High-resolution X-ray structures of Photosystem II reveal several potential sub- strate binding sites at the water-oxidizing/oxygen-evolving 4MnCa cluster. Aspartate- 61 of the D1 protein hydrogen bonds with one such water (W1), which is bound to the dangler Mn4A of the oxygen-evolving complex. Comparison of pulse EPR spectra of 14NH3 and 15NH3 bound to wild-type Synechocystis PSII and a D1-D61A mutant lacking this H-bonding interaction demonstrates that ammonia binds as a terminal NH3 at this dangler Mn4A site, and not as a partially deprotonated bridge between two metal centers. The implications of this finding on identifying the binding sites of the substrate and the subsequent mechanism of dioxygen formation are discussed

J. Am. Chem. Soc., 2015, DOI: 10.1021/jacs.5b04768

http://pubs.acs.org/doi/pdf/10.1021/jacs.5b04768