A review of the binding-change mechanism for proton-translocating transhydrogenase

摘要

Proton-translocating transhydrogenase is found in the inner membranes of animal mitochondria, and in the cytoplasmic membranes of many bacteria. It catalyses hydride transfer from NADH to NADP + coupled to inward proton translocation. Evidence is reviewed suggesting the enzyme operates by a binding-change mechanism. Experiments with Escherichia coli transhydrogenase indicate the enzyme is driven between open and occluded states by protonation and deprotonation reactions associated with proton translocation. In the open states NADP +/NADPH can rapidly associate with, or dissociate from, the enzyme, and hydride transfer is prevented. In the occluded states bound NADP +/NADPH cannot dissociate, and hydride transfer is allowed. Crystal structures of a complex of the nucleotide-binding components of Rhodospirillum rubrum transhydrogenase show how hydride transfer is enabled and disabled at appropriate steps in catalysis, and how release of NADP +/NADPH is restricted in the occluded state. Thermodynamic and kinetic studies indicate that the equilibrium constant for hydride transfer on the enzyme is elevated as a consequence of the tight binding of NADPH relative to NADP +. The protonation site in the translocation pathway must face the outside if NADP + is bound, the inside if NADPH is bound. Chemical shift changes detected by NMR may show where alterations in protein conformation resulting from NADP + reduction are initiated.