Journal
FRONTIERS IN PLANT SCIENCE
Volume 14, Issue -, Pages -Publisher
FRONTIERS MEDIA SA
DOI: 10.3389/fpls.2023.1029674
Keywords
crystal structure; x-ray radiation damage; low barrier hydrogen bond; photosystem II; oxygen evolving complex; quantum mechanics/molecular mechanics (QM/MM)
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The Mn4CaO5 cluster in photosystem II forms hydrogen bonds with D1-His337 and a water molecule. The hydrogen bond distances differ between the two monomer units and can be attributed to protonation and overreduction states.
In photosystem II (PSII), the O3 and O4 sites of the Mn4CaO5 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-W539 hydrogen bond (similar to 2.5 angstrom) of the B monomer is reproduced when O4 is protonated in the S-1 state. The short O3-N epsilon(His337) 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-1 or S-2). It seems plausible that the oxidation state differs between the two monomer units in the crystal.
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