Biosynthesis of 7, 8-diaminopelargonic acid from 7-keto-8-aminopelargonic acid and S-adenosyl-L-methionine. The kinetics of the reaction.

The transamination of 7-keto-8-aminopelargonic acid by 7, 8-diaminopelargonic acid aminotransferase of Escherichia coli requires S-adenosyl-L-methionine as the amino donor. Initial velocity studies of this reaction revealed a parallel pattern of reciprocal plots characteristic of a ping-pong mechanism. m-Keto-8-aminopelargonic acid showed strong substrate inhibition which was competitive with S-adenosyl-L-methionine. The Michaelis constants determined for S-adenosyl-L-methionine and 7-keto-8-aminopelargonic acid were 0.20 mM and 1.2 muM, respectively. The Vmax of 0.16 mumol/mg/min corresponds to a turnover number for the enzyme of only 17 molecules/molecule enzyme/min. The Km values for the interaction of pyridoxal 5'-phosphate and pyridoxamine 5'-phosphate with the apoenzyme were determined to be 32 muM and 21 muM, respectively. Two classes of inhibitors were observed: (a) those which showed competitive inhibition with respect to S-adenosynd (b) those which showed noncompetitive inhibition with respect to both substrates. In the former group were S-adenosyl-L-(2-hydroxy-4-methylthio)butyric acid and adenosine. In the latter were S-adenosyl-L-ethionine, adenine, and 8-keto-7-aminopelargonic acid. L-Methionine, S-methyl-L-methionine, inosine, and hypoxanthine were not significantly inhibitory. Certain conformations of the substrates in the active site of the enzyme have been proposed which explain: (a) the requirement for the sulfonium ion of S-adenosyl-L-methionine for activity but not for binding to the enzyme, and (b) the ability of 7-keto-8-aminopelargonic acid to bind to the pyridoxal form of the enzyme as a potent substrate inhibitor.