A Numerical Approach for Kinetic Analysis of the Nonexponential Thermoinactivation Process of Uricase

Prior to the exponential decrease of activity of a uricase from Candida sp. during storage at 37 °C, there was a plateau period of about 4 days at pH 7.4, 12 days at pH 9.2, and about 22 days in the presence of 30 μM oxonate at pH 7.4 or 9.2, but no degradation of polypeptides and no activity of resolved homodimers. To reveal determinants of the plateau period, a dissociation model involving a serial of conformation intermediates of homotetramer were proposed for kinetic analysis of the thermoinactivation process. In the dissociation model, the roles of interior noncovalent interactions essential for homotetramer integrity were reflected by an equivalent number of the artificial weakest noncovalent interaction; to avoid covariance among parameters, the rate constant for disrupting the artificial weakest noncovalent interaction was fixed at the minimum for physical significance of other parameters; among thermoinactivation curves simulated by numerical integration with different sets of parameters, the one for least-squares fitting to an experimental one gave the solution. Results found that the equivalent number of the artificial weakest noncovalent interaction primarily determined the plateau period; kinetics rather than thermodynamics for homotetramer dissociation determined the thermoinactivation process. These findings facilitated designing thermostable uricase mutants.