PARENT SESSION
Posters P7A Mechanisms of water oxidation. Abstracts (347-381)

The influence of the D1-H332 residue on the energetics of the S₂ state. Imre Vass^*,1, Yagut Allakhverdieva¹, Krisztián Cser¹, Zsuzsanna Deák¹, Bruce Diner², Peter Nixon³, ¹ Institute of Plant Biology, Biological Research Center, Szeged, Hungary² CR&D, Experimental Station, E.I. du Pont de Nemours and Co., Wilmington, DE, USA³ Department of Biological Sciences, Imperial College London, Biochemistry Building, South Kensington campus, London, UK

ABSTRACT- The His332 residue of the D1 protein has been proposed to participate in ligating the catalytic Mn ions of the water-oxidizing complex. Here we used thermoluminescence and flash-induced chlorophyll fluorescence measurements to characterize the effect of the D1-H333E, D1-H332D and D1-H332S mutations on the electron transport properties of PSII in intact cells of the cyanobacterium Synechocystis 6803. Although the mutants are not photoautotrophic and, with the exception of D1-H332S, do not evolve oxygen they all show flash-induced thermoluminescence and chlorophyll fluorescence, demonstrating charge stabilization in the of the S₂ state of the water oxidizing complex. The thermoluminescence Q and B bands, which originate from the S₂Q_A^- and S₂Q_B^- recombinations, respectively are shifted to higher temperature in the mutants when compared to the wild-type cells indicating the increased stability of the S₂ state. This is confirmed by the increased time constant of fluorescence relaxation in the presence of DCMU. These findings demonstrate that the S₂ state can be formed when D1-H332 is replaced by Asp, Glu and Ser. However, the energetic stability of the S₂ state increases due to a 10-70mV decrease in E_m(S₂/S₁). It is concluded that D1-H332 is essential for the functioning of the Mn cluster and has a critical role in maintaining the proper redox potential of the S₂ state.

KEY WORDS: Mn cluster, S-states, water oxidation, oxygen evolution