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Substrate-water exchange in photosystem II is arrested before dioxygen formation.

ABSTRACT: Light-driven oxidation of water into dioxygen, catalysed by the oxygen-evolving complex (OEC) in photosystem II, is essential for life on Earth and provides the blueprint for devices for producing fuel from sunlight. Although the structure of the OEC is known at atomic level for its dark-stable state, the mechanism by which water is oxidized remains unsettled. Important mechanistic information was gained in the past two decades by mass spectrometric studies of the H2(18)O/H2(16)O substrate-water exchange in the four (semi) stable redox states of the OEC. However, until now such data were not attainable in the transient states formed immediately before the O-O bond formation. Using modified photosystem II complexes displaying up to 40-fold slower O2 production rates, we show here that in the transient S3YZ state the substrate-water exchange is dramatically slowed as compared with the earlier S states. This further constrains the possible sites for substrate-water binding in photosystem II.

SUBMITTER: Nilsson H

PROVIDER: S-EPMC4102119 | biostudies-literature | 2014

REPOSITORIES: biostudies-literature

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Substrate-water exchange in photosystem II is arrested before dioxygen formation.

Nilsson Håkan H Rappaport Fabrice F Boussac Alain A Messinger Johannes J

Nature communications 20140704

Light-driven oxidation of water into dioxygen, catalysed by the oxygen-evolving complex (OEC) in photosystem II, is essential for life on Earth and provides the blueprint for devices for producing fuel from sunlight. Although the structure of the OEC is known at atomic level for its dark-stable state, the mechanism by which water is oxidized remains unsettled. Important mechanistic information was gained in the past two decades by mass spectrometric studies of the H2(18)O/H2(16)O substrate-water ...[more]

PMID: 24993602

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Project description:Highlights • We derive a model that provides an exact solution to the substrate-water exchange kinetics in a double-conformation system.• This model is used to interpret recently published data for Ca2+- and Sr2+-containing PSII in the S2 state, in which the g = 2.0 and g = 4.1 conformations coexist.• The component concentrations derived from the kinetic model provide an analytic description of the substrate-water exchange kinetics.• Contrary to previous reports, there is no significant effect of substituting Sr2+ for Ca2+ on any of the exchange rate constants.• The exchange rate of the slowly-exchanging water (Ws) in the S2 state g = 4.1 conformation is much faster than that in the g = 2.0 conformation, consistent with the assignment of Ws to W1 or W2 bound as terminal ligands to Mn4. We derive a model that provides an exact solution to the substrate-water exchange kinetics in a double-conformation system and use this model to interpret recently published data for Ca2+- and Sr2+-containing PSII in the S2 state, in which the g = 2.0 and g = 4.1 conformations coexist. The component concentrations derived from the kinetic model provide an analytic description of the substrate-water exchange kinetics, allowing us to more accurately interpret the results. Based on this model and the previously reported data on the S2 state g = 2.0 conformation, we obtain the substrate-water exchange rates of the g = 4.1 conformation and the conformational change rates. Two conclusions are made from the analyses. First, contrary to previous reports, there is no significant effect of substituting Sr2+ for Ca2+ on any of the exchange rate constants. Second, the exchange rate of the slowly-exchanging water (Ws) in the S2 state g = 4.1 conformation is much faster than that in the S2 state g = 2.0 conformation. The second conclusion is consistent with the assignment of Ws to W1 or W2 bound as terminal ligands to Mn4; Mn4 has been proposed to undergo an oxidation state change from Mn(IV) in the g = 2.0 conformation to Mn(III) in the g = 4.1 conformation.

Dataset Information

Substrate-water exchange in photosystem II is arrested before dioxygen formation.

Publications

Substrate-water exchange in photosystem II is arrested before dioxygen formation.

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