THE PLASTOQUINONE DIFFUSION-COEFFICIENT IN CHLOROPLASTS AND ITS MECHANISTIC IMPLICATIONS

Pyrene fluorescence quenching by plastoquinone (PQ) was used to estimate lateral diffusion coefficients for PQ-9, decyl PQ and PQ-2 in soybean phosphatidylcholine liposomes and in spinach thylakoid and subthylakoid membranes. All three PQs have diffusion coefficients in the range of (0.1-3) 10(-9) cm(2)/s in thylakoids and subthylakoids when measured by the pyrene fluorescence quenching technique, values that are at least two orders of magnitude lower than those measured in phosphatidylcholine liposomes. Our values of PQ diffusion coefficients in thylakoids and subthylakoids are at least one order of magnitude below the minimal values estimated by Mitchell et al. [R. Mitchell et al. (1990) Biophys. J. 58, 1011-1024] if the rate of quinol oxidation by the cytochrome bf complex is determined by quinol binding rather than quinol diffusion. Our results, together with those of Mitchell et al., provide evidence that PQ lateral mobility within the thylakoid membrane determines the rate of quinol oxidation by the cytochrome bf complex. We rationalize our results in terms of a percolation model in which PQ mobility is restricted by thylakoid membrane proteins and conclude that plastocyanin rather than PQ must be responsible for rapid electron transport between PS II and PS I, in agreement with a similar proposal by Lavergne, Joliot and their co-workers [Joliot et al. (1992) Biochim. Biophys. Acta 1101, 1-12; Lavergne et al. (1992) Biochim. Biophys. Acta 1101, 13-22].