Calcium depletion modifies the structure of the photosystem II O2-evolving complex

Abstract

The role of Ca2+ in regulating accessibility of the PSII Mn cluster to the external solvent was probed under conditions where Ca2+ was extracted by exposure to citric acid (pH 3) for 5 min., a condition which allows retention of extrinsic 33, 23 and 17 kDa polypeptides. Inhibition of O2 evolution was utilized as a probe for accessibility of the Mn cluster to a range of reducing agents (hydroquinone, NH2OH and its N-methyl derivatives). Preparations of PSII were exposed to reductants for various periods of time; loss of activity was monitored polarographically. Polypeptide retaining, Ca2+ extracted centers are sensitive to inhibition by NH2OH and N-methyl NH2OH. In the case of NH2OH, inhibition sensitivity is equivalent to that of PSII stripped of the extrinsic polypeptides, even though the citrate preparation retains these species. On the other hand, dimethyl NH2OH and hydroquinone are not inhibitory under the same conditions. To determine whether PSII structure was altered globally (and irreversibly), or whether Ca2+ exerts control over a specific channel into the Mn cluster, Ca2+-reconstituted PSII was subjected to reductant probing. The results of these experiment show that Ca2+ rebinding to PSII restores O2 evolution activity and that NH2OH access to the Mn cluster is impaired. These data suggest that Ca2+ might bind to PSII in a H2O channel between the solvent and the Mn cluster. This would be consistent with the proposed role for Ca2+ as a site for ligation of substrate H2O within the O2-evolving complex.