Interaction of bicarbonate with the manganese-stabilizing protein of photosystem II

The effect of reversible removal of HCO3- on structural re-arrangements in the Mn-stabilizing protein (MSP) of photosystem II, isolated from pea leaves, was studied using measurements of characteristic alterations in fluorescence of hydrophobic probe 8-anilino-1-naphthalene-sulfonic acid (ANS). It was shown that the treatments capable of removal of HCO3- (or CO2) from possible binding sites in MSP (pH lowering from 6.5 to 3.5, addition of a structurally similar anion HCO3- in concentration 1–20 mM or air evacuation at pH 3.5) result in a significant (up to 370%) increase of ANS fluorescence (indicative of structural changes in MSP), whereas HCO3- lowers the ANS fluorescence to the initial level observed in untreated protein at pH 6.5. Since the effects are revealed at (sub)micromolar concentrations of HCO3-, the specific high-affinity binding of HCO3- (or CO2) to MSP (required for its native structure preservation) is proposed. Possible bicarbonate binding sites and its physiological role within the water-oxidizing complex of photosystem II are discussed.