Stability and folding of endoglucanase I (Cel7b) from Humicola insolens

Cellulases are of economic significance, particularly in the detergent and textile industries, where they are subjected to a wide range of operating c onditions affecting their stability. To increase our insight into the prope rties of this class of enzymes, we have carried out a study of the stabilit y and folding behavior of the 413-residue endoglucanase I (Cel7B) from Humi cola insolens. Data from chemical denaturation in guanidinium. chloride agr ee satisfactorily with calorimetric measurements, revealing an optimum stab ility of ca. 20 kcal mol(-1) around pH 7 and a peak half-width of 3-4 pH un its. Stability and activity show very similar pH-profiles, but this is prob ably fortuitous. Judging from equilibrium m-values (the dependence of the l og of the equilibrium unfolding constant on the denaturant concentration), the denatured state becomes significantly more compact outside pH 6-9. Fold ing and unfolding proceed very slowly with relaxation half times up to 6 h. Single- and double-jump kinetic data at pH 7 suggest a folding scheme invo lving two intermediates with native-like secondary structure but varying de grees of tertiary structure.