Calcium retards NH2OH inhibition of O2 evolution activity by stabilization of Mn2+ binding to photosystem II.

Calcium is required for oxidation of water to molecular oxygen by photosystem II; the Ca2+ demand of the reaction increases upon removal of 23- and 17-kDa extrinsic polypeptides from detergent-derived preparations of the photosystem. Employing the manganese reductant NH2OH as a probe to examine the function of Ca2+ in photosystem II reveals that (1) Ca2+ slows the rate of NH2OH inhibition of O2 evolution activity, but only in photosystem II membranes depleted of extrinsic proteins, (2) other divalent cations (Sr2+, Cd2+) that compete for the Ca2+ site also slow NH2OH inhibition, (3) Ca2+ is noncompetitive with respect to NH2OH, (4) in order to slow inhibition, Ca2+ must be present prior to the initiation of NH2OH reduction of manganese, and (5) Ca2+ appears not to interfere with NH2OH reduction of manganese. We conclude that the ability of Ca2+ to slow the rate of NH2OH inhibition arises from the site in photosystem II where Ca2+ normally stimulates O2 evolution and that the mechanism of this phenomenon arises from the ability of Ca2+ or certain surrogate metals to stabilize the ligation environment of the manganese complex.