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Photosystem II oxygen-evolving complex photoassembly displays an inverse H/D solvent isotope effect under chloride-limiting conditions.

ABSTRACT: Photosystem II (PSII) performs the solar-driven oxidation of water used to fuel oxygenic photosynthesis. The active site of water oxidation is the oxygen-evolving complex (OEC), a Mn4CaO5 cluster. PSII requires degradation of key subunits and reassembly of the OEC as frequently as every 20 to 40 min. The metals for the OEC are assembled within the PSII protein environment via a series of binding events and photochemically induced oxidation events, but the full mechanism is unknown. A role of proton release in this mechanism is suggested here by the observation that the yield of in vitro OEC photoassembly is higher in deuterated water, D2O, compared with H2O when chloride is limiting. In kinetic studies, OEC photoassembly shows a significant lag phase in H2O at limiting chloride concentrations with an apparent H/D solvent isotope effect of 0.14 ± 0.05. The growth phase of OEC photoassembly shows an H/D solvent isotope effect of 1.5 ± 0.2. We analyzed the protonation states of the OEC protein environment using classical Multiconformer Continuum Electrostatics. Combining experiments and simulations leads to a model in which protons are lost from amino acid that will serve as OEC ligands as metals are bound. Chloride and D2O increase the proton affinities of key amino acid residues. These residues tune the binding affinity of Mn2+/3+ and facilitate the deprotonation of water to form a proposed ?-hydroxo bridged Mn2+Mn3+ intermediate.

SUBMITTER: Vinyard DJ

PROVIDER: S-EPMC6754581 | biostudies-literature | 2019 Sep

REPOSITORIES: biostudies-literature

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Photosystem II oxygen-evolving complex photoassembly displays an inverse H/D solvent isotope effect under chloride-limiting conditions.

Vinyard David J DJ Badshah Syed Lal SL Riggio M Rita MR Kaur Divya D Fanguy Annaliesa R AR Gunner M R MR

Proceedings of the National Academy of Sciences of the United States of America 20190904 38

Photosystem II (PSII) performs the solar-driven oxidation of water used to fuel oxygenic photosynthesis. The active site of water oxidation is the oxygen-evolving complex (OEC), a Mn<sub>4</sub>CaO<sub>5</sub> cluster. PSII requires degradation of key subunits and reassembly of the OEC as frequently as every 20 to 40 min. The metals for the OEC are assembled within the PSII protein environment via a series of binding events and photochemically induced oxidation events, but the full mechanism is ...[more]

PMID: 31484762

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Project description:Prolyl hydroxylase domain 2 (PHD2) is deemed a primary oxygen sensor in humans, yet many details of its underlying mechanism are still not fully understood. (Fe(2+) + ?KG)PHD2 is 6-coordinate, with a 2His/1Asp facial triad occupying three coordination sites, a bidentate ?-ketoglutarate occupying two sites, and an aquo ligand in the final site. Turnover is thought to be initiated upon release of the aquo ligand, creating a site for O(2) to bind at the iron. Herein we show that steady-state turnover is faster under acidic conditions, with k(cat) exhibiting a kinetic pK(a) = 7.22. A variety of spectroscopic probes were employed to identify the active-site acid, through comparison of (Fe(2+) + ?KG)PHD2 at pH 6.50 with pH 8.50. The near-UV circular dichroism spectrum was virtually unchanged at elevated pH, indicating that the secondary structure did not change as a function of pH. UV-visible and Fe X-ray absorption spectroscopy indicated that the primary coordination sphere of Fe(2+) changed upon increasing the pH; extended X-ray absorption fine structure analysis found a short Fe-(O/N) bond length of 1.96 Å at pH 8.50, strongly suggesting that the aquo ligand was deprotonated at this pH. Solvent isotope effects were measured during steady-sate turnover over a wide pH-range, with an inverse solvent isotope effect (SIE) of k(cat) observed ((D(2)O)k(cat) = 0.91 ± 0.03) for the acid form; a similar SIE was observed for the basic form of the enzyme ((D(2)O)k(cat) = 0.9 ± 0.1), with an acid equilibrium offset of ?pK(a) = 0.67 ± 0.04. The inverse SIE indicated that aquo release from the active site Fe(2+) immediately precedes a rate-limiting step, suggesting that turnover in this enzyme may be partially limited by the rate of O(2) binding or activation, and suggesting that aquo release is relatively slow. The unusual kinetic pK(a) further suggested that PHD2 might function physiologically to sense both intracellular pO(2) as well as pH, which could provide for feedback between anaerobic metabolism and hypoxia sensing.

Dataset Information

Photosystem II oxygen-evolving complex photoassembly displays an inverse H/D solvent isotope effect under chloride-limiting conditions.

Publications

Photosystem II oxygen-evolving complex photoassembly displays an inverse H/D solvent isotope effect under chloride-limiting conditions.

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