Crystallographic and Computational Analysis of the Barrel Part of the PsbO Protein of Photosystem II: Carboxylate–Water Clusters as Putative Proton Transfer Relays and Structural Switches
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- 作者：Martin Bommer ; Ana-Nicoleta Bondar ; Athina Zouni ; Holger Dobbek ; Holger Dau
- 刊名：Biochemistry
- 出版年：2016
- 出版时间：August 23, 2016
- 年：2016
- 卷：55
- 期：33
- 页码：4626-4635
- 全文大小：561K
- 年卷期：0
- ISSN：1520-4995
文摘
In all organisms that employ oxygenic photosynthesis, the membrane-extrinsic PsbO protein is a functionally important component of photosystem II. To study the previously proposed proton antenna function of carboxylate clusters at the protein–water interface, we combined crystallography and simulations of a truncated cyanobacterial (Thermosynechococcus elongatus) PsbO without peripheral loops. We expressed the PsbO β-barrel heterologously and determined crystal structures at resolutions of 1.15–1.5 Å at 100 K at various pH values and at 297 K and pH 6. (1) Approximately half of the 177 surface waters identified at 100 K are resolved at 297 K, suggesting significant occupancy of specific water sites at room temperature, and loss of resolvable occupancy for other sites. (2) Within a loop region specific to cyanobacterial PsbO, three residues and four waters coordinating a calcium ion are well ordered even at 297 K; the ligation differs for manganese. (3) The crystal structures show water–carboxylate clusters that could facilitate fast Grotthus-type proton transfer along the protein surface and/or store protons. (4) Two carboxylate side chains, which are part of a structural motif interrupting two β-strands and connecting PsbO to photosystem II, are within hydrogen bonding distance at pH 6 (100 K). Simulations indicate coupling between protein structure and carboxylate protonation. The crystal structure determined at 100 K and pH 10 indicates broken hydrogen bonding between the carboxylates and local structural change. At pH 6 and 297 K, both conformations were present in the crystal, suggesting conformational dynamics in the functionally relevant pH regime. Taken together, crystallography and molecular dynamics underline a possible mechanism for pH-dependent structural switching.