Network of Hydrogen Bonds near the Oxygen-Evolving Mn4CaO5 Cluster of Photosystem II Probed with FTIR Difference Spectroscopy

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• 作者：Rachel J. Service ; Warwick Hillier ; Richard J. Debus
• 刊名：Biochemistry
• 出版年：2014
• 出版时间：February 18, 2014
• 年：2014
• 卷：53
• 期：6
• 页码：1001-1017
• 全文大小：825K
• ISSN：1520-4995

文摘

We previously provided experimental evidence that an extensive network of hydrogen bonds exists near the oxygen-evolving Mn₄CaO₅ cluster in photosystem II and that elements of this network form part of a dominant proton-egress pathway leading from the Mn₄CaO₅ cluster to the thylakoid lumen. The evidence was based on (i) the elimination of the same 谓(C鈺怬) mode of a protonated carboxylate group in the S₂-minus-S₁ FTIR difference spectrum of wild-type PSII core complexes from the cyanobacterium Synechocystis sp. PCC 6803 by the mutations D1-E65A, D2-E312A, and D1-E329Q and (ii) the substantial decrease in the efficiency of the S₃ to S₀ transition caused by the mutations D1-D61A, D1-E65A, and D2-E312A. The eliminated 谓(C鈺怬) mode corresponds to an unidentified carboxylate group whose pK_a value decreases in response to the increased charge that develops on the Mn₄CaO₅ cluster during the S₁ to S₂ transition. In the current study, we have extended our work to include the 谓(C鈺怬) regions of other S_n+1-minus-S_n FTIR difference spectra and to additional mutations of residues inferred to participate in networks of hydrogen bonds near the Mn₄CaO₅ cluster or leading from the Mn₄CaO₅ cluster to the thylakoid lumen. Our data suggest that a different carboxylate group has its pK_a value increased during the S₂ to S₃ transition and that a third carboxylate group experiences a change in its environment during the S₀ to S₁ transition. The pK_a values that shift during the S₁ to S₂ and S₂ to S₃ transitions appear to be restored during the S₃ to S₀ transition. The D1-R334A mutation decreases or eliminates the same 谓(C鈺怬) modes from the S₂-minus-S₁ and S₃-minus-S₂ spectra as mutations D1-E65A, D2-E312A, and D1-E329Q and substantially decreases the efficiency of the S₃ to S₀ transition. We conclude that D1-R334 participates in the same dominant proton-egress pathway that was identified in our previous study. The D1-Q165E mutation leaves the 谓(C鈺怬) region of the S₂-minus-S₁ FTIR difference spectrum intact, but it eliminates a mode from this region of the S₃-minus-S₂ spectrum. We conclude that D1-Q165 participates in an extensive network of hydrogen bonds that that extends across the Mn₄CaO₅ cluster to the D1-E65/D2-E312 dyad and that includes D1-E329 and several water molecules including the W2 and W3 water ligands of the Mn₄CaO₅ cluster鈥檚 dangling Mn_A4 and Ca ions, respectively. The D2-E307Q, D2-D308N, D2-E310Q, and D2-E323Q mutations alter the 谓(C鈺怬) regions of none of the FTIR difference spectra. We conclude that these four residues are located far from the three unidentified carboxylate groups that give rise to the 谓(C鈺怬) features observed in the FTIR difference spectra.