The role of carboxyl group in the catalytic action of endoglucanase I (Mr 70 2000)from Trichoderma paseudokiningii S-38 was delineated through chemical modification using Woodword reagent K and kinetic studies. The pH profile gave the kinetic apparent pK values of 4. 2 and 5. 8 for free enzyme and 3. 7 and 5. 5 for enzyme-substrate complex, respectively. The kinetic of inactivation indicated that one carboxyl residue was essectial for the enzyme with a second order rate constant of 395 min/mol. The kinetic analysis of the WRK modified enzyme gave valuse of Km similar to that of native enzyme. but Kcat values was lower. It indicated that the catalytic hydrolysis and not the substrate binding was affected by chemical modification. Treatment of the enzyme with diethyl pyrocarbonate reslted in the modification of 3. 2 residues of 3. 5 histidyl residues with the retention of 92% of the original enzymatic activity. Tritation of the enzyme with dithiobis (2-nitrobenzoic acid) showed only one free sulfhydryl group and five disufide inkages present in the enayme. and 10% loss of activity was observed when the free sulfhydrl
group was modified. Reaction with tetranitromethane did not affect any of the enzymatic activity.