Consequences of the structure of the cytochrome b6f complex for its charge transfer pathways. | Semantic Scholar (2024)

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.

Functional studies are favored by the absence of antennae and the simple photosynthetic reaction chain but are hampered by the high oxygen sensitivity of the organism, its chlorophyll, and lipids.

The cytochrome b 6 f complex is a shared, central component of both photosynthetic and respiratory electron- and proton transport in cyanobacteria and its function within the cyanobacterial electron transport network is reviewed.

Early evidence for the presence of this unusual cytochrome, originally called G (Lavergne), is re-examine.

A systematic redox titration analysis based on three independent and comprehensive low-temperature spectroscopies allowed for unambiguous assignment of spectral components of hemes in cytochrome b6f and revealed that Em of heme bn is unexpectedly low.

Important advances include: the high-resolution structures of Cyt b 6 f and Cyt bc 1 complexes; evidence for domain movement of the Rieske iron-sulfur protein subunit during catalysis; discovery of a previously undetected c-type heme required for transmembrane electron flow and activity of the plastoquinone-reductase site of Cyr 6 f complexes.

Key issues surrounding the mechanisms of the RB complexes are discussed, including a series of currently debated models for the avoidance of deleterious side reactions within the Qo site, the mechanism of stabilization of semiquinone intermediates within the Qi site, and the role of the pivoting iron-sulfur protein subunit.

The ethyl methanesulfonate-induced nuclear mutant hcf208 is a hom*olog of the Chlamydomonas reinhardtii CCB2 protein, which is a factor mediating attachment of heme c(n) to the cytochrome b(6) polypeptide as part of a novel heme biogenesis pathway, called system IV.

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The presence of a unique heme, hemex, that is covalently linked by a single thioether bond to a Cys residue on the electrochemically negative (n) side of the cytochrome b(6) polypeptide implies that there is at least one more function of heme x that is related to axial binding of a physiological ligand.

The dimeric b6f complex from the thermophilic cyanobacterium Mastigocladus laminosus reveals a large quinone exchange cavity, stabilized by lipid, in which plastoquinone, a quin one-analog inhibitor, and a novel heme are bound.

Examination of the redox properties, the site of action of the inhibitor NQNO, and the question of interheme transfer in the chloroplast cytochrome b6 have been examined, providing no evidence for efficient electron transfer from heme bp to heme ba as specified by the Q cycle model.

The spectrum of c(i) matches the absorption changes previously observed in vivo for an unknown redox center denoted "G," and the effect of the membrane potential on the electron transfer equilibrium between G and heme b(H) found in earlier experiments is discussed.

The X-ray structure of cytochrome b6f from the alga Chlamydomonas reinhardtii is presented and it is shown that this haem is atypical as it is covalently bound by one thioether linkage and has no axial amino acid ligand, which may be the missing link in oxygenic photosynthesis.

A model of quinone oxidation in the bc1 complex is proposed, which incorporates fixed and loose states of the ISP as features important for electron transfer and, possibly, also proton transport.

Recent advances in the understanding of the mechanism of the bc1 complex and their relation to physiologically important issues are reviewed in the context of the structural information available.

X-ray crystal structures of the cytochrome bc1 complex from chicken, cow and rabbit in both the presence and absence of inhibitors of quinone oxidation, reveal two different locations for the extrinsic domain of one component of the enzyme, an iron–sulphur protein.

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