AUTHOR=Saito Keisuke , Nakao Shu , Ishikita Hiroshi 

TITLE=Identification of the protonation and oxidation states of the oxygen-evolving complex in the low-dose X-ray crystal structure of photosystem II

JOURNAL=Frontiers in Plant Science

VOLUME=14

YEAR=2023

URL=https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2023.1029674

DOI=10.3389/fpls.2023.1029674

ISSN=1664-462X

ABSTRACT=<p>In photosystem II (PSII), the O3 and O4 sites of the Mn<sub>4</sub>CaO<sub>5</sub> cluster form hydrogen bonds with D1-His337 and a water molecule (W539), respectively. The low-dose X-ray structure shows that these hydrogen bond distances differ between the two homogeneous monomer units (A and B) [Tanaka et al., J. Am Chem. Soc. 2017, 139, 1718]. We investigated the origin of the differences using a quantum mechanical/molecular mechanical (QM/MM) approach. QM/MM calculations show that the short O4-O<sub>W539</sub> hydrogen bond (~2.5 Å) of the B monomer is reproduced when O4 is protonated in the S<sub>1</sub> state. The short O3-Nε<sub>His337</sub> hydrogen bond of the A monomer is due to the formation of a low-barrier hydrogen bond between O3 and doubly-protonated D1-His337 in the overreduced states (S<sub>−1</sub> or S<sub>−2</sub>). It seems plausible that the oxidation state differs between the two monomer units in the crystal.</p>