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Ubiquinol:cytochrome c oxidoreductase
Cytochrome bc1 complex of Paracoccus denitrificans

Proton pumping cytochrome bc1 complex of 3 dissimilar subunits, PetABC.  The pathway of transmembrane electron transfer has been determined and compared with that of the B6f complex from the same organism which is strikingly different (Bhaduri et al. 2017).

Cytochrome bc1 Complex of Rhodobacter capsulatus
Cytochrome b, PetB, of 437 aas and 10 TMSs in a 5 + 5 TMS arrangement (P0CY47)
Cytochrome c1, PetA, of 279 aas and 2 TMSs, N- and C-terminal (D5ANZ4)
Iron-sulfur subunit of 191 aas and 1 TMS (P0CY48)

Ubiquinol:cytochrome c oxidoreductase

Cytochrome bc1 complex of Bos taurus

Proton-translocating cytochrome bc1/Rieske complex. Trophozoites of P. falciparum are inhibited by inhibitors such as atovaquone, buparvaquone and decoquinate (Meier et al. 2018).

Cytbc1 complex of Plasmodium falciparum
Cytb of 376 aas and 9 TMSs
cytc1 of 394 aas and 2 TMSsd
Rieske of 355 aas and 1 or 2 TMSs

Ubiquinol:cytochrome c oxidoreductase.  The cytochrome bc1 complex resides in the inner membrane of mitochondria and transfers electrons from ubiquinol to cytochrome c. This electron transfer is coupled to the translocation of protons across the membrane by the protonmotive Q cycle mechanism. This mechanism topographically separates reduction of quinone and reoxidation of quinol at sites on opposite sites of the membrane, referred to as the center N (Qn site) and the center P (Qp site), respectively. Both are located on cytochrome b, a transmembrane protein of the bc1 complex that is encoded on the mitochondrial genome (Ding et al. 2006).

Cytochrome bc1 complex of Saccharomyces cerevisiae

Menaquinone:cytochrome c oxidoreductase
Cytochrome bc1 complex of Bacillus subtilis

Plastoquinol:plastocyanine reductase
Cytochrome b6f complex of Synechocystis PCC6803

Cytochrome b6f complex, PetB, PetC, PetD, PetM, Hcf164. The dimeric photosynthetic cytochrome b6f complex, a 16-mer of eight distinct subunits and 26 transmembrane helices, catalyzes transmembrane proton-coupled electron transfer for energy storage. Using a 2.5 A crystal structure of the dimeric complex, Hasan and Cramer 2014 identified 23 distinct lipid-binding sites per monomer. Annular lipids provide a connection for super-complex formation with the photosystem-I reaction center and the LHCII kinase for transmembrane signaling. Internal lipids mediate crosslinking to stabilize the domain-swapped iron-sulfur protein subunit, dielectric heterogeneity within intermonomer and intramonomer electron transfer pathways, and dimer stabilization through lipid-mediated intermonomer interactions.  In the cytochrome b6f complex with the quinol analog, stigmatellin, which partitions in the Qp portal of the bc1 complex, but not of b6f, the Qp portal is partially occluded in the b6f complex relative to bc1. Occlusion of the Qp portal is attributed to the presence of the chlorophyll phytyl tail, which increases the quinone residence time within the Qp portal and is inferred to be a cause of enhanced superoxide production (Hasan et al. 2014).  PetD subunit integration into the thylakoid membrane is a post-translational and an SRP-dependent process that requires the formation of a cpSRP-cpFtsY-ALB3-PetD complex (Króliczewski et al. 2017). The role of the b6f complex in trans-membrane signal transduction from reductant generated on The effect of the p-side of the electron transport chain to the regulation of light energy to the two photosystems by trans-side phosphorylation of the light-harvesting chlorophyll protein has been discussed (Cramer 2018).

Cytochrome b6f complex of Arabidopsis thaliana

Cytochrome b6/f complex, PetABCD of (cyt f; cyt b6, iron sulfur protein, and subunit 4, respectively, of 324, 215, 179, and 160 aas, respectively (Soo et al. 2017).

PetABCD of Synechococcus elongatus (strain PCC 7942) (Anacystis nidulans R2)