Electron Transfer on the Donor and Acceptor Sides of Photosystem II Explored by Site-Directed Mutagenesis and Kinetic Spectroscopy

Abstract

H252Q and six donor-side mutants became the important tools to investigate a new model of neutral tyrosine radical, $\rm Y\sb{Z}\sp\cdot,$ introduced by Gilchrist et al. (1995) and Hoganson et al. (1995). In this new model, $\rm Y\sb{Z}$ is not simply a redox active residue in the electron transfer reactions, but is also directly involved in the water-oxidation. The models are critically-dependent on the presence of a neutral radical form of $\rm Y\sb{Z}\ (Y\sb{Z}\sp{\cdot}).$ The donor-side mutants, $\rm E189 > L,Q,$ and $\rm N191 > A,D,L,$ had only small effects on the pH-dependence of the rate of Q$\rm\sb{A}\sp-$ and oxidized $\rm Y\sb{Z}$ recombination. On the other hand, E189D mutant showed a pH-independent in rate of $\rm Q\sb{A}\sp-$ and oxidized $\rm Y\sb{Z}$ recombination. Based on these data, a simple thermodynamic model was introduced to explain the results and the model of $\rm Y\sb{Z}\sp{\cdot}$ as an oxidized form.