Transamination

 Transamination

In this reaction the nitrogen is transferred as NH, from labile nitrogen pool to other compound which is catalyzed by specific enzymes originally termed as transaminases and recently referred as aminotransferases. It is reversible process as well as combined process of deamination and amination. In most cases there is transference of amino group from amino acid to keto acid which is derived from either amino acid or carbohydrate and fat. Pyridoxal phosphate (a vitamin B derivative) acts as the coenzyme.

This mechanism plays a part both in the breakdown of amino acids to yield keto acids and in the formation of new amino acids (non-essential) from keto acids. The keto acids may go to form carbohydrate (neoglucogenesis). All natural amino acids, viz., alanine, leucine, aspartic acid, glutamic acid, histidine, proline, tyrosine, etc. take part in the Transamination reaction readily where the glutamic acid and aspartic acid being most reactive. Lysine and threonine do not take part in Transamination reaction.

Transamination is a chemical reaction that transfers an amino group to a keto acid to form new amino acids. This pathway is responsible for the deamination of most amino acids. This is one of the major degradation pathways which convert  essential amino acids to non-essential amino acids (amino acids that can be synthesized de novo by the organism).

Transamination is accomplished by some enzymes called transaminases or aminotransferases. α-ketoglutarate acts as the predominant amino-group acceptor and produces glutamate as the new amino acid.

Amino acid + α-ketoglutarate ↔ α-keto acid + Glutamate

Glutamate's amino group, in turn, is transferred to oxaloacetate in a second Transamination reaction yielding aspartate.

Glutamate + oxaloacetate ↔ α-ketoglutarate + aspartate

Transamination catalyzed by aminotransferase occurs in two stages.

In the first stage, α- amino group of an amino acid is transferred to the enzyme, producing the corresponding α-keto acid and the aminated enzyme.

During the second stage, the amino group is transferred to the keto acid acceptor, forming the amino acid product while regenerating the enzyme. The chirality of an amino acid is determined during Transamination.

For the reaction to complete, aminotransferases require participation of aldehyde containing coenzyme, pyridoxal-5'-phosphate (PLP), a derivative of Pyridoxine (Vitamin B₆). The amino group is accommodated by conversion of this coenzyme to pyridoxamine-5'-phosphate (PMP). PLP is covalently attached to the enzyme via a Schiff Base linkage formed by the condensation of its aldehyde group with the ε-amino group of an enzymatic Lys residue. The Schiff base, which is conjugated to the enzymes pyridinium ring, is the focus of the coenzyme activity.

The product of Transamination reactions depend on the availability of α-keto acids. The products usually are one of alanine, aspartate or glutamate, since their corresponding alpha-keto acids are produced through metabolism of fuels. Being a major degradative amino acid pathway, lysine, proline and threonine are the only three amino acids that do not always undergo Transamination and rather use respective dehydrogenase.