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Transamination and Transaminases
Q.- Give a brief description of the process of Transamination, highlight the role of B6 phosphate in this process.
Answer- Transamination interconverts pairs of α -amino acids and α -keto acids. During Transamination, the amino group of an amino acid (amino acid R 1) is transferred to a keto acid (keto acid R 2), this produces a new keto acid while from the original keto acid, a new amino acid is formed (Figure-1)
Figure-1- showing the transfer of amino group from a donor amino acid to a keto acid for the formation of a new amino acid and a new keto acid
The general process of transamination is reversible and is catalyzed by a transaminase, also called amino transferase that require B6-Phosphate as a coenzyme.
Most of the amino acids act as substrate for the transaminases but the amino acids like lysine, threonine, proline, and hydroxyproline do not participate in transamination reactions.
Transamination is not restricted to α -amino groups. The δ-amino group of ornithine and the ε-amino group of lysine—readily undergoes transamination.
Role of B6 Phosphate as a coenzyme
The coenzyme pyridoxal phosphate (PLP) is present at the catalytic site of aminotransferases and of many other enzymes that act on amino acids. PLP, is a derivative of vitamin B6 (Figure-2).
Figure-2- Showing the structure of B6-Phosphate
1) B6-P forms an enzyme-bound Schiff base intermediate that can rearrange in various ways-
Figure-3-In the “resting” state, the aldehyde group of pyridoxal phosphate is in a Schiff base linkage to the ε-amino group of an enzyme lysine side-chain.
2) During transamination, bound PLP serves as a carrier of amino groups (Figure-5 and 6)
3) Rearrangement forms an α -keto acid and enzyme-bound Pyridoxamine phosphate(Figure-4, 5 and 6), which forms a Schiff base with a second keto acid (Figure-5).
Figure-4-The α-amino group of a substrate amino acid displaces the enzyme lysine, to form a Schiff base linkage to PLP.
Figure-5- A different a-keto acid reacts with PMP (Pyridoxamine phosphate) and the process reverses, to complete the reaction.
Figure-6 -Overall reaction showing the role of B6-Phosphate, the transfer of α-amino group from donor amino acid to Pyridoxal phosphate forms Pyridoxamine phosphate, and a keto acid. The α-amino group is finally passed on to an acceptor an α-keto acid to form a new amino acid.
Significance of Transamination – Transamination is used both for the catabolic as well as anabolic processes. The resultant α-Keto acid can be completely oxidized to provide energy, glucose, fats or ketone bodies depending upon the cellular requirement. Since it is a reversible process, it is also used or the synthesis of non-essential amino acids. Some points of significance are as follows-
- Once the keto acids have been formed from the appropriate amino acids by transamination, they may be used for several purposes. The most obvious is the complete metabolism into carbon dioxide and water by the citric acid cycle.
- However, if there are excess proteins in the diet those amino acids that are converted into pyruvic acid and acetyl CoA can be converted into lipids by the lipogenesis process. If carbohydrates are lacking in the diet or if glucose cannot get into the cells (as in diabetes), then those amino acids converted into pyruvic acid and oxaloacetic acids can be converted into glucose or glycogen.
- The most usual and major keto acid involved with transamination reactions is alpha-ketoglutaric acid, an intermediate in the citric acid cycle.
- All of the amino acids can be converted through a variety of reactions and transamination into a keto acid which is a part of or feeds into the citric acid cycle.
- In addition to the catabolic function of transamination reactions, these reactions can also be used to synthesize amino acids needed or not present in the diet. An amino acid may be synthesized if there is an available “root” keto acid with a synthetic connection to the final amino acid. Since an appropriate “root” keto acid does not exist for eight amino acids, (lys, leu, ile, met, thr, try, val, phe), they are essential and must be included in the diet because they cannot be synthesized. Transaminases equilibrate amino groups among available α-keto acids. This permits synthesis of non-essential amino acids, using amino groups derived from other amino acids and carbon skeletons synthesized in the cell. Thus a balance of different amino acids is maintained, as proteins of varied amino acid contents are synthesized.
- Glutamic acid usually serves as the source of the amino group in the transamination synthesis of new amino acids. The reverse of the reactions are the most obvious methods for producing the amino acids alanine and aspartic acid.
- In addition to equilibrating amino groups among available α-keto acids, transaminases funnel amino groups from excess dietary amino acids to those amino acids (e.g., glutamate) that can be deaminated. Carbon skeletons of deaminated amino acids can be catabolized for energy or used to synthesize glucose or fatty acids for energy storage.
Figure-7 – Glutamate is the ultimate collector of amino groups of amino acids, In the liver it is rapidly deaminated, ammonia thus released is detoxified by forming urea
Q.- Discuss the clinical significance of transaminases.
Answer– The enzymes catalyzing transamination process 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-ketoglutarate ( α-KG), which participate in reactions with many different aminotransferases. Serum aminotransferases such as serum glutamate-oxaloacetate-aminotransferase (SGOT) (also called aspartate aminotransferase, AST) and serum glutamate-pyruvate aminotransferase (SGPT) (also called alanine transaminase, ALT) have been used as clinical markers of tissue damage, with increasing serum levels indicating an increased extent of damage.
1) AST is found in the liver, cardiac muscle, skeletal muscle, kidneys, brain, pancreas, lungs, leukocytes, and erythrocytes in decreasing order of concentration.
Reaction catalyzed can be represented as follows-
Figure-8- Showing the reaction catalyzed by AST (Aspartate amino transferase)
Normal serum activity is 0-41 IU/L. The concentration of the enzyme is very high in myocardium. The enzyme is both cytoplasmic as well as mitochondrial in nature.
2) ALT is found primarily in the liver.
Reaction catalyzed can be represented as follows-
Figure-9 – Showing the reaction catalyzed by ALT(Alanine amino transferase)
The normal serum activity ranges between 0-45 IU/L.
Diagnostic significance of amino transferases-
I) Liver Diseases- The aminotransferases are normally present in the serum in low concentrations. These enzymes are released into the blood in greater amounts when there is damage to the liver cell membrane resulting in increased permeability.These are sensitive indicators of liver cell injury and are most helpful in recognizing acute hepatocellular diseases such as hepatitis. Any type of liver cell injury can cause modest elevations in the serum aminotransferases.
- Levels of up to 300 U/L are nonspecific and may be found in any type of liver disorder.
- Striking elevations—i.e., aminotransferases > 1000 U/L—occur almost exclusively in disorders associated with extensive hepatocellular injury such as (1) viral hepatitis, (2) ischemic liver injury (prolonged hypotension or acute heart failure), or (3) toxin- or drug-induced liver injury.
- In most acute hepatocellular disorders, the ALT is higher than or equal to the AST.
- An AST: ALT ratio > 2:1 is suggestive while a ratio > 3:1 is highly suggestive of alcoholic liver disease.
- The AST in alcoholic liver disease is rarely >300 U/L and the ALT is often normal. A low level of ALT in the serum is due to an alcohol-induced deficiency of Pyridoxal phosphate.
- In obstructive jaundice the aminotransferases are usually not greatly elevated. One notable exception occurs during the acute phase of biliary obstruction caused by the passage of a gallstone into the common bile duct. In this setting, the aminotransferases can briefly be in the 1000–2000 U/L range. However, aminotransferase levels decrease quickly, and the liver function tests rapidly evolve into one typical of cholestasis.
2) Acute myocardial infarction- In acute MI the serum activity rises sharply within the first 12 hours, with a peak level of 24 hours or over and returns to normal within 3 to 5 days.
- Levels > 350 IU/L are usually fatal and signify massive infarction
- Levels < 50 IU/L are associated with low mortality
- The rise depends upon the size of infarction
- There is no rise of ALT in acute MI
- Reinfarction results in secondary rise of AST
3) Extra cardiac and extra hepatic conditions-
- Elevation of AST can also be seen in Muscle disorders like muscular dystrophies- myositis etc.
- Increase activity of AST is also observed in acute pancreatitis, leukemias and acute hemolytic anemias
- In normal health slight rise of AST level can be observed after prolonged exercise
4) Glucose Alanine cycle- 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.
Figure-10- Glucose Alanine cycle functions to transport amino group of amino acids in the form of alanine from skeletal muscle to liverPlease help "Biochemistry for Medics" by CLICKING ON THE ADVERTISEMENTS above!