Structure–Function of the Cytochrome b₆f Complex^†

Abstract

The structure and function of the cytochrome b6 f complex is considered in the context of recent crystal structures of the complex as an eight subunit, 220 kDa symmetric dimeric complex obtained from the thermophilic cyanobacterium, Mastigocladus laminosus, and the green alga, Chlamydomonas reinhardtii. A major problem confronted in crystallization of the cyanobacterial complex, proteolysis of three of the subunits, is discussed along with initial efforts to identify the protease. The evolution of these cytochrome complexes is illustrated by conservation of the hydrophobic heme‐binding transmembrane domain of the cyt b polypeptide between b6 f and bc1 complexes, and the rubredoxin‐like membrane proximal domain of the Rieske [2Fe‐2S] protein. Pathways of coupled electron and proton transfer are discussed in the framework of a modified Q cycle, in which the heme cn, not found in the bc1 complex, but electronically tightly coupled to the heme bn of the b6 f complex, is included. Crystal structures of the cyanobacterial complex with the quinone analogue inhibitors, NQNO or tridecyl‐stigmatellin, show the latter to be ligands of heme cn, implicating heme cn as an n‐side plastoquinone reductase. Existing questions include (a) the details of the shuttle of: (i) the [2Fe‐2S] protein between the membrane‐bound PQH2 electron/H+ donor and the cytochrome f acceptor to complete the p‐side electron transfer circuit; (ii) PQ/PQH2 between n‐ and p‐sides of the complex across the intermonomer quinone exchange cavity, through the narrow portal connecting the cavity with the p‐side [2Fe‐2S] niche; (b) the role of the n‐side of the b6 f complex and heme cn in regulation of the relative rates of noncyclic and cyclic electron transfer. The likely presence of cyclic electron transport in the b6 f complex, and of heme cn in the firmicute bc complex suggests the concept that hemes bn‐cn define a branch point in bc complexes that can support electron transport pathways that differ in detail from the Q cycle supported by the bc1 complex.

The DnaJ-Like Zinc-Finger Protein HCF222 Is Required for Thylakoid Membrane Biogenesis in Plants

“…The monomeric complex in higher plants consists of eight subunits: the plastomeencoded PetA (cytochrome f), PetB (cytochrome b 6 ), PetD, PetG, PetL, and PetN, and the nucleus-encoded PetC (Rieske iron-sulfur protein), and PetM. These subunits bound seven prosthetic groups, comprising two b-type hemes and atypically bound heme c n in cytochrome b 6 , a c-type heme in cytochrome f, a [2Fe-2S] cluster in the Rieske protein, and chlorophyll a and b-carotene (Baniulis et al, 2008). In a subpopulation of the Cytb6f complex, the ferredoxin-NADP + -reductase (FNR) was identified as a stoichiometric component (Zhang et al, 2001).…”

Inducible Repression of Nuclear-Encoded Subunits of the Cytochrome b6f Complex in Tobacco Reveals an Extraordinarily Long Lifetime of the Complex

“…The cyt-bf is the smallest multisubunit complex of the linear electron transport chain. Its active form is a dimer composed of eight different subunits with a total molecular mass of about 220 kD (Cramer et al, 2006;Baniulis et al, 2008). High-resolution structures of the cyt-bf have been obtained in the filamentous thermophilic cyanobacterium Mastigocladus laminosus (Kurisu et al, 2003;Hasan et al, 2013), in Nostoc sp.…”

“…In addition to the four large subunits, four small subunits with molecular masses of 3 to 4 kD are bound to each monomer of the cyt-bf (Baniulis et al, 2008). They form single transmembrane helices (hydrophobic sticks) and do not participate in any of the redox reactions of the complex.…”

Early Evolution of Photosynthesis

“…The evolutionary origin of this complex is not yet clear. While the cytochrome bc 1 and b 6 f complexes are similar in many ways, the cytochrome c 1 and f subunits are very different and are almost certainly of distinct evolutionary origin (Baniulis et al, 2008).…”