Stability and identification of active-site residues of carboxymethylcellulases from Aspergillus niger and Cellulomonas biazotea.

Determination of the apparent pKa's of purified carboxymethylcellulases from Aspergillus niger and Cellulomonas biazotea at different temperatures and in the presence of dioxane indicated two side chain carboxyl groups which controlled the limiting rate in both organisms. The thermostability of both enzymes slightly decreased with increasing pH from 5 to 75 but was unaffected in the presence of 0.5 mmol/L Mn2+. The CMCase from C. biazotea had an activation energy of 35 kJ/mol and a half-life of 89 min in the presence of 8 mol/L urea at 40 degrees C. The half-life of CMCase from A. niger in 8 mol/L urea and at 37 degrees C was 125 min as determined by a 0-9 mol/L transverse urea gradient PAGE. The CMCases from A. niger and C. biazotea had the same thermostabilities in the absence of CMC although the enzyme from the former was more thermostable in the presence of the substrate. The CMCase from A. niger was also more efficient in hydrolyzing CMC than the enzyme from C. biazotea.