Crystal structure of the lectin from Dioclea grandiflora complexed with core trimannoside of asparagine-linked carbohydrates

The seed lectin from Dioclea grandiflora (DGL) has recently been shown to p ossess high affinity for 3,6-di-O-(alpha-D-mannopyranosyl)-alpha-D-mannopyr anose, the core trimannoside of asparagine-linked carbohydrates, but lower affinity for biantennary complex carbohydrates. In the previous paper, the thermodynamics of DGL binding to deoxy analogs of the core trimannoside and to a biantennary complex carbohydrate were determined by isothermal titrat ion microcalorimetry. The data suggest that DGL recognizes specific hydroxy l groups of the trimannoside similar to that of the jack bean lectin concan avalin A (ConA) (Gupta, D. Dam, T. K., Oscarson, S., and Brewer, C. F. (199 7) J. Biol. Chem. 272, 6388-6392). However, the thermodynamics of DGL bindi ng to certain deoxy analogs and to the complex carbohydrate are different f rom that of ConA, In the present paper, the x-ray crystal structure of DGL complexed to the core trimannoside was determined to a resolution of 2.6 An gstrom. The overall structure of the DGL complex is similar to the structur e of the ConA-trimannoside complex (Naismith, J. H., and Field, R. A. (1996 ) J. Biol. Chem. 271, 972-976), The location and conformation of the bound trimannoside as well as its hydrogen-bonding interactions in both complexes are nearly identical. However, differences exist in the location of two lo ops outside of the respective binding sites containing residues 114-125 and 222-227, The latter residues affect the location of a network of hydrogen- bonded water molecules that interact with the trisaccharide. Differences in the arrangement of ordered water molecules in the binding site and/or prot ein conformational differences outside of the binding site may account for the differences in the thermodynamics of binding of the two lectins to deox y analogs of the trimannoside. Molecular modeling studies suggest how DGL d iscriminates against binding the biantennary complex carbohydrate relative to ConA.