Forms of Dissolved Carbon Dioxide Required for Photosystem II Activity in Chloroplast Membranes

Abstract

High concentrations of both bicarbonate and formate inhibit photosynthetic O(2) evolution at pH 8.0. At this pH, only 2.4% of the total dissolved carbon dioxide exists as CO(2). At pH 7.3, where 11% of the total dissolved carbon dioxide exists as CO(2), HCO(3) (-) no longer inhibits. While formate still inhibits O(2) evolution at pH 7.3, its effect can be partially overcome if CO(2) is also present. The rate of binding of added (14)C-labeled inorganic carbon is nearly 10-fold more rapid when the internal pH of thylakoid membranes is at 6.0 than when it is at 7.8. These observations suggest that CO(2), not HCO(3) (-), is initially bound to the photosystem II reaction center and that the location of the binding site is on the inside thylakoid surface. However, additional data presented here suggest that, after binding, CO(2) is hydrated to HCO(3) (-) + H(+) in a pH-dependent reaction. Two possible explanations of the "bicarbonate effect" are presented.