An unusual carbohydrate binding site revealed by the structures of two Maackia amurensis lectins complexed with sialic acid-containing oligosaccharides

Seeds from the legume tree Maackia amurensis contain two lectins that can agglutinate different blood cell types. Their specificity toward sialylated oligosaccharides is unique among legume lectins; the leukoagglutinin preferentially binds to sialyllactosamine (alpha NeuAc(2-3)beta Gal(1-4)beta GlcNAc), whereas the hemagglutinin displays higher affinity for a disialylated tetrasaccharide (alpha NeuAc(2-3) beta Gal(1-3)[alpha NeuAc (2-6)]alpha GalNAc). The three-dimensional structure of the complex between M. amurensis leukoagglutinin and sialyllactose has been determined at 2.75-Angstrom resolution using x-ray crystallography. The carbohydrate binding site consists of a deep cleft that accommodates the three carbohydrate residues of the sialyllactose, The central galactose sits in the primary binding site in an orientation that has not been observed previously in other legume lectins. The carboxyl group of sialic acid establishes a salt bridge with a lysine side chain. The glucose residue is very efficiently docked between two tyrosine aromatic rings. The complex between M. amurensis hemagglutinin and a disialylated tetrasaccharide could be modeled from the leukoagglutinin/sialyllactose crystal structure. The substitution of one tyrosine by an alanine residue is responsible for the difference in fine specificity between the two isolectins. Comparison with other legume lectins indicates that oligosaccharide specificity within this family is achieved by the recycling of structural loops in different combinations.