Calcium modulates the photoassembly of photosystem II (Mn)4-clusters by preventing ligation of nonfunctional high-valency states of manganese.

The requirement for Ca2+ in the Mn(2+)-dependent photoactivation of oxygen evolution was re-evaluated using 17 kDa/24 kDa-less photosystem II (PSII) membranes depleted of (Mn)4-clusters by NH2OH extraction. At optimum conditions (1 mM Mn2+/10 microM 2,6-dichlorphenolindophenol (DCIP)/20 mM Ca2+), the light-induced increase of oxygen-evolution activity, the increase of membrane-bound Mn, and the B-band thermoluminescence emission intensity occurred in parallel. The extent of recovery of the oxygen-evolution activity was equivalent to 88% and 66% of the activity shown by parent NaCl-extracted PSII membranes and by PSII membranes, respectively. Neither photodamage of primary electron transport nor photoligation of nonfunctional Mn > or = 3+ occurred. Analyses of the Ca2+ concentration dependence for the maximum recovery of oxygen evolution activity gave evidence for Ca(2+)-binding site(s) having Km values of approximately 38 and approximately 1300 microM. Illumination of membranes in the strict absence of Ca2+ resulted in large increases (up to 18 Mn/200 chlorophyll) of EDTA nonextractable, EPR silent, nonfunctional membrane-bound Mn > or = 3+ and small increases of oxygen-evolution capability, dependent on pH and concentrations of Mn2+ and DCIP. No photodamage of primary electron transport and only approximately 17% decrease of AT-band thermoluminescence occurred during the photoligation of the Mn > or = 3%. In the strict absence of Ca2+, significant recovery of oxygen-evolution activity was obtained under a limited set of conditions permitting photoligation of a limited abundance of the nonfunctional Mn > or = 3+. Small (NH2-OH, H2O2) as well as bulky external reductants readily reduced and dissociated the Mn > or = 3+ from the membranes. Reillumination of these membranes under optimal conditions for photoactivation (plus Ca2+) gave a high yield of (Mn)4-clusters and oxygen-evolution capability. Similarly, simple addition of Ca2+ to membranes containing nonfunctional Mn > or = 3+ followed by reillumination resulted in the conversion of Mn > or = 3+ to (Mn)4-clusters. It is argued that Ca2+ promotes the conformational change involved in the conversion of the Mn2+ mononuclear intermediate to the Mn(3+)-Mn2+ binuclear intermediate in the photoactivation mechanism, thereby permitting photoassembly of (Mn)4-clusters and preventing photo-inactivation by Mn > or = 3+ ions.