Multifrequency High-Field EPR Study of the Interaction between the Tyrosyl Z Radical and the Manganese Cluster in Plant Photosystem II

The light-driven oxidation of water to dioxygen is catalyzed by the enzyme photosystem II. A four-manganese ion cluster and a tyrosine, Y_Z, are present in the catalytic site. In preparations inhibited by addition of acetate or removal of the calcium cofactor, it is possible to trap the tyrosyl radical in interaction with the metal cluster. The coupled species is characterized by a broad split EPR signal at 9 GHz. In this work, high-field EPR has been used for further characterization of the coupling. The 285, 190 and 95 GHz EPR spectra of the interacting system are reported. Analysis of these spectra yielded exchange and dipolar couplings of the same magnitude as those found with 9 GHz EPR. However, the high-field spectra show that the coupling between the radical and the manganese cluster has opposite sign in acetate-treated compared to calcium-depleted samples. The sign difference indicates differences in the electronic structure of the radical−metal center pair. Comparisons are made between photosystem II and other enzymes containing radicals interacting with metal centers. Possible explanations for the difference in sign are proposed. The difficulty in obtaining reliable structural information for the spin coupled system is addressed.