Energetics of a possible proton exit pathway for water oxidation in photosystem II.
The crystal structure of photosystem II (PSII) at 3.0-A resolution suggests that titratable residues on the lumenal side of D1/D2 and PsbO form a polar channel, which might serve as a proton exit pathway associated with water oxidation on the Mn-cluster. With full account of protein environment, we calculated the pK(a) of these residues by solving the linearized Poisson-Boltzmann equation. Along the prospective proton 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 the Mn-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(+), suggesting that this residue is coupled to the redox states of the Mn-cluster and the tyrosine Y(Z). The calculated pK(a) values of 4.2-4.4 for D2-Glu312 and those of approximately 8-10.9 for D1-Asp59 and D1-Arg64 are indicative of the experimentally determined pK(a) values for inhibition of S-state transitions. Upon removal of the atomic coordinates of PsbO, the pK(a) of these residues are dramatically affected, indicating a significant role of PsbO in tuning the pK(a) of those residues in the proton exit pathway.
Institute of Chemistry and Biochemistry, Department of Biology, Chemistry, and Pharmacy, Free University of Berlin, Takustrasse 6, D-14195 Berlin, Germany.
Hiroshi
H
Ishikita
Wolfram
W
Saenger
Bernhard
B
Loll
Jacek
J
Biesiadka
Ernst-Walter
EW
Knapp
D001665:Binding Sites; D002104:Cadmium / Q000737:chemistry; D010084:Oxidation-Reduction; D045332:Photosystem II Protein Complex / Q000737:chemistry*; D011522:Protons; D014867:Water / Q000737:chemistry*
D045332:Photosystem II Protein Complex; D011522:Protons; D002104:Cadmium; D014867:Water