A model for the kinetics of activation and catalysis of ribulose 1,5-bisphosphate carboxylase

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Abstract
Further evidence for time-dependent interconversions between active and inactive states of ribulose 1,5-bisphosphate carboxylase [photosynthetic enzyme] is presented. It was found that ribulose bisphosphate oxygenase and ribulose bisphosphate carboxylase could be totally inactivated by excluding CO2 and Mg2+ during dialysis of the enzyme at 4.degree. C. When initially inactive enzyme was assayed, the rate of reaction continually increased with time, and the rate was inversely related to the ribulose bisphosphate concentration. The initial rate of fully activated enzyme showed normal Michaelis-Menten kinetics with respect to ribulose bisphosphate (Km = 10 .mu.M). Activation was shown to depend on both CO2 and Mg2+ concentrations, with equilibrium constants for activation of about 100 .mu.M and 1 mM, respectively. In contrast with activation, catalysis appeared to be independent of Mg2+ concentration, but dependent on CO2 concentration, with a Km (CO2) of about 10 .mu.M. By studying activation and de-activation of ribulose bisphosphate carboxylases as a function of CO2 and Mg2+ concentrations, the values of the kinetic constants for these actions have been determined. A model is proposed for activation and catalysis of ribulose bisphosphate carboxylase: .**GRAPHIC**. where E represents free inactive enzyme; complex in parentheses, activated enzyme; R, ribulose bisphosphate; M, Mg2+; C, CO2; P, the product. Ribulose bisphosphate probably can bind to both the active and inactive forms of the enzyme, and slow interconversion between the 2 states occurs.