title: Comparison of the cytochrome bc1 complex with the anticipated structure of the cytochrome b6f complex: Le plus ça change le plus c'est la même chose.
abstract: Structural alignment of the integral cytochrome b6-SU IV subunits with the solved structure of the mitochondrial bc1 complex shows a pronounced asymmetry. There is a much higher homology on the p-side of the membrane, suggesting a similarity in the mechanisms of intramembrane and interfacial electron and proton transfer on the p-side, but not necessarily on the n-side. Structural differences between the bc1 and b6f complexes appear to be larger the farther the domain or subunit is removed from the membrane core, with extreme differences between cytochromes c1 and f. A special role for the dimer may involve electron sharing between the two hemes b(p), which is indicated as a probable event by calculations of relative rate constants for intramonomer heme b(p) --> heme b(n), or intermonomer heme b(p) --> heme b(p) electron transfer. The long-standing observation of flash-induced oxidation of only approximately 0.5 of the chemical content of cyt f may be partly a consequence of the statistical population of ISP bound to cytfon the dimer. It is proposed that the p-side domain of cyt f is positioned with its long axis parallel to the membrane surface in order to: (i) allow its large and small domains to carry out the functions of cyt c1 and suVIII, respectively, of the bc1 complex, and (ii) provide maximum dielectric continuity with the membrane. (iii) This position would also allow the internal water chain ("proton wire") of cyt f to serve as the p-side exit port for an intramembrane H+ transfer chain that would deprotonate the semiquinol located in the myxothiazol/MOA-stilbene pocket near heme b(p). A hypothesis is presented for the identity of the amino acid residues in this chain.
authors.affiliation: Department of Biological Sciences, Purdue University, West Lafayette, Indiana 47907-1392, USA.
authors.forename: G M
authors.initials: GM
authors.lastname: Soriano
authors.forename: M V
authors.initials: MV
authors.lastname: Ponamarev
authors.forename: C J
authors.initials: CJ
authors.lastname: Carrell
authors.forename: D
authors.initials: D
authors.lastname: Xia
authors.forename: J L
authors.initials: JL
authors.lastname: Smith
authors.forename: W A
authors.initials: WA
authors.lastname: Cramer
mesh_terms: D000818:Animals; D001426:Bacterial Proteins / Q000737:chemistry; D003573:Cytochrome b Group / Q000737:chemistry*; D045346:Cytochrome b6f Complex; D004024:Dicyclohexylcarbodiimide / Q000494:pharmacology; D019281:Dimerization; D004579:Electron Transport; D014450:Electron Transport Complex III / Q000737:chemistry*; D006418:Heme / Q000737:chemistry; D008958:Models, Molecular; D010084:Oxidation-Reduction; D011487:Protein Conformation; D017434:Protein Structure, Tertiary; D011522:Protons; D013329:Structure-Activity Relationship; D014451:Ubiquinone / Q000737:chemistry
chemical_list: D001426:Bacterial Proteins; D003573:Cytochrome b Group; D011522:Protons; D014451:Ubiquinone; D006418:Heme; D004024:Dicyclohexylcarbodiimide; D045346:Cytochrome b6f Complex; D014450:Electron Transport Complex III