Effect of subunit IV on superoxide generation by Rhodobacter sphaeroides cytochrome bc1 complex

Abstract

Previous studies indicate that the three-subunit cytochrome bc(1) core complex of Rhodobacter sphaeroides contains a fraction of the electron transfer activity of the wild-type enzyme. Addition of subunit IV to the core complex increases electron transfer activity to the same level as that of the wild-type complex. This activity increase may result from subunit IV preventing electron leakage, from the low potential electron transfer chain, and reaction with molecular oxygen, producing superoxide anion. This suggestion is based on the following observations: (1) the extent of cytochrome b reduction in the three-subunit core complex, by ubiquinol, in the presence of antimycin A, never reaches the same level as that in the wild-type complex; (2) the core complex produces 4 times as much superoxide anion as does the wild-type complex; and (3) when the core complex is reconstituted with subunit IVs having varying reconstitutive activities, the activity increase in reconstituted complexes correlates with superoxide production decrease and extent of cytochrome b reduction increase.

Effect of Mutations of Arginine 94 on Proton Pumping, Electron Transfer, and Superoxide Anion Generation in Cytochrome b of the bc1 Complex from Rhodobacter sphaeroides

“…4) increased the extent and decreased the rate of heme b reduction in the wild type and mutants, and these results are consistent with a previous report (44). However, the degree of this effect varied in these complexes.…”

Mechanism of Enhanced Superoxide Production in the Cytochrome b6f Complex of Oxygenic Photosynthesis

“…O 2 ∸ generation has also been measured with bc 1 complex obtained from the purple photosynthetic bacterium, Rb. sphaeroides (29, 46) , for which the rate of O 2 ∸ production relative to the electron transport rate was not reported. In the present experiments with the yeast bc 1 complex, the specific rate of O 2 ∸ production, normalized to the electron transport rate, is, however, increased greatly (> 40-fold) in the presence of antimycin A (Table 1), in qualitative agreement with data obtained previously for the bc 1 complex (4, 5, 8, 27-29) .…”

“…Thus, the specific n-side quinone analogue inhibitor, antimycin A, causes a large increase in the specific rate of formation of O 2 ∸, relative to the electron transport rate, by the bc 1 complex of yeast or bovine mitochondria (4, 7, 8, 27, 28) , and the photosynthetic bacterium, Rb. sphaeroides (29) . No n-side inhibitor comparable in efficacy to that of antimycin.…”

Generation, characterization and crystallization of a cytochrome c1-subunit IV fused cytochrome bc1 complex from Rhodobacter sphaeroides

“…Recently, we have observed several interesting results [11], including (i) in the three-subunit core complex, the extent of cytochrome b reduction by ubiquinol in the presence of antimycin A cannot reach the same level as that in the wild-type complex; (ii) the core complex produces 4 times more superoxide than the wild-type complex; and (iii) the activity of reconstituted complexes, formed from the core complex and subunit IVs, is inversely proportional to its superoxide production activity. Although these findings may suggest that subunit IV increases the bc 1 complex activity [11] by preventing electron leakage from the low potential electron transfer chain and thereby keeping electrons from reacting with molecular oxygen to produce superoxide anions [12–15], the mechanism through which subunit IV performs these functions is yet to be elucidated.…”

“…Although these findings may suggest that subunit IV increases the bc 1 complex activity [11] by preventing electron leakage from the low potential electron transfer chain and thereby keeping electrons from reacting with molecular oxygen to produce superoxide anions [12–15], the mechanism through which subunit IV performs these functions is yet to be elucidated.…”