DIGESTION OF PROTEINS.
GENERAL PATHWAYS OF AMINO ACIDS TRANSFORMATION.
Detoxification of ammonia and biosynthesis of urea. Specific pathways of amino acid metabolism. Mechanisms of hormonal regulation and pathologies of protein metabolism.
Proteins are essential nutrients for the human body. They are one of the building blocks of body tissue, and can also serve as a fuel source. As fuel, proteins contain 4 kcal per gram, just like carbohydrates and unlike lipids, which contain 9 kcal per gram.
Proteins are polymer chains made of amino acids linked together by peptide bonds. In nutrition, proteins are broken down in the stomach during digestion by enzymes known as proteases into smaller polypeptides to provide amino acids for the body, including the essential amino acids that cannot be biosynthesized by the body itself.
Amino acids can be divided into three categories: essential amino acids, non-essential amino acids and conditional amino acids. Essential amino acids cannot be made by the body, and must be supplied by food. Non-essential amino acids are made by the body from essential amino acids or in the normal breakdown of proteins. Conditional amino acids are usually not essential, except in times of illness, stress or for someone challenged with a lifelong medical condition.
Essential amino acids are leucine, isoleucine, valine, lysine, threonine, tryptophan, methionine, phenylalanine and histidine. Non-essential amino acids include alanine, asparagine, aspartic acid and glutamic acid. Conditional amino acids include arginine, cysteine, glutamine, glycine, proline, serine, and tyrosine.
Amino acids are found in animal sources such as meats, milk, fish and eggs, as well as in plant sources such as whole grains, pulses, legumes, soy, fruits, nuts and seeds. Vegetarians and vegans can get enough essential amino acids by eating a variety of plant proteins.
Protein is a nutrient needed by the human body for growth and maintenance. Aside from water, proteins are the most abundant kind of molecules in the body. Protein can be found in all cells of the body and is the major structural component of all cells in the body, especially muscle. This also includes body organs, hair and skin. Proteins also are utilized in membranes, such as glycoproteins. When broken down into amino acids, they are used as precursors to nucleic acid, co-enzymes, hormones, immune response, cellular repair and molecules essential for life. Finally, protein is needed to form blood cells.
Proteins are very important molecules in our cells. They are involved in virtually all cell functions. Each protein within the body has a specific function. Some proteins are involved in structural support, while others are involved in bodily movement, or in defense against germs. Proteins vary in structure as well as function. They are constructed from a set of 20 amino acids and have distinct three-dimensional shapes. Below is a list of several types of proteins and their functions.
Antibodies - are specialized proteins involved in defending the body from antigens (foreign invaders). They can travel through the blood stream and are utilized by the immune system to identify and defend against bacteria, viruses, and other foreign intruders. One way antibodies counteract antigens is by immobilizing them so that they can be destroyed by white blood cells.
Contractile Proteins - are responsible for movement. Examples include actin and myosin. These proteins are involved in muscle contraction and movement.
Enzymes - are proteins that facilitate biochemical reactions. They are often referred to as catalysts because they speed up chemical reactions. Examples include the enzymes lactase and pepsin. Lactase breaks down the sugar lactose found in milk. Pepsin is a digestive enzyme that works in the stomach to break down proteins in food.
Hormonal Proteins - are messenger proteins which help to coordinate certain bodily activities. Examples include insulin, oxytocin, and somatotropin. Insulin regulates glucose metabolism by controlling the blood-sugar concentration. Oxytocin stimulates contractions in females during childbirth. Somatotropin is a growth hormone that stimulates protein production in muscle cells.
Structural Proteins - are fibrous and stringy and provide support. Examples include keratin, collagen, and elastin. Keratins strengthen protective coverings such as hair, quills, feathers, horns, and beaks. Collagens and elastin provide support for connective tissues such as tendons and ligaments.
Storage Proteins - store amino acids. Examples include ovalbumin and casein. Ovalbumin is found in egg whites and casein is a milk-based protein.
Transport Proteins - are carrier proteins which move molecules from one place to another around the body. Examples include hemoglobin and cytochromes. Hemoglobin transports oxygen through the blood. Cytochromes operate in the electron transport chain as electron carrier proteins.
Proteins are one of the key nutrients for success in terms of sports. Amino acids, the building blocks of proteins, are used for building tissue, including muscle, as well as repairing damaged tissues. Proteins usually only provide a small source of fuel for the exercising muscles, being used as fuel typically only when carbohydrates and lipid resources are low.
Animal sources of protein.
A wide range of foods are a source of protein. The best
combination of protein sources depends on the region of the world, access,
cost, amino acid types and nutrition balance, as well as acquired tastes. Some
foods are high in certain amino acids, but their digestibility and the anti-nutritional factors present in these
foods make them of limited value in human nutrition. Therefore, one must
consider digestibility and secondary nutrition profile such as calories,
cholesterol, vitamins and essential mineral density of the protein source. On a
worldwide basis, plant protein foods contribute over 60 percent of the per
capita supply of protein, on average. In
Meat, eggs and fish are sources of complete protein. Milk and milk-derived foods are also good sources of protein.
Whole grains and cereals are another source of proteins. However, these tend to be limiting in the amino acid lysine or threonine, which are available in other vegetarian sources and meats. Examples of food staples and cereal sources of protein, each with a concentration greater than 7 percent, are (in no particular order) buckwheat, oats, rye, millet, maize (corn), rice, wheat, spaghetti, bulgar, sorghum, amaranth, and quinoa.
Vegetarian sources of proteins include legumes, nuts, seeds and fruits. Legumes, some of which are called pulses in certain parts of the world, have higher concentrations of amino acids and are more complete sources of protein than whole grains and cereals. Examples of vegetarian foods with protein concentrations greater than 7 percent include soybeans, lentils, kidney beans, white beans, mung beans, chickpeas, cowpeas, lima beans, pigeon peas, lupines, wing beans, almonds, Brazil nuts, cashews, pecans, walnuts, cotton seeds, pumpkin seeds, sesame seeds, and sunflower seeds.
The amount of protein required in a person's diet is determined in large part by overall energy intake, the body's need for nitrogen and essential amino acids, body weight and composition, rate of growth in the individual, physical activity level, individual's energy and carbohydrate intake, as well as the presence of illness or injury. Physical activity and exertion as well as enhanced muscular mass increase the need for protein. Requirements are also greater during childhood for growth and development, during pregnancy or when breast-feeding in order to nourish a baby, or when the body needs to recover from malnutrition or trauma or after an operation.
If enough energy is not taken in through diet, as in the process of starvation, the body will use protein from the muscle mass to meet its energy needs, leading to muscle wasting over time. If the individual does not consume adequate protein in nutrition, then muscle will also waste as more vital cellular processes (e.g. respiration enzymes, blood cells) recycle muscle protein for their own requirements.
According to US & Canadian Dietary Reference Intake guidelines, women
aged 19–70 need to consume
Several studies have concluded that active people and athletes may require elevated protein intake (compared to 0.8 g/kg) due to increase in muscle mass and sweat losses, as well as need for body repair and energy source. Suggested amounts vary between 1.6 g/kg and 1.8 g/kg, while a proposed maximum daily protein intake would be approximately 25% of energy requirements i.e. approximately 2 to 2.5 g/kg. However, many questions still remain to be resolved.
The result of limited synthesis and normal rates of protein degradation is that the balance of nitrogen intake and nitrogen excretion is rapidly and significantly altered. Normal, healthy adults are generally in nitrogen balance, with intake and excretion being very well matched. Young growing children, adults recovering from major illness, and pregnant women are often in positive nitrogen balance. Their intake of nitrogen exceeds their loss as net protein synthesis proceeds. When more nitrogen is excreted than is incorporated into the body, an individual is in negative nitrogen balance. Insufficient quantities of even one essential amino acid is adequate to turn an otherwise normal individual into one with a negative nitrogen balance. The biological value of dietary proteins is related to the extent to which they provide all the necessary amino acids. Proteins of animal origin generally have a high biological value; plant proteins have a wide range of values from almost none to quite high. In general, plant proteins are deficient in lysine, methionine, and tryptophan and are much less concentrated and less digestible than animal proteins. The absence of lysine in low-grade cereal proteins, used as a dietary mainstay in many underdeveloped countries, leads to an inability to synthesize protein (because of missing essential amino acids) and ultimately to a syndrome known as kwashiorkor, common among children in these countries.
Protein deficiency and malnutrition can lead to variety of ailments including mental retardation and kwashiorkor. Symptoms of kwashiorkor include apathy, diarrhea, inactivity, failure to grow, flaky skin, fatty liver, and edema of the belly and legs. This edema is explained by the action of lipoxygenase on arachidonic acid to form leukotrienes and the normal functioning of proteins in fluid balance and lipoprotein transport.
Although protein energy malnutrition is more common in low-income countries, children from higher-income countries are also affected, including children from large urban areas in low socioeconomic neighborhoods. This may also occur in children with chronic diseases, and children who are institutionalized or hospitalized for a different diagnosis. Risk factors include a primary diagnosis of mental retardation, cystic fibrosis, malignancy, cardiovascular disease, end stage renal disease, oncologic disease, genetic disease, neurological disease, multiple diagnoses, or prolonged hospitalization. In these conditions, the challenging nutritional management may get overlooked and underestimated, resulting in an impairment of the chances for recovery and the worsening of the situation.
Deficiency of protein leads to following:
1. Shortage of protein leads to retardation of growth and in extreme cases failure of growth. This is manifested as marasmus and kwashiorkor among infants and children.
2. Protein deficiency affects the intestinal mucosa and the gland that secret digestive enzymes. This results in the failure to digest and absorb the food, consequently leading to diarrhea and loss of fluid and electrolyte.
3. The normal structure and function of liver is disturbed leading fat accumulation and fatty livers. Liver fails to synthesis plasma albumin thus leading to Oedema.
4. Muscle wasting and anemia due to the shortage of hemoglobin are common feature due to the deficiency of protein.
6. The amino acids presents in the protein help in tissue synthesis during growth period e.g. infancy childhood and adolescence. The body goes into negative N2 -balance due to the shortage of protein in the diet. This results in muscle wastage
7. Proteins from important constituents of hormones. How-ever the deficiency of proteins leads to no marked and characteristic changes in the functioning of endocrine glands.
8. Proteins furnish 10-12per cent of calories required daily. However the major part of proteins is essentially for body-building purposes only.
Digestion of proteins starts in the stomach and accomplishes in the small intestine. Several hormones take part in protein digestion. They include trypsin, chymotrypsin, pepsin, etc.
Digestion of Protein in Stomach
Protein digestion does not start with chewing of food in the mouth. It begins in the stomach. The stomach is especially designed for the purpose of digestion of foods. Its walls are composed of strong muscles. These muscles mix and churn the ingested food. They do it with the help of rhythmic contractions, occurring at the average rate of 3 per min.
The lining of the stomach contains glands. Their function is to secrete gastric juice. It is a colorless and strong acidic liquid at a pH of 1-3. The main components of gastric juice are digestive enzymes, hydrochloric acid and mucus.
Hydrochloric acid produced in the stomach is a very strong acid. It is produced by the type of epithelial cells called parietal cells present in the lining of the stomach. HCl is so strong that it can easily digest the stomach itself. But such a destructive process is prevented from occurring by another secretion of the stomach called mucus. It protects the delicate cell lining of the stomach as well as moistens the food present there. However, the cells in the stomach lining keep getting destroyed by hydrochloric acid. It gets replaced by newer cells. According to studies, the lining of the stomach gets completely replaced every third day. Protein digestion in the stomach occurs mainly by the action of hydrochloric acid (HCl) and enzyme called pepsin. The enzyme pepsin forms in the stomach when its precursor pepsinogen reacts with HCl. Pepsin and HCl breaks the protein bonds. The foods containing proteins are separated from each other. The proteins get separated out, which is necessary for the action of enzymes. The enzymes needed for digesting proteins are proteinases and proteases. These enzymes break down the molecules of proteins into its constituents, amino acids by a depolymerisation process called hydrolysis. It is described as a chemical reaction wherein a water molecule breaks down into hydrogen cations and hydroxide anions. The rate of action of these protein digestive enzymes is influenced by a number of factors. Some of them are concentration and amount of the enzyme, amount of protein food needed to be digested, temperature of the food, acidity of the food, acidity of the stomach and presence of antacids or other inhibitors of digestion. The task of enzymes is to breakdown of protein molecules into simpler structures called peptones and proteose. They leave the stomach and enter the small intestine with the help of peristalsis movement of the body. It is called chyme. The entire process of protein digestion in the stomach takes about 4 hours.
Digestion of Protein in Small Intestine
The chyme first enters duodenum, which
is a part of small intestine. It is a C-shaped structure about
Trypsin is a serine protease found in the digestive system of many vertebrates, where it hydrolyses proteins. Trypsin is produced in the pancreas as the inactive proenzyme trypsinogen. Trypsin cleaves peptide chains mainly at the carboxyl side of the amino acids lysine or arginine, except when either is followed by proline. It is used for numerous biotechnological processes. The process is commonly referred to as trypsin proteolysis or trypsinisation, and proteins that have been digested/treated with trypsin are said to have been trypsinized.
In the duodenum, trypsin catalyzes the hydrolysis of peptide bonds, breaking down proteins into smaller peptides. The peptide products are then further hydrolyzed into amino acids via other proteases, rendering them available for absorption into the blood stream. Tryptic digestion is a necessary step in protein absorption as proteins are generally too large to be absorbed through the lining of the small intestine.
Trypsin is produced in the pancreas, in the form of the inactive zymogen trypsinogen. When the pancreas is stimulated by cholecystokinin, it is then secreted into the first part of the small intestine (the duodenum) via the pancreatic duct. Once in the small intestine, the enzyme enteropeptidase activates it into trypsin by proteolytic cleavage. Auto catalysis does not happen with trypsin since trypsinogen is a poor substrate for trypsin. This activation mechanism is common for most serine proteases, and serves to prevent autodegradation of the pancreas.
Activation of trypsin from proteolytic cleavage of trypsinogen in the pancreas can lead to a series of events that cause pancreatic self-digestion, resulting in pancreatitis. One consequence of the autosomal recessive disease cystic fibrosis is a deficiency in transport of trypsin and other digestive enzymes from the pancreas. This leads to the disorder termed meconium ileus. This disorder involves intestinal obstruction (ileus) due to overly thick meconium, which is normally broken down by trypsins and other proteases, then passed in faeces.
Trypsin is available in high quantity in pancreases, and can be purified rather easily. Hence it has been used widely in various biotechnological processes.
In a tissue culture lab, trypsins are used to re-suspend cells adherent to the cell culture dish wall during the process of harvesting cells. Some cell types have a tendency to "stick" - or adhere - to the sides and bottom of a dish when cultivated in vitro. Trypsin is used to cleave proteins bonding the cultured cells to the dish, so that the cells can be suspended in fresh solution and transferred to fresh dishes.
Trypsin can also be used to dissociate dissected cells (for example, prior to cell fixing and sorting).
Trypsins can be used to break down casein in breast milk. If trypsin is added to a solution of milk powder, the breakdown of casein will cause the milk to become translucent. The rate of reaction can be measured by using the amount of time it takes for the milk to turn translucent.
Trypsin is commonly used in biological
research during proteomics experiments to digest proteins into peptides for
mass spectrometry analysis, e.g. in-gel
digestion. Trypsin is particularly suited for this, since it has a
very well defined specificity, as it hydrolyzes only the peptide bonds in which
the carbonyl group is contributed either by an Arg or
Trypsin can also be used to dissolve blood clots in its microbial form and treat inflammation in its pancreatic form.
THE KINDS OF GASTRIC JUICE ACIDITY.
Gastric acid is a digestive fluid, formed in the stomach. It has a pH of 1.5 to 3.5 and is composed of hydrochloric acid (HCl) (around 0.5%, or 5000 parts per million) as high as 0.1 N, and large quantities of potassium chloride (KCl) and sodium chloride (NaCl). The acid plays a key role in digestion of proteins, by activating digestive enzymes, and making ingested proteins unravel so that digestive enzymes break down the long chains of amino acids.
Gastric acid is produced by cells lining the stomach, which are coupled to systems to increase acid production when needed. Other cells in the stomach produce bicarbonate, a base, to buffer the fluid, ensuring that it does not become too acidic. These cells also produce mucus, which forms a viscous physical barrier to prevent gastric acid from damaging the stomach. Cells in the beginning of the small intestine, or duodenum, further produce large amounts of bicarbonate to completely neutralize any gastric acid that passes further down into the digestive tract.
Gastric acid is produced by parietal cells (also called oxyntic cells) in the stomach. Its secretion is a complex and relatively energetically expensive process. Parietal cells contain an extensive secretory network (called canaliculi) from which the gastric acid is secreted into the lumen of the stomach. These cells are part of epithelial fundic glands in the gastric mucosa. The pH of gastric acid is 1.35 to 3.5  in the human stomach lumen, the acidity being maintained by the proton pump H+/K+ ATPase. The parietal cell releases bicarbonate into the blood stream in the process, which causes a temporary rise of pH in the blood, known as alkaline tide.
The resulting highly acidic environment in the stomach lumen causes proteins from food to lose their characteristic folded structure (or denature). This exposes the protein's peptide bonds. The chief cells of the stomach secrete enzymes for protein breakdown (inactive pepsinogen and rennin). Hydrochloric acid activates pepsinogen into the enzyme pepsin, which then helps digestion by breaking the bonds linking amino acids, a process known as proteolysis. In addition, many microorganisms have their growth inhibited by such an acidic environment, which is helpful to prevent infection.
In hypochlorhydria and achlorhydria, there is low or no gastric acid in the stomach, potentially leading to problems as the disinfectant properties of the gastric lumen are decreased. In such conditions, there is greater risk of infections of the digestive tract (such as infection with Vibrio or Helicobacter bacteria).
Absorption of Amino Acids and Peptides
Dietary proteins are, with very few exceptions, not absorbed. Rather, they must be digested into amino acids or di- and tripeptides first. In previous sections, we've seen two sources secrete proteolytic enzymes into the lumen of the digestive tube:
· the stomach secretes pepsinogen, which is converted to the active protease pepsin by the action of acid.
· the pancreas secretes a group of potent proteases, chief among them trypsin, chymotrypsin and carboxypeptidases.
Through the action of these gastric and pancreatic proteases, dietary proteins are hydrolyzed within the lumen of the small intestine predominantly into medium and small peptides (oligopeptides).
The brush border of the small intestine is equipped with a family of peptidases. Like lactase and maltase, these peptidases are integral membrane proteins rather than soluble enzymes. They function to further the hydrolysis of lumenal peptides, converting them to free amino acids and very small peptides. These endproducts of digestion, formed on the surface of the enterocyte, are ready for absorption.
Absorption of Amino Acids
The mechanism by which amino acids are absorbed is conceptually identical to that of monosaccharides. The lumenal plasma membrane of the absorptive cell bears at least four sodium-dependent amino acid transporters - one each for acidic, basic, neutral and amino acids. These transporters bind amino acids only after binding sodium. The fully loaded transporter then undergoes a conformational change that dumps sodium and the amino acid into the cytoplasm, followed by its reorientation back to the original form.
Thus, absorption of amino acids is also absolutely dependent on the electrochemical gradient of sodium across the epithelium. Further, absorption of amino acids, like that of monosaccharides, contributes to generating the osmotic gradient that drives water absorption.
The basolateral membrane of the enterocyte contains additional transporters which export amino acids from the cell into blood. These are not dependent on sodium gradients.
Absorption of Peptides
There is virtually no absorption of peptides longer than four amino acids. However, there is abundant absorption of di- and tripeptides in the small intestine. These small peptides are absorbed into the small intestinal epithelial cell by cotransport with H+ ions via a transporter called PepT1.
Once inside the enterocyte, the vast bulk of absorbed di- and tripeptides are digested into amino acids by cytoplasmic peptidases and exported from the cell into blood. Only a very small number of these small peptides enter blood intact.
Absorption of Intact Proteins
As emphasized, absorption of intact proteins occurs only in a few circumstances. In the first place, very few proteins get through the gauntlet of soluble and membrane-bound proteases intact. Second, "normal" enterocytes do not have transporters to carry proteins across the plasma membrane and they certainly cannot permeate tight junctions.
One important exception to these general statements is that for a very few days after birth, neonates have the ability to absorb intact proteins. This ability, which is rapidly lost, is of immense importance because it allows the newborn animal to acquire passive immunity by absorbing immunoglobulins in colostral milk.
In constrast to humans and rodents, there is no significant transfer of antibodies across the placenta in many animals (cattle, sheep, horses and pigs to name a few), and the young are born without circulating antibodies. If fed colostrum during the first day or so after birth, they absorb large quantities of immunoglobulins and acquire a temporary immune system that provides protection until they generate their own immune responses.
The small intestine rapidly loses the capacity to absorb intact proteins - a process called closure - and consequently, animals that do not receive colostrum within the first few days after birth will likely die due to opportunistic infections.
General pathways of amino acids transformation
Deamination of amino acids, it kinds. The role of vitamins in deamination of amino acids.
Removal of an amino group from a molecule.
the elimination of an amino group (NH2) from organic compounds. Deamination is accompanied by the substitution of some other group, such as H, OH, OR, or Hal, for the amino group or by the formation of a double bond. In particular, deamination is brought about by the action of nitrous acid on primary amines. In this reaction, acyclic amines yield alcohols (I) and olefins (II), for example:
The deamination of alicyclic amines is accompanied by ring expansion or contraction. In the presence of strong inorganic acids, aromatic amines and nitrous acid yield diazonium salts. Such reactions as hydrolysis, hydrogenolysis, decomposition of quaternary ammonium salts, and pyrolytic reactions also result in deamination.
Deamination plays an important part in the life processes of animals, plants, and microorganisms. Oxidative deamination, with the formation of ammonia and α-keto acids, is characteristic of d-amino acids. Amines also undergo oxidative deamination. Except for glutamate dehydrogenase, which deaminates L-glutamic acid, oxidases of natural amino acids are not very active in animal tissues. Therefore, most L-amino acids undergo indirect deamination by means of prior transamination, with the formation of glutamic acid, which then undergoes oxidative deamination or other transformations. Other types of deamination are reductive, hydrolytic (deamination of amino derivatives of purines, pyrimidines, and sugars), and intramolecular (histidine deamination), which occur mainly in microorganisms.
Oxidative Deamination Reaction
Deamination is also an oxidative reaction that occurs under aerobic conditions in all tissues but especially the liver. During oxidative deamination, an amino acid is converted into the corresponding keto acid by the removal of the amine functional group as ammonia and the amine functional group is replaced by the ketone group. The ammonia eventually goes into the urea cycle.
Oxidative deamination occurs primarily on glutamic acid because glutamic acid was the end product of many transamination reactions.
The glutamate dehydrogenase is allosterically controlled by ATP and ADP. ATP acts as an inhibitor whereas ADP is an activator.
Central Role for Glutamic Acid:
Apparently most amino acids may be deaminated but this is a significant reaction only for glutamic acid. If this is true, then how are the other amino acids deaminated? The answer is that a combination of transamination and deamination of glutamic acid occurs which is a recycling type of reaction for glutamic acid. The original amino acid loses its amine group in the process. The general reaction sequence is shown on the left.
Synthesis of New Amino Acids:
Transamination of amino acids, mechanism, role of enzymes and coenzymes.
In the degradation of most standard amino acids, an early step in degradation consists in transamination, which is the transfer of the α-amino group from the amino acid to an α-keto acid. There are several different aminotransferases, each of which is specific for an individual amino acid or for a group of chemically similar ones, such as the branched amino acids leucine, isoleucine, and valine. The α-keto acid that accepts the amino group is always α-ketoglutarate (Figure). Transamination is freely reversible; therefore, both glutamate and α-ketoglutarate are substrates of every single transaminase. If amino groups are to be transferred between two amino acids other than glutamate, this will still occur by transient formation of glutamate (Figure).
Transamination reactions. a: Glutamate pyruvate transaminase (also called alanine amino transferase) transfers the α-amino group from alanine to α-ketoglutarate, which yields glutamate and pyruvate. b: All transaminases have α-ketoglutarate as one of their substrates. Transfer of amino groups between arbitrary amino and α-keto acids (here: alanine and oxaloacetate) occurs by transient transfer to α-ketoglutarate.
1. Transfer of the amino group from alanine to the enzyme, which releases pyruvate, and
2. Transfer of the amino group from the enzyme to α-ketoglutarate, which releases glutamate.
Decarboxylisation of amino acids, role of enzymes and co-enzymes.
Decarboxylation is a chemical reaction that removes a carboxyl group and releases carbon dioxide (CO2). Usually, decarboxylation refers to a reaction of carboxylic acids, removing a carbon atom from a carbon chain. The reverse process, which is the first chemical step in photosynthesis, is called carboxylation, the addition of CO2 to a compound. Enzymes that catalyze decarboxylations are called decarboxylases or, the more formal term, carboxy-lyases.
Amino acid Amine Function
serine → ethanolamine → Conversion of phosphatidyl
serine to phosphatidyl
ethanolamine in bacteria
lysine → cadaverine
arginine/ornithine → putrescine→ leads to spermine and
S-adenosylmethionine → aminopropane donor→decarboxylase contains pyruvate in place of PLP; leads to spermine and spermidine
histidine→histamine→vasodilator, inflammatory agent, stimulates acid secretion in stomach; formed and stored for secretion in granulocytes, e.g. mast cells
glutamate →gamma-aminobutyrate, GABA →important neurotransmitter in brain
3,4-dihydroxyphenylalanine (Dopa) →dopamine, hydroxytyramine→ inhibitory neurotransmitter in brain, precursor of catecholamines, melanin
5-hydroxytryptophan →serotonin →precursor of melatonin
phenylalanine→phenylethylamine → antidepressant, mild amphetamine-like stimulant, present in chocolate
Histamine mediates a wide variety of physiological and pathological responses, such as inflammation, gastric acid secretion, neurotransmission and immune modulation.
Histamine is synthesised through decarboxylation of l‐histidine by histidine decarboxylase and is catabolised through oxidative deamination or methylation.
Tissue histamine levels are transiently increased by degranulation of mast cells and basophils while they are upregulated by de novo synthesis in gastric enterochromaffin‐like cells, and neurons.
Histamine exerts its functions by acting on its specific receptors consisting of H1, H2, H3 and H4 receptors.
Histamine is a paracrine mediator, of which actions are generally limited in the local microenvironment.
Histamine H1 receptor antagonists have brought successful therapeutic approaches for immediate allergy, because histamine evokes vasodilation and increased vascular permeability by acting on the H1 receptor.
In the central nervous system, histamine is involved in awakening, appetite, maintenance of circadian rhythm, learning and memory, which are regulated by the H1 and H3 receptors.
Histamine stimulates parietal cells to induce gastric acid secretion by acting on the H2 receptor, of which antagonists drastically improved therapeutic approaches for peptic ulcer.
Pre‐synaptic histamine H3 receptor regulates release of various neurotransmitters including histamine itself, and is expected as a potential drug target for the treatment of cognitive dysfunctions.
Histamine H4 receptor is expressed exclusively in blood cells and mediates their chemotaxis in response to histamine.
Gamma-aminobutyric acid (GABA), a major inhibitory neurotransmitter in the mammalian central nervous system, is produced from glutamic acid in a reaction catalysed by glutamic acid decarboxylase. The sequential actions of GABA-transaminase (converting GABA to succinic semialdehyde) and succinic semialdehyde dehydrogenase (oxidizing succinic semialdehyde to succinic acid) allow oxidative metabolism of GABA through the tricarboxylic acid cycle.
Glutamate decarboxylase or glutamic acid decarboxylase (GAD) is an enzyme that catalyzes the decarboxylation of glutamate to GABA and CO2. GAD uses PLP as a cofactor. The reaction proceeds as follows:
5-Hydroxytryptophan is decarboxylated to serotonin (5-hydroxytryptamine or 5-HT) by the enzyme aromatic-L-amino-acid decarboxylase with the help of vitamin B6. This reaction occurs both in nervous tissue and in the liver. 5-HTP crosses the blood–brain barrier, while 5-HT does not. Excess 5-HTP, especially when administered with Vitamin B6, is thought to be metabolized and excreted.
The main functions of serotonin are:
the regulation of mood, appetite, sleep, muscle contraction, and some cognitive functions including memory and learning. Modulation of serotonin at synapses is thought to be a major action of several classes of pharmacological antidepressants.
Dopamine is synthesized in the body from within cells (mainly by neurons and cells in the medulla of the adrenal glands) and can be created from any one of the following three amino acids:
L-Tyrosine (L-4-hydroxyphenylalanine; TYR)
L-DOPA (L-3,4-dihydroxyphenylalanine; DOPA)
Diagnostic role of determination of AlAT and AsAT
Aspartate transaminase (AST), also called aspartate aminotransferase (AspAT/ASAT/AAT) or serum glutamic oxaloacetic transaminase (SGOT), is a pyridoxal phosphate (PLP)-dependent transaminase enzyme. AST catalyzes the reversible transfer of an α-amino group between aspartate and glutamate and, as such, is an important enzyme in amino acid metabolism. AST is found in the liver, heart, skeletal muscle, kidneys, brain, and red blood cells, and it is commonly measured clinically as a marker for liver health.
Two isoenzymes are present in a wide variety of eukaryotes. In humans:
These isoenzymes are thought to have evolved from a common ancestral AST via gene duplication, and they share a sequence homology of approximately 45%.
AST has also been found in a number of microorganisms, including E. coli, H. mediterranei, and T. thermophilus. In E. coli, the enzyme is encoded by the aspCgene and has also been shown to exhibit the activity of an aromatic-amino-acid transaminase.
AST is similar to alanine transaminase (ALT) in that both enzymes are associated with liver parenchymal cells. The difference is that ALT is found predominantly in the liver, with clinically negligible quantities found in the kidneys, heart, and skeletal muscle, while AST is found in the liver, heart (cardiac muscle), skeletal muscle, kidneys, brain, and red blood cells. As a result, ALT is a more specific indicator of liver inflammation than AST, as AST may be elevated also in diseases affecting other organs, such as myocardial infarction, acute pancreatitis, acute hemolytic anemia, severe burns, acute renal disease, musculoskeletal diseases, and trauma.
AST was defined as a biochemical marker for the diagnosis of acute myocardial infarction in 1954. However, the use of AST for such a diagnosis is now redundant and has been superseded by the cardiac troponins.
AST (SGOT) is commonly measured clinically as a part of diagnostic liver function tests, to determine liver health.
Alanine transaminase or ALT is a transaminase It is also called serum glutamic pyruvic transaminase (SGPT) or alanine aminotransferase (ALAT).
It is commonly measured clinically as a part of a diagnostic evaluation of hepatocellular injury, to determine liver health. When used in diagnostics, it is almost always measured in international units/liter (U/L). While sources vary on specific normal range values for patients, 10-40 U/L is the standard normal range for experimental studies. Alanine transaminase shows a marked diurnal variation.
Significantly elevated levels of ALT(SGPT) often suggest the existence of other medical problems such as viral hepatitis, diabetes, congestive heart failure, liver damage, bile duct problems, infectious mononucleosis, or myopathy. For this reason, ALT is commonly used as a way of screening for liver problems. Elevated ALT may also be caused by dietary choline deficiency. However, elevated levels of ALT do not automatically mean that medical problems exist. Fluctuation of ALT levels is normal over the course of the day, and ALT levels can also increase in response to strenuous physical exercise.
Investigation of detoxification processes and biosynthesis of urine. Hormonal adjusting and pathologies of proteins metabolism.
Ammonia the highly toxic product of protein catabolism, is rapidly inactivated by a variety of reactions. Some product of these reactions are utilized for other purposes (thus salvaging a portion of the amino nitrogen), while others are excreted. The excreted form varies quite widely among vertebrate and invertebrate animals. The development of a pathway for nitrogen disposal in a species appears to depend chiefly on the availability of water.
Humans are totally dependent on other organisms for converting atmospheric nitrogen into forms available to the body. Nitrogen fixation is carried out by bacterial nitrogenases forming reduced nitrogen, NH4+ which can then be used by all organisms to form amino acids.
Overview of the flow of nitrogen in the biosphere. Nitrogen, nitrites and nitrates are acted upon by bacteria (nitrogen fixation) and plants and we assimilate these compounds as protein in our diets. Ammonia incorporation in animals occurs through the actions of glutamate dehydrogenase and glutamine synthase. Glutamate plays the central role in mammalian nitrogen flow, serving as both a nitrogen donor and nitrogen acceptor.
Reduced nitrogen enters the human body as dietary free amino acids, protein, and the ammonia produced by intestinal tract bacteria. A pair of principal enzymes, glutamate dehydrogenase and glutamine synthatase, are found in all organisms and effect the conversion of ammonia into the amino acids glutamate and glutamine, respectively. Amino and amide groups from these 2 substances are freely transferred to other carbon skeletons by transamination and transamidation reactions.
Aminotransferases exist for all amino acids except threonine and lysine. The most common compounds involved as a donor/acceptor pair in transamination reactions are glutamate and α-KG, which participate in reactions with many different aminotransferases. Serum aminotransferases such as aspartate aminotransferase, AST and alanine transaminase, ALT have been used as clinical markers of tissue damage, with increasing serum levels indicating an increased extent of damage. Alanine transaminase has an important function in the delivery of skeletal muscle carbon and nitrogen (in the form of alanine) to the liver. In skeletal muscle, pyruvate is transaminated to alanine, thus affording an additional route of nitrogen transport from muscle to liver. In the liver, alanine transaminase transfers the ammonia to α-KG and regenerates pyruvate. The pyruvate can then be diverted into gluconeogenesis. This process is referred to as the glucose-alanine cycle.
The glucose-alanine cycle is used primarily as a mechanism for skeletal muscle to eliminate nitrogen while replenishing its energy supply. Glucose oxidation produces pyruvate which can undergo transamination to alanine. This reaction is catalyzed by alanine transaminase, ALT. Additionally, during periods of fasting, skeletal muscle protein is degraded for the energy value of the amino acid carbons and alanine is a major amino acid in protein. The alanine then enters the blood stream and is transported to the liver. Within the liver alanine is converted back to pyruvate which is then a source of carbon atoms for gluconeogenesis. The newly formed glucose can then enter the blood for delivery back to the muscle. The amino group transported from the muscle to the liver in the form of alanine is converted to urea in the urea cycle and excreted.
The reaction catalyzed by glutamate dehydrogenase is:
The glutamate dehydrogenase utilizes both nicotinamide nucleotide cofactors; NAD+ in the direction of nitrogen liberation and NADP+ for nitrogen incorporation. In the forward reaction as shown above glutamate dehydrogenase is important in converting free ammonia and α-KG to glutamate, forming one of the 20 amino acids required for protein synthesis. However, it should be recognized that the reverse reaction is a key anapleurotic process linking amino acid metabolism with TCA cycle activity. In the reverse reaction, glutamate dehydrogenase provides an oxidizable carbon source used for the production of energy as well as a reduced electron carrier, NADH. As expected for a branch point enzyme with an important link to energy metabolism, glutamate dehydrogenase is regulated by the cell energy charge. ATP and GTP are positive allosteric effectors of the formation of glutamate, whereas ADP and GDP are positive allosteric effectors of the reverse reaction. Thus, when the level of ATP is high, conversion of glutamate to α-KG and other TCA cycle intermediates is limited; when the cellular energy charge is low, glutamate is converted to ammonia and oxidizable TCA cycle intermediates. Glutamate is also a principal amino donor to other amino acids in subsequent transamination reactions. The multiple roles of glutamate in nitrogen balance make it a gateway between free ammonia and the amino groups of most amino acids.
The Glutamine Synthetase Reaction:
The glutamine synthetase reaction is also important in several respects. First it produces glutamine, one of the 20 major amino acids. Second, in animals, glutamine is the major amino acid found in the circulatory system. Its role there is to carry ammonia to and from various tissues but principally from peripheral tissues to the kidney, where the amide nitrogen is hydrolyzed by the enzyme glutaminase (reaction below); this process regenerates glutamate and free ammonium ion, which is excreted in the urine.
Note that, in this function, ammonia arising in peripheral tissue is carried in a non-ionizable form which has none of the neurotoxic or alkalosis-generating properties of free ammonia.
Liver contains both glutamine synthetase and glutaminase but the enzymes are localized in different cellular segments. This ensures that the liver is neither a net producer nor consumer of glutamine. The differences in cellular location of these two enzymes allows the liver to scavenge ammonia that has not been incorporated into urea. The enzymes of the urea cycle are located in the same cells as those that contain glutaminase. The result of the differential distribution of these two hepatic enzymes makes it possible to control ammonia incorporation into either urea or glutamine, the latter leads to excretion of ammonia by the kidney.
When acidosis occurs the body will divert more glutamine from the liver to the kidney. This allows for the conservation of bicarbonate ion since the incorporation of ammonia into urea requires bicarbonate (see below). When glutamine enters the kidney, glutaminase releases one mole of ammonia generating glutamate and then glutamate dehydrogenase releases another mole of ammonia generating α-KG. The ammonia will ionizes to ammonium ion (NH4+) which is excreted. The net effect is a reduction in the concentration of hydrogen ion, [H+], and thus an increase in the pH.
Earlier it was noted that kidney glutaminase was responsible for converting excess glutamine from the liver to urine ammonium. However, about 80% of the excreted nitrogen is in the form of urea which is also largely made in the liver, in a series of reactions that are distributed between the mitochondrial matrix and the cytosol. The series of reactions that form urea is known as the Urea Cycle or the Krebs-Henseleit Cycle.
1: carbamoyl phosphate synthetase-I (CPS-I)
2: ornithine transcarbamoylase (OTC)
3: argininosuccinate synthetase
The essential features of the urea cycle reactions and their metabolic regulation are as follows: arginine from the diet or from protein breakdown is cleaved by the cytosolic enzyme arginase, generating urea and ornithine. In subsequent reactions of the urea cycle a new urea residue is built on the ornithine, regenerating arginine and perpetuating the cycle.
Ornithine arising in the cytosol is transported to the mitochondrial matrix, where ornithine transcabamoylase catalyzes the condensation of ornithine with carbamoyl phosphate, producing citrulline. The energy for the reaction is provided by the high-energy anhydride of carbamoyl phosphate.
The product, citrulline, is then transported to the cytosol, where the remaining reactions of the cycle take place. The synthesis of citrulline requires a prior synthesis of carbamoyl phosphate (CP).
The activation step requires 2 equivalents of ATP and the mitochondrial matrix enzyme carbamoyl phosphate synthetase-I (CPS-I) (see reaction mechanism).There are two CP synthetases: a mitochondrial enzyme, CPS-I, which forms CP destined for inclusion in the urea cycle, and a cytosolic CP synthatase (CPS-II), which is involved in pyrimidine nucleotide biosynthesis. CPS-I is positively regulated by the allosteric effector N-acetyl-glutamate, while the cytosolic enzyme is acetylglutamate independent.
In a 2-step reaction, catalyzed by cytosolic argininosuccinate synthetase, citrulline is converted to argininosuccinate.
The reaction involves the addition of AMP (from ATP) to the amido carbonyl of citrulline, forming an activated intermediate on the enzyme surface (AMP-citrulline), and the subsequent addition of aspartate to form argininosuccinate.
Arginine and fumarate are produced from argininosuccinate by the cytosolic enzyme argininosuccinate lyase.
In the final step of the cycle arginase cleaves urea from aspartate, regenerating cytosolic ornithine, which can be transported to the mitochondrial matrix for another round of urea synthesis.
Beginning and ending with ornithine, the reactions of the cycle consumes 3 equivalents of ATP and a total of 4 high-energy nucleotide phosphates. Urea is the only new compound generated by the cycle; all other intermediates and reactants are recycled.
The energy consumed in the production of urea is more than recovered by the release of energy formed during the synthesis of the urea cycle intermediates. Ammonia released during the glutamate dehydrogenase reaction is coupled to the formation of NADH. In addition, when fumarate is converted back to aspartate, the malate dehydrogenase reaction used to convert malate to oxaloacetate generates a mole of NADH. These two moles of NADH, thus, are oxidized in the mitochondria yielding 6 moles of ATP.
Regulation of the Urea Cycle
The urea cycle operates only to eliminate excess nitrogen. On high-protein diets the carbon skeletons of the amino acids are oxidized for energy or stored as fat and glycogen, but the amino nitrogen must be excreted. To facilitate this process, enzymes of the urea cycle are controlled at the gene level. With long-term changes in the quantity of dietary protein, changes of 20-fold or greater in the concentration of cycle enzymes are observed. When dietary proteins increase significantly, enzyme concentrations rise. On return to a balanced diet, enzyme levels decline. Under conditions of starvation, enzyme levels rise as proteins are degraded and amino acid carbon skeletons are used to provide energy, thus increasing the quantity of nitrogen that must be excreted.
Short-term regulation of the cycle occurs principally at CPS-I, which is relatively inactive in the absence of its allosteric activator N-acetylglutamate. The steady-state concentration of N-acetylglutamate is set by the concentration of its components acetyl-CoA and glutamate and by arginine, which is a positive allosteric effector of N-acetylglutamate synthetase (glutamate transacylase).
A complete lack of any one of the enzymes of the urea cycle will result in death shortly after birth. However, deficiencies in each of the enzymes of the urea cycle, including N-acetylglutamate synthase, have been identified. These disorders are referred to as urea cycle disorders or UCDs. A common thread to most UCDs is hyperammonemia leading to ammonia intoxication with the consequences described below. Deficiencies in arginase do not lead to symptomatic hyperammonemia as severe or as commonly as in the other UCDs.
Clinical symptoms are most severe when the UCD is at the level of carbamoyl phosphate synthetase I. Symptoms of UCDs usually arise at birth and encompass, ataxia, convulsions, lethargy, poor feeding and eventually coma and death if not recognized and treated properly. In fact, the mortality rate is 100% for UCDs that are left undiagnosed. Several UCDs manifest with late-onset such as in adulthood. In these cases the symptoms are hyperactivity, hepatomegaly and an avoidance of high protein foods.
In general, the treatment of UCDs has as common elements the reduction of protein in the diet, removal of excess ammonia and replacement of intermediates missing from the urea cycle. Administration of levulose reduces ammonia through its action of acidifying the colon. Bacteria metabolize levulose to acidic byproducts which then promotes excretion of ammonia in the feces as ammonium ions, NH4+. Antibiotics can be administered to kill intestinal ammonia producing bacteria. Sodium benzoate and sodium phenylbutyrate can be administered to covalently bind glycine (forming hippurate) and glutamine (forming phenylacetylglutamine), respectively. These latter compounds, which contain the ammonia nitrogen, are excreted in the feces. Dietary supplementation with arginine or citrulline can increase the rate of urea production in certain UCDs.
AMINO ACID BIOSYNTHESIS
Glutamate and Aspartate
Glutamate and aspartate are synthesized from their widely distributed α-keto acid precursors by simple transamination reactions. The former catalyzed by glutamate dehydrogenase and the latter by aspartate aminotransferase, AST. Aspartate is also derived from asparagine through the action of asparaginase. The importance of glutamate as a common intracellular amino donor for transamination reactions and of aspartate as a precursor of ornithine for the urea cycle.
Alanine and the Glucose-Alanine Cycle
Aside from its role in protein synthesis, alanine is second only to glutamine in prominence as a circulating amino acid. In this capacity it serves a unique role in the transfer of nitrogen from peripheral tissue to the liver. Alanine is transferred to the circulation by many tissues, but mainly by muscle, in which alanine is formed from pyruvate at a rate proportional to intracellular pyruvate levels. Liver accumulates plasma alanine, reverses the transamination that occurs in muscle, and proportionately increases urea production. The pyruvate is either oxidized or converted to glucose via gluconeogenesis. When alanine transfer from muscle to liver is coupled with glucose transport from liver back to muscle, the process is known as the glucose-alanine cycle. The key feature of the cycle is that in 1 molecule, alanine, peripheral tissue exports pyruvate and ammonia (which are potentially rate-limiting for metabolism) to the liver, where the carbon skeleton is recycled and most nitrogen eliminated. There are 2 main pathways to production of muscle alanine: directly from protein degradation, and via the transamination of pyruvate by glutamate-pyruvate aminotransferase.
The sulfur for cysteine synthesis comes from the essential amino acid methionine. A condensation of ATP and methionine catalyzed by methionine adenosyltransferase S-adenosylmethionine (SAM or AdoMet).
Biosynthesis of S-adenosylmethionine, SAM
SAM serves as a precurosor for numerous methyl transfer reactions the conversion of norepinephrine to epinenephrine. The result of methyl transfer is the conversion of SAM to S-adenosylhomocysteine. S-adenosylhomocysteine is then cleaved by adenosylhomocyteinase to yield homocysteine and adenosine. Homocysteine can be converted back to methionine by methionine synthase, a reaction that occurs under methionine-sparing conditions and requires N5-methyl-tetrahydrofolate as methyl donor. This reaction was discussed in the context of vitamin B12-requiring enzymes. Transmethylation reactions employing SAM are extremely important, but in this case the role of S-adenosylmethionine in transmethylation is secondary to the production of homocysteine (essentially a by-product of transmethylase activity). In the production of SAM all phosphates of an ATP are lost: one as Pi and two as PPi. It is adenosine which is transferred to methionine and not AMP. In cysteine synthesis, homocysteine condenses with serine to produce cystathionine, which is subsequently cleaved by cystathionase to produce cysteine and α-ketobutyrate.
The sum of the latter two reactions is known as trans-sulfuration. Cysteine is used for protein synthesis and other body needs, while the α-ketobutyrate is decarboxylated and converted to propionyl-CoA. While cysteine readily oxidizes in air to form the disulfide cystine, cells contain little if any free cystine because the ubiquitous reducing agent, glutathione effectively reverses the formation of cystine by a non-enzymatic reduction reaction.
Utilization of methionine in the synthesis of cysteine
The 2 key enzymes of this pathway, cystathionine synthase and cystathionase (cystathionine lyase), both use pyridoxal phosphate as a cofactor, and both are under regulatory control. Cystathionase is under negative allosteric control by cysteine, as well, cysteine inhibits the expression of the cystathionine synthase gene. Genetic defects are known for both the synthase and the lyase. Missing or impaired cystathionine synthase leads to homocystinuria and is often associated with mental retardation, although the complete syndrome is multifaceted and many individuals with this disease are mentally normal. Some instances of genetic homocystinuria respond favorably to pyridoxine therapy, suggesting that in these cases the defect in cystathionine synthase is a decreased affinity for the cofactor. Missing or impaired cystathionase leads to excretion of cystathionine in the urine but does not have any other untoward effects. Rare cases are known in which cystathionase is defective and operates at a low level. This genetic disease leads to methioninuria with no other consequences.
Tyrosine is produced in cells by hydroxylating the essential amino acid phenylalanine. This relationship is much like that between cysteine and methionine. Half of the phenylalanine required goes into the production of tyrosine; if the diet is rich in tyrosine itself, the requirements for phenylalanine are reduced by about 50%. Phenylalanine hydroxylase is a mixed-function oxygenase: one atom of oxygen is incorporated into water and the other into the hydroxyl of tyrosine. The reductant is the tetrahydrofolate-related cofactor tetrahydrobiopterin, which is maintained in the reduced state by the NADH-dependent enzyme dihydropteridine reductase.
Biosynthesis of tyrosine from phenylalanine
Missing or deficient phenylalanine hydroxylase leads to the genetic disease known as phenlyketonuria (PKU), which if untreated leads to severe mental retardation. The mental retardation is caused by the accumulation of phenylalanine, which becomes a major donor of amino groups in aminotransferase activity and depletes neural tissue of α-ketoglutarate. This absence of α-ketoglutarate in the brain shuts down the TCA cycle and the associated production of aerobic energy, which is essential to normal brain development.
Ornithine and Proline Biosynthesis
Glutamate is the precursor of both proline and ornithine, with glutamate semialdehyde being a branch point intermediate leading to one or the other of these 2 products. While ornithine is not one of the 20 amino acids used in protein synthesis, it plays a significant role as the acceptor of carbamoyl phosphate in the urea cycle. Ornithine serves an additional important role as the precursor for the synthesis of the polyamines. The production of ornithine from glutamate is important when dietary arginine, the other principal source of ornithine, is limited. The fate of glutamate semialdehyde depends on prevailing cellular conditions. Ornithine production occurs from the semialdehyde via a simple glutamate-dependent transamination, producing ornithine.
Ornithine synthesis from glutamate
The main pathway to serine starts with the glycolytic intermediate 3-phosphoglycerate.
An NADH-linked dehydrogenase converts 3-phosphoglycerate into a keto acid, 3-phosphopyruvate, suitable for subsequent transamination. Aminotransferase activity with glutamate as a donor produces 3-phosphoserine, which is converted to serine by phosphoserine phosphatase.
The main pathway to glycine is a 1-step reaction catalyzed by serine hydroxymethyltransferase.
This reaction involves the transfer of the hydroxymethyl group from serine to the cofactor tetrahydrofolate (THF), producing glycine and N5,N10-methylene-THF. Glycine produced from serine or from the diet can also be oxidized by glycine cleavage complex, GCC, to yield a second equivalent of N5,N10-methylene-tetrahydrofolate as well as ammonia and CO2.
Glycine is involved in many anabolic reactions other than protein synthesis including the synthesis of purine nucleotides, heme, glutathione, creatine and serine.
Aspartate/Asparagine and Glutamate/Glutamine Biosynthesis
Glutamate is synthesized by the reductive amination of α-ketoglutarate catalyzed by glutamate dehydrogenase; it is thus a nitrogen-fixing reaction. In addition, glutamate arises by aminotransferase reactions, with the amino nitrogen being donated by a number of different amino acids. Thus, glutamate is a general collector of amino nitrogen. Aspartate is formed in a transamintion reaction catalyzed by aspartate transaminase, AST. This reaction uses the aspartate α-keto acid analog, oxaloacetate, and glutamate as the amino donor. Aspartate can also be formed by deamination of asparagine catalyzed by asparaginase. Asparagine synthetase and glutamine synthetase, catalyze the production of asparagine and glutamine from their respective α-amino acids. Glutamine is produced from glutamate by the direct incorporation of ammonia; and this can be considered another nitrogen fixing reaction. Asparagine, however, is formed by an amidotransferase reaction. Aminotransferase reactions are readily reversible. The direction of any individual transamination depends principally on the concentration ratio of reactants and products. By contrast, transamidation reactions, which are dependent on ATP, are considered irreversible. As a consequence, the degradation of asparagine and glutamine take place by a hydrolytic pathway rather than by a reversal of the pathway by which they were formed. As indicated above, asparagine can be degraded to aspartate.
PATHWAYS OF AMINO ACID DEGRADATION
There are 20 standard amino acids in proteins, with a variety of carbon skeletons. Correspondingly, there are 20 different catabolic pathways for amino acid degradation. In humans, these pathways taken together normally account for only 10 to 15% of the body's energy production. Therefore, the individual amino acid degradative pathways are not nearly as active as glycolysis and fatty acid oxidation. In addition, the activity of the catabolic pathways can vary greatly from one amino acid to the next, depending upon the balance between requirements for biosynthetic processes and the amounts of a given amino acid available. For this reason, we shall not examine them all in detail. The 20 catabolic pathways converge to form only five products, all of which enter the citric acid cycle. From here the carbons can be diverted to gluconeogenesis or ketogenesis, or they can be completely oxidized to CO2 and H2O.
All or part of the carbon skeletons of ten of the amino acids are ultimately broken down to yield acetyl-CoA. Five amino acids are converted into α-ketoglutarate, four into succinyl-CoA, two into fumarate, and two into oxaloacetate. The individual pathways for the 20 amino acids will be summarized by means of flow diagrams, each leading to a specific point of entry into the citric acid cycle. In these diagrams the amino acid carbon atoms that enter the citric acid cycle are shown in color. Note that some amino acids appear more than once, reflecting the fact that different parts of their carbon skeletons have different fates. Some of the enzymatic reactions in these pathways that are particularly noteworthy for their mechanisms or their medical significance will be singled out for special discussion.
The catabolism of arginine begins within the context of the urea cycle. It is hydrolyzed to urea and ornithine by arginase.
The Arginine Metabolic Pathways: Nitric Oxide Synthase and Arginase
Nitric Oxide Synthases (NOS)
Is an important cellular-signaling molecule, a potent vasodilatator due to the smooth muscle relaxation. It also inhibits platelet adherence and aggregation, reduces adherence of leukocytes to the endothelium. Furthermore, NO has been shown to inhibit DNA synthesis and mitogenesis, and the proliferation of vascular smooth muscle cells. These antiproliferative effects are likely to be mediated by cyclic GMP.
Nitric Oxide Synthases from the biochemical point of view, are a family of complex enzymes catalyzing the oxidation of L-arginine to form NO and L-citrulline. The three human NOS isoforms identified to date are: eNOS (endothelial NOS), nNOS (neuronal NOS), and iNOS (inducible NOS). Their genes are found on human chromosomes 7, 12, and 17, respectively, and so they were named for the tissue in which they were first cloned and characterized. vasculoprotective effect of individual NOS isoforms in human organism is not sufficiently clarified yet. Endothelial NOS (eNOS) and neuronal NOS (nNOS) are constitutively expressed, mainly in endothelial cells and nitrergic nerves, respectively, synthesizing a small amount of NO under basal conditions and on stimulation by various agonists. By contrast, inducible NOS (iNOS) is expressed when stimulated by inflammatory stimuli, synthesizing a large amount of NO in a transient manner. The knowledge of nitric oxide synthases (NOSs) is of extreme scientific importance, not only for understanding new pathophysiological mechanisms but also as a target for therapeutic intervention.
The role of NO in regulating vascular tone and mediating platelet function is attributable to the ongoing activity of eNOS. It is pharmacologically identical with previously isolated EDRF (endothelium-derived releasing factor), exprimed by the intact endothelium. Inactivation of the eNOS pathway limits the contribution of NO to vessel homeostasis and results in increased vascular tone and platelet adhesion and aggregation. The activity of eNOS is regulated by the intracellular free calcium concentration and calcium- calmodulin complexes. Endothelial NOS is a constitutively expressed protein predominantly associated with the particulate subcellular fraction, suggesting that the native enzyme is a membrane-bound protein. A detailed analysis of the membrane association of eNOS showed that this enzyme is localized to the Golgi apparatus as well as to specific structures in the plasmalemmal membrane called caveolae. The association of eNOS with a region of the plasma membrane in which several key signal-transducing complexes are concentrated (such as G-proteins) is likely to have profound repercussions on enzyme activity as well as on its accessibility to intracellular mechanisms of the pathway release, including mechanisms independent of intracellular calcium release.
Glycine is classified as a glucogenic amino acid, since it can be converted to serine by serine hydroxymethyltransferase, and serine can be converted back to the glycolytic intermediate, 3-phosphoglycerate or to pyruvate by serine/threonine dehydratase.
Nevertheless, the main glycine catabolic pathway leads to the production of CO2, ammonia, and one equivalent of N5,N10-methyleneTHF by the mitochondrial glycine cleavage complex.
Hyperglycinemia refers to a condition where glycine is elevated in the blood.
- Propionic acidemia, also known as "ketotic glycinemia"
- Glycine encephalopathy, also known as "non-ketotic hyperglycinemia".
Glycine encephalopathy (also known as non-ketotic hyperglycinemia or NKH) is a rare autosomal recessive disorder of glycine metabolism. After phenylketonuria, glycine encephalopathy is the second most common disorder of amino acid metabolism. The disease is caused by defects in the glycine cleavage system, an enzyme responsible for glycine catabolism. There are several forms of the disease, with varying severity of symptoms and time of onset. The symptoms are exclusively neurological in nature, and clinically this disorder is characterized by abnormally high levels of the amino acid glycine in bodily fluids and tissues, especially the cerebral spinal fluid.
Glutamine/Glutamate and Asparagine/Aspartate Catabolism
Glutaminase is an important kidney tubule enzyme involved in converting glutamine (from liver and from other tissue) to glutamate and NH3+, with the NH3+ being excreted in the urine. Glutaminase activity is present in many other tissues as well, although its activity is not nearly as prominent as in the kidney. The glutamate produced from glutamine is converted to -ketoglutarate, making glutamine a glucogenic amino acid. Asparaginase is also widely distributed within the body, where it converts asparagine into ammonia and aspartate. Aspartate transaminates to oxaloacetate, which follows the gluconeogenic pathway to glucose. Glutamate and aspartate are important in collecting and eliminating amino nitrogen via glutamine synthetase and the urea cycle, respectively. The catabolic path of the carbon skeletons involves simple 1-step aminotransferase reactions that directly produce net quantities of a TCA cycle intermediate. The glutamate dehydrogenase reaction operating in the direction of -ketoglutarate production provides a second avenue leading from glutamate to gluconeogenesis.
Alanine is also important in intertissue nitrogen transport as part of the glucose-alanine cycle. Alanine's catabolic pathway involves a simple aminotransferase reaction that directly produces pyruvate. Generally pyruvate produced by this pathway will result in the formation of oxaloacetate, although when the energy charge of a cell is low the pyruvate will be oxidized to CO2 and H2O via the PDH complex and the TCA cycle. This makes alanine a glucogenic amino acid.
Glutamine is converted to glutamate by glutaminase or several other enzymes by the removal of the amide nitrogen. Proline is first converted to a Schiff base and then converted by hydrolysis to glutamate-5-semialdehyde. All of these changes occur on the same carbon. Arginine and histidine contain 5adjacent carbons and a sixth carbon attached through a nitrogen attom. The catabolism of these amino acids is thus slightly more complicated than glutamine or proline. Arginine is converted to ornithine and urea. Ornithine is furthere transaminated to produce glutamate-5-semialdehyde. Glutamate-5-semialdehyde is converted to glutamate. The enzymes involved in the steps of the histidine pathway are listed in the box in the lower right corner of the diagram. Tetrahydrofolate is the cofactor in the final step converting histidine to glutamate. Transamination or deamination of glutamate produces a-ketoglutarate which feeds into the citric acid cycle.
Glutamate semialdehyde can serve as the precursor for proline biosynthesis as described above or it can be converted to glutamate. Proline catabolism is a reversal of its synthesis process. The glutamate semialdehyde generated from ornithine and proline catabolism is oxidized to glutamate by an ATP-independent glutamate semialdehyde dehydrogenase. The glutamate can then be converted to ketoglutarate in a transamination reaction. Thus arginine, ornithine and proline, are glucogenic.
Hyperprolinemia is an excess of a particular protein building block (amino acid), called proline, in the blood. This condition generally occurs when proline is not broken down properly by the body. There are two inherited forms of hyperprolinemia, called type I and type II.
Valine, Leucine and Isoleucine Catabolism
This group of essential amino acids are identified as the branched-chain amino acids, BCAAs. Because this arrangement of carbon atoms cannot be made by humans, these amino acids are an essential element in the diet. The catabolism of all three compounds initiates in muscle and yields NADH and FADH2 which can be utilized for ATP generation. The catabolism of all three of these amino acids uses the same enzymes in the first two steps. The first step in each case is a transamination using a single BCAA aminotransferase, with α-ketoglutarate as amine acceptor. As a result, three different α-keto acids are produced and are oxidized using a common branched-chain α-keto acid dehydrogenase, yielding the three different CoA derivatives. Subsequently the metabolic pathways diverge, producing many intermediates. The principal product from valine is propionylCoA, the glucogenic precursor of succinyl-CoA.
Isoleucine catabolism terminates with production of acetylCoA and propionylCoA; thus isoleucine is both glucogenic and ketogenic. Leucine gives rise to acetylCoA and acetoacetylCoA, and is thus classified as strictly ketogenic. There are a number of genetic diseases associated with faulty catabolism of the BCAAs. The most common defect is in the branched-chain α-keto acid dehydrogenase.
Since there is only one dehydrogenase enzyme for all three amino acids, all three α-keto acids accumulate and are excreted in the urine. The disease is known as Maple syrup urine disease because of the characteristic odor of the urine in afflicted individuals. Mental retardation in these cases is extensive. Unfortunately, since these are essential amino acids, they cannot be heavily restricted in the diet; ultimately, the life of afflicted individuals is short and development is abnormal The main neurological problems are due to poor formation of myelin in the CNS.
There are at least 3 pathways for threonine catabolism. One involves a pathway initiated by threonine dehydrogenase yielding a-amino-b-ketobutyrate. The α-amino-β-ketobutyrate is either converted to acetyl-CoA and glycine or spontaneously degrades to aminoacetone which is converted to pyruvate. The second pathway involves serine/threonine dehydratase yielding α-ketobutyrate which is further catabolized to propionyl-CoA and finally the TCA cycle intermediate, succinyl-CoA. The third pathway utilizes threonine aldolase. The products of this reaction are both ketogenic (acetyl-CoA) and glucogenic (pyruvate).
The principal fates of the essential amino acid methionine are incorporation into polypeptide chains, and use in the production of α-ketobutyrate and cysteine via SAM as described above. The transulfuration reactions that produce cysteine from homocysteine and serine also produce α-ketobutyrate, the latter being converted to succinyl-CoA. Regulation of the methionine metabolic pathway is based on the availability of methionine and cysteine.
If both amino acids are present in adequate quantities, SAM accumulates and is a positive effector on cystathionine synthase, encouraging the production of cysteine and α-ketobutyrate (both of which are glucogenic). However, if methionine is scarce, SAM will form only in small quantities, thus limiting cystathionine synthase activity. Under these conditions accumulated homocysteine is remethylated to methionine, using N5-methylTHF and other compounds as methyl donors.
There are several pathways for cysteine catabolism. The simplest, but least important pathway is catalyzed by a liver desulfurase and produces hydrogen sulfide, (H2S) and pyruvate. The more important catabolic pathway is via a cytochrome-P450-coupled enzyme, cysteine dioxygenase that oxidizes the cysteine sulfhydryl to sulfinate, producing the intermediate cysteinesulfinate. Cysteinesulfinate can serve as a biosynthetic intermediate undergoing decarboxylation and oxidation to produce taurine. Catabolism of cysteinesulfinate proceeds through transamination to b-sulfinylpyruvate which is in undergoes desulfuration yielding bisulfite, (HSO3-) and the glucogenic product, pyruvate. The enzyme sulfite oxidase uses O2 and H2O to convert HSO3- to sulfate, (SO4-) and H2O2. The resultant sulfate is used as a precursor for the formation of 3'-phosphoadenosine-5'-phosphosulfate,PAPS.
PAPS is used for the transfer of sulfate to biological molecules such as the sugars of the glycosphingolipids.Other than protein, the most important product of cysteine metabolism is the bile salt precursor taurine, which is used to form the bile acid conjugates taurocholate and taurochenodeoxycholate.The enzyme cystathionase can also transfer the sulfur from one cysteine to another generating thiocysteine and pyruvate. Transamination of cysteine yields -mercaptopyruvate which then reacts with sulfite, (SO32-), to produce thiosulfate, (S2O32-) and pyruvate. Both thiocysteine and thiosulfate can be used by the enzyme rhodanese to incorporate sulfur into cyanide, (CN-), thereby detoxifying the cyanide to thiocyanate.
Phenylalanine and Tyrosine Catabolism
Phenylalanine normally has only two fates: incorporation into polypeptide chains, and production of tyrosine via the tetrahydrobiopterin-requiring phenylalanine hydroxylase. Thus, phenylalanine catabolism always follows the pathway of tyrosine catabolism. The main pathway for tyrosine degradation involves conversion to fumarate and acetoacetate, allowing phenylalanine and tyrosine to be classified as both glucogenic and ketogenic. Tyrosine is equally important for protein biosynthesis as well as an intermediate in the biosynthesis of several physiologically important metabolites e.g. dopamine, norepinephrine and epinephrine.
As in phenylketonuria (deficiency of phenylalanine hydroxylase), deficiency of tyrosine transaminase leads to urinary excretion of tyrosine and the intermediates between phenylalanine and tyrosine.
Phenylketonuria (commonly known as PKU) is an inherited disorder that increases the levels of a substance called phenylalanine in the blood. Phenylalanine is a building block of proteins (an amino acid) that is obtained through the diet. It is found in all proteins and in some artificial sweeteners. If PKU is not treated, phenylalanine can build up to harmful levels in the body, causing intellectual disability and other serious health problems.
The signs and symptoms of PKU vary from mild to severe. The most severe form of this disorder is known as classic PKU. Infants with classic PKU appear normal until they are a few months old. Without treatment, these children develop permanent intellectual disability. Seizures, delayed development, behavioral problems, and psychiatric disorders are also common. Untreated individuals may have a musty or mouse-like odor as a side effect of excess phenylalanine in the body. Children with classic PKU tend to have lighter skin and hair than unaffected family members and are also likely to have skin disorders such as eczema.
Less severe forms of this condition, sometimes called variant PKU and non-PKU hyperphenylalaninemia, have a smaller risk of brain damage. People with very mild cases may not require treatment with a low-phenylalanine diet.
Babies born to mothers with PKU and uncontrolled phenylalanine levels (women who no longer follow a low-phenylalanine diet) have a significant risk of intellectual disability because they are exposed to very high levels of phenylalanine before birth. These infants may also have a low birth weight and grow more slowly than other children. Other characteristic medical problems include heart defects or other heart problems, an abnormally small head size (microcephaly), and behavioral problems. Women with PKU and uncontrolled phenylalanine levels also have an increased risk of pregnancy loss.
The adverse neurological symptoms are the same for the two diseases. Genetic diseases (such as various tyrosinemias and alkaptonuria) are also associated with other defective enzymes of the tyrosine catabolic pathway. The first genetic disease ever recognized, alcaptonuria, is caused by defective homogentisic acid oxidase. Homogentisic acid accumulation is relatively innocuous, causing urine to darken on exposure to air, but no life-threatening effects accompany the disease.
The other genetic deficiencies lead to more severe symptoms, most of which are associated with abnormal neural development, mental retardation, and shortened life span.
Albinism is a congenital disorder characterized by the complete or partial absence of pigment in the skin, hair and eyes due to absence or defect of tyrosinase, a copper-containing enzyme involved in the production of melanin. Albinism results from inheritance of recessive gene alleles and is known to affect all vertebrates, including humans. While an organism with complete absence of melanin is called an albino an organism with only a diminished amount of melanin is described as albinoid.
Albinism is associated with a number of vision defects, such as photophobia, nystagmus and astigmatism. Lack of skin pigmentation makes for more susceptibility to sunburn and skin cancers. In rare cases such as Chédiak–Higashi syndrome, albinism may be associated with deficiencies in the transportation of melanin granules. This also affects essential granules present in immune cells leading to increased susceptibility to infection.
Lysine catabolism is unusual in the way that the α-amino group is transferred to α-ketoglutarate and into the general nitrogen pool. The reaction is a transamination in which the α-amino group is transferred to the α-keto carbon of α-ketoglutarate forming the metabolite, saccharopine. Unlike the majority of transamination reactions, this one does not employ pyridoxal phosphate as a cofactor. Saccharopine is immediately hydrolyzed by the enzyme α-aminoadipic semialdehyde synthase in such a way that the amino nitrogen remains with the α-carbon of α-ketoglutarate, producing glutamate and α-aminoadipic semialdehyde. Because this transamination reaction is not reversible, lysine is an essential amino acid. The ultimate end-product of lysine catabolism is acetoacetyl-CoA Genetic deficiencies in the enzyme α-aminoadipic semialdehyde synthase have been observed in individuals who excrete large quantities of urinary lysine and some saccharopine. The lysinemia and associated lysinuria are benign. Other serious disorders associated with lysine metabolism are due to failure of the transport system for lysine and the other dibasic amino acids across the intestinal wall. Lysine is essential for protein synthesis; a deficiencies of its transport into the body can cause seriously diminished levels of protein synthesis. Probably more significant however, is the fact that arginine is transported on the same dibasic amino acid carrier, and resulting arginine deficiencies limit the quantity of ornithine available for the urea cycle. The result is severe hyperammonemia after a meal rich in protein. The addition of citrulline to the diet prevents the hyperammonemia. Lysine is also important as a precursor for the synthesis of carnitine, required for the transport of fatty acids into the mitochondria for oxidation. Free lysine does not serve as the precursor for this reaction, rather the modified lysine found in certain proteins. Some proteins modify lysine to trimethyllysine using SAM as the methyl donor to transfer methyl groups to the α -amino of the lysine side chain. Hydrolysis of proteins containing trimethyllysine provide the substrate for the subsequent conversion to carnitine.
SPECIALIZED PRODUCTS OF AMINO ACIDS
The majority of tyrosine that does not get incorporated into proteins is catabolized for energy production. One other significant fate of tyrosine is conversion to the catecholamines. The catecholamine neurotransmitters are dopamine, norepinephrine, and epinephrine (see also Biochemistry of Nerve Transmission).Norepinephrine is the principal neurotransmitter of sympathetic postganglionic endings. Both norepinephrine and the methylated derivative, epinephrine are stored in synaptic knobs of neurons that secrete it, however, epinephrine is not a mediator at postganglionic sympathetic endings.Tyrosine is transported into catecholamine-secreting neurons and adrenal medullary cells where catechaolamine synthesis takes place. The first step in the process requires tyrosine hydroxylase, which like phenylalanine hydroxylase requires tetrahydrobiopterin as cofactor. The hydroxylation reaction generates DOPA (3,4-dihydrophenylalanine). DOPA decarboxylase converts DOPA to dopamine, dopamine b-hydroxylase converts dopamine to norepinephrine and phenylethanolamine N-methyltransferase converts norepinephrine to epinephrine. This latter reaction is one of several in the body that uses SAM as a methyl donor generating S-adenosylhomocysteine. Within the substantia nigra and some other regions of the brain, synthesis proceeds only to dopamine. Within the adrenal medulla dopamine is converted to norepinephrine and epinephrine.
Synthesis of the catecholamines from tyrosine
Once synthesized, dopamine, norepinephrine and epinephrine are packaged in granulated vesicles. Within these vesicles, norepinephrine and epinephrine are bound to ATP and a protein called chromogranin A. Metabolism of the catecholemines occurs through the actions of catecholamine-O-methyltransferase, (COMT) and monoamine oxidase, (MAO). Both of these enzymes are widley distributed throughout the body. However, COMT is not found in nerve endings as is MAO.
Tryptopan serves as the precursor for the synthesis of serotonin (5-hydroxytryptamine, 5-HT, see also Biochemistry of Nerve Transmission) and melatonin (N-acetyl-5-methoxytryptamine).
Serotonin is synthesized through 2-step process involving a tetrahydrobiopterin-dependent hydroxylation reaction (catalyzed by tryptophan-5-monooxygenase) and then a decarboxylation catalyzed by aromatic L-amino acid decarboxylase. The hydroxylase is normally not saturated and as a result, an increased uptake of tryptophan in the diet will lead to increased brain serotonin content. Serotonin is present at highest concentrations in platelets and in the gastrointestinal tract. Lesser amounts are found in the brain and the retina. Serotonin containing neurons have their cell bodies in the midline raphe nuclei of the brain stem and project to portions of the hypothalamus, the limbic system, the neocortex and the spinal cord. After release from serotonergic neurons, most of the released serotonin is recaptured by an active reuptake mechanism. The function of the antidepressant, Prozac is to inhibit this reuptake process, thereby, resulting in prolonged serotonin presence in the synaptic cleft. The function of serotonin is exerted upon its interaction with specific receptors. Several serotonin receptors have been cloned and are identified as 5HT1, 5HT2, 5HT3, 5HT4, 5HT5, 5HT6, and 5HT7. Within the 5HT1 group there are subtypes 5HT1A, 5HT1B, 5HT1D, 5HT1E, and 5HT1F. There are three 5HT2 subtypes, 5HT2A, 5HT2B, and 5HT2C as well as two 5HT5 subtypes, 5HT5a and 5HT5B. Most of these receptors are coupled to G-proteins that affect the activities of either adenylate cyclase or phospholipase C (PLC). The 5HT3 class of receptors are ion channels.Some serotonin receptors are presynaptic and others postsynaptic. The 5HT2A receptors mediate platelet aggregation and smooth muscle contraction. The 5HT2C receptors are suspected in control of food intake as mice lacking this gene become obese from increased food intake and are also subject to fatal seizures. The 5HT3 receptors are present in the gastrointestinal tract and are related to vomiting. Also present in the gastrointestinal tract are 5HT4 receptors where they function in secretion and peristalsis. The 5HT6 and 5HT7 receptors are distributed throughout the limbic system of the brain and the 5HT6 receptors have high affinity for antidepressant drugs. Melatonin is derived from serotonin within the pineal gland and the retina, where the necessary N-acetyltransferase enzyme is found. The pineal parenchymal cells secrete melatonin into the blood and cerebrospinal fluid. Synthesis and secretion of melatonin increases during the dark period of the day and is maintained at a low level during daylight hours. This diurnal variation in melatonin synthesis is brought about by norepinephrine secreted by the postganglionic sympathetic nerves that innervate the pineal gland. The effects of norepinephrine are exerted through interaction with b-adrenergic receptors. This leads to increased levels of cAMP, which in turn activate the N-acetyltransferase required for melatonin synthesis. Melatonin functions by inhibiting the synthesis and secretion of other neurotransmitters such as dopamine and GABA.
Creatine is synthesized in the liver by methylation of guanidoacetate using SAM as the methyl donor. Guanidoacetate itself is formed in the kidney from the amino acids arginine and glycine.
Synthesis of creatine and creatinine
Creatine is used as a storage form of high energy phosphate. The phosphate of ATP is transferred to creatine, generating creatine phosphate, through the action of creatine phosphokinase. The reaction is reversible such that when energy demand is high (e.g. during muscle exertion) creatine phosphate donates its phosphate to ADP to yield ATP.Both creatine and creatine phosphate are found in muscle, brain and blood. Creatinine is formed in muscle from creatine phosphate by a nonenzymatic dehydration and loss of phosphate. The amount of creatinine produced is related to muscle mass and remains remarkably constant from day to day. Creatinine is excreted by the kidneys and the level of excretion (creatinine clearance rate) is a measure of renal function.
Glutathione (abbreviated GSH) is a tripeptide composed of glutamate, cysteine and glycine that has numerous important functions within cells. It serves as a reductant, is conjugated to drugs to make them more water soluble, is involved in amino acid transport across cell membranes (the γ-glutamyl cycle), is a part of the peptidoleukotrienes, serves as a cofactor for some enzymatic reactions and as an aid in the rearrangement of protein disulfide bonds.
The role of GSH as a reductant is extremely important particularly in the highly oxidizing environment of the erythrocyte. The sulfhydryl of GSH can be used to reduce peroxides formed during oxygen transport. The resulting oxidized form of GSH consists of two molecules disulfide bonded together (abbreviated GSSG).
The enzyme glutathione reductase utilizes NADPH as a cofactor to reduce GSSG back to two moles of GSH. Hence, the pentose phosphate pathway is an extremely important pathway of erythrocytes for the continuing production of the NADPH needed by glutathione reductase. In fact as much as 10% of glucose consumption, by erythrocytes, may be mediated by the pentose phosphate pathway.
Several mechanisms exist for the transport of amino acids across cell membranes. Many are symport or antiport mechanisms that couple amino acid transport to sodium transport. The γ-glutamyl cycle is an example of a group transfer mechanism of amino acid transport. Although this mechanism requires more energy input, it is rapid and has a high capacity. The cycle functions primarily in the kidney, particularly renal epithelial cells. The enzyme γ-glutamyl transpeptidase is located in the cell membrane and shuttles GSH to the cell surface to interact with an amino acid.
Synthesis of glutathione (GSH) Structure of GSSG
Reaction with an amino acid liberates cysteinylglycine and generates a γ-glutamyl-amino acid which is transported into the cell and hydrolyzed to release the amino acid. Glutamate is released as 5-oxoproline and the cysteinylglycine is cleaved to its component amino acids. Regeneration of GSH requires an ATP-dependent conversion of 5-oxoproline to glutamate and then the 2 additional moles of ATP that are required during the normal generation of GSH.
. Polyamnine Biosynthesis
One of the earliest signals that cells have entered their replication cycle is the appearance of elevated levels of mRNA for ornithine decarboxylase (ODC), and then increased levels of the enzyme, which is the first enzyme in the pathway to synthesis of the polyamines. Because of the latter, and because the polyamines are highly cationic and tend to bind nucleic acids with high affinity, it is believed that the polyamines are important participants in DNA synthesis, or in the regulation of that process.
The key features of the pathway are that it involves putrescine, an ornithine catabolite, and S-adenosylmethionine (SAM) as a donor of 2 propylamine residues. The first propylamine conjugation yields spermidine and addition of another to spermidine yields spermine.The function of ODC is to produce the 4-carbon saturated diamine, putrescine. At the same time, SAM decarboxylase cleaves the SAM carboxyl residue, producing decarboxylated SAM (S-adenosymethylthiopropylamine), which retains the methyl group usually involved in SAM methyltransferase activity. SAM decarboxylase activity is regulated by product inhibition and allosterically stimulated by putrescine. Spermidine synthase catalyzes the condensation reaction, producing spermidine and 5'-methylthioadenosine. A second propylamine residue is added to spermidine producing spermine.The signal for regulating ODC activity is unknown, but since the product of its activity, putrescine, regulates SAM decarboxylase activity, it appears that polyamine production is principally regulated by ODC concentration.The butylamino group of spermidine is used in a posttranslational modification reaction important to the process of translation. A specific lysine residue in the translational initiation factor eIF-4D is modified. Following the modification the residue is hydroxylated yielding a residue in the protein termed hypusine.
Nitric Oxide Synthesis and Function
Vasodilators, such as acetylcholine, do not exert their effects upon the vascular smooth muscle cell in the absence of the overlying endothelium. When acetylcholine binds its receptor on the surface of endothelial cells, a signal cascade, coupled to the activation phospholipase C (PLC), is initiated. The PLCg-mediated release of inositol trisphosphate, IP3 (from membrane associated phosphatidylinositol-4,5-bisphosphate, PIP2), leads to the release of intracellular stores of Ca2+. In turn, the elevation in Ca2+ leads to the liberation of endothelium-derived relaxing factor (EDRF) which then diffuses into the adjacent smooth muscle. Within smooth muscle cells, EDRF reacts with the heme moiety of a soluble guanylyl cyclase, resulting in activation of the latter and a consequent elevation of intracellular levels of cGMP. The net effect is the activation of cGMP-responsive enzymes which lead to smooth muscle cell relaxation. The coronary artery vasodilator, nitroglycerin, acts to increase intracellular release of EDRF and thus of cGMP. Quite unexpectedly, EDRF was found to be the free radical diatomic gas, nitric oxide, NO. NO is formed by the action of NO synthase, (NOS) on the amino acid arginine.
Nitric oxide is involved in a number of other important cellular processes in addition to its impact on vascular smooth muscle cells. Events initiated by NO that are important for blood coagulation include inhibition of platelet aggregation and adhesion and inhibition of neutrophil adhesion to platelets and to the vascular endothelium. NO is also generated by cells of the immune system and as such is involved in non-specific host defense mechanisms and macrophage-mediated killing. NO also inhibits the proliferation of tumor cells and microorganisms. Additional cellular responses to NO include induction of apoptosis (programmed cell death), DNA breakage and mutation. NOS is a very complex enzyme, employing five redox cofactors: NADPH, FAD, FMN, heme and tetrahydrobiopterin. NO can also be formed from nitrite, derived from vasodilators such as glycerin trinitrate during their metabolism. The half-life of NO is extremely short, lasting only 2-4 seconds. This is because it is a highly reactive free radical and interacts with oxygen and superoxide. NO is inhibited by hemoglobin and other heme proteins which bind it tightly.Chemical inhibitors of NOS are available and can markedly decrease production of NO. The effect is a dramatic increase in blood pressure due to vasoconstriction. Another important cardiovascular effect of NO is exerted through the production of cGMP, which acts to inhibit platelet aggregation.