Chapter 20: Structure–Function of the Cytochrome b₆f Complex: A Design that has Worked for Three Billion Years

The 3.0–3.1 Å X-ray structures of the cytochrome b₆ f complex from the thermophilic cyanobacterium Mastigocladus laminosus and from the green alga Chlamydomonas reinhardtii are very similar. Eight natural prosthetic groups, four hemes, one [2Fe-2S] cluster, one Chl, one β-carotene, and one n-side plastoquinone are embedded in the eight polypeptide subunits of the complex, four large (18–33 kDa) and four small (∼4 kDa). The complex is organized as a dimer with a molecular weight of 217 kDa in M. laminosus. Other subunits such as ferredoxin: NADP⁺ reductase may bind transiently and more weakly to the n-side of the complex. Major features of the structure are: (i) a large inter-monomer lipophilic “quinone exchange cavity” that exchanges plastoquinone/quinol with the quinone pool in the lipid bilayer membrane; (ii) a labyrinthine pathway of plastoquinone movement between n- and p-electron exchange sites through the 11 × 12 Å portal at the roof of the cavity; (iii) three prosthetic groups with unknown function, a novel high-spin heme (c_n) close to heme b_n, a chlorophyll a, and a β-carotene; (iv) a proposed function of heme c_n is in PS I-linked cyclic electron transport, although the presumed binding site of a “sometime” inhibitor of cyclic ET, antimycin A, is occluded by heme c_n; (v) the single Chl a molecule in the monomer is characterized by a short (200 ps) fluorescence lifetime and large anisotropy of fluorescence; and (vi) transfer of energy from the Chl triplet state to the β-carotene occurs despite the 14 Å separation of the pigments – it is proposed that this transfer operates through an intraprotein, interpigment O₂ channel.