Arrested Substrate Binding Resolves Catalytic Intermediates in Higher‐Plant Water Oxidation

Abstract

Among the intermediate catalytic steps of the wateroxidizing Mn 4 CaO 5 cluster of photosystem II (PSII), the final metastable S 3 state is critically important because it binds one substrate and precedes O 2 evolution. Herein, we combine Xand Q-band EPR experiments on native and methanol-treated PSII of Spinacia oleracea and show that methanol-treated PSII preparations of the S 3 state correspond to a previously uncharacterized high-spin (S = 6) species. This is confirmed as a major component also in intact photosynthetic membranes, coexisting with the previously known intermediate-spin conformation (S = 3). The high-spin intermediate is assigned to a water-unbound form, with a Mn IV 3 subunit interacting ferromagnetically via anisotropic exchange with a coordinatively unsaturated Mn IV ion. These results resolve and define the structural heterogeneity of the S 3 state, providing constraints on the S 3 to S 4 transition, on substrate identity and delivery pathways, and on the mechanism of OÀO bond formation.

Successes, challenges, and opportunities for quantum chemistry in understanding metalloenzymes for solar fuels research

“…4), which must have a high spin state (S = 6) as predicted by the quantum chemical analysis of Retegan et al 109 This has received very strong experimental support by a recent multifrequency EPR experimental study of the S 3 state in spinach PSII, which directly identified an S = 6 population with high effective anisotropy deriving from anisotropic exchange coupling between the coordinatively unsaturated Mn4(IV) ion and the Mn(IV) 3 Ca cubane subunit. 108 Importantly, the S = 6 component was found to be the exclusive species in methanol-treated spinach PSII and to form a major part of S 3 even in the native system. 108 Although the water bound intermediate-spin (S = 3) isomers of Fig.…”

“…108 Importantly, the S = 6 component was found to be the exclusive species in methanol-treated spinach PSII and to form a major part of S 3 even in the native system. 108 Although the water bound intermediate-spin (S = 3) isomers of Fig. 4 have been experimentally confirmed as distinct populations of the S 3 state, Krewald et al showed that water binding is not required to oxidize the cluster to the S 4 state.…”

“…4). 108,109 Similar structure-magnetism correlations have been used to examine the effect of protonation of oxo bridges on specific exchange coupling pathways. It is known that in general an oxo bridge enables stronger superexchange and thus enhances antiferromagnetic coupling between two Mn ions compared to a hydroxo bridge.…”

Orientational Jahn–Teller Isomerism in the Dark‐Stable State of Nature's Water Oxidase

“…[24] Theemergence of valence isomers in the S 2 state may be crucial for controlling access to the last experimentally observable S 3 state. [25][26][27] Thev alence isomers are part of agating mechanism in which the majority form (S 2 A )ismore easily deprotonated yet the minority form (S 2 B )c an be more easily oxidized to an all-Mn IV species before binding an additional water molecule. [26] This water-unbound form of the S 3 state (Figure 1) was recently identified by EPR spectroscopy on spinach PSII as ahigh-spin (S = 6) species with high effective anisotropy, [27] in contrast to water-bound S 3 intermediates that have intermediate-spin S = 3ground states.…”

“…[25][26][27] Thev alence isomers are part of agating mechanism in which the majority form (S 2 A )ismore easily deprotonated yet the minority form (S 2 B )c an be more easily oxidized to an all-Mn IV species before binding an additional water molecule. [26] This water-unbound form of the S 3 state (Figure 1) was recently identified by EPR spectroscopy on spinach PSII as ahigh-spin (S = 6) species with high effective anisotropy, [27] in contrast to water-bound S 3 intermediates that have intermediate-spin S = 3ground states. [27][28][29][30][31][32] Thedetails of the S 2 -S 3 transition and the precise composition of the heterogeneous S 3 state are subjects of intense current research efforts because of their direct implications for the mechanism of the final unresolved steps of O 2 formation and evolution.…”