Mass spectrometric study of Photosystem II heterogeneity in oxygen and nitrogen production: Effects of magnesium and of phosphorylation of pea thylakoids

Photosystem II (PS II) is capable of the oxidation of both water and hydroxylamine with the production of O₂- and N₂-production, respectively. The resulting changes in the partial pressure of the respective gases can be measured by an appropriate mass spectrometric set-up. Analysis of single turn-over flash saturation curves of O₂- and N₂-production has been performed to determine the relative optical cross sections of the competent PS II units and absolute amounts of their fractions in pea thylakoids. We studied the changes of these parameters upon Mg²⁺-induced transition of thylakoid membrane from unstacked to stacked configuration and upon protein phosphorylation of the stacked samples. The results showed a 2.5-fold increase of effective antenna size of PS II units competent in either O₂- or N₂-production after addition of 10 mM MgCl₂ to cation-depleted thylakoids, which indicates a potential capability of both - and -units to carry out these alternative reactions. However, we observed a significant difference in the amounts of PS II units competent in O₂- or N₂-production, with a ratio of 1:4 in unstacked thylakoids, and reciprocal alterations in stacked ones. This represents an increase by about 20% and a 2-fold decrease of O₂- and N₂-evolving units, respectively, yielding a ratio of 1:1.5, which implies a heterogeneity of PS II with respect to these reactions, the capabilities of - and -units being distinct. The phosphorylation of stacked thylakoids did not essentially influence the antenna size of O₂- and N₂-evolving PS II units but caused opposite and reciprocal changes in their amounts, approximately 30% decrease and increase, respectively, to a ratio of 1:3. The relationship of the structure-function heterogeneity in PS II with implications for current models of photosynthetic regulation mechanisms is discussed.