Ubiquinone reduction

The negative band at 275 nm and the positive bands in the 320-450 nm region of the difference spectrum reflect reduction of ubiquinone. In addition, the difference spectrum also consists of several band shifts the red shift of the absorption bands of BO at 535 and 760 nm, and small blue shift of the 800- and 865-nm bands ofP870. These band shifts are attributed to the influence of local electric fields produced by photochemical charge separation. They are ascribed neither to ubiquinone reduction itself nor to redox changes of the pigment molecules. Note that the ordinate is expressed in differential molar extinction coefficient. As, in mM em", as it is readily obtainable since P870 and Qa are present in equimolar quantities. 

In benzoquinone-treated algae, the re-reduction rate of Cyt c could be accelerated by increasing the concentration of reduced DAD (not shown). In this case, addition of myxothiazol also blocked the reaction. Therefore, the inhibition caused by the treatment is not due to its impairing the b-c complex, but rather affects an upstream step, such as ubiquinone reduction. This was actually verified by Prof. R. Douce (personal communication), who found that p-benzoquinone was an inhibitor of the NADH-ubiquinone- reductase in isolated plant mitochondria. The slow re-reduction of Cyt c in Fig. 1 depends on the duration and concentration of the benzoquinone treatment, and as mentioned above, on the added reductant. In this experiment sufficient reduction occurs within each flashing period so that the saturation effect observed in the myxothiazol curve of Fig.2 does not occur. As shown in [l], under such conditions, the mitochondrial response remains linear. 

Rotenone, the active ingredient of derris powder, an insecticide prepared from the roots of the leguminous plant Lonchocarpus nicou. It is an inhibitor of complex I (NADH — ubiquinone reduction). The same effect is seen in the presence of amytal (amobarbital), a barbiturate sedative drug, which again inhibits complex I. These two compounds inhibit oxidation of malate, which requires complex I, but not succinate, which reduces ubiquinone directly. The addition of the uncoupler... 

ADH can oxidize ubiquinol in vitro using an artificial electron acceptor, but the physiological acceptor is not yet clarified. Although this activity can be thought to be involved in regulation of redox balance of the membranous ubiquinone, the yet physiological role of this activity is not fully established. ADH has ubiquincme as a bound form in the enzyme (Matsushita et al. 2008). We anticipate that the bound quinone functions in the catalysis of ubiquinone reduction (Yakushi and Matsushita 2010). 

As described in the ADH section, the glucose oxidase respiratory chain has been reconstituted in proteoliposomes composed of GDH, ubiquinone, terminal oxidase, and phospholipids (Matsushita et al. 1989b). Although GDH has a transmembrane region with five hydrophobic segments, the enzyme itself does not produce membrane potential (Yamada et al. 1993). GDH of Escherichia coli is well characterized in biochemical aspects. GDH has bound quinone, which is likely involved in catalysis of ubiquinone reduction (Elias et al. 2004). An intramolecular electron transport pathway is proposed by pulse radiolysis experiments (Mustafa et al. 2008). 

Matsushita K, Yakushi T, Toyama H, Shinagawa E, Adachi O (1996) Function of multiple heme c moieties in intramolecular electron transport and ubiquinone reduction in the quinohemoprotein alcohol dehydrogenase-cytochrome c complex of Gluconobacter suboxydans. J Biol Chem 271(9) 4850-4857... 

Matsushita K, Yakushi T, Toyama H, Adachi O, Miyoshi H, Tagami E, Sakamoto K (1999) The quinohemoprotein alcohol dehydrogenase of Gluconobacter suboxydans has ubiquinol oxidation activity at a site different from the ubiquinone reduction site. Biochim Biophys Acta 1409... 

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