Pyridoxamine-pyruvate aminotransferase

In the other, the amino acid is rotated 180° so that the a-hydrogen protrudes behind the plane of the paper. Dunathan studied pyridoxamine pyruvate aminotransferase, an enzyme closely related to PLP-requiring aminotransferases and which catalyzes the transamination of pyridoxal with L-alanine to form pyridox-amine and pyruvate. The same reaction is catalyzed by the apoenzyme of aspartate aminotransferase. In both cases, when the alanine contained 2H in the a position the 2H was transferred stereospecifically into... 

Pyridoxamine pyruvate aminotransferase, stereochemistry of 748 Pyridoxine (vitamin B6) 305s, 738... 

Inhibits alanine racemase, pyridoxamine-pyruvate aminotransferase. 

The experiments conclusively prove that the addition of hydrogen to C-4 of the coenzyme occurs at the Si face of the Schiff base. Evidence has already been provided for the syn nature of the tautomeric process in the reaction catalysed by pyridoxamine-pyruvate aminotransferase [107]. If the same precedent is extended to aspartate aminotransferase it then follows that the bond to C that is formed and broken in this case must also be located on the Si face at C-4, in the catalytic complex, as shown in structure 2 (Fig. 53). In other words, the alternative arrangement for syn proton transfer shown in 1 (Fig. 53) is precluded by these experiments. The direction of hydrogen addition to C-4 of the coenzyme in the half-reaction has also been studied using several other L-amino add requiring aminotransferases and in every case the medium hydrogen was shown to add to the Si face at C-4 (Table 5). These experiments have led to the generaUsed view that in B -dependent reactions... 

Yoshikane, Y., Yokochi, N., Ohnishi, K., Hayashi, H., and Yagi, T., 2006. Molecular cloning, expression and characterization of pyridoxamine-pyruvate aminotransferase. Biochemical Journal. 396 499 507. 

A bacterial aminotransferase [46] promotes a decarboxylative transamination reaction with a-aminoisobutyrate in the presence of pyruvate. The reaction occurs via the sequence of Fig. 12 involving an initial cleavage of the Q-COjH bond in the substrate pyridoxal-P Schiff base complex (Fig. 12, 1) followed by reprotonation at C-4 of the coenzyme to give the pyridoxamine-P-enzyme complex (Fig. 12, 4) that participates in the transamination of pyruvate. However, the enzyme will also transform L-alanine at a significant rate by a half-transamination reaction into pyruvate, thereby implying that it is now the C -H bond of the amino acid that is first broken. 

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