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

•An extensive and interconnected hydrogen-bonding network surrounds the OEC in PSII.•This network stabilizes intermediates of the OEC during water oxidation.•Three distinct channels have been identified that transport water, protons, and O2.•Changing second-shell residues of the OEC affects the efficiency of water oxidation. 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.