Plastoquinone Diffusion in Proteoliposomes Studied by Pyrene Fluorescence Quenching

Summary

The rate limiting step of linear electron flow in chloroplasts appears to be the oxidation of plastoquinol by the cytochrome b/f complex. Thus, it is important to know whether long-range diffusion of plastoquinone from photosystem II to the cytochrome b/f complex can ever determine the rate of plastoquinol oxidation. Plastoquinone diffusion coefficients in phosphatidylcholine and thylakoid lipid vesicles are of the order 1–2 × 10^–7 cm²s^-1 and this would be unlikely to restrict the reaction rate. However, the effect that high (50% or greater) weight percentages of protein occurring in natural membranes would have on lateral diffusion remains unknown. The rate of plastoquinone diffusion was therefore studied in phosphatidylcholine proteoliposomes containing the following integral membrane proteins: gramicidin D, spinach cytochrome b/f complex, reaction centres from Rhodobacter sphaeroides and spinach cytochrome f. The rate of plastoquinone diffusion decreased by up to an order of magnitude when the protein composition of proteoliposomes exceeded 30–50 weight per cent. Based on the expectation of a protein composition of 60–65% in thylakoid membranes, we estimate that the plastoquinone diffusion coefficient in thylakoid membranes will be of the order 10^-8 cm²s^-1. In our view, the question remains open as to whether or not plastoquinone diffusion ever limits the rate of electron transport.