The sunlight-powered oxidation of water by photosystem II (PSII) of algae, plants, and cyanobacteria underpins the energy conversion processes that sustain most life on our planet. Understanding the structure and function of the "engine of life", the oxygen-evolving complex (OEC) in the active site of PSII, has been one of the great and persistent challenges of modern science. Immense progress has been achieved in recent years through combined contributions of diverse disciplines and research approaches, yet the challenge remains. The improved understanding of the tetramanganese-calcium cluster of the OEC for the experimentally accessible catalytic states often creates a more complex picture of the system than previously imagined, while the various strands of evidence cannot always be unified into a coherent model. This review focuses on selected current problems that relate to structural electronic features of the OEC, emphasizing conceptual aspects and highlighting topics of structure and function that remain uncertain or controversial. The Mn4CaOx cluster of the OEC cycles through five redox states (S-0-S-4) to store the oxidizing equivalents required for the final step of dioxygen evolution in the spontaneously decaying S-4 state. Remarkably, even the dark-stable state of the OEC, the S-1 state, is still incompletely understood because the available structural models do not fully explain the complexity revealed by spectroscopic investigations. In addition to the nature of the dioxygen-evolving S-4 state and the precise mechanism of O-O bond formation, major current open questions include the type and role of structural heterogeneity in various intermediate states of the OEC, the sequence of events in the highly complex S-2-S-3 transition, the heterogeneous nature of the S-3 state, the accessibility of substrate or substrate analogues, the identification of substrate oxygen atoms, and the role of the protein matrix in mediating proton removal and substrate delivery. These open questions and their implications for understanding the principles of catalytic control in the OEC must be convincingly addressed before biological water oxidation can be understood in its full complexity on both the atomic and systemic levels.
