PARENT SESSION
Posters P7A Mechanisms of water oxidation. Abstracts (347-381)

Mechanism of the tyrosine Y_Z oxidation in Photosystem II studied by density functional theory. Masami Kusunoki^*,1, Atsuhiro Adachi¹, ¹ 1-1-1, Kawasaki, Kanagawa, Japan

ABSTRACT- Photosysten II (PSII) uses four light quanta to oxidize water to oxygen at a catalytic site that consists of Mn₄CaCl cluster, tyrosine Y_Z, and reaction center P680. Each charge separation generates a high-potential cation radical, P680^+⋅, which can oxidize Y_Z and water. The oxidized form of Y_Z is evidently a neutral radical. Where does this Y_Z proton transfer ? Recently, a crystal structure of PSII from thermophilic cyanobacteria at 3.5 resolution (Ferreira et al. 2004) was reported. According to this model, Y_Z can make a hydrogen bond directly with either His-190-N, His-190-NH or Gln-165-O, but not with any water molecule bound to Mn. Electron transfer from Y_Z to P680^+⋅ can be mediated by Phe-182. This model is adopted to investigate the Y_Z oxidation processes from the energetic point of view obtained by DFT computations. It follows: (1) when His-190 is neutral (pH > pK_H), a H-bonded species of Y_Z-His-190-N is most stable, but when it is protonated (pH < pK_H) Y_Z forms H-bonds both with His-190 NH and with Gln-165-O, (2) when Y_Z is oxidized by P680^+⋅, the Y_Z proton transfers to His-190-N at pH > pK_H with no activation barrier by substantially reducing the H-bond length from 2.85 to 2.67 , while it does to Gln-165-O owing to the positive charge of protonated His-190 at pH < pK_H with an activation barrier to form a short H-bond of 2.52 , and (3) the oxidation of the H-bonded Y_Z-H190 species by a monomeric chlorophyll-a^+⋅ ligated by His-198 at pH > pK_H requires more than ca.10 kcal/mol bias energy which could be generated by polar amino acids, protons and Mn₄CaCl cluster. This model seems to well explain essential features of Y_Z oxidation in inactive and oxygen-evolving PSII under variations of the pH, the temperature, and the H/D isotopic ratio.

KEY WORDS: Yz oxidation, P680 reduction, photosystem 2, density functional theory