THE PLASTOQUINONE DIFFUSION-COEFFICIENT IN CHLOROPLASTS AND ITS MECHANISTIC IMPLICATIONS

Pyrene fluorescence quenching by plastoquinone (PQ) was used to estima te lateral diffusion coefficients for PQ-9, decyl PQ and PQ-2 in soybe an phosphatidylcholine liposomes and in spinach thylakoid and subthyla koid membranes. All three PQs have diffusion coefficients in the range of (0.1-3) 10(-9) cm(2)/s in thylakoids and subthylakoids when measur ed by the pyrene fluorescence quenching technique, values that are at least two orders of magnitude lower than those measured in phosphatidy lcholine liposomes. Our values of PQ diffusion coefficients in thylako ids and subthylakoids are at least one order of magnitude below the mi nimal values estimated by Mitchell et al. [R. Mitchell et al. (1990) B iophys. J. 58, 1011-1024] if the rate of quinol oxidation by the cytoc hrome bf complex is determined by quinol binding rather than quinol di ffusion. Our results, together with those of Mitchell et al., provide evidence that PQ lateral mobility within the thylakoid membrane determ ines the rate of quinol oxidation by the cytochrome bf complex. We rat ionalize our results in terms of a percolation model in which PQ mobil ity is restricted by thylakoid membrane proteins and conclude that pla stocyanin rather than PQ must be responsible for rapid electron transp ort between PS II and PS I, in agreement with a similar proposal by La vergne, Joliot and their co-workers [Joliot et al. (1992) Biochim. Bio phys. Acta 1101, 1-12; Lavergne et al. (1992) Biochim. Biophys. Acta 1 101, 13-22].