title: A review of the binding-change mechanism for proton-translocating transhydrogenase.
abstract: 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. This article is part of a Special Issue entitled: 17th European Bioenergetics Conference (EBEC 2012).
authors.affiliation: University of Birmingham, Birmingham, UK. j.b.jackson@bham.ac.uk
authors.forename: J Baz
authors.initials: JB
authors.lastname: Jackson
mesh_terms: D000818:Animals; D018360:Crystallography, X-Ray; D004926:Escherichia coli / Q000201:enzymology; D029968:Escherichia coli Proteins / Q000737:chemistry / Q000378:metabolism; D006801:Humans; D017136:Ion Transport / Q000502:physiology; D051336:Mitochondrial Membranes / Q000201:enzymology*; D024101:Mitochondrial Proteins / Q000737:chemistry / Q000378:metabolism; D009249:NADP / Q000737:chemistry / Q000378:metabolism; D009250:NADP Transhydrogenases / Q000737:chemistry / Q000378:metabolism; D011522:Protons; D012247:Rhodospirillum rubrum / Q000201:enzymology
chemical_list: D029968:Escherichia coli Proteins; D024101:Mitochondrial Proteins; D011522:Protons; D009249:NADP; D009250:NADP Transhydrogenases