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 Mn2+-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 mu M 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-greater than or equal to 3+ occurred. Analyses of the Ca2+ concentration dependence for the maximum recovery of oxygen evolution activity gave evidence for Ca2+-binding site(s) having K-m values of similar to 38 and similar to 1300 mu M. 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-greater than or equal to 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 similar to 17% decrease of A(T)-band thermoluminescence occurred during the photoligation of the Mn-greater than or equal to 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-greater than or equal to 3+. Small (NH2OH, H2O2) as well as bulky external reductants readily reduced and dissociated the Mn-greater than or equal to 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-greater than or equal to 3+ followed by reillumination resulted in the conversion of Mn-greater than or equal to 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 Mn3+-Mn2+ binuclear intermediate in the photoactivation mechanism, thereby permitting photoassembly of (Mn)(4)-clusters and preventing photoinactivation by Mn-greater than or equal to 3+ ions.