Coupling of electron and proton transfer in oxidative water cleavage in photosynthesis

Abstract

This minireview addresses questions on the mechanism of oxidative water cleavage with special emphasis on the coupling of electron (ET) and proton transfer (PT) of each individual redox step of the reaction sequence and on the mode of OO bond formation. The following topics are discussed: (1) the multiphasic kinetics of Y Z ox formation by P680 + originate from three different types of rate limitations: (i) nonadiabatic electron transfer for the “fast” ns reaction, (ii) local “dielectric” relaxation for the “slow” ns reaction, and (iii) “large-scale” proton shift for the μs kinetics; (2) the ET/PT-coupling mode of the individual redox transitions within the water oxidizing complex (WOC) driven by Y Z ox is assumed to depend on the redox state S i : the oxidation steps of S 0 and S 1 comprise separate ET and PT pathways while those of S 2 and S 3 take place via proton-coupled electron transfer (PCET) analogous to Jerry Babcock's hydrogen atom abstractor model [Biochim. Biophys. Acta, 1458 (2000) 199]; (3) S 3 is postulated to be a multistate redox level of the WOC with fast dynamic equilibria of both redox isomerism and proton tautomerism. The primary event in the essential OO bond formation is the population of a state S 3(P) characterized by an electronic configuration and nuclear geometry that corresponds with a complexed hydrogen peroxide; (4) the peroxidic type S 3(P) is the entatic state for formation of complexed molecular oxygen through S 3 oxidation by Y Z ox; and (5) the protein matrix itself is proposed to exert catalytic activity by functioning as “PCET director”. The WOC is envisaged as a supermolecule that is especially tailored for oxidative water cleavage and acts as a molecular machine.