Chemical modification results in hyperactivation and thermostabilization of Fusarium solani glucoamylase

Chemical modification of carboxyl groups of glucoamylase from a mesophilic fungus, Fusarium solani, was carried out using ethylenediamine as nucleophile in the presence of water-soluble 1-ethyl-3(3-dimethylaminopropyl)carbodiimide. Modification brought about a dramatic enhancement of catalytic activity and thermal stability of glucoamylase. Temperature and pH optima of ethylenediamine-coupled glucoamylase (ECG) increased as compared with those of native enzyme. The specificity constant (k(cat)/K(m)) of native, ECG-2, ECG-11, and ECG-17 was 136, 173, 225, and 170, respectively, at 55 degrees C. The enthalpy of activation (Delta H*) and free energy of activation (Delta G*) for soluble starch hydrolysis were lower for the chemically modified forms. All of the modified forms were stable at higher temperatures and possessed high Delta G* against thermal unfolding. The effects of alpha-chymotrypsin and subtilisin on the modified forms were activating as compared with native. Moreover, denaturation of ECG-2, ECG-11, and ECG-17 in urea at 4 mol x L(-1) also showed an activation trend. A possible explanation for the thermal denaturation of native and increased thermal stability of ECG-2, ECG-11, and ECG-17 at higher temperatures is also discussed.