Proton Equilibria in the Manganese Cluster of Photosystem II Control the Intensities of the S0 and S2 State g ~ 2 Electron Paramagnetic Resonance Signals

• We have studied the pH effect on the S0 and S2 multiline electron paramagnetic resonance (EPR) signals from the water-oxidizing complex of photosystem II. Around pH 6, the maximum signal intensities were detected. On both the acidic and alkaline sides of pH 6, the intensities of the EPR signals decreased. Two pKs were determined for the S0 multiline signal; pK1 = 4.2 ± 0.2 and pK2 = 8.0 ± 0.1, and for the S2 multiline signal the pKs were pK1 = 4.5 ± 0.1 and pK2 = 7.6 ± 0.1. The intensity of the S0-state EPR signal was partly restored when the pH was changed from acidic or alkaline pH back to pH ~ 6. In the S2 state we observed partial recovery of the multiline signal when going from alkaline pH back to pH 6, whereas no significant recovery of the S2 multiline signal was observed when the pH was changed from acidic pH back to pH 6. Several possible explanations for the intensity changes as a function of pH are discussed. Some are ruled out, such as disintegration of the Mn cluster or decay of the S states and formal Cl- and Ca2+ depletion. The altered EPR signal intensities probably reflect the protonation/deprotonation of ligands to the Mn cluster or the oxo bridges between the Mn ions. Also, the possibility of decreased multiline signal intensities at alkaline pH as an effect of changed redox potential of YZ is put forward.

Subject headings

NATURVETENSKAP  -- Biologi -- Biokemi och molekylärbiologi (hsv//swe)

NATURAL SCIENCES  -- Biological Sciences -- Biochemistry and Molecular Biology (hsv//eng)

Publication and Content Type

art (subject category)

ref (subject category)