Ubiquinol:Cytochrome c Oxidoreductase

Abstract

The effects of inhibitors on the reduction of the bisheme cytochrome b of ubiquinol: cytochrome c oxidoreductase (complex III, bc 1 complex) has been studied in bovine heart submitochondrial particles (SMP) when cytochrome b was reduced by NADH and succinate via the ubiquinone (Q) pool or by ascorbate plus N,N,N,Ntetramethyl-p-phenylenediamine via cytochrome c 1 and the iron-sulfur protein of complex III (ISP). The inhibitors used were antimycin (an N-side inhibitor), ␤-methoxyacrylate derivatives, stigmatellin (P-side inhibitors), and ethoxyformic anhydride, which modifies essential histidyl residues in ISP. In agreement with our previous findings, the following results were obtained: (i) When ISP/cytochrome c 1 were prereduced or SMP were treated with a P-side inhibitor, the high potential heme b H was fully and rapidly reduced by NADH or succinate, whereas the low potential heme b L was only partially reduced. with an EPR signal centered at g ϭ 1.90, and a cytochrome c 1 (E m ϭ ϩ 230 mV) (1, 7). In the mitochondrial respiratory chain, complex III transfers electrons from the ubiquinone pool to cytochrome c in a reaction that is coupled to outward proton translocation with a stoichiometry of H ϩ /e ϭ 2. When properly isolated, purified bovine heart complex III catalyzes the reduction of cytochrome c by ubiquinol-2 at a rate of 2500 -3000 s Ϫ1 at 38°C (8, 9).Recently, x-ray crystallographic data have been published at 2.9 Å resolution by Yu and co-workers (2, 10 -12) for about 80% of the bovine enzyme, at 3.0 Å resolution by Zhang et al. We have shown recently in energized submitochondrial particles (SMP) that reverse electron transfer from ISP/c 1 to cytochrome b is inhibited more by antimycin, which binds near b H , than by myxothiazol, which binds near b L (13). Antimycin also inhibited reverse electron transfer from ISP/c 1 to b in Q-extracted SMP, which contained Յ0.06 mol Q/mol cytochrome b or c 1 (200-fold less than the unextracted SMP) and was incapable of oxidizing NADH or succinate by molecular oxygen (13). We have also shown that when SMP were treated with antimycin, KCN, and ascorbate plus TMPD to reduce the high potential centers of complex III, subsequent addition of NADH or succinate resulted in rapid and complete reduction of b H , and only the reduction of b L became slow and partial when ISP/c 1 were prereduced (14). These and other results reported in Refs. 13 and 14 are not compatible with the Q cycle hypothesis, but they are fully consistent with the x-ray diffraction data of the three different groups mentioned, especially because these

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Roles of Bound Quinone in the Single Subunit NADH-Quinone Oxidoreductase (Ndi1) from Saccharomyces cerevisiae

“…Bovine heart mitochondria were kindly provided by Prof. Chang-An Yu (Oklahoma State University, Stillwater, OK), and bovine heart SMP were prepared according to Ref. 26. Piericidin A 1 was a generous gift from Prof. Dale.…”

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