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The Activation of Muscle Adenylate Deaminase by Substrate

From the ¹ From the Department of Chemistry, Yale University, New Haven, Connecticut 06520

The kinetics of the deamination of 5'-adenosine monophosphate by rabbit muscle adenylate deaminase has been studied over a wide range of substrate and enzyme concentrations at pH 6.5. The kinetics in the presence of concentrations of the cationic activator K⁺ of 0.1 m or higher appears to be of normal Michaelis-Menten character, except that the apparent enthalpy of activation has the unusually high value of 19 kcal mole^-1. At low K⁺ concentrations, the apparent specific activity of the enzyme depends on the concentrations of substrate, K⁺, and enzyme. In the absence of K⁺, a lag phase in the deamination reaction, observable by the stopped flow technique, indicates activation by substrate with an apparent second order rate constant of 7000 m^-1 sec^-1 at 25°. It is shown that a quantitatively consistent interpretation of all the data presented in this paper can be based on the allosteric model of Monod, Wyman, and Changeux (Monod, J., Wyman, J., and Changeux, J.-P., J. Mol. Biol., 12, 88 (1965)), with the assumption that the enzyme is composed of six or more identical subunits. Equally satisfactory interpretations could undoubtedly be based on other models for apparent allosteric effects which have been proposed by other authors. A decrease in specific activity observed at very low enzyme concentrations, also in the absence of K⁺, suggests that the enzyme undergoes dissociation, with only the intact oligomer having catalytic activity.

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