Structures of the complexes of peanut lectin with methyl-β-galactose and N-acetyllactosamine and a comparative study of carbohydrate binding in Gal/GalNAc-specific legume lectins

Abstract

The crystal structures of complexes of peanut lectin with methyl-β-galactose and N-acetyllactosamine have been determined at 2.8 and 2.7 Å, respectively. These, and the complexes involving lactose and the T-antigenic disaccharide reported previously, permit a detailed characterization of peanut-lectin–carbohydrate association and the role of water molecules therein. The water molecules in the combining site are substantially conserved in the four complexes. The role of interacting sugar hydroxyl groups, when absent, are often mimicked by ordered water molecules not only at the primary combining site, but also at the site of the second sugar ring. The similarity of peanut-lectin–sugar interactions with those in other galactose/N-acetylgalactosamine-specific lectins also extend to a substantial degree to water bridges. The comparative study provides a structural explanation for the exclusive specificity of peanut lectin for galactose at the monosaccharide level, compared with that of the other lectins for galactose as well as N-acetylgalactosamine. The complexes also provide a qualitative structural rationale for differences in the strengths of binding of peanut lectin to different sugars.

Lectins: Tools for the Molecular Understanding of the Glycocode

“…44 Interestingly, as already reported for other carbohydrate-binding proteins, 49 ordered water molecules can be found in the unligated form at positions corresponding to hydroxyl groups in the ligated form (for instance, a water molecule occupies the position of the acetamido carbonyl O in the GalNAc moiety of the T-antigen). In general, water molecules in the carbohydrate-binding region mimic the ligand to a substantial extent not only at the primary site, but also in the regions adjacent to it.…”

“…Thus, water plays a significant role in carbohydrate recognition, imparting in some cases exquisite specificity. Comparison of crystal structures of PNA complexed with different ligands (T-antigen, Tant, 43 methyl-b-galactoside, MeGal, 44 N-acetyllactosamine, LacNAc 44 and lactose, Lac 45 ) has shown that water bridges involving water molecules W1 and W2 (Fig. 2) occur in all four complexes.…”

“…[42][43][44][45] There is a key stacking interaction between the aromatic ring of Tyr125 and the hydrophobic a-face patch of the galactose unit of the disaccharide. Further hydrogen bonds are Asp80 Od2-Gal O6 and that involving Ser211 Oc.…”

Crystal Structure of Anti-polysialic Acid Antibody Single Chain Fv Fragment Complexed with Octasialic Acid

“…Peanut lectin prefers T-antigen (Gal␤1-3GalNAc) to lactose (Gal␤1-4Glc). The 20-fold difference in affinity is attributed to the presence of two water molecules that make hydrogen bonds with the protein and the N-acetyl group of the T-antigen (39,40). In the case of family 15 and 29 carbohydrate-binding modules (CBM15 and -29), the removal of amino acid residues engaging in water-mediated interactions with ligands had comparatively little effect on carbohydrate binding (41,42).…”

Mutational Analysis at Asn-41 in Peanut Agglutinin

“…solution of the complex of PNA with T-antigen confirmed the importance of this residue and revealed the participation of the same water molecule in linking both Leu-212 and Asn-41 to T-antigen (15,17). We extend this study further on the role of Asn-41 in defining the anti-T specificity of PNA more so in relation to the role of the water-mediated hydrogen bond in imparting the lectin its exquisite specificity for the tumorassociated antigen.…”