Calcium reconstitutes high rates of oxygen evolution in polypeptide depleted Photosystem II preparations

Exposure of highly resolved Photosystem II preparations to 2 M NaCl produces an 80% inhibition of oxygen-evolution activity concomitant with extensive loss of two water-soluble polypeptides (23 and 17 kDa). Addition of Ca 2+ to salt-washed PS II membranes causes an acceleration in the decay of Z ⨥, the primary donor to P-680 +, and we show here that this acceleration is due to reconstitution of oxygen-evolution activity by Ca 2+. Other cations (Mg 2+, Mn 2+, Sr 2+) are much less effective in restoring oxygen evolution. On the basis of these observations we propose that Ca 2+, perhaps in concert with the 23 kDa polypeptide, is an essential cofactor for electron transfer from the ‘S’-states to Z on the oxidizing side of PS II.