Effect of substituents of the benzoquinone ring on electron-transfer activities of ubiquinone derivatives.
The effect of substituents on the 1,4-benzoquinone ring of ubiquinone on its electron-transfer activity in the bovine heart mitochondrial succinate-cytochrome c reductase region is studied by using synthetic ubiquinone derivatives that have a decyl (or geranyl) side-chain at the 6-position and various arrangements of methyl, methoxy and hydrogen in the 2, 3 and 5 positions of the benzoquinone ring. The reduction of quinone derivatives by succinate is measured with succinate-ubiquinone reductase and with succinate-cytochrome c reductase. Oxidation of quinol derivatives is measured with ubiquinol-cytochrome c reductase. The electron-transfer efficacy of quinone derivatives is compared to that of 2,3-dimethoxy-5-methyl-6-decyl-1,4-benzoquinone. When quinone derivatives are used as the electron acceptor for succinate-ubiquinone reductase, the methyl group at the 5-position is less important than are the methoxy groups at the 2- and 3-positions. Replacing the 5-methyl group with hydrogen causes a slight increase in activity. However, replacing one or both of 2- and 3-methoxy groups with a methyl completely abolishes electron-acceptor activity. Replacing the 3-methoxy group with hydrogen results in a complete loss of electron-acceptor activity, while replacing the 2-methoxy with hydrogen results in an activity decrease by 70%, suggesting that the methoxy group at the 3-position is more specific than that at the 2-position. The structural requirements for quinol derivatives to be oxidized by ubiquinol-cytochrome c reductase are less strict. All 1,4-benzoquinol derivatives examined show partial activity when used as electron donors for ubiquinol-cytochrome c reductase. Derivatives that possess one unsubstituted position at 2, 3 or 5, with a decyl group at the 6-position, show substrate inhibition at high concentrations. Such substrate inhibition is not observed when fully substituted derivatives are used. The structural requirements for quinone derivatives to be reduced by succinate-cytochrome c reductase are less specific than those for succinate-ubiquinone reductase. Replacing one or both of the 2- and 3-methoxy groups with a methyl and keeping the 5-position unsubstituted (plastoquinone derivatives) yields derivatives with no acceptor activity for succinate-Q reductase. However, these derivatives are reducible by succinate in the presence of succinate-cytochrome c reductase. This reduction is antimycin-sensitive and requires endogenous ubiquinone, suggesting that these (plastoquinone) derivatives can only accept electrons from the ubisemiquinone radical at the Qi site of ubiquinol-cytochrome c reductase, and cannot accept electrons from the QPs of succinate-ubiquinone reductase.
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Department of Biochemistry, OAES, Oklahoma State University, Stillwater 74078.
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D016227:Benzoquinones; D004579:Electron Transport; D042963:Electron Transport Complex II; D014450:Electron Transport Complex III / Q000378:metabolism; D007700:Kinetics; D008823:Micelles; D008928:Mitochondria / Q000201:enzymology; D015394:Molecular Structure; D009097:Multienzyme Complexes / Q000378:metabolism; D010084:Oxidation-Reduction; D010088:Oxidoreductases / Q000378:metabolism; D010971:Plastoquinone / Q000378:metabolism; D011809:Quinones / Q000378:metabolism*; D013384:Succinate Cytochrome c Oxidoreductase / Q000378:metabolism; D013385:Succinate Dehydrogenase / Q000378:metabolism; D014451:Ubiquinone / Q000031:analogs & derivatives* / Q000138:chemical synthesis / Q000378:metabolism
D016227:Benzoquinones; D008823:Micelles; D009097:Multienzyme Complexes; D011809:Quinones; D014451:Ubiquinone; C004532:quinone; D010088:Oxidoreductases; D013384:Succinate Cytochrome c Oxidoreductase; D042963:Electron Transport Complex II; D013385:Succinate Dehydrogenase; D014450:Electron Transport Complex III; D010971:Plastoquinone