Menaquinol oxidation

The site of menaquinol oxidation on the diheme cytochrome b subunit of W. succinogenes QFR is not known. No convincing density for... 

The fumarate molecule is in van der Waals contact with the isoallox-azine ring of FAD. The linear arrangement of the prosthetic groups in one QFR complex therefore provides one straightforward pathway by which electrons could be transferred efficiently from the menaquinol oxidizing site via the two heme groups, the three iron—sulfur clusters, and the FAD to the site of fumarate reduction. 

In addition to the transfer of electrons, two protons are bound on fumarate reduction (see reaction 1) and two protons are liberated on menaquinol oxidation (see reaction 2). The protons consumed on fumarate reduction are undoubtably bound from the cytoplasm (see Fig. 8a). The experimental results on intact bacteria, with inverted vesicles or liposomes containing W. succinogenes QFR (Kroger et al., 2002 Biel et al., 2002), suggest that the oxidation of menaquinol by fumarate as catalyzed by W. succinogenes QFR is an electroneutral process. The protons formed by menaquinol oxidation have therefore been assumed to be released to the cytoplasmic side of the membrane where they balance the protons consumed by fumarate reduction. 

Oxidative demethylation. Kapoport el al. examined the oxidation of the dimethyl ether of menaquinol-1(1) with silver(ll) oxide with the expectation of effecting oxidation... 

A cytochrome having hemes D and B in the molecule is known cytochrome bd. This cytochrome was first purified from Photobacterium phosphoreum (Watanabe et al., 1979). It is a quinol oxidase and functions as the terminal oxidase in some bacteria. It utilizes menaquinol in vivo as the electron donor, while it catalyzes the oxidation of both menaquinol and ubiquinol in vitro, but it does not catalyze the oxidation of ferrocytochrome c. It does not have copper, but still reduces molecular oxygen at the site of heme D (Mogi et al., 1998). 

In the heterotrophic nitrate respiration, in most cases hydrogen atoms derived from organic compounds ([H], mostly in the form of NADH in the case of heterotrophic denitriliers) are first oxidized with nitrate. The reaction is catalyzed by nitrate reductase. The enzyme contains Mo, [Fe4S4] and [Fe2S2] clusters (Fe/S), and cytochrome b (Chaudhry and MacGregor, 1983). Mo is present in the enzyme as molybdenum cofactors combining with molybdopterin or molybdopterin guanine dinucleotide. The enzyme catalyzes the reduction of nitrate to nitrite with ubiquinol or menaquinol (QH2) as the electron donor. 

Suharti, Strampraad MJ, Schroder I, de Vries S (2001) A novel copper A containing menaquinol NO reductase from Bacillus azotoformans. Biochemistry 40 2632-2639 Sumino T, Isaka K, Ikuta H, Saiki Y, Yokota T (2006) Nitrogen removal from waste-water using simultaneous nitrate reduction and anaerobic ammonium oxidation in single reactor. J Biosci Bioeng 102 346-351... 

The blue oxidases like ascorbate oxidase, laccase, and ceruloplasmin, and the terminal oxidases of aerobic respiratory chains like cytochrome oxidases and quinol oxidases are the only enzymes so far known that catalyze the direct four-electron reduction of molecular oxygen to water. Thereby, the reducing substrates like ascorbate, quinol, Fe " ", and cytochrome c are oxidized in one-electron transfer steps. The substrates of quinol oxidases, ubiquinol, or menaquinol, may be oxidized in two-electron transfer steps. For the two cases the following general reaction formulae can be defined ... 

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