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Abstract

Photosystem II is the enzyme that catalyzes the thermodynamically demanding splitting of water, yielding dioxygen, protons, and electrons, a sunlight‐driven reaction that forms the foundation of oxygenic photosynthesis. The latest results from high‐resolution crystallographic models have led to a refined view of the overall structure of the enzyme and the arrangement of the cofactors involved in light harvesting, charge separation, and electron transfer. In addition, combined efforts from protein crystallography, spectroscopy, and computational chemistry have greatly improved the understanding of the tetramanganese–calcium cluster of the oxygen‐evolving complex, the site of water oxidation. The most important features of the geometric and electronic structures of the oxygen‐evolving complex for the earlier reaction cycle intermediates are now sufficiently well understood such that connections between several structural features and spectroscopic observables can be made with confidence. Advanced spectroscopic techniques have also identified possible sites where the substrate water binds. Although the details of the actual mechanism of biological water oxidation remain elusive, experimental and theoretical studies impose an increasing number of constraints that significantly limit the number of mechanistic possibilities for O–O bond formation.