Proton pumping and electron transfer in the cytochrome [formula omitted] complex of algae

Abstract

The electron-transfer reactions and the slow transmembrane electrogenic phase (phase b) have been studied in Photosystem-II-depleted mutant strains of Chlorella sorokiniana under anaerobic conditions. The dark reduction of cytochrome b previously oxidized by a strong illumination is a biphasic process. The first phase, completed in about 20 s, is associated with the reduction of the high-potential form of cytochrome b; a slower phase, completed in about 10 min, is associated with the reduction of the low-potential form of cytochrome b. The α-bands of these two forms differ slightly. Under repetitive weak flash illumination, i.e., when a fraction of cytochrome b is oxidized prior to each flash, phase b is associated with minor changes in cytochrome b redox changes. In the presence of 2- n-nonyl-4-hydroxyquinoline N-oxide (NQNO), a large reduction of cytochrome b ( t 1 2 ≈ 2 ms ) is followed by a slow reoxidation. Under these conditions, phase b becomes biphasic, a first phase, insensitive to the inhibitor, is associated with cytochrome b reduction while the second phase, dependent upon the inhibitor concentration, is related to cytochrome b oxidation. These results are interpreted in terms of a modified Q-cycle model. A single weak flash given to dark-adapted algae, i.e., when the two b cytochromes are in their reduced state, induces a large cytochrome b oxidation with which is associated a phase b of longer duration but larger amplitude than the one observed under repetitive-flash illumination. Addition of 2- n-nonyl-4-hydroxyquinoline N-oxide inhibits neither the oxidation of cytochrome b nor phase b. These results suggest that a proton pump is coupled to the redox changes of plastoquinone occurring at a site, Z, close to the inner face of the membrane. This model implies that site Z is connected to the outer face of the thylakoid by a proton channel. Thus, depending upon the experimental conditions, two different mechanisms might be involved in the process of proton-pumping by the cytochrome b f complex.