Structural foundation for the design of receptor antagonists targeting Escherichia coli heat-labile enterotoxin

Abstract

The bound conformations of these receptor antagonist compounds preserve the toxin-galactose interactions previously observed for toxin-sugar complexes, but gain additional favorable interactions. The highest affinity compound, MNPG, is notable in that it displaces a water molecule that is observed to be well-ordered in all other previous and current crystal structures of toxin-sugar complexes. This could be a favorable entropic factor contributing to the increased affinity. The highest affinity members of the present set of antagonists (MNPG and TDG) bury roughly half (400 A2) of the binding-site surface covered by the full receptor GM1 pentasaccharide, despite being considerably smaller. This provides an encouraging basis for the creation of subsequent generations of derived compounds that can compete effectively with the natural receptor.

Binding affinity of lactose permease is not altered by the H + electrochemical gradient

“…Recent fluorescence studies support the contention that Trp-151 is located in the hydrophilic cavity, and phosphorescence experiments demonstrate hydrophobic stacking between the galactopyranosyl and indole rings (17). The binding site in the N-terminal domain bears a striking similarity to that of many other sugar-binding proteins (18,19). Glu-269 in helix VIII in the C-terminal domain, another irreplaceable residue, forms a salt bridge with Arg-144 as well as an H bond with Trp-151.…”

Sequence Alignment and Homology Threading Reveals Prokaryotic and Eukaryotic Proteins Similar to Lactose Permease

“…30 Trp151 (helix V) stacks hydrophobically with the galactopyranosyl ring, 5,31,32 as observed in a number of sugar binding proteins. [33][34][35][36] As indicated by the X-ray structure of LacY, Met23 (helix I) is close to the C 6 of the galactopyranosyl ring, and Phe20 is in close proximity to Trp151. 5 Although the F20A mutant exhibits a 50-fold decrease in affinity, as judged by sugar protection against alkylation of Cys148, replacement of Met23 with Ala has no effect on affinity.…”

Probing of the rates of alternating access in LacY with Trp fluorescence

“…However, functionally important Trp residues are found in the binding sites of different transport proteins where they are involved in stacking or -interactions (23)(24)(25)(26)(27)(28)(29). Soluble sugar-binding proteins (30)(31)(32), as well as two sugar transport proteins, contain Trp (8) or Tyr (33) residues that undergo aromatic interactions with sugar in the binding site. The fluorescent properties of Trp make it valuable as an intrinsic reporter group in proteins (34,35).…”