Electroneutral And Electrogenic Catalysis By Diheme-Containing Succinate:Quinone Oxidoreductases

Abstract

Membrane proteins can support both the generation and utilisation of a transmembrane electrochemical proton potential (“proton-motive force”), either by transmembrane electron transfer coupled to protolytic reactions on opposite sides of the membrane or by transmembrane proton transfer. In the case of the diheme-containing quinol:fumarate reductase (QFR) of Wolinella succinogenes [1], both theoretical and experimental results, reviewed in [2], supported, but did not prove, a previous hypothesis [3] that both of these mechanisms are combined in a single membrane protein complex, so as to facilitate transmembrane electron transfer by transmembrane proton transfer. Results of measurements on proteoliposomes will be presented, providing evidence for the presence of this unprecedented transmembrane proton transfer pathway (“E-pathway”) in the wild-type enzyme and its non-functionality in a variant QFR where a key glutamate residue has been replaced [4]. The “E-pathway”, discussed on the basis of the 1.78-A-resolution crystal structure of QFR, is essential for life under the conditions of fumarate respiration. Results will be compared to those obtained with the diheme-containing succinate:menaquinone reductase from the Gram-positive bacterium Bacillus licheniformis [5,6].