Reduced Occupancy of the Oxygen-Evolving Complex of Photosystem II Detected in Cryo-Electron Microscopy Maps

Computational simulations of electrostatic potentials (ESPs), based on atomistic models and independent atomic scattering factors, have remained challenging when applied to the oxygen-evolving complex (OEC) of photosystem II (PSII). Here, we overcome that challenge by using an ESP function obtained with density functional theory and atomic coordinates for the OEC of PSII obtained by optimization of the dark-adapted S₁ state. We find that the ESP is much higher for the OEC than for the nearby reference side chain of amino acid residue D1-H190. In contrast, experimental ESP maps recently published for two PSII-light-harvesting complex II super-complexes show that the ESP of the OEC is approximately half the value of the D1-H190 side chain. The apparent disparity is attributed to a reduced 31-38% occupancy of the OEC, likely associated with its reduction by electron scattering.