Energetics of a Possible Proton Exit Pathway for Water Oxidation in Photosystem II

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• 作者：Hiroshi Ishikita ; Wolfram Saenger ; Bernhard Loll ; Jacek Biesiadka ; and Ernst-Walter Knapp
• 刊名：Biochemistry
• 出版年：2006
• 出版时间：February 21, 2006
• 年：2006
• 卷：45
• 期：7
• 页码：2063 - 2071
• 全文大小：342K
• 年卷期：v.45,no.7(February 21, 2006)
• ISSN：1520-4995

文摘

The crystal structure of photosystem II (PSII) at 3.0-Å resolution suggests that titratable residueson the lumenal side of D1/D2 and PsbO form a polar channel, which might serve as a proton exit pathwayassociated with water oxidation on the Mn-cluster. With full account of protein environment, we calculatedthe pK_a of these residues by solving the linearized Poisson-Boltzmann equation. Along the prospectiveproton channel, the calculated pK_a of titratable residues (namely via D1-Asp61, D1-Glu65, D2-Glu312,D2-Lys317 D1-Asp59, D1-Arg64, PsbO-Arg152, and PsbO-Asp224) monotonically increase from theMn-cluster to the lumenal bulk side. We suggest that these residues form the exit pathway guiding protons,which are released at the Mn-cluster as a product of water oxidation, in an exergonic process out of PSII.Upon the S2 to S3 transition, CP43-Arg357 showed a dramatic deprotonation of ca. one H⁺, suggestingthat this residue is coupled to the redox states of the Mn-cluster and the tyrosine Y_Z. The calculated pK_avalues of 4.2-4.4 for D2-Glu312 and those of ~8-10.9 for D1-Asp59 and D1-Arg64 are indicative ofthe experimentally determined pK_a values for inhibition of S-state transitions. Upon removal of the atomiccoordinates of PsbO, the pK_a of these residues are dramatically affected, indicating a significant role ofPsbO in tuning the pK_a of those residues in the proton exit pathway.