The pH dependence of the donor side reactions in Ca2+-depleted Photosystem II: A role for Ca in the proton expulsion pathway?

Abstract

Removal of Ca2+ from PSII lead to impaired electron transfer from YZ to P680+ and to inhibition of S state transitions which results in a modified stable S2 state (probed by a stable multiline EPR signal) and the induction of the S2YZox state ("split" signal) by illumination at 0° C. We have performed an EPR study on the pH dependence of these reactions in Ca2+-depleted PSII (prepared by citrate treatment at pH 3.0). The stable multiline, the "split" signal and the induction and decay kinetics of YZox over a pH range from 3.5 to 8.0 were investigated. The stable S2 multiline signal showed no significant pH dependence. In contrast, the induction of the "split" signal (two forms of 110 G and 160 G wide were studied) was decreased by 50-60% with an apparent pK of 4.5 when the pH was decreased. The decay of the induced YZox at optimal pH values (pH 6-7) shows two kinetic phases with half times of about 0.5 and 20 s. This decay phases became of about 0.1 and 10 s at pH 4.0. Our results indicate that YZox is easier to reduce at low pH values in Ca2+- depleted PSII. Possible donors and the relevance to the proton translocation reactions during S cycle are discussed. *This work was entirely done in collaboration with G.T. Babcock who passed away in December 2000.