Abstract
The photosystem II reaction center mediates the light-induced transfer of electrons from water to plastoquinone, with concomitant production of O2. Water oxidation chemistry occurs in the oxygen-evolving complex (OEC), which consists of an inorganic Mn4CaO5 cluster and its surrounding protein matrix. Light-induced Fourier transform infrared (FTIR) difference spectroscopy has been successfully used to study the molecular mechanism of photosynthetic water oxidation. This powerful technique has enabled the characterization of the dynamic structural changes in active water molecules, the Mn4CaO5 cluster, and its surrounding protein matrix during the catalytic cycle. This mini-review presents an overview of recent important progress in FTIR studies of the OEC and implications for revealing the molecular mechanism of photosynthetic water oxidation.
Highlights
Photosynthetic water oxidation is catalyzed by a Mn4Ca cluster and its surrounding protein matrix in photosystem II (PSII; Ferreira et al, 2004; Loll et al, 2005; Yano et al, 2006; Umena et al, 2011)
More recent studies have suggested that the structure of the X-ray diffraction (XRD) model of PSII is modified by radiation-induced reduction of the Mn cluster (Luber et al, 2011; Grundmeier and Dau, 2012)
These observations are consistent with a recent Fourier transform infrared (FTIR) study which concluded that the proton release pattern from the substrate water on the oxygen-evolving complex (OEC) is in 1:0:1:2 stoichiometry for the S0 → S1 → S2 → S3 → S0 transition (Suzuki et al, 2009)
Summary
Photosynthetic water oxidation is catalyzed by a Mn4Ca cluster and its surrounding protein matrix in photosystem II (PSII; Ferreira et al, 2004; Loll et al, 2005; Yano et al, 2006; Umena et al, 2011). One very important development in FTIR studies of the OEC were reports of high-frequency spectra (3700–3500 cm−1) of the OEC, which contain information on structural changes of the weakly H-bonded OH-stretching of active water molecules during S-state transitions of the OEC (Noguchi and Sugiura, 2000, 2002a,b). In contrast to the S1 → S2 transition, the S2 → S3, S3 → S0, and S0 → S1 transitions all showed a negative OH-stretching mode at different frequencies, which indicates that these water (or hydroxide) molecules were involved in proton release reactions of the OEC or formed strong hydrogen-bonding interactions during these transitions (Noguchi and Sugiura, 2002a,b) These observations are consistent with a recent FTIR study which concluded that the proton release pattern from the substrate water on the OEC is in 1:0:1:2 stoichiometry for the S0 → S1 → S2 → S3 → S0 transition (Suzuki et al, 2009).
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