading contributory factor in diseases of the coronary arteries.

Biosynthesis of Cholesterol


Slightly less than half of the cholesterol in the body derives from biosynthesis de novo. Biosynthesis in the liver accounts for approximately 10%, and in the intestines approximately 15%, of the amount produced each day. Cholesterol synthesis occurs in the cytoplasm and microsomes (ER) from the two-carbon acetate group of acetyl-CoA.

The acetyl-CoA utilized for cholesterol biosynthesis is derived from an oxidation reaction (e.g., fatty acids or pyruvate) in the mitochondria and is transported to the cytoplasm by the same process as that described for fatty acid synthesis (see the Figure below). Acetyl-CoA can also be synthesized from cytosolic acetate derived from cytoplasmic oxidation of ethanol which is initiated by cytoplasmic alcohol dehydrogenase (ADH3). All the reduction reactions of cholesterol biosynthesis use NADPH as a cofactor. The isoprenoid intermediates of cholesterol biosynthesis can be diverted to other synthesis reactions, such as those for dolichol (used in the synthesis of N-linked glycoproteins, coenzyme Q (of the oxidative phosphorylation pathway) or the side chain of heme-a. Additionally, these intermediates are used in the lipid modification of some proteins.

Pathway for the movement of acetyl-CoA units from within the mitochondrion to the cytoplasm for use in lipid and cholesterol biosynthesis. Note that the cytoplasmic malic enzyme catalyzed reaction generates NADPH which can be used for reductive biosynthetic reactions such as those of fatty acid and cholesterol synthesis.

The process of cholesterol synthesis has five major steps:

1. Acetyl-CoAs are converted to 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA)

2. HMG-CoA is converted to mevalonate

3. Mevalonate is converted to the isoprene based molecule, isopentenyl pyrophosphate (IPP), with the concomitant loss of CO2

4. IPP is converted to squalene

5. Squalene is converted to cholesterol.

Pathway of cholesterol biosynthesis.

Synthesis begins with the transport of acetyl-CoA from the mitochondrion to the cytosol. The rate limiting step occurs at the 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) reducatase, HMGR catalyzed step. The phosphorylation reactions are required to solubilize the isoprenoid intermediates in the pathway. Intermediates in the pathway are used for the synthesis of prenylated proteins, dolichol, coenzyme Q and the side chain of heme a. The abbreviation "PP" (e.g. isopentenyl-PP) stands for pyrophosphate. Place mouse over intermediate names to see structure.

Acetyl-CoA units are converted to mevalonate by a series of reactions that begins with the formation of HMG-CoA. Unlike the HMG-CoA formed during ketone body synthesis in the mitochondria, this form is synthesized in the cytoplasm. However, the pathway and the necessary enzymes are similar to those in the mitochondria. Two moles of acetyl-CoA are condensed in a reversal of the thiolase reaction, forming acetoacetyl-CoA. The cytoplasmic thiolase enzyme involved in cholesterol biosynthesis is acetoacetyl-CoA thiolase encoded by the ACAT2 gene. Although the bulk of acetoacetyl-CoA is derived via this process, it is possible for some acetoacetate, generated during ketogenesis, to diffuse out of the mitochondria and be converted to acetoacetyl-CoA in the cytosol via the action of acetoacetyl-CoA synthetase (AACS). Acetoacetyl-CoA and a third mole of acetyl-CoA are converted to HMG-CoA by the action of HMG-CoA synthase.

HMG-CoA is converted to mevalonate by HMG-CoA reductase, HMGR (this enzyme is bound in the endoplasmic reticulum, ER). HMGR absolutely requires NADPH as a cofactor and two moles of NADPH are consumed during the conversion of HMG-CoA to mevalonate. The reaction catalyzed by HMGR is the rate limiting step of cholesterol biosynthesis, and this enzyme is subject to complex regulatory controls as discussed below.

Mevalonate is then activated by two successive phosphorylations (catalyzed by mevalonate kinase, and phosphomevalonate kinase), yielding 5-pyrophosphomevalonate. In humans, mevalonate kinase resides in the cytosol indicating that not all the reactions of cholesterol synthesis are catalyzed by membrane-associated enzymes as originally described. After phosphorylation, an ATP-dependent decarboxylation yields isopentenyl pyrophosphate, IPP, an activated isoprenoid molecule. Isopentenyl pyrophosphate is in equilibrium with its isomer, dimethylallyl pyrophosphate, DMPP. One molecule of IPP condenses with one molecule of DMPP to generate geranyl pyrophosphate, GPP. GPP further condenses with another IPP molecule to yield farnesyl pyrophosphate, FPP. Finally, the NADPH-requiring enzyme, squalene synthase catalyzes the head-to-tail condensation of two molecules of FPP, yielding squalene. Like HMGR, squalene synthase is tightly associated with the ER. Squalene undergoes a two step cyclization to yield lanosterol. The first reaction is catalyzed by squalene monooxygenase. This enzyme uses NADPH as a cofactor to introduce molecular oxygen as an epoxide at the 2,3 position of squalene. Through a series of 19 additional reactions, lanosterol is converted to cholesterol.

The terminal reaction in cholesterol biosynthesis is catalyzed by the enzyme 7-dehydrocholesterol reductase encoded by the DHCR7 gene. Functional DHCR7 protein is a 55.5 kDa NADPH-requiring integral membrane protein localized to the microsomal membrane. Deficiency in DHCR7 (due to gene mutations) results in the disorder called Smith-Lemli-Opitz syndrome, SLOS. SLOS is characterized by increased levels of 7-dehydrocholesterol and reduced levels (15% to 27% of normal) of cholesterol resulting in multiple developmental malformations and behavioral problems.


There are three stage in cholesterol synthesis. (1) acetic acid is converted to mevalonic acid, (2) mevalonic acid is converted into squalene, and (3) squalene is con­verted into cholesterol.

Mevalonic acid is formed by condensation of three mole­cules of acetyl-CoA. The key intermediate in this process is b-hydroxy-b-methylglutaryl-CoA (HMG-CoA), which is formed as follows:

Acetyl-CoA        Acetyl-CoA           Acetoacetyl-CoA




The enzyme is called b-hydroxy-b-methylglutaryl-CoA synthase.

The b-hydroxy-b-methylglutaryl-CoA undergoes an irreversible two-step reduction of one of its carboxyl groups to an alcohol group, with concomitant loss of CoA, by the action of hydroxymethylglutaryl-CoA reductase, to yield mevalonate:




Mevalonate is phosphorylated by ATP, first to the 5-monophosphate ester and then to the 5-pyrophosphomevalonic acid:


5-pyrophosphomevalonic acid



A third phosphorylation, at carbon atom 3, yields a very unstable intermediate which loses phosphoric acid and decarboxylates to form 3-isopentenyl pyrophosphate, which isomerizes to 3,3-dimethylallyl pyrophosphate.


3,3-dimethylallyl pyrophosphate


In the next several reactions 3,3-dimethylallyl pyrophosphate is converted to squalene. In the last stage of cholesterol biosynthesis, squalene un­dergoes attack by molecular oxygen, undergoes cyclization and lanosterol  is formed. Lanosterol is transformed to cholesterol.


Transport forms of cholesterol in blood, content of cholesterol in blood, biological role of cholesterol.

LDL are formed in liver and transport cholesterol from liver to peripheral tissue. LDL is taken up by various tissues and provides cholesterol, which the tissue utilize.

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.

The content of cholesterol in blood plasma – 3-8 mmol/l.

Biological role of cholesterol:

-         building blocks of membranes;

-         synthesis of steroid hormones;

-         synthesis of bile acids;

-         synthesis of vitamin D;

-         cholesterol is often deposited on the inner walls of blood vessels, together with other lipids, a condition known as atherosclerosis, which often leads to occlusion of blood vessels in the heart and the brain, re­sulting in heart attacks and strokes, respectively.


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.

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.


Component           Chylomicrons      VLDL        IDL            LDL          HDL

Triacylglycerol            85%               55%              26%             10%          8%

Protein                          2%                 9%              11%              20%          45%

Type                 B,C,E              B,C,E             B,E               B          A,C,E

Cholesterol                  1%                  7%                8%              10%            5%

Cholesterol ester        2%                 10%               30%              35%         15%

Phospholipid              8%                 20%               23%             20%          25%

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.

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.

Metabolism of ketonе bodies


In many vertebrates the liver has the enzymatic capacity to divert some of the acetyl-CoA derived from fatty acid or pyruvate oxidation, presumably during periods of excess for­mation, into free acetoacetate and b-hydroxybutyrate, which are transported via the blood to the peripheral tissues, where they may be oxidized via the tricarboxylic acid cyrcle.

These compounds, together with acetone, are collectively called the ketone bodies.


acetoacetate      b-hydroxybutyrate   acetone

Ketogenesis: mechanism, localization, biological role.

Free acetoacetate, which is the primary source of the other ketone bodies, is formed from acetoacetyl-CoA. Some of the acetoacetyl-CoA arises from the last four carbon atoms of a long-chain fatty acid after oxidative removal of successive acetyl-CoA resi­dues in the mitochondrial matrix. However, most of the ace­toacetyl-CoA formed in the liver arises from the head-to-tail condensation of two molecules of acetyl-CoA derived from fatty acid oxidation by the action of acetyl-CoA acetyltransferase:

acetyl-CoA     acetyl-CoA        ace­toacetyl-CoA

The acetoacetyl-CoA formed in these reactions then un­dergoes loss of CoA, a process called deacylation, to yield free acetoacetate in a special pathway taking place in the mi­tochondrial matrix. It involves the enzymatic formation and cleavage of b-hydroxy-b-methylglutaryl-CoA, an intermediate which also serves as a precursor of sterols.

acetoacetyl-CoA   acetyl-CoA          b-hydroxy-b-methylglutaryl-CoA

b-hydroxy-b-methylglutaryl-CoA   acetoacetate    acetyl-CoA

The free acetoacetate so produced is enzymatically reduced to D-b-hydroxybutyrate by the NAD-linked b-hydroxybutyrate dehydrogenase, which is located in the inner mi­tochondrial membrane.

acetoacetate                                         b-hydroxybutyrate

The mixture of free acetoacetate and b-hydroxybutyrate re­sulting from these reactions may diffuse out of the liver cells into the bloodstream, to be transported to the peripheral tissues.

The mechanism of acetoacetate utilizing in tissues (ketolysis).

In the peripheral tissues the b-hydroxybutyrate is ox­idized to acetoacetate, which is then activated by transfer of CoA from succinyl-CoA. The succinyl-CoA required arises from the oxidation of a-ketoglutarate.

Another way of acetoacetate activation in peripheral tissues is the direct interaction of acetoacetate with ATP and CoA-SH:

The acetoacetyl-CoA formed in the peripheral tissues by these reactions then undergoes thiolytic cleavage to two molecules of acetyl-CoA, which then may enter the tricarboxylic acid cycle.

The mechanism of the increase of ketone bodies content in blood at diabetus mellitus and starvation.

Normally the concentration of ketone bodies in the blood is rather low (10-20 mg/l), but in fasting or in the disease diabetes mellitus, it may reach very high levels. This condition, known as ketosis, arises when the rate of formation of the ke­tone bodies by the liver exceeds the capacity of the periph­eral tissues to utilize them, with a resulting accumulation in the blood and excretion via the kidneys (in normal the content of ketone bodies in urine is up to 50 mg/day).

The utilization of acetyl-CoA in tricarboxylic acid cycle depends on the availability of oxaloacetate in cell. The formation of oxaloacetate depends on quantity of pyruvate, which is formed from glucose. In fasting or diabetus mellitus the entering of glucose into cells is inhibited, oxaloacetate enters the gluconeogenesis process and is not available for the interaction with acetyl CoA in citrate synthase reaction. In this metabolic state acetyl-CoA is used for the ketone bodies formation. The accumulation of ketone bodies is also promotted by b-oxidation of fatty acids due to the stimulation of lipolysis in adipose tissue in glucose starvation conditions.

The effect of nervous system on lipid metabolism.

Sympathetic nervous system activates the splitting of triacylglycerol (lipolysis) and oxidation of fatty acids.

Parasympathetic nervous system promotes the synthesis of lipids and cholesterol in organism.

Endocrine regulation of lipid metabolism.

The effect of somatotropic hormone on lipid metabolism:

-          stimulates lipolysis;

-          stimulates the oxidation of fatty acids.


- stimulates synthesis of lipids in mammary glands.

Lipotropic hormone.

- stimulates the mobilization of lipids from depot.

Thyroxine and triiodthyronine.

-         activate the lipid oxidation and mobilization.


- enhances the synthesis of lipids;

- promotes the lipid storage activating the carbohydrate decomposition;

-         inhibits the gluconeogenesis.


- activates the lipolisis;


- activates the formation of phospholipids in liver and stimulates the action of lipotropic alimentary factors;

-         activates the oxidation of fatty acids in liver.


-          activates the tissue lipase, mobilization of lipids and oxidation of fatty acids.


-         promote the absorption of lipids in intestine;

-         activate lipolysis;

-         activate the conversion of fatty acids in carbohydrates.

Sex hormones.

-         enhance the oxidation of lipids;

-         inhibit the synthesis of cholesterol.


Interrelationship of carbohydrate and lipid metabolism.

Interrelationship of carbohydrate metabolism with lipid metabolism has two directions: 1-st – convertion of carbohydrates to lipids, 2-nd - convertion of lipids to carbohydrates.

Transformation of carbohydrates to lipids.

1. Glycerol phosphate can be produced from dihydroxiacetone monophosphate. The last is the common intermediate product of lipid and carbohydrate metabolism. Glycerol phosphate is the  active form of glycerol and can react with active forms of fatty acids resulting in synthesis of triacylglycerols and complex lipids.

2. Biosynthesis of fatty acids takes place from acetyl-CoA which is formed in oxidative decarboxilation of pyruvate. Pyruvate is the central intermediate product of carbohydrate metabolism.

3. Carbohydrates are also source of hydrogen atoms, which are necessary for fatty acids synthesis. For this purpose the hydrogen atoms of reduced coenzymes NADPH2 are used. NADPH2 are usually produced in pentose phosphate cycle.

Transformation of lipids to carbohydrates.

The formation of carbohydrates from other compounds is called gluconeogenesis.

1. In b-oxidation of fatty acids the acetyl-CoA is formed. Acetyl-CoA can’t be converted directly to pyruvate. But it enter the tricarboxilic acid cycle and some intermediates of this cycle can be used for gluconeogenesis.

2. Small amount of carbohydrates can be also synthesised from glycerol by means of its oxidation to dihydroxiacetone monophosphate and glycerolaldehyde phosphate, which are the intermediates metabolites of glycolysis.



Hyperlipidemia, hyperlipoproteinemia, or hyperlipidaemia (British English) involves abnormally elevated levels of any or all lipids and/or lipoproteins in the blood. It is the most common form of dyslipidemia (which also includes any decreased lipid levels).

Lipids (fat-soluble molecules) are transported in a protein capsule. The size of that capsule, or lipoprotein, determines its density. The lipoprotein density and type of apolipoproteins it contains determines the fate of the particle and its influence on metabolism.

Hyperlipidemias are divided in primary and secondary subtypes. Primary hyperlipidemia is usually due to genetic causes (such as a mutation in a receptor protein), while secondary hyperlipidemia arises due to other underlying causes such as diabetes. Lipid and lipoprotein abnormalities are common in the general population, and are regarded as a modifiable risk factor for cardiovascular disease due to their influence on atherosclerosis. In addition, some forms may predispose to acute pancreatitis.


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