Proton-Translocating Transhydrogenase in Photosynthetic Bacteria

Transhydrogenase uses the proton-motive force across a membrane to drive the reduction of NADP+ by NADH. It is found in the inner mitochondrial membrane of animal cells and in the cytoplasmic membrane of bacteria, including many photosynthetic bacteria. The enzyme has three components: dI, which binds NADH, and dIII, which binds NADP+, protrude from the membrane, whereas dII spans the membrane. Within this organization, the polypeptide arrangement varies among species. The Rhodospirillum (Rsp.) rubrum transhydrogenase was the first to be shown to have a dissociable dI component, a finding that opened up a new approach in studies on the kinetics and the structure of the enzyme. A complex of dI and dIII from Rsp. rubrum transhydrogenase catalyzes very fast hydride transfer from NADH to NADP+. Crystal structures of the complex show how the dihydronicotinamide ring of the NADH and the nicotinamide ring of the NADP+ are brought together to effect this redox reaction. They also indicate that the dihydronicotinamide ring of the NADH can move within its binding site. This is thought to gate the hydride-transfer reaction and thus prevent a slip in the coupling mechanism. Short polypeptide loops in dI and dIII are seen to bind and to position the (dihydro)nicotinamide rings. Longer loops regulate nucleotide and solvent access to the hydride-transfer site. Asymmetry in the structure of the dI-dIII complex suggests that the intact enzyme may operate by an alternating-site mechanism in which one monomer runs 180° out-of-phase with the other.