A Proposed Mechanism for the Stimulatory Effect of Bicarbonate Ions on ATP Synthesis in Isolated Chloroplasts

The effect of bicarbonate ions on induction of Mg(2+)-ATPase activity, on the N-ethylmaleimide inhibition of phosphorylation and on energy-dependent adenine nucleotide exchange has been examined with pea seedling chloroplasts. Incubation of chloroplasts with N-ethylmaleimide in the presence of 15 millimolar bicarbonate in the light results in enhanced inhibition of ATP synthesis when the preillumination pH is maintained between 7.0 and 7.5. Bicarbonate also enhances Mg(2+)-ATPase activity when it is included in the light-triggering stage at pH 7.0. The conditions (medium pH, bicarbonate concentration, etc.) for demonstrating the bicarbonate-induced enhancement of the N-ethylmaleimide inhibition and ATPase activity are similar to those required for the direct effect of bicarbonate on phosphorylation. Bicarbonate, under the same conditions, does not affect adenine nucleotide exchange (binding or release). It is concluded that the stimulatory effect of bicarbonate on ATP synthesis may be related to its ability to alter directly the conformation of the chloroplast coupling factor under conditions (suboptimal pH) where the enzyme shows minimal activity.