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Water oxidation in Photosystem II is dependent on a particular amino acid residue, TyrosineZ. This is a redox intermediate in steady state oxygen evolution and transfers electrons from the water splitting CaMn4 cluster to the central chlorophyll radical P680+. This Perspective discusses the functional principles of TyrosineZ as a proton-coupled redox active link, as well as mechanistic studies of synthetic model systems and implications for artificial photosynthesis. Experimental studies of temperature dependence and kinetic isotope effects are important tools to understand these reactions. We emphasize the importance of proton transfer distance and hydrogen bond dynamics that are responsible for variation in the rate of PCET by several orders of magnitude. The mechanistic principles discussed and their functional significance are not limited to tyrosine and biological systems, but are important to take into …