Consequences of the structure of the cytochrome b6f complex for its charge transfer pathways

At least two features of the crystal structures of the cytochrome b(6)f complex from the thermophilic cyanobacterium, Mastigocladus laminosus and a green alga, Chlamydomonas reinhardtii, have implications for the pathways and mechanism of charge (electron/proton) transfer in the complex: (i) The narrow 11 x 12 angstrom portal between the p-side of the quinone exchange cavity and p-side plastoquinone/quinol binding niche, through which all Q/QH(2) must pass, is smaller in the b(6)f than in the bc(1) complex because of its partial occlusion by the phytyl chain of the one bound chlorophyll a molecule in the b(6)f complex. Thus, the pathway for trans-membrane passage of the lipophilic quinone is even more labyrinthine in the b(6)f than in the bc(1) complex. (ii) A unique covalently bound heme, heme c(n), in close proximity to the n-side b heme, is present in the b(6)f complex. The b(6)f structure implies that a Q cycle mechanism must be modified to include heme c(n) as an intermediate between heme b(n) and plastoquinone bound at a different site than in the bc(1) complex. In addition, it is likely that the heme b(n)-c(n) couple participates in photosytem I-linked cyclic electron transport that requires ferredoxin and the ferredoxin: NADP(+) reductase. This pathway through the n-side of the b(6)f complex could overlap with the n-side of the Q cycle pathway. Thus, either regulation is required at the level of the redox state of the hemes that would allow them to be shared by the two pathways, and/or the two different pathways are segregated in the membrane. (c) 2006 Elsevier B.V. All rights reserved.