Binding stoichiometry and affinity of the manganese-stabilizing protein affects redox reactions on the oxidizing side of photosystem II.

It has been reported previously that the two subunits of PsbO, the photosystem II (PSII) manganese stabilizing protein, have unique functions in relation to the Mn, Ca(2+), and Cl(-) cofactors in eukaryotic PSII [Popelkova; (2008) Biochemistry 47, 12593]. The experiments reported here utilize a set of N-terminal truncation mutants of PsbO, which exhibit altered subunit binding to PSII, to further characterize its role in establishing efficient O(2) evolution activity. The effects of PsbO binding stoichiometry, affinity, and specificity on Q(A)(-) reoxidation kinetics after a single turnover flash, S-state transitions, and O(2) release time have been examined. The data presented here show that weak rebinding of a single PsbO subunit to PsbO-depleted PSII repairs many of the defects in PSII resulting from the removal of the protein, but many of these are not sustainable, as indicated by low steady-state activities of the reconstituted samples [Popelkova; (2003) Biochemistry 42 , 6193]. High affinity binding of PsbO to PSII is required to produce more stable and efficient cycling of the water oxidation reaction. Reconstitution of the second PsbO subunit is needed to further optimize redox reactions on the PSII oxidizing side. Native PsbO and recombinant wild-type PsbO from spinach facilitate PSII redox reactions in a very similar manner, and nonspecific binding of PsbO to PSII has no significance in these reactions.