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Biophys. J. BioFAST: First Published September 10, 2004. doi:10.1529/biophysj.104.042945
© 2004 by the Biophysical Society.

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BIOENERGETICS

Calculated Coupling of Transmembrane Electron and Proton Transfer in Dihemic Quinol:Fumarate Reductase

Alexander H. Haas 1 and C. Roy D. Lancaster 1*

1 Max Planck Institute of Biophysics

* To whom correspondence should be addressed. E-mail: roy.lancaster{at}mpibp-frankfurt.mpg.de.

Submitted on March 19, 2004
Revised on May 27, 2004
Accepted on 25 August 2004


  Abstract
The quinol:fumarate reductase (QFR) of Wolinella succinogenes binds a low- and a high-potential heme b group in its transmembrane subunit C. Both hemes are part of the electron transport chain between the two catalytic sites of this redox enzyme. The oxidation-reduction midpoint potentials of the hemes are well established but their assignment in the structure has not yet been determined. By simulating redox titrations, using continuum electrostatics calculations, it was possible to achieve an unequivocal assignment of the low- and high-potential hemes to the ''distal'' and ''proximal'' positions in the structure, respectively. Prominent features governing the differences in midpoint potential between the two hemes are the higher loss of reaction field energy for the proximal heme and the stronger destabilization of the oxidized form of the proximal heme due to several buried Arg and Lys residues. According to the so-called ''E-pathway hypothesis'', QFR has previously been postulated to exhibit a novel coupling of transmembrane electron and proton transfer. Simulation of heme b reduction indicates that the protonation state of the conserved residue Glu C180, predicted to play a key role in this process, indeed depends on the redox state of the hemes. This result clearly supports the ''E-pathway hypothesis''.

Key Words: E-Pathway hypothesis, Multiconformation continuum electrostatics, Wolinella succinogenes, diheme cytochrome b, respiratory complex II, succinate:quinone oxidoreductase




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Copyright © 2004 by the Biophysical Society.