Crystal structure of cyanobacterial photosystem II at 3.2 angstrom resolution: a closer look at the Mn-cluster

In the crystal structure of photosystent II (PSII) from the cyanobacterium Thermosynechococcus elongatus at 3.2 Angstrom resolution, several loop regions of the principal protein subunits are now defined that were not interpretable previously at 3.8 Angstrom resolution. The head groups and side chains of the organic cofactors of the electron transfer chain and of antenna chlorophyll a (Chl a) have been modeled, coordinating and hydrogen bonding amino acids identified and the nature of the binding pockets derived. The orientations of these cofactors resemble those of the reaction center from anoxygenic purple bacteria, but differences in hydrogen bonding and protein environment modulate their properties and provide the unique high redox potential (1.17 V) of the primary donor. Coordinating amino acids of manganese cluster, redox-active Tyr(Z) and non-haem Fe2+ have been determined, and an all-trans beta-carotene connects cytochrome b-559, Chl(Z) and primary electron donor (coordinates are available under PDB-code 1W5C).