Mutation of non-conserved amino acids surrounding catalytic site to shift pH optimum of Bacillus circulans xylanase

Improvement of enzyme function by engineering pH dependence of enzymatic activity is of importance for industrial application of Bacillus circulans xylanases. Target mutation sites were selected by structural alignment between B. circulans xylanase and other xylanases having different pH optima. We selected non-conserved mutant sites within 8 Å from the catalytic residues, to see whether these residues have some role in modulating pKas of the catalytic residues. We hypothesized that the non-conserved residues which may not have any role in enzyme catalysis might perturb pKas of the catalytic residues. Change in pKa of a titratable group due to change in electrostatic potential of a mutation was calculated and the change in pH optimum was predicted from the change in pKa of the catalytic residues. Our strategy is proved to be useful in selection of promising mutants to shift the pH optimum of the xylanases towards desired side.