摘要
The molecular mechanism of the water oxidation reaction in photosystem II (PSII) of green plants remains a great mystery in life science. This reaction is known to take place in the oxygen evolving complex (OEC) incorporating four manganese, one calcium and one chloride cofactors, that is light-driven to cycle four intermediates, designated S0 through S4, to produce four protons, five electrons and lastly one molecular oxygen, for indispensable resources in biosphere. Recent advancements of X-ray crystallography models established the existence of a catalytic Mn4Ca cluster ligated by seven protein amino acids, but its functional structure is not yet resolved. The 18O exchange rates of two substrate water molecules were recently measured for four Si-state samples (i = 0–3) leading to 34O2 and 36O2 formations, revealing asymmetric substrate binding sites significantly depending on the Si-state. In this paper, we present a chemically complete model for the Mn4Ca cluster and its surrounding enzyme field, which we found out from some possible models by using the hybrid density functional theoretic geometry optimization method to confirm good agreements with the 3.0 Å resolution PSII model [B. Loll, J. Kern, W. Saenger, A. Zouni , J. Biesiadka, Nature 438 (2005) 1040–1044] and the S-state dependence of 18O exchange rates [W. Hillier and T. Wydrzynski, Phys. Chem. Chem. Phys. 6 (2004) 4882–4889]. Furthermore, we have verified that two substrate water molecules are bound to asymmetric cis-positions on the terminal Mn ion being triply bridged (μ-oxo, μ-carboxylato, and a hydrogen bond) to the Mn3CaO3(OH) core, by developing a generalized theory of 18O exchange kinetics in OEC to obtain an experimental evidence for the cross exchange pathway from the slow to the fast exchange process. Some important experimental data will be discussed in terms of this model and its possible tautomers, to suggest that a cofactor, Cl− ion, may be bound to CP43-Arg357 nearby Ca2 ion and that D1-His337 may be used to trap a released proton only in the S2-state.