title: Wolinella succinogenes quinol:fumarate reductase-2.2-A resolution crystal structure and the E-pathway hypothesis of coupled transmembrane proton and electron transfer.
abstract: The structure of the respiratory membrane protein complex quinol:fumarate reductase (QFR) from Wolinella succinogenes has been determined by X-ray crystallography at 2.2-A resolution [Nature 402 (1999) 377]. Based on the structure of the three protein subunits A, B, and C and the arrangement of the six prosthetic groups (a covalently bound FAD, three iron-sulfur clusters, and two haem b groups), a pathway of electron transfer from the quinol-oxidising dihaem cytochrome b in the membrane to the site of fumarate reduction in the hydrophilic subunit A has been proposed. The structure of the membrane-integral dihaem cytochrome b reveals that all transmembrane helical segments are tilted with respect to the membrane normal. The "four-helix" dihaem binding motif is very different from other dihaem-binding transmembrane four-helix bundles, such as the "two-helix motif" of the cytochrome bc(1) complex and the "three-helix motif" of the formate dehydrogenase/hydrogenase group. The gamma-hydroxyl group of Ser C141 has an important role in stabilising a kink in transmembrane helix IV. By combining the results from site-directed mutagenesis, functional and electrochemical characterisation, and X-ray crystallography, a residue was identified which was found to be essential for menaquinol oxidation [Proc. Natl. Acad. Sci. U. S. A. 97 (2000) 13051]. The distal location of this residue in the structure indicates that the coupling of the oxidation of menaquinol to the reduction of fumarate in dihaem-containing succinate:quinone oxidoreductases could in principle be associated with the generation of a transmembrane electrochemical potential. However, it is suggested here that in W. succinogenes QFR, this electrogenic effect is counterbalanced by the transfer of two protons via a proton transfer pathway (the "E-pathway") in concert with the transfer of two electrons via the membrane-bound haem groups. According to this "E-pathway hypothesis", the net reaction catalysed by W. succinogenes QFR does not contribute directly to the generation of a transmembrane electrochemical potential.
authors.affiliation: Max-Planck-Institut für Biophysik, Abteilung Molekulare Membranbiologie, Heinrich-Hoffmann-Str. 7, D-60528 Frankfurt am Main, Germany. Roy.Lancaster@mpibp-frankfurt.mpg.de
authors.forename: C Roy D
authors.initials: CR
authors.lastname: Lancaster
mesh_terms: D001665:Binding Sites; D003460:Crystallization; D018360:Crystallography, X-Ray; D003573:Cytochrome b Group / Q000737:chemistry; D004579:Electron Transport; D003576:Electron Transport Complex IV / Q000737:chemistry; D007425:Intracellular Membranes / Q000737:chemistry; D008565:Membrane Proteins / Q000737:chemistry; D008956:Models, Chemical; D008958:Models, Molecular; D015394:Molecular Structure; D010088:Oxidoreductases / Q000737:chemistry / Q000378:metabolism*; D011487:Protein Conformation; D024482:Vitamin K 2 / Q000737:chemistry; D016968:Wolinella / Q000201:enzymology / Q000378:metabolism*
chemical_list: D003573:Cytochrome b Group; D008565:Membrane Proteins; D024482:Vitamin K 2; D010088:Oxidoreductases; C082578:menaquinol oxidase; C424907:quinol fumarate reductase; D003576:Electron Transport Complex IV