pK(a) of a Proton-Conducting Water Chain in Photosystem II

Recent high-resolution crystal structures of the water-oxidizing enzyme photosystem II (PSII) show that O4 of the catalytic Mn4CaO5 cluster forms an H-bond with a water molecule W539, which belongs to a chain of water molecules (O4-water chain). Oxidation of Mn4CaO5 to S1 resulted in elongation of the O-H bonds and decrease in pKa(O-H/O(-)) in the [O4-H···OW539-H···OW538-H···OW393] region along the O4-water chain. In S1, removal of all water molecules from the O4-water chain, except W539, resulted in a significant pKa upshift at O4; this suggests that the proton-conducting water chain serves as a conducting media for protons and significantly decreases the donor pKa, leading to a downhill proton transfer. The absence of a corresponding proton-conducting channel is disadvantageous for release of protons from the proton-releasing site, as in the case of O5 that has no H-bond partner.