Mechanism of hydride transfer during the reduction of 3-acetylpyridine adenine dinucleotide by NADH catalyzed by the pyridine nucleotide transhydrogenase of Escherichia coli.

The pyridine nucleotide transhydrogenase is a proton pump which catalyzes the reversible transfer of a hydride ion equivalent between NAD+ and NADP+ coupled to translocation of protons across the cytoplasmic membrane. The enzyme also catalyzes the reduction of the NAD+ analog 3-acetylpyridine adenine dinucleotide (AcPyAD+) by NADH. It has been proposed (Hutton et al. (1994) Eur. J. Biochem. 219, 1041-1051) that this reaction requires NADP(H) as an intermediate. Thus, NADP+ bound at the NADP(H)-binding site on the transhydrogenase would be reduced by NADH and reoxidized by AcPyAD+ binding alternately to the NAD(H)-binding site. The reduction of AcPyAD+ by NADPH would be a partial reaction in the reduction of AcPyAD+ by NADH. Using cytoplasmic membrane vesicles from mutants having elevated activities for transhydrogenation of AcPyAD+ by NADH in the absence of added NADP(H), the kinetics of reduction of AcPyAD+ by NADH and NADPH have been compared. The Km values for the reductants NADPH and NADH over a range of mutants, and for the non-mutant enzyme, differed to a much lesser degree than the Km for AcPyAD+ in the two reactions. The Km(AcPyAD) values for the transhydrogenation of AcPyAD+ by NADH were over an order of magnitude greater than those for the transhydrogenation of AcPyAD+ by NADPH. It is unlikely that AcPyAD+ binds at the same site in both reactions. A plausible explanation is that this substrate binds to the NADP(H)-binding site for transhydrogenation by NADH. Thus, a hydride equivalent can be transferred directly between NADH and AcPyAD+ under these conditions.