Inhibition of photosynthetic oxygen evolution by calmodulin-type inhibitors and other calcium-antagonists.

Several studies have recently implicated a role for Ca2+ in photosynthetic oxygen evolution (9-11). Our previous study indicated that Ca2+ was likely acting at the level of the Cl- cofactor requirement in photosystem II (9). We now demonstrate, through the use of calmodulin-type inhibitors ( calmidazolium and trifluoperazine) and metal Ca2+-antagonists (e.g., Tb+3 and La+3), the function of Ca2+ on the oxidizing side of photosystem II. In addition, the peroxide (H2O2) electron donation site was differentiated from the electron donation site of NH2OH, Mn2+ and diphenyl carbazide in the photosystem II complex.