Abstract
Photosystem II uses water as an enzymatic substrate. It has been hypothesized that this water is vectored to the active site for water oxidation via water channels that lead from the surface of the protein complex to the Mn4O5Ca metal cluster. The radiolysis of water by synchrotron radiation produces amino acid residue-modifying OH(•) and is a powerful technique to identify regions of proteins that are in contact with water. In this study, we have used this technique to oxidatively modify buried amino acid residues in higher plant Photosystem II membranes. Fourier transform ion cyclotron resonance mass spectrometry was then used to identify these oxidized amino acid residues that were located in several core Photosystem II subunits (D1, D2, CP43, and CP47). While, as expected, the majority of the identified oxidized residues (≈75%) are located on the solvent-exposed surface of the complex, a number of buried residues on these proteins were also modified. These residues form groups which appear to lead from the surface of the complex to the Mn4O5Ca cluster. These residues may be in contact with putative water channels in the photosystem. These results are discussed within the context of a number of largely computational studies that have identified putative water channels in Photosystem II.
Highlights
Substrate water must reach the buried Mn4O5Ca cluster in Photosystem II
As we have reported previously, even in the absence of synchrotron radiation a number of amino acid residues are found to be oxidatively modified in Photosystem II (PS II) [30, 35]
Seventy-three oxidatively modified amino acid residues were identified in the D1, D2, CP43, and CP47 proteins of spinach PS II at time 0, representing Ϸ4% of the residues found in these four subunits
Summary
Substrate water must reach the buried Mn4O5Ca cluster in Photosystem II. Results: OH1⁄7 produced by radiolysis modified buried amino acid residues. The radiolysis of water by synchrotron radiation produces amino acid residue-modifying OH1⁄7 and is a powerful technique to identify regions of proteins that are in contact with water. As expected, the majority of the identified oxidized residues (≈75%) are located on the solvent-exposed surface of the complex, a number of buried residues on these proteins were modified. These residues form groups which appear to lead from the surface of the complex to the Mn4O5Ca cluster. These residues may be in contact with putative water channels in the photosystem.
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