Cell Surface Receptors
Interactions between cellular membrane receptors and carbohydrate binding proteins lead to intercellular contact, to endocytosis of the glycoconjugates, or to modulation of the cellular signaling pathways. The recognition of carbohydrate cell surface receptors is highly specific, and the ligands adopt a conformations of a wide range of complexity. Oligosaccharide moieties serve as cell surface receptors for a variety of proteins.
 Bacterial Lectins
The Cholera Toxin of Vibrio cholerae is an AB5 hexamer, consisting of 5 identical B subunits and a single A subunit. The saccharide moiety of GM1 binds the complete CT-AB5 hexamer and the B-pentamer alone, but not individual B subunits. GM1 binds each B subunit and is hydrogen bonded to Gly33 of an adjacent subunit via a solvent molecule. The binding of GM1 pentasaccharide to the 5 sites is known to be cooperative. Both terminal sugars, galactoes and sialic acid, of the branched GM1 interact witht the binding site residues of the toxin.
Cholera Toxin protomer B5, bound to ganglioside GM1



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