Role of methionine-239, an amino acid residue in the mobile-loop region of the NADH-binding domain (domain I) of proton-translocating transhydrogenase.

Transhydrogenase couples the transfer of hydride equivalents between NAD(H) and NADP(H) to proton translocation across a membrane. The one-dimensional proton NMR spectrum of the recombinant NAD(H)-binding domain (domain I) of transhydrogenase from Rhodospirillum rubrum reveals well-defined resonances, several of which arise from a mobile loop at the protein surface. Four have been assigned to Met residues (MetA-MetD). Substitution of Met239 with either Ile (dI.M239I) or Phe (dI.M239F) resulted in loss of MetA from the NMR spectrum. Broadening and shifting of the mobile loop resonances consequent on NAD(H) binding indicate that the loop closes down on the protein surface. More NAD(H) had to be added to mutant domain I than to wild type to give comparable resonance broadening. The Kd of domain I for NADH, measured by equilibrium dialysis, was increased about three-fold by the Met239 mutations. Mutant and wild-type domain I were reconstituted with domain I-depleted membranes from R. rubrum, and with recombinant domain III of transhydrogenase. With membranes, the Km for acetylpyridine adenine dinucleotide during reverse transhydrogenation was 5x and > 6x greater in dI.M239I and dI.M239F, respectively, than in wild-type. Cyclic transhydrogenation (in membranes and the recombinant system) was substantially more inhibited (70% in dI.M239I, and 84% in dI.M239F) than either forward or reverse transhydrogenation. The docking affinities of dI.M239I and dI.M239F to the depleted membranes were similar to those of wild-type. It is concluded that Met239 is MetA in the mobile loop of domain I, and that in proteins with amino acid substitutions at this position, the binding affinity of NAD(H) is decreased, and the hydride transfer step is inhibited.