Reaction of serine-glyoxylate aminotransferase with the alternative substrate ketomalonate indicates rate-limiting protonation of a quinonoid intermediate.

William E Karsten, Takashi Ohshiro, Yoshikazu Izumi, Paul F Cook

Department of Chemistry and Biochemistry, University of Oklahoma, 620 Parrington Oval, Norman, Oklahoma 73019, USA.

Biochemistry 2005 Dec 6

filter terms:

Serine-glyoxylate aminotransferase (SGAT) from Hyphomicrobium methylovorum is a pyridoxal 5'-phosphate (PLP) enzyme that catalyzes the interconversion of L-serine and glyoxylate to hydroxypyruvate and glycine. The primary deuterium isotope effect using L-serine 2-D is one on (V/K)serine and V in the steady state. Pre-steady-state experiments also indicate that there is no primary deuterium isotope effect with L-serine 2-D. The results suggest there is no rate limitation by abstraction of the alpha proton of L-serine in the SGAT reaction. In the steady-state a solvent deuterium isotope effect of about 2 was measured on (V/K)L-serine and (V/K)ketomalonate and about 5.5 on V. Similar solvent isotope effects were observed in the pre-steady-state for the natural substrates and the alternative substrate ketomalonate. In the pre-steady-state, no reaction intermediates typical of PLP enzymes were observed with the substrates L-serine, glyoxylate, and hydroxypyruvate. The data suggest that breakdown and formation of the ketimine intermediate is the primary rate-limiting step with the natural substrates. In contrast, using the alternative substrate ketomalonate, pre-steady-state experiments display the transient formation of a 490 nm absorbing species typical of a quinonoid intermediate. The solvent isotope effect results also suggest that with ketomalonate as substrate protonation at C(alpha) is the slowest step in the SGAT reaction. This is the first report of a rate-limiting protonation of a quinonoid at C(alpha) of the external Schiff base in an aminotransferase reaction.

Citation

William E Karsten, Takashi Ohshiro, Yoshikazu Izumi, Paul F Cook. Reaction of serine-glyoxylate aminotransferase with the alternative substrate ketomalonate indicates rate-limiting protonation of a quinonoid intermediate. Biochemistry. 2005 Dec 6;44(48):15930-6