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Ammonia Binds to the Dangler Manganese of the Photosystem II Oxygen-Evolving Complex

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posted on 2015-07-15, 00:00 authored by Paul H. Oyala, Troy A. Stich, Richard J. Debus, R. David Britt
High-resolution X-ray structures of photosystem II reveal several potential substrate 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 hydrogen-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.

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