THE MECHANISM OF HYDRIDE TRANSFER BETWEEN NADH AND 3-ACETYLPYRIDINE ADENINE-DINUCLEOTIDE BY THE PYRIDINE-NUCLEOTIDE TRANSHYDROGENASE OF ESCHERICHIA-COLI

The pyridine nucleotide transhydrogenase of Escherichia coli catalyzes the reversible transfer of hydride ion equivalents between NAD(+) and NADP(+) coupled to translocation of protons across the cytoplasmic me mbrane. Recently, transhydrogenation of 3-acetylpyridine adenine dinuc leotide (AcPyAD(+)), an analog of NAD(+), by NADH has been described u sing a solubilized preparation of E. coli transhydrogenase [Hutton, M. , Day, J.M., Bizouarn, T., and Jackson, J.B. (1994) fur. J. Biochem. 2 19, 1041-1051]. This reaction depended on the presence of NADP(H). We show that (a) this reaction did not require NADP(H) at pH 6 in contras t to pH 8; (b) the reaction occurred at pH 8 in the absence of NADP(H) in the mutant PH91K and in a mutant in which six amino acids of the c arboxy-terminus of the alpha subunit had been deleted; (c) the mutant transhydrogenases contained bound NADP(+) and were in a conformation i n which the beta subunit was digestible by uypsin; (d) the conformatio n of the beta subunit of the wild-type enzyme was made susceptible to trypsin digestion by NADP(H) or by placing the enzyme at pH 6 in the a bsence of NADP(H). It is concluded that reduction of AcPyAD(+) by NADH does not involve NADPH as an intermediate and that the role of NADP(H ) in this reaction at pH 8 is to cause the transhydrogenase to adopt a conformation favouring transhydrogenation between NADH and AcPyAD(+).