Analysis of the P680+˙ reduction pattern and its temperature dependence in oxygen-evolving PS II core complexes from thermophilic cyanobacteria and higher plants

Abstract

The multiphasic P680⁺˙ reduction kinetics by Y_Z and their temperature dependence were investigated in PS II core complexes with high oxygen evolution capacity, isolated from a thermophilic cyanobacterium (Thermosynechococcus elongatus) and a higher plant (Spinacea oleracea). Measurements and kinetic analyses of laser flash induced 820 nm absorption changes (reflecting the turnover of P680) led to the following results: (a) the pattern of multiphasic P680⁺˙ reduction is basically the same in both species, (b) the activation energy of the “fast” nanosecond kinetics is 20 ± 5 kJ mol⁻¹ and 14 ± 5 kJ mol⁻¹ for the samples from T. elongatus and S. oleracea, respectively, (c) the activation energies of this reaction are nearly the same in complexes with water oxidizing complex (WOC) in redox states S₁ and S₂, (d) the activation energy of the “slow” nanosecond kinetics ascribed to “local” relaxation processes is larger by almost a factor of two compared to that of the “fast” nanosecond kinetics, and (e) the normalized amplitudes of the “fast” and “slow” nanosecond kinetics are virtually independent of temperature in the physiological range for PS II core complexes from both organisms. Based on these findings the energetics and kinetics of P680⁺˙ reduction in fully competent PS II is briefly discussed within the framework of a dynamic model of sequential relaxation processes. The protein dynamics are inferred to provide the major contribution to the driving force of the redox reaction.