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 a gglutinate different blood cell types. Their specificity toward sialylated oligosaccharides is unique among legume lectins; the leukoagglutinin prefer entially binds to sialyllactosamine (alpha NeuAc(2-3)beta Gal(1-4)beta GlcN Ac), whereas the hemagglutinin displays higher affinity for a disialylated tetrasaccharide (alpha NeuAc(2-3) beta Gal(1-3)[alpha NeuAc (2-6)]alpha Gal NAc). The three-dimensional structure of the complex between M. amurensis l eukoagglutinin and sialyllactose has been determined at 2.75-Angstrom resol ution using x-ray crystallography. The carbohydrate binding site consists o f a deep cleft that accommodates the three carbohydrate residues of the sia lyllactose, The central galactose sits in the primary binding site in an or ientation that has not been observed previously in other legume lectins. Th e carboxyl group of sialic acid establishes a salt bridge with a lysine sid e chain. The glucose residue is very efficiently docked between two tyrosin e aromatic rings. The complex between M. amurensis hemagglutinin and a disi alylated tetrasaccharide could be modeled from the leukoagglutinin/sialylla ctose crystal structure. The substitution of one tyrosine by an alanine res idue is responsible for the difference in fine specificity between the two isolectins. Comparison with other legume lectins indicates that oligosaccha ride specificity within this family is achieved by the recycling of structu ral loops in different combinations.