Manganese plays a variety of roles in enzymes [1], such as for example arginase, which catalyzes the hydrolysis of arginine, forming urea and ornithine as products [2–4]. Given the biological accessibility of at least three oxidation states — MnII, MnIII, and MnIV — it is not surprising that manganese is also involved in important redox reactions [5]. Beside the well-known manganese superoxide dismutase and manganese catalase enzymes where Mn plays a role that can also be served by Fe or other metals, manganese exclusively acts as an oxidizer of water in the pentanuclear Mn4Ca cluster of Photosystem II. This cluster carries out the fourelectron oxidation of two H2O molecules to O2, providing nearly all of the free molecular oxygen in our atmosphere.
Photosystem II is a large transmembrane protein/cofactor complex with approximate C2 symmetry that utilizes light energy to oxidize water to molecular oxygen and to reduce membrane diffusible plastoquinone. Two tyrosines called YD and YZ are sidechains of two polypeptides D1 and D2, positioned on both sides of the symmetry axis. Four successive photons are absorbed by the chlorophyll (Chl) pigment P680; pheophytin (Ph) and two plastoquinones, QA and QB, participate in the electron transfer, while the pentanuclear Mn4Ca moiety is used to build charges necessary for a subsequent electron transfer.