Comparison of the cytochrome bc(1) complex with the anticipated structure of the cytochrome b(6)f complex: De plus ca change de plus c'est la meme chose

Structural alignment of the integral cytochrome b(6)-SU IV subunits with th e solved structure of the mitochondrial bc(1) complex shows a pronounced as ymmetry. There is a much higher homology on the p-side of the membrane, sug gesting a similarity in the mechanisms of intramembrane and interfacial ele ctron 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 fo r 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) --> he me b(p) electron transfer. The long-standing observation of flash-induced o xidation of only similar to 0.5 of the-chemical content of cyt f may be par tly a consequence of the statistical population of ISP bound to cyt f on th e 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 l arge and small domains to carry out the functions of cyt c(1) and suVII, re spectively, of the bc(1) complex, and (ii) provide maximum dielectric conti nuity 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 lo cated in the myxothiazol/MOA-stilbene pocket near heme b(p). A hypothesis i s presented for the identity of the amino acid residues in this chain.