The heterotrimer of the membrane-peripheral components of transhydrogenase and the alternating-site mechanism of proton translocation

Jamie Venning, Daniel Rodrigues, Christopher Weston, Nicholas Cotton, Philip Quirk, N Errington, S Finet, Scott White, John Jackson

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28 Citations (Scopus)

Abstract

Transhydrogenase undergoes conformational changes to couple the redox reaction between NAD(H) and NADP(H) to proton translocation across a membrane. The protein comprises three components: dI, which binds NAD(H); dIII, which binds NADP(H); and dII, which spans the membrane. Experiments using isothermal titration calorimetry, analytical ultracentrifugation, and small angle x-ray scattering show that, as in the crystalline state, a mixture of recombinant dI and dIII from Rhodospirillum rubrum transhydrogenase readily forms a dI(2)dIII(1) heterotrimer in solution, but we could find no evidence for the formation of a dI(2)dIII(2) tetramer using these techniques. The asymmetry of the complex suggests that there is an alternation of conformations at the nucleotide-binding sites during proton translocation by the complete enzyme. The characteristics of nucleotide interaction with the isolated dI and dIII components and with the dI(2)dIII(1) heterotrimer were investigated. (a) The rate of release of NADP(+) from dIII was decreased 5-fold when the component was incorporated into the heterotrimer. (b) The binding affinity of one of the two nucleotide-binding sites for NADH on the dI dimer was decreased about 17-fold in the dI(2)dIII(1) complex; the other binding site was unaffected. These observations lend strong support to the alternating-site mechanism.
Original languageEnglish
Pages (from-to)30678-30685
Number of pages8
JournalJournal of Biological Chemistry
Volume276
Issue number33
Early online date8 Jun 2001
DOIs
Publication statusPublished - 10 Aug 2001

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