Deprotonation of the 33-kDa, extrinsic, manganese-stabilizing subunit accompanies photooxidation of manganese in photosystem II

Photosystem II catalyzes photosynthetic water oxidation. The oxidation of w ater to molecular oxygen requires four sequential oxidations; the sequentia lly oxidized forms of the catalytic site are called the S states. An extrin sic subunit, the manganese-stabilizing protein (MSP), promotes the efficien t turnover of the S states. MSP can be removed and rebound to the reaction center; removal and reconstitution is associated with a decrease in and the n a restoration of enzymatic activity. We have isotopically edited MSP by u niform C-13 labeling of the Escherichia coli-expressed protein and have obt ained the Fourier transform infrared spectrum associated with the S-1 to S- 2 transition in the presence either of reconstituted C-12 or C-13 MSP, C-13 labeling of MSP is shown to cause 30-60 cm(-1) shifts in a subset of vibra tional lines. The derived, isotope-edited vibrational spectrum is consisten t with a deprotonation of glutamic/ aspartic acid residues on MSP during th e S, to S, transition; the base, which accepts this proton(s), is not locat ed on MSP. This finding suggests that this subunit plays a role as a stabil izer of a charged transition state and, perhaps, as a general acid/base cat alyst of oxy gen evolution. These results provide a molecular explanation f or known MSP effects on oxygen evolution.