Cryo-EM Structure of Monomeric Photosystem II from Synechocystis sp. PCC 6803 Lacking the Water-Oxidation Complex

Summary The photosystem II (PSII) enzyme provides nearly all the oxygen on Earth by catalyzing water oxidation via its oxygen-evolving complex (OEC), a heterocubane metal cluster in the PSII active site. The overall biogenesis of PSII, and especially the mechanism of photoactivation where the OEC is assembled, is currently unclear. To investigate this mechanism, we solved the cryo-EM structure of apo-PSII lacking extrinsic and peripheral subunits from a mesophilic cyanobacterium, Synechocystis sp. PCC 6803. The lumenal surface is negatively charged to attract cations, and the configuration of the OEC-binding site is found to be quite different from that observed in mature PSII. We use these observations to suggest important characteristics of apo-PSII prior to a structural rearrangement involved in photoactivation that has previously remained elusive, giving insight into its mechanism. This provides a platform for future studies that aim to understand OEC assembly using structural and computational techniques.