Rabu, 21 Desember 2011


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.

Reactions catalyzed by glutamate dehydrogenase

Aspartate is formed in a transamination 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.

Reaction catalyzed by aspartate transaminase (AST)

Reaction 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.

Reaction catalyzed by asparagine synthetase

Reaction catalyzed by glutamine synthetase

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

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