Determination of the Binding Constant of H14 Co−3 to the Photosystem Ii Complex in Maize Chloroplasts: Effects of Inhibitors and Light

The binding (dissociation) constant for HCO−3 to the photosystem II complex in maize chloroplasts is approximately 80 μM. One HCO−3 binds per 500–600 chlorophyll molecules. In the dark, formate is a competitive inhibitor of HCO−3 binding, while 3‐(3′,4′‐dichlorophenyl)‐1, 1‐dimethylurea (DCMU) inhibits HCO−3 binding non‐competitively. Light decreases HCO−3 binding in the presence of formate. Light increases the binding of HCO−3 in the presence of DCMU. The high binding constant for HCO, discriminates strongly among the various hypotheses attempting to explain the “bicarbonate‐effect” on photosystem II. The proposal by Stemler and Jursinic (Arch. Biochem. Biophys. 221, 227–237 1983), that HCO−3 is one of a class of monovalent anionic inhibitors of photosystem II, is favored. These anions compete for a specific binding site on the photosystem II complex.