Comparison of the cytochrome bc1 complex with the anticipated structure of the cytochrome b6f complex:: De plus ca change de plus c'est la meme chose

Structural alignment of the integral cytochrome b(6)-SU IV subunits with the solved structure of the mitochondrial bc(1) 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 bc(1) and b(6)f complexes appear to be larger the farther the domain or subunit is removed from the membrane core, with extreme differences between cytochromes c(1) 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 similar to 0.5 of the-chemical content of cyt f may be partly a consequence of the statistical population of ISP bound to cyt f on 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 c(1) and suVII, respectively, of the bc(1) 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.