Photooxidation of cytochromes in reaction center preparations from Chromatium and Rhodopseudomonas viridis

Abstract

In a reaction center preparation from Rhodopseudomonas viridis, which contains as its only bacteriochlorophyll a single reactive trimer, two bound cytochromes under-go photooxidation. Cytochrome C558 photooxidation occurs at redox potentials above +100 mV, with a half-time of approx. 1 μsec. Cytochrome C 553 photooxidation occurs at potentials below o mV, with a rate that is presently too fast to measure. Similarly, a subchomatophore fraction from Chromatium exhibits photooxidation of bound cytochrome C555 with a half-time of 2.3 μsec at potentials above +100 mV, and cytochrome C552 with a half-time of 1.1 μsec at potentials below o mV. The changeover from one cytochrome to the other fits a one-electron titration curve with E m = +25 mV, which is independent of the pH. At all potentials, the rate of P883 + (P870+) reduction is the same as that of cytochrome oxidation. Measurements of P883+ reduction at both 882 and 785 nm allow the conclusion that this reactive bacteriochlorophyll species oxidizes both cytochromes. The similarities between the light-induced reactions in Chromatium and Rps. viridis suggest that a general feature of bacterial photosynthesis may be the oxidation of both high- and low-potential cytochromes by a single photochemical system. The cytochrome oxidation kinetics in the Chromatium fraction are essentially identical with those in Chromatium chromatophores; however, the reaction between the primary and secondary electron acceptors is slower by a factor of 1×10 3. The purified material appears to lack the secondary acceptor, Y. N-Methylphenazonium methosulfate (PMS) or methylene blue can replace Y in a reaction which, unlike the in vivo reaction, is insensitive to 1,10-phenanthroline.