Abstract
A ubiquinone derivative, 3-chloro-5-hydroxyl-2-methyl-6-decyl- 1,4-benzoquinone (3-CHMDB), which shows different effects on the mitochondrial cytochrome b-c1 complex and chloroplast cytochrome b6-f complex, has been synthesized and characterized. When the cytochrome b-c1 complex is treated with varying concentrations of 3-CHMDB and assayed at constant substrate (Q2H2) concentration, a 50% inhibition is observed when 2 mol of 3-CHMDB per mol of enzyme are used. The degree of inhibition is dependent on the substrate concentration. When ubiquinol-cytochrome c reductase is treated with 2 mol of 3-CHMDB per mol of enzyme, less inhibition is observed with a lower substrate concentration, suggesting the possible existence of two forms of reductases: one with a high affinity for ubiquinone and another with a low affinity. 2-Chloro-5-hydroxyl-3-methyl-6-decyl-1,4-benzoquinone (2-CHMDB), an isomer of 3-CHMDB, shows much less inhibition of the mitochondrial cytochrome b-c1 complex, suggesting that the quinone binding site in this complex is highly specific. In contrast to the inhibition observed with the cytochrome b-c1 complex, 3-CHMDB causes no inhibition of the plastoquinol-plastocyanin reductase activity of chloroplast cytochrome b6-f complex, regardless of whether plastoquinol-2 or ubiquinol-2 is used as substrate. 3-CHMDB restores the dibromothymoquinone-altered EPR spectra of iron-sulfur protein in both complexes. In the case of the cytochrome b6-f complex, 3-CHMDB also partially restores the dibromothymoquinone-inhibited activity. Reduced form 3- or 2-CHMDB is oxidizable by the cytochrome b6-f complex, but not by the cytochrome b-c1 complex. These results suggest that the quinol oxidizing sites in the cytochrome b6-f complex may differ from those in the mitochondrial cytochrome b-c1 complex.
Highlights
Dation andthe generation of proton gradients andmembrane methyl-6-decyl-l,4-benzoquinon(e3-CHMDB),which potential
When ubiquinol-cytochrome c reductase is treated with2 mol of 3-CHMDB per mol of enzyme, less inhibition is observed with a components, these two quinol oxidizing complexes differ in some important aspects, such as the redox properties of cytochromes, lipid composition, electron acceptor, and sensitivity to electron transfer inhibitors
Antimycin blocks electron transfer from ubichrome b-cl complex, 3-CHMDB causes no inhibition quinol to cytochrome c in the mitochondrial cytochrome b-cl of the plastoquinol-plastocyanin reductase activity of complex [6], buthas no effect on electron transfer from chloroplastcytochrome be-f complex, regardless of plastoquinol to plastocyanin in the chloroplast cytochrome whether plastoquinol-2 or ubiquinol-2 is used as sub- b6-f complex
Summary
Dation andthe generation of proton gradients andmembrane methyl-6-decyl-l,4-benzoquinon(e3-CHMDB),which potential. Treated with 3-CHMDB, an inhibition curve similar to that unit, the difference in the inhibitory potency of these two observed with the mitochondrial cytochrome b-cl complex is isomers to the cytochrome b-cl complex cannot be explained obtained.
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