Kinetic studies of electron transfer in a hybrid system constructed from the cytochrome bf complex and Photosystem I

Abstract

(1) A hybrid electron-transfer system has been constructed from purified cytochrome bf complex and Photosystem I preparations which are active in plastocyanin oxidation. Donor and acceptor characteristics of these purified preparations are described. (2) The use of this simplified system enabled the application of a rigorous method of component deconvolution from spectral kinetic data. This relied on matrix analysis of the extinction coefficients of all components at each wavelength of measurement. (3) Kinetic analyses of flash-induced redox changes of cytochromes b-563, cytochrome f, plastocyanin and P-700 were made with this method and were compared with computer simulations of predicted behaviour. These studies indicated that cytochrome b-563 behaviour was consistent with its being reduced by ‘oxidant-induced reduction’ at centre o, even although cytochrome b-563 reduction was apparently much faster than cytochrome f rereduction after a flash. However, in order to obtain good agreement between experiment and simulation, the midpoint potential of the Rieske centre had to be assumed to be +370 mV. (4) Multiple turnover behaviour of the cytochrome bf complex at high potentials was also consistent with the Q-cycle feature of oxidant-induced reduction of cytochrome b. However, a maximum reduction of only around 25% of total cytochrome b-563 could be achieved via this route, and this extent was neither affected by 2- n-nonyl-4-hydroxyquinoline N-oxide (NQNO) nor by added quinone. Similarly, in thylakoids in the presence of NQNO, less than half of the total cytochrome b-563 could be reduced by multiple flashes. (5) Continuous illumination experiments at a low potential indicated that cytochrome b oxidation occurred on turnover of the cytochrome bf complex even when the cytochromes b were fully prereduced. The oxidation was sensitive to NQNO.