Quinols, oxidation

This electron transport and proton flow is controlled by at least three other proteins on the cytosolic side of the membrane, P47, P67, and P21rac (Figure 5). Note that this is a much simpler enzyme complex than the complex III (cyt bc ) of mitochondria which drives proton export from the interior of mitochondria based on quinol oxidation. Despite its simplicity, the neutrophil enzyme may have similarities to the complex three, since it essentially carries out an oxidation of a protonated two electron flavin by a nonprotonated cytochrome b complex with two heme sites. This is the essence of the mitochondrial enzyme in that the two electron quinol is oxidized by a cytochrome b with two heme components. 

They can also catalyze the opposite reaction, the coupling of quinol oxidation to quinone to the reduction of fumarate to succinate (Lemma et al., 1991). The m-configuration isomer of fumarate, maleinate, is neither produced in the oxidation reaction nor consumed as a substrate in the reduction reaction, i.e, the reaction is stereospecific in both directions. Depending on the direction of the reaction catalyzed in vivo, the members of the superfamily of succinate quinone oxidoreductases... 

For the function of QFR, electrons have to be transferred from the quinol-oxidizing site in the membrane to the fumarate-reducing site, protruding into the cytoplasm. The arrangement of the prosthetic groups in the QFR dimer is displayed in Fig. 8a together with the... 

The third protein complex in this electron-transfer chain (complex 111) is ubiquinol cytochrome c oxidoreductase (E.C. 1.10.2.2), or commonly known as cytochrome be, complex named after the its b-type and c-type cytochrome subunits. Probably the best-understood one among the complexes, be, complex catalyses electron transfers between two mobile electron carriers the hydrophobic molecule ubiquinone (Q) and the small soluble haem-containing protein cytochrome c. Two protons are translocated across the membrane per quinol oxidized (Hinkel, 1991 Crofts, 1985 Mitchell, 1976). 

The loop between the E- and F-helices is adjacent to the Qq site and contains a highly conserved sequence, namely Pro-294/Glu-295/Trp-296/Tyr-297 (or PEWY ). Mutation of the PEWY residues would impair the function of the Qo site and is known to raise the redox potential of Cyt (LP). It also results in a 50-fold inhibition of the quinol oxidation rate. The trace from flash-induced absorption change in Fig. 13 (C, b) shows that no reduction of Cyt (HP) takes place in the Glu-295->Gln mutant even in the presence of antimycin. The behavior of this mutant is similar to that of the wild-type control in the presence of myxothiazol. Of course, the wild-type control shows an enhanced reduction of Cyt (HP) but no re-oxidation when antimycin is present (see lower trace in Fig. 13 (C, b). 

AR Crofts, S Hong, Z Zhang and EA Berry (1999) Physicochemical aspects of the movement of the Rieske iron sulfur protein during quinol oxidation by the bci complex from mitochondria and photosynthetic bacteria. Biochemistry 38 15827-15839... 

Hydroxyquinones from quinols Oxidative ring opening... 

The free energy gained from the quinol oxidation inthe cytochrome-6c, complex allows further proton transfer from the cytoplasm to the periplasm. The 6c,-complex also mediates ET to the periplasmic side. There, soluble cytochromes accept the electrons and transport them back to the RC to reduce D+. The electron transfer is cycUc and therefore does not cause transmembrane potential. This potential is generated by the electrogenic proton translocation in the cytochrome-6c, complex. The electrochemical proton gradient is utilized by the ATP-synthase to form adenosine triphosphate from adenosine diphosphate and phosphate. 

Mechanistically, the cyt bej complex is thought to contain two distinct catalytic domains located on each side of the membrane (8,9,10). The quinol oxidation site (called Qz in bacterial and Qo in mitochondrial systems) is on the outer side of the membrane. It converts a quinol molecule to a quinone by transferring an electron to the Rieske FeS center and another to the lower potential cyt b heme ( l) This second electron is subsequently transferred to the cyt bn which then reduces a quinone trapped at the quinone reduction site (called Qc in bacterial, Qi in mitochondrial systems) located in the vicinity of the inner negative face of the membrane (8,14). Several classes of inhibitors are known to affect the reactions catalyzed at these active sites of the cyt bci complex (11). Myxothiazol, mucidin and stigmatellin interfere with the electron transfer between ubiquinol, Rieske FeS protein and cyt bi at the Qz site (28). Although stigmatellin also affects the Photosystem II of... 

Isolation, characterization and classification of R. capsulatus mutants resistant to quinol oxidation (Qz)-inhibitors. 

The electron transfer and proton translocation mechanism of the mammalian and bacterial bc complexes is now relatively well understood. Their catalytic Q-cycle can occur in the monomeric enzyme and is always coupled to a net proton translocation. The key step is electro-genic electron transfer through the haems b from a site of quinol oxidation to one of quinone reduction. Associated (de)protonations are probably non-electrogenic but result in net proton translocation [1]. 

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