Merck

Mechanism of tyrosine D oxidation in Photosystem II.

Proceedings of the National Academy of Sciences of the United States of America (2013-04-20)
Keisuke Saito, A William Rutherford, Hiroshi Ishikita
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.

MATERIALS
Product Number
Brand
Product Description

Supelco
L-Tyrosine, Pharmaceutical Secondary Standard; Certified Reference Material
Sigma-Aldrich
L-Tyrosine, reagent grade, ≥98% (HPLC)
Sigma-Aldrich
L-Tyrosine, from non-animal source, meets EP, USP testing specifications, suitable for cell culture, 99.0-101.0%
Sigma-Aldrich
L-Tyrosine, BioUltra, ≥99.0% (NT)
SAFC
L-Tyrosine
Sigma-Aldrich
L-Tyrosine, FG
Supelco
L-Tyrosine, certified reference material, TraceCERT®