The pH dependence of the donor side reactions in Ca~(2+)-depleted Photosystem II: a role for Ca in the proton expulsion pathway?

摘要

During oxidation of water the photosystem II complex cycles through 5 intermediate S-states (S_0-S_4). The catalytic water oxidation is performed with the involvement of four Mn-ions, a redox active tyrosine, Y_z, situated 7-8 A away, and one essential Ca~(2+) ion (Diner & Babcock, 1996; Nugent, 2001). Depletion of Ca~(2+) inhibits oxygen evolution, alters the proton release in the S-cycle and perturbs the S_2 state leading to the generation of a stable modified S_2 multiline EPR signal from Mn. Inaddition, Ca~(2+) depletion stalls the S-state cycle in the S_2Y_z~(centre dot) state during illumination and generates a characteristic "plit" EPR signal (Ono & Inoue 1988; Boussac et. al., 1989, 1990; Sivaraja et al. 1989; MacLachlan & Nugent, 1993; Gilchrist et al., 1995; Lakshmi et. al., 1999). The Y_z~(centre dot) in this state is very long-lived (Andreasson et. al., 1995). The question then is what the function of this Ca ion is in photosynthetic water oxidation.