Stoichiometry, inhibitor sensitivity, and organization of manganese associated with photosynthetic oxygen evolution

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Abstract
Chloroplast thylakoid membranes isolated in the presence of EDTA retain high rates of O 2 evolution (≥340 μmol·h -1 ·mg chlorophyll -1 ) but contain no Mn 2+ that is detectable by electron paramagnetic resonance (EPR) at room temperature. The total Mn 2+ content of these preparations is 4.6 per 400 chlorophylls; 0.6 Mn 2+ can be released by addition of Ca 2+ , a treatment that does not affect O 2 evolution. The remaining Mn 2+ (4 per 400 chlorophylls) appears to be functionally associated with O 2 evolution activity. Inhibition by Tris, NH 2 OH, or heat will release a small fraction of Mn 2+ from these membranes (≈25% with Tris, for example). Addition of Ca 2+ further enhances Mn 2+ release so that for Tris and for NH 2 OH, 2 and 3, respectively, Mn 2+ per 400 chlorophylls are extracted from the O 2 -evolving complex. Based on the microwave power-saturation properties of the EPR signal IIf, which arises from an intermediate electron carrier in the water splitting process, it appears that one of the four Mn 2+ associated with photosystem II is uniquely sensitive to Tris. A new model is proposed for the organization and inhibitor sensitivity of manganese in the O 2 -evolving complex.

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