Characteristics of the Dark-Stable Multiline EPR Signal of Ca2+-Depleted Photosystem II

Abstract

Photosystem II (PSII), which produces molecular oxygen using energy from light absorption, requires Ca2+ and Cl- ions as inorganic cofactors. PSII shows two electron paramagnetic resonance signals that have been associated with the depletion or disruption of Ca2+ at the catalytic Mn4Ca cluster: a dark-stable multiline signal from an S2 state that decays very slowly, and a broad metalloradical signal from an S2YZ state that is unable to proceed to higher oxidation states. The conditions for their formation were explored to help clarify how they are correlated. The dark-stable multiline signal was found to form in PSII prepared at pH 5.5 using itaconic acid buffer, a relative of citrate. The signal was very similar to the previously reported signal that is observed after EDTA treatment of PSII lacking the PsbP and PsbQ subunits. Both of these treatments, which employ Ca2+ chelators, also resulted in formation of the S2YZ signal when PSII was illuminated in the presence of an electron acceptor. Treatment of intact PSII with fluoride, which is a competitor of Cl- activation, resulted in formation of the S2YZ signal, but not the dark-stable multiline signal. Fluoride may also interfere with Ca2+ function as a result of the high stability of the CaF2 complex. These findings are examined in relation to the requirements of PSII for Ca2+ and Cl-. (Supported by UNCG Office of Research).