The role of the site 342 in catalytic efficiency and pH optima of endoglucanase II from Trichoderma reesei as probed by saturation mutagenesis

摘要

The role of site 342 of endoglucanase II from Trichoderma reesei in catalytic efficiency and pH optima was investigated by site saturation mutagenesis. The mutations identified in this study can be divided into three separate classes according to their amino acid features. When Asn342 was substituted by hydrophobic and non-polar amino acids, most variants exhibited an up-shift in pH optimum and their catalytic efficiency was similar to that of the wild-type at their optimal pH. N342R variant had a pH optimum at 6.2. N342K variant did not give an up-shift in pH optimum, although K and R are both amino acids carrying positive charges. Molecular modelling indicated that residue 342 was located at the C-terminus of one of the a-helices near two catalytic residues. Hydrophobic side chains and more H-bonds would make the helix more rigid, which might affect the stability and activity of the enzyme at higher pH.