Structure of a legume lectin with an ordered N-linked carbohydrate in complex with lactose

The three-dimensional structure of the lactose complex of the Erythrina corallodendron Iectin (EcorL), a dimer of N-glycosylated subunits, was determined crystallographically and refined at 2.0 angstrom resolution to an R value of 0.19. The tertiary structure of the subunit is similar to that of other legume lectins, but interference by the bulky N-linked heptasaccharide, which is exceptionally well ordered in the crystal, forces die EcorL dimer into a drastically different quaternary structure. Only the galactose moiety of the lactose ligand resides within the combining site. The galactose moiety is oriented differently from ligands in the mannose-glucose specific legume lectins and is held by hydrophobic interactions with Ala⁸⁸, Tyr¹⁰⁶, Phe¹³¹, and Ala²¹⁸ and by seven hydrogen bonds, four of which are to the conserved Asp⁸⁹, Asn¹³³, and NH of Gly¹⁰⁷. The specificity of legume lectins toward the different C-4 epimers appears to be associated with extensive variations in the outline of the variable parts of the binding sites.