Menaquinone reduction

HQNO is known to inhibit menaquinone reduction by formate or H2 in IV. succinogenes [41-43]. The midpoint potential of at least one heme b group in the membrane fraction of IV. succinogenes was shifted from —190 to -230 mM upon the addition of HQNO [44]. Therefore, HQNO probably interacts at the site of quinone reduction of hydrogenase and of formate dehydrogenase of W. succinogenes. 

According to this mechanism, the Ap is exclusively generated by MMb reduction with H2 (Fig. 8). In this process, two protons are released on the periplasmic side from H2 oxidation, and one proton disappears from the cytoplasmic side for MMb reduction. Consistently, the site of quinone reduction on HydC is probably located close to the cytoplasmic membrane surface (Fig. 5). Furthermore, menaquinone reduction by H2 catalyzed by hydrogenase in the membrane of W. succinogenes or in proteoUposomes was found to generate a Ap [9,25]. 

Saffarini DA, SL Blumerman, KJ Mansoorabadi (2002) Role of menaquinones in Fe(III) reduction by membrane fractions of Shewanella putrefaciens. J Bacteriol 184 846-848. 

The reduction is mediated by a PCE reductase and hydrogenase, both of which are associated with the cytoplasmic membrane. The hydrogenase faces the periplasm of the cell and thus releases the protons into the periplasm (Schumacher and Holliger 1996). Two electrons are transferred across the membrane to the PCE reductase, via cytochrome b and menaquinone (Fig. 9.13). Since dihydrogen is the only electron donor used by Dehalobacter restrictus, the hydrogenase is a crucial enzyme of this... 

A simple and effective chemical method was developed for quantitatively reducing quinones, based on their reaction with metallic zinc and zinc ions [248]. Comparison of this method with conventional electrochemical reduction [249-252] revealed the chemical method to be considerably superior. A reduction reaction of vitamin Kj and other quinones in the presence of Zn° and Zn2+ eliminates the need to apply large negative potentials and may also be performed in the absence of any applied electrochemical potential. Some quinones used, such as UQ-10, menadione, and vitamin K, of the menaquinone series (MKs 4-10) could all be reduced to their corresponding hydroquinones in these conditions. 

FIGURE 12. Stereo diagram of the complete fimiarate reductase complex. The FAD-binding subunit is at the top, the iron-sulfur subunit is in the center and die two membrane anchoring subunits that provide die binding sites for two molecules of menaquinone are at the bottom. In this molecule electron h ansfer occiffs from menaquinone at die bottom to FAD at die top during reduction of fumarate by menaquinone. Skeletal models of two molecules of menaquinone, a 3Fe-4S, a 4Fe-4S, a 2Fe-2S, an FAD molecule and one molecule of oxalate are included. 

Fig. 4.2. (C and D) Intermediate electron acceptor (A,) hacteriopheophytin. (C) Flash-induced optical changes compared to the sum of the spectra of D, oxidation and A, reduction (from Ref. 18) ESR spectrum (recorded at 2.1 °K and 10 W power) of the hacteriopheophytin anion radical trapped in RC in which ubiquinone was substituted by menaquinone. The doublet disappears if the spectrum is recorded at low power (10 W) (from Ref. 23).

Photosynthetically active quinones include plastoquinone of green-plant photosystem II, ubiquinone and menaquinone in photosynthetic bacteria, and phylloquinone in photosystem I. Plastoquinone is present in green-plant photosystem II both as a tightly-bound and a loosely-bound electron carrier, designated Qa and Qb, respectively. Qa is photoreduced only to the semiquinone (PQ ) but Qb can accept two electrons, forming the plastohydroquinone (PQ-Hj) [see Chapters 5, 6 and 16 for further discussion]. Plastohydroquinone PQb H2 is the final reduction product of photosystem II and goes on to reduce the cytochrome bj complex as part of the electron transport and proton translocation processes [see Chapter 35 for detailed discussions]. 

Fig. 9. (A) and (B) Absorbance difference spectrum of Cf. aurantiacus reaction centers measured 10 ms after a 13-/rs saturating xenon flash was applied sample absorbance at 865 nm was 9x10. (C) and (D) are the absorbance difference spectra measured the same way as in (A) and (B), except that the reaction-center sample contained Asc and PMS as an exogenous reductant. The dashed spectrum in (C) is the in vitro difference spectrum of vitamin K-1 in methanol obtained by EJ Land by pulse radiolysis. Figure source Vasmel and Amesz (1983) Photoreduction of menaquinone in the reaction centers of the green photosynthetic bacterium Chloroflexus aurantiacus. Biochim Biophys Acta 724 119-121.

Cytochrome Cjsa (Table XX) and the b and d cytochromes reported in Desulfovibrio could be components of an energy-storing ETS II. If ETS II is a mitochondria-like chain, it might provide the role for the plentiful menaquinone molecule, which resembles the plastoquinones of photosynthesis or ubiquinone of mitochondrial respiration. Wagner et al. (4 8) have suggested this role for menaquinone and pointed out that its reduction potential is compatible with this idea. 

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