O2 evolution and recovery of the water-oxidizing enzyme

Abstract

In photosystem II, light-induced water oxidation occurs at the Mn4CaO5 cluster. Here we demonstrate proton releases, dioxygen formation, and substrate water incorporation in response to Mn4CaO5 oxidation in the protein environment, using a quantum mechanical/molecular mechanical approach and molecular dynamics simulations. In S2, H2O at the W1 site forms a low-barrier H-bond with D1-Asp61. In the S2-to-S3 transition, oxidation of OW1H– to OW1•–, concerted proton transfer from OW1H– to D1-Asp61, and binding of a water molecule Wn-W1 at OW1•– are observed. In S4, Wn-W1 facilitates oxo-oxyl radical coupling between OW1•– and corner μ-oxo O4. Deprotonation via D1-Asp61 leads to formation of OW1=O4. As OW1=O4 moves away from Mn, H2O at W539 is incorporated into the vacant O4 site of the O2-evolved Mn4CaO4 cluster, forming a μ-oxo bridge (Mn3–OW539–Mn4) in an exergonic process. Simultaneously, Wn-W1 is incorporated as W1, recovering the Mn4CaO5 cluster.

Theoretical and computational investigations of geometrical, electronic and spin structures of the CaMn4OX (X = 5, 6) cluster in the Kok cycle Si (i = 0–3) of oxygen evolving complex of photosystem II

“…A. On the other hand, the nucleophilic attack of H 2 O(W1) coordinated to Mn 4 to the electrophilic O (4) site (Yamanaka et al ) is compatible with the QM/MM computational results (Kawashima et al ) as illustrated in Fig. B.…”

Mimicking the Catalytic Center for the Water-Splitting Reaction in Photosystem II

“…Based on recent crystallographic studies [39,42,46,[50][51][52][53], spectroscopic investigations, and theoretical calculations [17,18,[66][67][68][69][70][71][72][73][74][75], different proposals for the O-O bond formation have been suggested [6,49,66,[75][76][77][78][79][80][81]. Figures 3-6 show four typical proposals from different groups.…”

“…The second proposal for the mechanism of the water-splitting reaction was suggested by Ishikita's group, as shown in Figure 4 [79]. In this mechanism, the µ 2 -oxide bridge (O4) and one water molecule (W1) were suggested to provide the oxygen atoms to form the O-O bond.…”

Capturing structural changes of photosystem II using time-resolved serial femtosecond crystallography with proper flash excitations

“…Dioxygen is then likely formed between O5 and O6, or between O5 and another water molecule, by an oxyl/oxo coupling mechanism, and in the latter case, O6 re lls the empty substrate site. The changes of waters in the O1-channel support the mechanism of water delivery from the Ca side 8,29,30 , rather than the O4-channel [22][23][24] . Further structural analysis will be required to reveal how the OEC incorporates those waters in the O1-channel and how protons egress during the subsequent S-state transitions.…”

“…Structural changes in the O4 and O1 channels Among the four channels, the O4-channel has been suggested to function as the pathway of proton release during the S 0 -to-S 1 transition based on theoretical calculations 21 . On the other hand, other groups argued that it serves as the source of substrate water by a "pivot" or "carousel" mechanism in the transition of S 2 -to-S 3 [22][23][24] . As described above, upon transition to the S 2 state, W665 in the O4-channel became highly disordered.…”