Flash-induced turnover of the cytochrome bc1 complex in chromatophores of Rhodobacter capsulatus: binding of Zn2+ decelerates likewise the oxidation of cytochrome b, the reduction of cytochrome c1 and the voltage generation.
The effect of Zn2+ on the rates of electron transfer and of voltage generation in the cytochrome bc1 complex (bc1) was investigated under excitation of Rhodobacter capsulatus chromatophores with flashing light. When added, Zn2+ retarded the oxidation of cytochrome b and allowed to monitor (at 561-570 nm) the reduction of its high potential heme b(h) (in the absence of Zn2+ this reaction was masked by the fast re-oxidation of the heme). The effect was accompanied by the deceleration of both the cytochrome c(1) reduction (as monitored at 552-570 nm) and the generation of transmembrane voltage (monitored by electrochromism at 522 nm). At Zn2+ <100 microM the reduction of heme b(h) remained 10 times faster than other reactions. The kinetic discrepancy was observed even after an attenuated flash, when bc1 turned over only once. These observations (1) raise doubt on the notion that the transmembrane electron transfer towards heme b(h) is the main electrogenic reaction in the cytochrome bc1 complex, (2) imply an allosteric link between the site of heme b(h) oxidation and the site of cytochrome c1 reduction at the opposite side of the membrane, and (3) indicate that the internal redistribution of protons might account for the voltage generation by the cytochrome bc1 complex.