title: Mechanism of tyrosine D oxidation in Photosystem II.
abstract: Using quantum mechanics/molecular mechanics calculations and the 1.9-Å crystal structure of Photosystem II [Umena Y, Kawakami K, Shen J-R, Kamiya N (2011) Nature 473(7345):55-60], we investigated the H-bonding environment of the redox-active tyrosine D (TyrD) and obtained insights that help explain its slow redox kinetics and the stability of TyrD(•). The water molecule distal to TyrD, located ~4 Å away from the phenolic O of TyrD, corresponds to the presence of the tyrosyl radical state. The water molecule proximal to TyrD, in H-bonding distance to the phenolic O of TyrD, corresponds to the presence of the unoxidized tyrosine. The H(+) released on oxidation of TyrD is transferred to the proximal water, which shifts to the distal position, triggering a concerted proton transfer pathway involving D2-Arg180 and a series of waters, through which the proton reaches the aqueous phase at D2-His61. The water movement linked to the ejection of the proton from the hydrophobic environment near TyrD makes oxidation slow and quasiirreversible, explaining the great stability of the TyrD(•). A symmetry-related proton pathway associated with tyrosine Z is pointed out, and this is associated with one of the Cl(-) sites. This may represent a proton pathway functional in the water oxidation cycle.
authors.affiliation: Career-Path Promotion Unit for Young Life Scientists, Graduate School of Medicine, Kyoto University, Yoshida-Konoe-cho, Sakyo-ku, Kyoto 606-8501, Japan.
authors.forename: Keisuke
authors.initials: K
authors.lastname: Saito
authors.forename: A William
authors.initials: AW
authors.lastname: Rutherford
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Ishikita
mesh_terms: D019295:Computational Biology; D018360:Crystallography, X-Ray; D000458:Cyanobacteria / Q000737:chemistry; D004583:Electrons; D006860:Hydrogen Bonding; D008958:Models, Molecular; D008968:Molecular Conformation; D010084:Oxidation-Reduction; D010100:Oxygen / Q000737:chemistry*; D045332:Photosystem II Protein Complex / Q000737:chemistry*; D011522:Protons; D012984:Software; D014443:Tyrosine / Q000737:chemistry*; D014867:Water / Q000737:chemistry
chemical_list: D045332:Photosystem II Protein Complex; D011522:Protons; D014867:Water; D014443:Tyrosine; D010100:Oxygen
keywords: controlling electron transfer rate; hydrogen bond direction switching; oxygen-evolving complex; proton-coupled electron transfer; reaction center evolution