Binding of alkyl beta-D-xylopyranosides, containing branched-chain, cyclic, and substituted aglycon groups, to beta-D-xylosidase from Bacillus pumilus PRL B12.

For a number of alkyl beta-D-xylopyranosides having branched-chain, cyclic, and substituted aglycon groups, and binding to beta-D-xylosidase from B. pumilus PRL B12, the binding constant Ki and (for some of them) the thermodynamic equilibrium parameters delta H0, delta S0, and delta G0 have been determined. Although the aglycon is bound through hydrophobic forces, no simple relationships between the binding parameters and the relative hydrophobicity of the alkyl beta-D-xylopyranosides could be demonstrated. All of the available evidence suggests that the aglycon sub-site has a highly specific structure which forces the atoms of the aglycon group to occupy well-defined positions. The supplementary energy-requirements resulting from the imposed restrictions seem to be the main reason for the irregular way in which the binding parameters depend on the aglycon structure.