Binding of alkyl 1-thio-beta-D-galactopyranosides to beta-D-galactosidase from E. coli.

The binding of a series of alkyl 1-thio-beta-D-galactopyranosides to beta-D-galactosidase from E. coli has been investigated. The inhibition constants were compared to the partition coefficients for the transfer of these substrate-analogues from water to 1-octanol. The relationships between the observed binding-constants and the partition coefficients indicate that part of the aglycon group binds to a hydrophobic area that is limited in relation to the length of hydrocarbon chain that can be accomodated. Outside this area, the hydrocarbon chain is only partially desolvated. The main driving-force for binding of the aglycon group is the increase in entropy resulting from the return of water molecules from the more-organized layer around the solute molecule to the bulk-water phase.