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Transamination and Transaminases – A quick revision
Transamination interconverts pairs of α-amino acids and α-keto acids. During transamination, the alpha amino group of an amino acid (amino acid R1) is transferred to a α keto acid (keto acid R2), this produces a new keto acid while from the original keto acid, a new amino acid is formed (Figure-1)
Figure-1- 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 transaminases, also called amino transferases that require B6-Phosphate as coenzyme.
- Most of the amino acids act as substrate for the transaminases but the amino acids like lysine, threonine, proline, and hydroxy proline 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.
- 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.
Role of B6 Phosphate as a coenzyme
The coenzyme pyridoxal phosphate (PLP) is present at the catalytic site of aminotransferases, PLP, is a derivative of vitamin B6.
During transamination, bound PLP serves as a carrier of amino groups. Rearrangement forms an α-keto acid and enzyme-bound Pyridoxamine phosphate, which forms a Schiff base with a second keto acid (Figure-2).
Figure-2 – 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 acceptor 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 for the synthesis of non-essential amino acids.
Clinical significance of transaminases.
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 (figure-3) can be represented as follows-
Figure-3- Reaction catalyzed by AST
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.
Diagnostic significance of Aspartate amino transferase
Marker of Acute myocardial infarction- In acute MI the serum activity of AST 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.
Extra cardiac 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
- Alcohol consumption increases AST levels (Alcohol induces AST synthesis)
2) ALT (Alanine amino transferase) is found primarily in the liver.
Reaction catalyzed (figure-4) can be represented as follows-
Figure-4- Reaction catalyzed by ALT
The normal serum activity ranges between 0-45 IU/L
Diagnostic significance of Alanine amino transferase
1) 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. ALT is a better indicator of liver cell injury as compared to AST.
2) Alcoholic liver cell injury- Both AST and ALT levels are increased.
3) 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 (Figure-5).
Figure-5- 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 a-KG, forming Glutamate and pyruvate. Glutamate is oxidatively deaminated forming ammonia that is detoxified to form urea. The pyruvate can then be diverted into the pathway of gluconeogenesis. This process is referred to as the glucose-alanine cycle.
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