5G38: PsbO subunit of Photosystem II, beta barrel domain at 100K, pH 6

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 beta-barrel heterologously and determined crystal structures at resolutions of 1.15-1.5 A 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 beta-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.
PDB ID: 5G38Download
MMDB ID: 142105
PDB Deposition Date: 2016/4/24
Updated in MMDB: 2016/10
Experimental Method:
x-ray diffraction
Resolution: 1.15  Å
Source Organism:
Similar Structures:
Biological Unit for 5G38: monomeric; determined by author and by software (PISA)
Molecular Components in 5G38
Label Count Molecule
Protein (1 molecule)
Photosystem II Manganese-stabilizing Polypeptide
Molecule annotation
Chemicals (3 molecules)
* Click molecule labels to explore molecular sequence information.

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