Abp is a family 27 glycoside hydrolase from the bacterium Geobacillus stearothermophilus T-6 that possess β-L-arabinopyranosidase activity. We have recently shown that Abp is capable of removing arabinopyranose residues from natural arabinose-containing polysaccharides. The crystal structure of Abp was determined at 2.28 Å resolution, and the crystal forms obtained belong to the primitive orthorombic space group P212121, with unit cell dimensions of a =107.7 Å, b= 202.2 Å, c=287.3 Å. Based on gel filtration experiments and bioinformatics analysis, it seems that the biological unit of Abp is a tetramer.
Most enzymes from GH27 family exhibit α-D-galactosidase activity, while some others demonstrate β-L-arabinopyranosidase activity. Abp is highly specific to β-L-arabinopyranose and exhibits only marginal activity on α-D-galactopyranose. Multiple sequence alignment of Abp with representative GH27 family enzymes revealed a single residue (Ile67 in Abp) that is probably involved in substrate specificity. Galactosidases in GH27 family have an Asp residue in that position, which forms a hydrogen bond with Gal-O6, thus stabilizing the binding of galactose. In Abp the corresponding residue is Ile67 which interferes with the binding of galactose. A single replacement of Ile67 to Asp, resulted in three orders of magnitude increase in specificity towards α-D-galactopyranose with only 2.7-fold decrease in specificity towards β-L-arabinopyranose
