The acid-catalyzed hydrolysis of urea has been investigated, using 0.5 N hydrochloric acid in aqueous solution. The activation energy was 24.6 kcal. and the entropy of activation -14.0 e. u.2. The urease-catalyzed hydrolysis of urea was investigated in phosphate-buffered solutions over a range of urea concentrations and temperatures. At low urea concentrations the activation energy was 12.5 kcal. and the entropy of activation 7.5 e. u. The activation energy decreases markedly with increasing urea concentration.3. The results are interpreted quantitatively on the basis of an enzyme model involving the formation of a urea-urease-water complex, the urease being assumed to displace the water reversibly at high concentrations. The heat and the entropy of reaction for complex formation are 3.25 kcal. and 14.9 e. u. at pH 6.6, while the heat and the entropy of activation for complex decomposition into the products of reaction are 9.25 kcal. and -7.0 e. u. 4. It is suggested, in order to account for the positive value of the entropy of formation of the complex, and the increase in entropy in forming the activated state from the initial reactants, that a reversible structural change in the enzyme occurs, the molecule opening out, as in a deactivation, during complex formation.5. The results are shown to be consistent with a decrease in activation energy with increasing temperature, the change being a gradual one and not a sharp one as had been suggested on the basis of certain previous experiments.
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