Branched chain amino acid glutamate transaminase

Branched-chain amino acid glutamate transaminase... 

Transaminase enzymes (also called aminotransferases) specifically use 2-oxoglutarate as the amino group acceptor to generate glutamate but some have a wide specificity with respect to the amino donor. For example, the three branched-chain amino acids leucine, isoleucine and valine, all serve as substrates for the same enzyme, branched-chain amino acid transaminase, BCAAT ... 

Finally, transamination with the branched-chain amino acid transaminase (EC 2.6.1.42), the coenzyme for which is pyridoxal and the source of the amino group from which pyridoxal transfers the nitrogen is glutamate (Glu, E), occurs. 2-Oxoglutarate forms in the process. 

Scheme 12.16. A representation of the final stages in the biosynthesis of isoleucine (He, I). In the late stages shown here, the migration of the ethyl group is accompanied by the reduction of the carbonyl generated in that migration (the enzyme is a reductoisomerase (EC 1.1.1.86). Dehydration to an enol (a dihydroxy acid dehydratase, EC4.2.1.9), tautomeriza-tion to the corresponding ketone, and a final transamination ( branched-chain amino acid transaminase with pyridoxal as a cofactor) from glutamate (Glu,E) produces isoleuciue (He, I). EC numbers and some graphic materials provided in this scheme have been taken from appropriate links in a URL starting with http //www.chem.qmul.ac.uk/iubmb/enzyme/.

Rudman and Meister IJ ) first showed the presence of a transaminase in cell-free extracts of E. colt that catalyze transamination reactions between glutamate and isoleucine, valine, leucine, norleucine, and norvaline. These monocarboxylic amino acids transaminated with each other as well as with glutamine. Preparations of an E. cdi mutant which did not respond to a-keto- 8-methylvalerate was unable to transaminate isoleucine or valine. The transaminase responsible for activity with the branched-chain amino acids was separated from other transaminases and considerably purified by standard methods of protein purification. It was shown to... 

L-Amino acid transaminases are ubiquitous in nature and are involved, be it directly or indirectly, in the biosynthesis of most natural amino acids. All three common types of the enzyme, aspartate, aromatic, and branched chain transaminases require pyridoxal 5 -phosphate as cofactor, covalently bound to the enzyme through the formation of a Schiff base with the e-amino group of a lysine side chain. The reaction mechanism is well understood, with the enzyme shuttling between pyridoxal and pyridoxamine forms [39]. With broad substrate specificity and no requirement for external cofactor regeneration, transaminases have appropriate characteristics to function as commercial biocatalysts. The overall transformation is comprised of the transfer of an amino group from a donor, usually aspartic or glutamic acids, to an a-keto acid (Scheme 15). In most cases, the equilibrium constant is approximately 1. 

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