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 around the chloroplast cytochrome bf complex were studied using a reconstituted system consisting of Photosystem I (PS I) particles, cytochrome bf complex and plastocyanin (PC), all derived from pea chloroplasts. There 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-coefficients of increasing pressure were observed on the reduction of P700(+) by PC(I), on the reduction of PC(II) by ascorbate, and on the oxidation of decyl plastoquinol by the bf complex. The volumes of activation ΔV(#) were determined from the dependence of the rate-coefficient on pressure using: [Formula: see text] The volume of activation is the difference in partial molar volume 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(+) by PC(I) and the oxidation of decyl plastoquinol by the bf complex, the rate coefficient decreased with increase in pressure, whilst for the reduction of PC(II) by ascorbate it increased. The corresponding volumes of activation were 9.6±0.6×10(-6) m(3) mol(-1), 18±2×10(-6) m(3) mol(-1) and -14±1×10(-6) m(3) mol(-1), respectively. Much of the pressure-dependence of PC(II) reduction by ascorbate was ascribed to an increase in ascorbate ionisation with increase in pressure. There was little effect of pressure on the kinetics of oxidation of ferrocytochrome f by PC(II), or on the equilibrium constant of the redox pair ferrocytochrome f/ferricytochrome f: PC(II)/PC(I). Possible physical bases for these activation volumes are discussed, and they are compared with literature values.