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

Studies on the mechanism of oxidative water cleavage in cyanobacteria and plants. Gernot Renger^*,1, ¹ Max Volmer Laboratorium für Biophysikalische Chemie, Berlin, Berlin, Germany

ABSTRACT- Oxidative photosynthetic water cleavage into molecular oxygen and four protons occurs via a sequence of redox steps energetically driven by the strongly oxidizing cation radical P680^+⋅ that is generated by light-induced charge separation. The reaction pattern has been analyzed by comparative studies of individual reactions in thermophilic cyanobacteria (Thermosynechococcus elongatus) and higher plants (spinacea oleacea) with special emphasis on the temperature dependence and kinetic H/D exchange effects. Based on the results of flash-induced absorption changes and oxygen evolution in combination with theoretical considerations the following results were obtained: 1. The multiphasic kinetics of P680^+⋅ reduction by Y_Z originates from at least three types of rate limitations depending on the levels of sequential conformational states in a ladder of relaxation processes: (i) nonadiabatic electron transfer in the initial state, (ii) local "dielectric" relaxation, and (iii) "large scale" proton shift within hydrogen bond network(s) in the individual oxidation steps (1). 2. The mode of coupling between electron (ET) and proton transfer (PT) in the individual oxidation steps is assumed to depend on the redox state S_i of the water-oxidizing complex (WOC): oxidation of the WOC in S₀ and S₁ comprises separate ET and PT pathways, whereas S₂ and S₃ undergo proton-coupled electron transfer (PCET) redox reactions according to the postulations of Babcock's hydrogen abstractor model (2). 3. The primary event in the essential O-O bond formation is postulated to occur within a multistate equilibrium at the redox level of S₃, comprising both redox isomerism and proton tautomerism. One state S₃(P) is assumed to obtain an electronic configuration and nuclear geometry that corresponds with a hydrogen bonded peroxide and acts as the entatic state for the generation of complexed molecular oxygen through S₃(P) oxidation by Y_Z (3). 4. The protein matrix of the WOC is discussed to play an active catalytic function, predominantly via directed proton shifts. References: (1) Renger, G. Biochim. Biophys. Acta 1503 (2001) 210-228 (2) Tommos, C. and Babcock, G.T. Biochim. Biophys. Acta 1458 (2000) 199-219 (3) Renger, G. Biochim. Biophys. Acta 1655 (2004) 195-204

KEY WORDS: P680 YZ electron transfer, H/D isotope effects and temperature dependence, photosystem II, hydrogen bond network