The structures of the Erythrina corallodendron lectin (EcorL) and of its complexes with galactose, N-acetylgalactosamine, lactose and N-acetyllactosamine were determined at a resolution of 1.9 to 1.95 A. The final R-values of the five models are in the range 0.169 to 0.181. The unusual, non-canonical, dimer interface of EcorL is made of beta-strands from the two monomers, which face one another in a "hand-shake" mode. The galactose molecule in the primary binding site is bound in an identical way in all four complexes. Features of the electrostatic potential of the galactose molecule match those of the potential in the combining site, thus probably pointing to the contribution of the electrostatic energy to determining the orientation of the ligand. No conformational change occurs in the protein upon binding the ligand. Subtle variations in the binding mode of the second monosaccharide (glucose in the complex with lactose and N-acetylglucosamine in the complex with N-acetyllactosamine) were observed. The mobility of Gln219 is lower in the complexes with the disaccharides than in the complexes with the monosaccharides, indicating further recruitment of this residue to ligand binding through more extensive hydrogen bonding in the former complexes. Water molecules that have been located in the combining sites of the five structures undergo rearrangement in response to binding of the different ligands. The new structural information is in qualitative agreement with thermodynamic data on the binding to EcorL.
Copyright 1998 Academic Press Limited.