A Q-cycle mechanism for the cyclic electron-transfer chain of Rhodopseudomonas sphaeroides.

Over the past few years, work from a number of laboratories has shown that the ubiquinol :cytochrome c, (QH, :cyt. c,) oxidoreductase of the cyclic photosynthetic electron-transport chain of R hodopseudomonas sphaeroides is essentially similar to the mitochondria1 complex 111. Scheme 1 summarizes our present knowledge of the thermodynamics, kinetics and stoichiometry of the components of the chain, shown operating as a Q-cycle (Mitchell, 1976). We have previously pointed out that several features of the kinetics following flash illumination appeared to be inconsistent with Q-cycle mechanisms proposed to operate in this system (Crofts et al., 1975, 1981). In reviewing a number of recent results from our own work and that of other laboratories, we have now been able to show that the data can be accounted for by a Q-cycle, but only if this is formulated within narrow constraints. Recent information pertinent to our new formulation is summarized below. (i) The Rieske-type FeS centre has been characterized, with the midpoint redox potential, stoichiometry and kinetics shown in Scheme 1 (Bowyer et al., 1979, 1980). (ii) A bound cyt. c analogous to mitochondria1 cyt. c, has been shown to be present in the chain (Wood, 1980; Bowyer et al., 1981). (iii) The kinetics, stoichiometry and Em (midpoint potential) values of cyt. c , and cyt. c, in chromatophores have been determined (Crofts et al., 1981), as shown in Scheme 1. (iv) The low potential b-type cytochrome (cyt. b,,,) has been shown to be a kinetically competent member of the chain (Bowyer & Crofts, 1981; Crofts et al., 1981). (v) The stoichiometries of the other components of the QH,:cyt. c, oxidoreductase complex in relation to the reaction centre (one complex per two reaction centres) have been assigned, assuming that the stoichiometries with respect to cyt. c, and cyt. b,,, are the same as those in mitochondria (Crofts et al., 198 1). The experimental results and conclusions which have led us to reevaluate the Q-cycle are discussed below.