MECHANISTIC STUDIES ON THE 8-AMINO-7-OXOPELARGONATE SYNTHASE, A PYRIDOXAL-5'-PHOSPHATE-DEPENDENT ENZYME INVOLVED IN BIOTIN BIOSYNTHESIS

The reaction mechanism of 8-amino-7-oxopelargonate (8-amino-7-oxononoa te) synthase from Bacillus sphaericus, an enzyme dependent on pyridoxa l 5'-phosphate (pyridoxal-P), which catalyzes the condensation of L-al anine with pimeloyl-CoA, the second step of biotin biosynthesis, has b een studied. To facilitate mechanistic studies, an improved over-expre ssion system in Escherichia coli, and a new continuous spectrophotomet ric assay for 8-amino-7-oxopelargonate synthase were designed. In orde r to discriminate between the two plausible basic mechanisms that can be put forth for this enzyme, that is: (a) formation of the pyridoxal- P-stabilized carbanion by abstraction of the C2-H proton of the alanin e-pyridoxal-P aldimine, followed by acylation and decarboxylation, and (b) formation of the carbanion by decarboxylation followed by acylati on, the fate of the C2-H proton of alanine during the course of the re action has been examined using H-1 NMR. Spectra of the 8-amino-7-oxope largonate formed using either L-[2-H-2]alanine in H2O or L-alanine in D2O, showed that the C2-H proton of alanine is lost during the reactio n and that the C8-H proton of 8-amino-7-oxopelargonate is derived from the solvent, a result that is only consistent with mechanism (a). Fur thermore 8-amino-7-oxopelargonate synthase catalyzes, in the absence o f pimeloyl-CoA, the stereospecific exchange, with retention of configu ration, of the C2-H proton of L-alanine with the solvent protons. Simi larly, 8-amino-7-oxopelargonate synthase catalyzes the exchange of the C8-H proton of 8-amino-7-oxopelargonate. In addition to these exchang e reactions, 8-amino-7-oxopelargonate synthase catalyzes an abortive t ransamination yielding an inactive pyridoxamine 5'-phosphate (pyridoxa mine-P) form of 8-amino-7-oxopelargonate synthase and pyruvate. Kineti c analysis gave a rate constant of k(exch) = 1.8 min(-1) for the excha nge reaction which is 10 times lower than the catalytic constant and a rate constant of k(trans) = 0.11 h(-1) for the transamination. Finall y deuterium kinetic isotope effects (KIE) were measured at position 2 of L-alanine (V-D = 1.3) and in D2O (V-D2O = 4.0). The magnitudes of t he KIE are consistent with a partially rate-limiting abstraction of th e C2-H proton of alanine and a partially rate-limiting reprotonation s tep. Taken together, all these results show that 8-amino-7-oxopelargon ate synthase utilizes mechanism (a). 8-Amino-7-oxopelargonate synthase and 5-aminolevulinate synthase, which has also been shown to use mech anism (a), belong to a class of pyridoxal-P-dependent enzymes that cat alyze the formation of alpha-oxoamines. Based on the fact that all the se alpha-oxoamine synthases share strong sequence similarities, we pos tulate that they also share the same reaction mechanism.