Comparison of the Cytochrome bc1 Complex with theAnticipated Structure of the Cytochrome b6f Complex:De Plus Ça Change de Plus C'est la Même Chose

Structural alignment of the integral cytochrome b6-SU IV subunits with the solved structure of themitochondrial bc1 complex shows a pronounced asymmetry. There is a much higher homology onthe p-side of the membrane, suggesting a similarity in the mechanisms of intramembrane andinterfacial electron and proton transfer on the p-side, but not necessarily on the n-side. Structuraldifferences between the bc1 and b6f complexes appear to be larger the farther the domain or subunitis removed from the membrane core, with extreme differences between cytochromes c1 and f. Aspecial role for the dimer may involve electron sharing between the two hemes bp, which is indicatedas a probable event by calculations of relative rate constants for intramonomer heme bp → hemebn, or intermonomer heme bp → heme bp electron transfer. The long-standing observation offlash-induced oxidation of only ∼0.5 of the chemical content of cyt f may be partly a consequence ofthe statistical population of ISP bound to cyt f on the dimer. It is proposed that the p-side domainof cyt f is positioned with its long axis parallel to the membrane surface in order to: (i) allow itslarge and small domains to carry out the functions of cyt c1 and suVIII, respectively, of the bc1complex, and (ii) provide maximum dielectric continuity with the membrane. (iii) This positionwould also allow the internal water chain (“proton wire”) of cyt f to serve as the p-side exit portfor an intramembrane H+ transfer chain that would deprotonate the semiquinol located in themyxothiazol/MOA-stilbene pocket near heme bp. A hypothesis is presented for the identity of theamino acid residues in this chain.