Transhydrogenase, in animal mitochondria and bacteria, couples hydride transfer between NADH and NADP(+) to proton translocation across a membrane. Within the protein, the redox reaction occurs at some distance from the proton translocation pathway and coupling is achieved through conformational changes. In an 'open' conformation of transhydrogenase, in which substrate nucleotides bind and product nucleotides dissociate, the dihydronicotinamide and nicotinamide rings are held apart to block hydride transfer; in an 'occluded' conformation, they are moved into apposition to permit the redox chemistry. In the two monomers of transhydrogenase, there is a reciprocating, out-of-phase alternation of these conformations during turnover.