beta-D-Galactoside transport in Escherichia coli: substrate recognition.

1. A number of galactosides and other sugar compounds were examined as inhibitors of facilitated or active transport by the lactose permease system of Escherichia coli. Efficient inhibition required an alpha- or beta-anomeric galactopyranosyl ring of D-configuration, a free 6-hydroxyl group, and a certain aglycone size which was reached, for example, by monosaccharide or nitrophenyl substituents. 2. Aromatic alpha-D-galactopyranosides acted as high-affinity inhibitors (Ki, below 50 micrometer). At least two of them were not transported, in contrast to alpha-galactoside disaccharides and to aromatic beta-D-galactopyranosides. 3. beta-D-Galactoside transport was not significantly inhibited by specific inhibitors and transitionstate analogues of beta-galactosidase (D-galactal, D-galactonolascone). 4. The beta-D-galactopyranoside, lactitol, and alpha-D-galactopyranoside, galactinol, were not efficiently bound by the lactose permease system, although the maximal rate of uptake of lacitol was similar to that of lactose. By comparison with several structurally related D-galactopyranosides, the decreased affinity was attributed to an effect of the membrane/water interface. A model for substrate recognition by the lactose permease system is presented.