Location of two Mn2+ affinity sites in photosystem II detected by pulsed electron–electron double resonance

In this study, we identified two Mn2+ sites in apo-Photosystem II (PSII) using the pulsed electron–electron double resonance (PELDOR). A Mn2+ ion was bound to apo-PSII on the deactivation of the oxygen-evolving complex. The electron–electron magnetic dipole interaction of the Mn2+ to YD· was estimated to be 2.4 MHz. The site was assigned at the position between His332 and Glu189 in the D1 polypeptide, which is close to the Mn1 site in mature PS II. Using recent structures observed under electron microscopes (EM), the location of the Mn2+ site on photoactivation was reevaluated. The position between Asp170 and Glu189 in the D1 polypeptide is a good candidate for the initial high-affinity site for photoactivation. Based on a comparison with the PELDOR results, the two EM structures were evaluated.