Membrane-bound transhydrogenases are conformationally driven proton-pumps which couple an inward proton translocation to the reversible reduction of NADP + by NADH(forward reaction). This reaction is stimulatedby an electrochemical protongradient,Dp, presumably throughan increased release of NADPH. The enzymes have three domains: domain II spans the membrane, while domain I and III are hydrophilic and contain the binding sites for NAD(H) andNADP(H), respectively.