The mechanism of irreversible inactivation of lysozyme at pH 4 and 100 degrees C.

The mechanism of irreversible inactivation of lysozyme at pH 4, 100 degrees C, was investigated. It was elucidated that the inactivation was caused by production of molecules in an irreversibly denatured state. From analyses of the mechanism of production of the inactive enzyme, the inactivation was not evoked by a single chemical reaction. The free energy change between the folded and unfolded states decreased by the accumulation of chemical reactions (isomerization of Asp-Gly, deamidation of Asn, racemization of Asp and Asn, and cleavage of the Asp-X-peptide bond) induced at high temperature. Thus, certain molecules were ultimately in the unfolded state even at low temperature and lost activity. Moreover, a good correlation between the stability (free energy change) and the averaged number of chemical reactions that leads to the inactivation was obtained on the basis of some assumptions.