Download MendeleyExperimental Support for the E-Pathway Hypothesis of Coupled Transmembrane Electron and Proton Transfer in Dihemic Quinol:Fumarate Reductase
Lancaster, C.R.D., Sauer, U.S., Gross, R., Haas, A.H., Graf, J., Schwalbe, H., Maentele, W., Simon, J., Madej, G.(2005) Proc Natl Acad Sci U S A 102: 18860
- PubMed: 16380425 Search on PubMedSearch on PubMed Central
- DOI: https://doi.org/10.1073/pnas.0509711102
- Primary Citation Related Structures:
2BS3, 2BS4 - PubMed Abstract:
Reconciliation of apparently contradictory experimental results obtained on the quinol:fumarate reductase, a diheme-containing respiratory membrane protein complex from Wolinella succinogenes, was previously obtained by the proposal of the so-called "E pathway hypothesis." According to this hypothesis, transmembrane electron transfer via the heme groups is strictly coupled to cotransfer of protons via a transiently established pathway thought to contain the side chain of residue Glu-C180 as the most prominent component. Here we demonstrate that, after replacement of Glu-C180 with Gln or Ile by site-directed mutagenesis, the resulting mutants are unable to grow on fumarate, and the membrane-bound variant enzymes lack quinol oxidation activity. Upon solubilization, however, the purified enzymes display approximately 1/10 of the specific quinol oxidation activity of the wild-type enzyme and unchanged quinol Michaelis constants, K(m). The refined x-ray crystal structures at 2.19 A and 2.76 A resolution, respectively, rule out major structural changes to account for these experimental observations. Changes in the oxidation-reduction heme midpoint potential allow the conclusion that deprotonation of Glu-C180 in the wild-type enzyme facilitates the reoxidation of the reduced high-potential heme. Comparison of solvent isotope effects indicates that a rate-limiting proton transfer step in the wild-type enzyme is lost in the Glu-C180 --> Gln variant. The results provide experimental evidence for the validity of the E pathway hypothesis and for a crucial functional role of Glu-C180.
- Department of Molecular Membrane Biology, Max Planck Institute of Biophysics, Max-von-Laue-Strasse 3, D-60438 Frankfurt am Main, Germany. roy.lancaster@mpibp-frankfurt.mpg.de
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