Oxidation-reduction kinetics of cytochrome f in whole cells

Abstract

The kinetic aspects of the cytochrome f function and its relation to the non-cyclic electron transport were investigated in photosynthetizing green algae under physiological conditions. Easy-to-use and low cost single- and double-beam spectrophotometers were constructed which enabled the measurement of light-induced absorption changes of cytochromes of highly scattering samples under different light—dark preconditioning regimes. The complexity of the light-induced absorption spectra in the 540 – 570 nm spectral range is discussed. The lack of separation of the components based on their kinetic behaviour indicated that different cytochrome f forms contributed to the spectra. By comparison with the time course of the variable part of the fluorescence at 744 nm at room temperature, it was demonstrated that the oxidation of cytochrome f is a pure photochemical process if the half-risetime is greater than 50 ms. The dark-reduction kinetics is monophasic after a single-flash excitation of dark-adapted algae ( t 1 2 = 2 ms) or after producing a photostationary state by means of a long (1 s) and relatively weak (half-risetime of the oxidation greater than 50 ms) flash ( t 1 2 = 500 ms, highly dependent on the systematic position and the states of the algae). The biphasicity (the existence of a mixture of fast and slow components) is evident in the reduction kinetics after excitation by a long and intense flash. The results are discussed within the framework of a simple linear pathway: the electrons from a pool of reduced plastoquinone are transferred to cytochrome f, to plastocyanin and then to the reaction centre of photosystem I. We distinguish basically two states of electron pressure in the plastoquinone pool: high if the half-time of dark reduction is less than 15 ms, and low if it is greater than 100 ms. The states of the pool can interconvert by preconditioning (dark—light regime) and excitation (oxidation). The majority of the data presented in this paper were obtained under photostationary conditions of living cells which fulfill the criteria for the efficient function of cytochrome f in non-cyclic electron transport of photosynthesis.