Oxygen inhibition and other properties of soybean ribulose 1,5-diphosphate carboxylase.

n-Ribulose-l , 5-di-P carboxylase, purified from soybeans, had K, values of 0.13 XnM for COS and 0.19 mM for ribulose1,5-di-P under a Nz atmosphere. O2 inhibited 14C02 incorporation by the enzyme and this inhibition was rapidly reversed by Ns. Inhibition was competitive with respect to CO2 and uncompetitive with respect to ribulose-1,5-di-P. The Ki for O2 was 0.8 mM. This O2 inhibition, together with the ribulose-1,5-di-P carboxylase-catalyzed oxidation of ribulose-l ,5-di-P to P-glycolate observed previously (Bowes, G., Ogren, W. L., and Hageman, R. H. (1971) Biochem. Biophys. Res. Commun. 45, 716-722), explains the “Warburg effect”: the rapidly reversible O2 inhibition of photosynthesis and stimulation of glycolate production seen in plants and isolated chloroplasts. In corn and soybean extracts, ribulose-1,5di-P carboxylase was inhibited by O2 but P-enolpyruvate carboxylase was unaffected. These data may explain the different response to O2 by plants which utilize ribulose-1,5-di-P carboxylase for the initial photosynthetic carboxylation and those utilizing P-enolpyruvate carboxylase. The optimum temperature for purified ribulose-1,5-di-P carboxylase was 55” and activation energies, in kilocalories per mole, were 18.4 in Nt and 20.4 in OZ. Phosphorylated compounds inhibited 14C02 incorporation by the enzyme. Nonphosphorylated sugars, including ribulose, did not inhibit. Fructose-1,6-di-P was a competitive inhibitor with respect to ribulose-l,S-di-P, the Ki being 0.88 mM. Fructose-l, 6-di-P was a more effective inhibitor than fructose-6-P, fructose-l-P, and ribulose-5-P suggesting that both phosphate groups of ribulose-1,5-di-P are involved in binding to the enzyme. HgCl, was a noncompetitive inhibitor with respect to COZ and a mixed inhibitor with respect to ribulose-1,5-di-P, suggesting that sulfhydryl groups are not involved in COZ binding but may be close to the site where ribulose-1,5-di-P binds to the enzyme.