Location of the calcium binding site in Photosystem II: A Mn2+ substitution study

The whereabouts of the Ca2+ site in Photosystem II (PSII) was investigated by experiments in which Mn2+ was substituted for Ca2+. When stoichiometric amounts of Mn2+ ions were added to Ca2+-depleted PSII, the Mn2+ was not detected by EPR. The titration of Ca2+ back into Ca2+-depleted/Mn2+-containing PSII resulted in the simultaneous release of the Mn2+ and the loss of the two EPR signals which are characteristic of the Ca2+-depleted enzyme (i.e., the stable, modified S2 multiline signal arising from the intrinsic Mn cluster and the split S3 signal from an organic radical interacting with the Mn cluster). These results indicate that the Mn2+ occupies the functional Ca2+ site. The S2 and S3 EPR signal characteristic of this kind of Ca2+-depleted preparation were unaffected by the binding of the Mn2+ Since, from earlier results, it seems likely that the modification and stability of S2 multiline signal in these PSII preparations is due to binding of chelator to or close to the Mn cluster, the present results indicate that the Ca2+ site (at least when occupied by Mn2+) does not overlap with the chelator binding site. Since Mn2+ binding does not effect the S2 EPR signal from the Mn cluster, it can be concluded that the Mn2+ is not involved in detectable magnetic interactions with the cluster. This result indicates that the Mn2+-occupied Ca2+ binding site is outside the first co-ordination sphere of the Mn cluster. The relaxation properties of TyrD. were enhanced by the presence of the Mn2+ when the Mn cluster was in the S1 state.