Transient kinetics of the reaction between cytochrome c-552 or plastocyanin and P-700 in subchloroplast particles

Abstract

We have investigated the kinetics of reduction of P-700 in Photosystem I-enriched subchlroplast particles by cytochrome c-552 from Euglena and by plastocyanin, and the effects of cations on these reactions. In both cases, the results can be explained in terms of a bimolecular reaction between P-700 + and the electron donor. Addition of low concentrations of mono-, di- or polyvalent cations stimulate, whereas addition of high concentrations of cations inhibit these reactions. The effects of cations can be interpreted as effects on the membrane surface potential of the Photosystem I particles, as shown previously (Tamura, N., Yamamoto, Y. and Nishimura, M. (1980) Biochim. Biophys. Acta 592, 536–545). The maximal second-order rate constants obtained for the reduction of P-700 + are 9 · 10 7 M −1 · s −1 if cytochrome c-552 is the electron donor, and 1.5 · 10 8 M −1 · s −1 when the donor is plastocyanin. Using the latter rate constant, we are able to demonstrate that the apparently biphasic kinetics of reduction of P-700 + observed in intact chloroplasts show a good fit to those expected for a bimolecular reaction between P-700 + and plastocyanin.