Photosystem II (PSII) uses light energy to split water into protons, electrons and oxygen. In this reaction, Nature has solved the difficult chemical problem of efficient four-electron oxidation of water to yield O2 without significant side reactions. In order to use Nature’s solution for the design of materials that split water for solar fuel production, it is important to understand the structure of the catalytic site and the mechanism of the reaction. Mechanistic studies have made use of site-directed mutagenesis to probe the function of amino-acid residues in and around the O2-evolving complex (OEC), a Mn4CaO5 cluster in PSII. A majority of the site-directed mutagenesis studies of have utilized the cyanobacterium Synechocystis sp. PCC 6803. However, without a high-resolution structure of PSII from Synechocystis 6803, interpretations of biophysical studies of mutated PSII complexes have relied on the structures of PSII from thermophilic cyanobacteria. Recently, we solved the cryo-electron microscopy structure of wild-type Synechocystis 6803 PSII to 1.93-Å resolution. This structure, which reveals significant differences around the OEC between thermophilic and mesophilic PSII, provides a platform for determination of structures of mutated Synechocystis 6803 PSII complexes. Our progress on the structures of three mutated Synechocystis 6803 PSII complexes (D1-D61A, D1-E189R and D1-D170E) will be described.

Structure-function studies of the O2-evolving complex in photosystem II from Synechocystis sp. PCC 6803

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