Aspartate-187 of Cytochrome b Is Not Needed for DCCD Inhibition of Ubiquinol: Cytochrome c Oxidoreductase in Rhodobacter sphaeroides Chromatophores

Publisher Website
Google Scholar
Abstract
N,N‘-dicyclohexylcarbodiimide (DCCD) has been reported to inhibit steady-state proton translocation by cytochrome bc1 and b6f complexes without significantly altering the rate of electron transport, a process referred to as decoupling. In chromatophores of the purple bacterium Rhodobacter sphaeroides, this has been associated with the specific labeling of a surface-exposed aspartate-187 of the cytochrome b subunit of the bc1 complex [Wang et al. (1998) Arch. Biochem. Biophys.352, 193−198]. To explore the possible role of this amino acid residue in the protonogenic reactions of cytochrome bc1 complex, we investigated the effect of DCCD modification on flash-induced electron transport and the electrochromic bandshift of carotenoids in Rb. sphaeroides chromatophores from wild type (WT) and mutant cells, in which aspartate-187 of cytochrome b (AspB187) has been changed to asparagine (mutant B187 DN). The kinetics and amplitude of phase III of the electrochromic shift of carotenoids, reflecting electrogenic reactions in the bc1 complex, and of the redox changes of cytochromes and reaction center, were similar (± 15%) in both WT and B187DN chromatophores. DCCD effectively inhibited phase III of the carotenoid bandshift in both B187DN and WT chromatophores. The dependence of the kinetics and amplitude of phase III of the electrochromic shift on DCCD concentration was identical in WT and B187DN chromatophores, indicating that covalent modification of AspB187 is not specifically responsible for the effect of DCCD-induced effects of cytochrome bc1 complex. Furthermore, no evidence for differential inhibition of electrogenesis and electron transport was found in either strain. We conclude that AspB187 plays no crucial role in the protonogenic reactions of bc1 complex, since its replacement by asparagine does not lead to any significant effects on either the electrogenic reactions of bc1 complex, as revealed by phase III of the electrochromic shift of carotenoids, or sensitivity of turnover to DCCD.

This publication has 13 references indexed in Scilit: