Phenazine methosulfate electron acceptor

Complex III - Complex III contains a diversity of electron carrying proteins. They include cytochrome b, iron sulfur centers, and cytochrome cl. Cytochrome b is the first of the heme-carrying proteins (Figure 15.6) involved in electron transport. Passage of electrons from cytochrome b to the iron sulfur centers can be blocked by antimycin A. Also, the artificial electron acceptor phenazine methosulfate can accept electrons from cytochrome b and 2,6-dichlorophenol-indophenol can accept electrons from the iron sulfur proteins (Figure 15.9). The crystal structure of the redox components of complex III from bovine heart mitochondria is shown in Figure 15.16... 

D-Lactate cytochrome c reductase can oxidize D-2-hydroxymonocar-boxylic acids, but only D-lactate and D-2-hydroxybutyrate are oxidized at appreciable rates. The enzyme exhibits a similar high specificity for electron acceptors. It reacts with cytochrome c and phenazine methosul-fate as electron acceptors, but not with ferricyanide, methylene blue, 2,6-dichloroindophenol, and menadione 308, 312, 313). With D-lactate as substrate and at Fmax with respect to acceptor, phenazine methosulfate is reduced at 30° eight times as fast as cytochrome c 308). The values at 30° and pH 7.5 are D-lactate, 0.29 mM n-2-hydroxybutyrate,... 

The activity of complex II (succinate dehydrogenase) is measured as the succinate-dependent reduction of decylubiquinone, which is in turn recorded spectro-photometrically through the reduction of dichlorophenol indophenol at 600 nm (e 19,100-M -cm Fig. 3.8.5). In order to ensure a linear rate for the activity, the medium is added with rotenone, ATP, and a high concentration of succinate. As noticed previously for complex I, decylubiquinone is not a perfect acceptor for electrons from the membrane-inserted complex II [70]. Malonate, a competitive inhibitor of the enzyme, is used to inhibit it. Rather than decylubiquinone, phenazine methosulfate can be utilized, which diverts the electrons from the complex before they are conveyed through subunits C and D, therefore allowing measurement of the activity of subunits A and B. 

Physical measurements support a molecular weight of approximately 200,000. This value is also in accord with gel exclusion studies on Sephadex G-200. Thus the enzyme contains 1 mole of flavin and 4 g-atoms of nonheme iron per mole.. . . The sedimentation velocity of the beef heart enzyme at 10-15 mg protein/ml is 6.5 S. . . This preparation could oxidize succinate in the presence of ferricyanide or phenazine methosulfate (PMS) as electron acceptor but was unable to transfer elec-to be unable to interact with the respiratory chain. 

Fig. 12.8. Schematic representation of events occurring during biogenesis of photosystem I reaction center. The subunits are designated as I to VII, the abbreviations are Ferr, ferredoxin P.C., plas-tocyanin A, Aj, Aj and A4, primary, secondary, tertiary and quaternary electron acceptors PMS, phenazine methosulfate DAD, diaminodurine.

Visual localization of electrophoretically separated LDH isoenzymes is accomplished by the reduction of nitro-tetrazolium blue as the electron acceptor (terminal) in a medium containing phenazine methosulfate and NAD. The linked reaction is as follows ... 

The movement of electrons through the electron carrying proteins of the inner mitochondrial membrane is shown in Figure 15.9. Also shown are inhibitors of electron movement at their point of action and the sites where artificial electron acceptors can accept electrons from the electron transport system. Specific inhibitors shown in Figure 15.9 are rotenone, amytal, antimycin A, cyanide, azide, and carbon monoxide. The artificial electron acceptors are methylene blue, phenazine methosulfate, 2,6-indophenol, tetramethyl-p-phenylene diamine, and ferricyanide. 

The failure of a generation of investigators to obtain soluble succinic dehydrogenase preparations was caused by the unusually fastidious requirements of this enzyme. Of the ordinary electron acceptors, including methylene blue and dichlorophenolindophenol, none supports the oxidation of succinate. Phenazine methosulfate was found to be reduced by solubilized preparations the enzyme is extracted from acetone powders and was undoubtedly present in many extracts, where it was not detected for lack of a suitable oxidant. A preparation from yeast was reported... 

The spectrophotometric methods described earlier were used to determine the optimum pH for the isoenzymes (Schabort et al., 1971). Cytochrome c [together with a small amount of phenazine methosulfate (PMS) as intermediate electron acceptor] was particularly useful for determinations below pH 6.4, because the absorbance of DCIP at 600 nm decreases rapidly below this value. The five isoenzymes showed the same optimum pH of 6.8 for both the dehydrogenation and the total conversion of jSCA into aCA. The temperature stability of the five isoenzymes was essentially the same. They lost all their activity after heat treatment for 10 min at 75.5°C, but retained 70% of their activity after 10 min at 55" C. 

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