Summary
Aspects of the crystal structures of the hetero-oligomeric cytochrome bc1 and b6f (“bc”) complexes relevant to their electron/proton transfer function and the associated redox reactions of the lipophilic quinones are discussed. Differences between the b6f and bc1 complexes are emphasized.
The cytochrome bc1 and b6f dimeric complexes diverge in structure from a core of subunits that coordinate redox groups consisting of two bis-histidine coordinated hemes, a heme bn and bp on the electrochemically negative (n) and positive (p) sides of the complex, the high potential [2Fe-2S] cluster and c-type heme at the p-side aqueous interface and aqueous phase, respectively, and quinone/quinol binding sites on the n- and p-sides of the complex. The bc1 and b6f complexes diverge in subunit composition and structure away from this core. b6f also contains additional prosthetic groups including a c-type heme cn on the n-side, and a chlorophyll a and β-carotene. Common structure aspects; functions of the symmetric dimer. (I) Quinone exchange with the bilayer. An inter-monomer protein-free cavity of approximately 30 Å along the membrane normal × 25 Å (central inter-monomer distance) × 15 Å (depth in the center), is common to both bc1 and b6f complexes, providing a niche in which the lipophilic quinone/quinol (Q/QH2) can be exchanged with the membrane bilayer. (II) Electron transfer. The dimeric structure and the proximity of the two hemes bp on the electrochemically positive side of the complex in the two monomer units allow the possibility of two alternate routes of electron transfer across the complex from heme bp to bn,: intra-monomer, and inter-monomer involving electron cross-over between the two hemes bp. A structure-based summary of inter-heme distances in seven bc complexes, representing mitochondrial, chromatophore, cyanobacterial, and algal sources, indicates that, based on the distance parameter, the intra-monomer pathway would be favored kinetically. (III) Separation of quinone binding sites. A consequence of the dimer structure and the position of the Q/QH2binding sites is that the p-side QH2 oxidation and n-side Q reduction sites are each well separated. Therefore, In the event of an overlap in residence time by QH2 or Q molecules at the two oxidation or reduction sites, their spatial separation would result in minimal steric interference between extended Q or QH2 isoprenoid chains. (IV) Trans-membrane QH2/Q transfer. (i) n/p side QH2/Q transfer may be hindered by lipid acyl chains; (ii) the shorter less hindered inter-monomer pathway across the complex would not pass through the center of the cavity, as inferred from the n-side antimycin site on one monomer and the p-side stigmatellin site on the other residing on the same surface of the complex. (V) Narrow p-Side portal for QH2/Q passage. The [2Fe-2S] cluster that serves as oxidant, and whose hisitidine ligand serves as a H+ acceptor in the oxidation of QH2, is connected to the inter-monomer cavity by a narrow extended portal, which is also occupied in the ...