QM/MM Models of the O_2-Evolving Complex of Photosystem II

摘要

This paper introduces structural models of the oxygen-evolving complex of photo-system II (PSII) in the dark-stable S_1 state,as well as in the reduced So and oxidized S_2 states,with complete ligation of the metal-oxo cluster by amino acid residues,water,hydroxide,and chloride.The models are developed according to state-of-the-art quantum mechanics/molecular mechanics (QM/MM) hybrid methods,applied in conjunction with the X-ray crystal structure of PSII from the cyanobacterium Thermosynechococcus elongatus,recently reported at 3.5 A resolution.Manganese and calcium ions are ligated consistently with standard coordination chemistry assumptions,supported by biochemical and spectroscopic data.Furthermore,the calcium-bound chloride ligand is found to be bound in a position consistent with pulsed electron paramagnetic resonance data obtained from acetate-substituted PSII.The ligation of protein ligands includes monodentate coordination of D1-D342,CP43-E354,and D1-D170 to Mn(1),Mn(3),and Mn(4),respectively;eta~2 coordination of D1-E333 to both Mn(3) and Mn(2);and ligation of D1-E189 and D1-H332 to Mn(2).The resulting QM/MM structural models are consistent with available mechanistic data and also are compatible with X-ray diffraction models and extended X-ray absorption fine structure measurements of PSII.It is,therefore,conjectured that the proposed QM/MM models are particularly relevant to the development and validation of catalytic water-oxidation intermediates.