Probing the topography of the photosystem II oxygen evolving complex: PsbO is required for efficient calcium protection of the manganese cluster against dark-inhibition by an artificial reductant

The photosystem II (PSII) manganese-stabilizing protein (PsbO) is known to be the essential PSII extrinsic subunit for stabilization and retention of the Mn and Cl- cofactors in the oxygen evolving complex (OEC) of PSII, but its function relative to Ca2+ is less clear. To obtain a better insight into the relationship, if any, between PsbO and Ca2+ binding in the OEC, samples with altered PsbO-PSII binding properties were probed for their potential to promote the ability of Ca2+ to protect the Mn cluster against dark-inhibition by an exogenous artificial reductant, N,N-dimethylhydroxylamine. In the absence of the PsbP and PsbQ extrinsic subunits, Ca2+ and its surrogates (Sr2+, Cd2+) shield Mn atoms from inhibitory reduction (Kuntzleman et al., Phys Chem Chem Phys 6:4897, 2004). The results presented here show that PsbO exhibits a positive effect on Ca2+ binding in the OEC by facilitating the ability of the metal to prevent inhibition of activity by the reductant. The data presented here suggest that PsbO may have a role in the formation of the OEC-associated Ca2+ binding site by promoting the equilibrium between bound and free Ca2+ that favors the bound metal.