β‐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 α‐ or β‐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 α‐D‐galactopyranosides acted as high‐affinity inhibitors (K, below 50 μM). At lest two of them were not transported, in contrast of α‐galactoside disaccharides and to aromatic β‐D‐galactopyranosides. 3. β‐D‐galactoside transport was not significantly inhibited by specific inhibitors and transition‐state analogues of β‐galactosidase (D‐galactal, D galactonolactone). 4. The β‐D‐galactopyranoside, lactitol, and the α‐D‐galactopyranoside, galactinol, were not efficiently bound by the lactose permease system, although the maximal rate of uptake of lactitol was similar to that of lactose. By comparison with several structrally 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. Copyright © 1977, Wiley Blackwell. All rights reserved