Mechanism and energy diagram for O-O bond formation in the oxygen-evolving complex in photosystem II.
The recent finding of a transition state with a significantly lower barrier than previously found, has made the mechanism for O-O bond formation in photosystem II much clearer. The full mechanism can be described in the following way. Electrons and protons are ejected from the oxygen-evolving complex (OEC) in an alternating fashion, avoiding unnecessary build-up of charge. The S0-S1 and S1-S2 transitions are quite exergonic, while the S2-S3 transition is only weakly exergonic. The strong endergonic S3-S4 transition is a key step in the mechanism in which an oxygen radical is produced, held by the dangling manganese outside the Mn3Ca cube. The O-O bond formation in the S4-state occurs by an attack of the oxygen radical on a bridging oxo ligand in the cube. The mechanism explains the presence of both a cube with bridging oxo ligands and a dangling manganese. Optimal orbital overlap puts further constraints on the structure of the OEC. An alternating spin alignment is necessary for a low barrier. The computed rate-limiting barrier of 14.7 kcal mol(-1) is in good agreement with experiments.
Siegbahn
Per E M
PE
Department of Biochemistry and Biophysics, Arrhenius Laboratory, Stockholm University, 106 91 Stockholm, Sweden. ps@physto.se
D008345:Manganese / Q000737:chemistry*; D008956:Models, Chemical; D010084:Oxidation-Reduction; D010100:Oxygen / Q000737:chemistry*; D045332:Photosystem II Protein Complex / Q000737:chemistry*; D011789:Quantum Theory; D013816:Thermodynamics
D045332:Photosystem II Protein Complex; D008345:Manganese; D010100:Oxygen