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Rational protein engineering was applied to improve the limited stability of the glycosyl hydrolase family 11 (GH11) endo-β-1,4-xylanase from Bacillus subtilis under acidic conditions. Since the pH dependence of protein stability is governed by the ionisation states of the side chains of its titrable amino acid residues, we explored the strategy of changing pH-stability profiles by altering pK_a values of key residues through in silico designed mutations. To this end, computational predictions and molecular modelling were carried out using the recently developed pKD software package. Four endoxylanase variants, in which the pK_a values of either Asp4 and Asp11 or His149 were targeted to shift downwards through incorporation of three to five point mutations, were generated and recombinantly expressed in the cytoplasm of Escherichia coli. All four mutants showed considerably increased functional stability at acid …