The proton enviroment about Yz as a function of metal content in photosystem II

Recent work indicates that YZ and the manganese cluster in Photosystem II form an integrated catalytic site for water oxidation (1–4). This realization, coupled with the idea that the function of amino-acid radicals in the emerging class of radical enzymes is to abstract hydrogen atoms from substrate, leads to a complete model for the S-state cycle for water oxidation in photosynthesis (5,6, see Figure 1). A key aspect of this mechanism is that, upon its oxidation, YZ sloughs a proton and that this H+ is released to bulk phase upon each of the S-state transitions. Although fast proton release is detected upon each S-state transition (7), the origin of this proton release remains controversial (5,6,8–10). Here, we summarize recent work that indicates that electro-neutrality is maintained at the YZ site upon each S-state transition and that provides insight into the accessibility of the YZ site to bulk solvent as a function of the integrity of the (Mn)4/Ca site in the oxygen-evolving complex.