Structural Changes in the Acceptor Site of Photosystem II upon Ca(2+)/Sr(2+) Exchange in the Mn(4)CaO(5) Cluster Site and the Possible Long-Range Interactions

The Mn(4)CaO(5) cluster site in the oxygen-evolving complex (OEC) of photosystem II (PSII) undergoes structural perturbations, such as those induced by Ca(2+)/Sr(2+) exchanges or Ca/Mn removal. These changes have been known to induce long-range positive shifts (between +30 and +150 mV) in the redox potential of the primary quinone electron acceptor plastoquinone A (Q(A)), which is located 40 Å from the OEC. To further investigate these effects, we reanalyzed the crystal structure of Sr-PSII resolved at 2.1 Å and compared it with the native Ca-PSII resolved at 1.9 Å. Here, we focus on the acceptor site and report the possible long-range interactions between the donor, Mn(4)Ca(Sr)O(5) cluster, and acceptor sites.