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.