Calcium, conformational selection, and redox-active tyrosine YZ in the photosynthetic oxygen-evolving cluster

Significance Photosynthetic water oxidation provides oxygen and maintains aerobic life on earth. The Mn 4 CaO 5 cluster is the catalytic site for this reaction in Photosystem II. During water oxidation, a redox-active tyrosine, YZ, is oxidized and reduced in a proton-coupled electron transfer reaction (PCET). We provide spectroscopic evidence that the essential calcium ion interacts with YZ in its radical and singlet states. In addition, multiple conformational states of the YZ radical/singlet are identified. Calcium is shown to select for a PCET reaction in a subset of these states. Conformational targeting could be a key to control of the PCET pathway and may occur in other enzymes, such as ribonucleotide reductase, which use tyrosine as a PCET cofactor.