Bacillus subtilis endoxylanase XynA was engineered by site-directed mutagenesis to alter its substrate selectivity, i.e. the ratio of its capacity to solubilise water-unextractable arabinoxylans (WU-AX) to its capacity to hydrolyse water-extractable AX (AE-AX). Four surface exposed aromatic residues were targeted for mutation to alanine, either individually (Y113 and W185) orjointly (F48, Y94, Y113 and W185), based on the general role of aromatic residues in the binding of endoxylanases to insoluble substrates and the hypothesis that they affect the activity of endoxylanases towards WU-AX. Determination of the substrate selectivity factors and WU-AX degradation profile of the wild type and mutant enzymes using an elaborate procedure requiring incubation of native wheat flour WU-AX and WE-AX with different enzyme levels showed that mutants W185A and 4mutA, the mutant containing all four mutations, displayed lower selectivity towards WU-AX than the wild type recombinant endoxylanase. Each of the endoxylanases had similar binding activity to wheat flour WU-AX and insoluble oat spelt xylan. (c) 2007 Elsevier Inc. All rights reserved.