EFFECTS OF HYDROSTATIC-PRESSURE ON THE KINETICS OF ELECTRON-TRANSFER IN AN ISOLATED SYSTEM OF CHLOROPLAST CYTOCHROME BF COMPLEX, PLASTOCYANIN AND P700

The effects of pressure on the kinetics of redox reactions in and arou nd the chloroplast cytochrome bf complex were studied using a reconsti tuted system consisting of Photosystem I (PS I) particles, cytochrome bf complex and plastocyanin (PC), all derived from pea chloroplasts. T here were no significant permanent effects of pressure in the range 0. 1-191 MPa on the reaction kinetics, or on the shape of the absorption spectra of components studied. Discernable effects on rate-coefficient s of increasing pressure were observed on the reduction of P700(+) by PCI, on the reduction of PCII by ascorbate, and on the oxidation of de cyl plastoquinol by the bf complex. The volumes of activation Delta V# were determined from the dependence of the rate-coefficient on pressu re using: (partial derivative lnk/partial derivative P)(T) = -Delta V# /RT. The volume of activation is the difference in partial molar volum e between the activated state and the reactants for the redox reaction . Such data was sought to help define in detail those redox reactions and the corresponding activated states. For the reduction of P700(+) b y PCI and the oxidation of decyl plastoquinol by the bf complex, the r ate coefficient decreased with increase in pressure, whilst for the re duction of PCII by ascorbate it increased. The corresponding volumes o f activation were 9.6 +/- 0.6 x 10(-6) m(3) mol(-1), 18 +/- 2 x 10(-6) m(3) mol(-1) and -14 +/- 1 x 10(-6) m(3) mol(-1), respectively. Much of the pressure-dependence of PCII reduction by ascorbate was ascribed to an increase in ascorbate ionisation with increase in pressure. The re was little effect of pressure on the kinetics of oxidation of ferro cytochrome f by PCII, or on the equilibrium constant of the redox pair ferrocytochrome f/ferricytochrome f: PCII/PCI. Possible physical base s for these activation volumes are discussed, and they are compared wi th literature values.