Ca²⁺ binding to the oxygen evolving enzyme varies with the redox state of the Mn cluster

Oxygen evolution by the mangano‐enzyme of photosystem II is inhibited by Ca 2+ depletion induced by NaCl washing and restored by Ca 2+ addition. The effectiveness of NaCl treatment in inhibiting oxygen evolution in photosystem II was studied after a series of preilluminating flashes. The susceptibility of the enzyme to NaCl treatment varied with the number of preilluminating flashes and this variation showed an oscillation pattern with a period of four. This pattern is characteristic of cycling through the four long‐lived intermediate states in the enzyme cycle (i.e. the states, S 0 , S 1 , S 2 , S 3 ). The relative extent of inhibition corresponding to each of the S states was as follows: S 3 > S 0 ≈S 2 > S 1 . From these results it is concluded that Ca 2+ binding is dependent on the S states and that Ca 2+ probably plays a fundamental role in the mechanism of water splitting. The results also help to explain the conflicting reports of the extent of inhibition induced by NaCl washing and the controversy over which electron transfer step is inhibited by Ca 2+ depletion.