Oxygen-evolving complex of Photosystem II: An analysis of second-shell residues and hydrogen-bonding networks

The oxygen-evolving complex (OEC) is a Mn₄O₅Ca cluster embedded in the Photosystem II (PSII) protein complex. As the site of water oxidation, the OEC is connected to the lumen by channels that conduct water, oxygen, and/or protons during the catalytic cycle. The hydrogen-bond networks found in these channels also serve to stabilize the oxidized intermediates, known as the S states. We review recent developments in characterizing these networks via protein mutations, molecular inhibitors, and computational modeling. On the basis of these results, we highlight regions of the PSII protein in which changes have indirect effects on the S₁, S₂, and S₃ oxidation states of the OEC while still allowing photosynthetic activity.