X-RAY CRYSTAL-STRUCTURE OF A PEA LECTIN-TRIMANNOSIDE COMPLEX AT 2.6 ANGSTROM RESOLUTION

The x-ray crystal structure of pea lectin, in complex with a methyl gl ycoside of the N-linked-type oligosaccharide trimannosyl core, methyl -(alpha-D-mannopyranosyl)-alpha-D-mannopyranoside, has been solved by molecular replacement and refined at 2.6-angstrom resolution. The R fa ctor is 0.183 for all data in the 8.0 to 2.6 angstrom resolution range with an average atomic temperature factor of 26.1 angstrom2. Strong e lectron density for a single mannose residue is found in the monosacch aride-binding site suggesting that the trisaccharide binds primarily t hrough one of the terminal alpha-linked mannose residues. The complex is stabilized by hydrogen bonds involving the protein residues Asp-81, Gly-99, Asn-125, Ala-217, and Glu-218, and the carbohydrate oxygen at oms O3, O4, O5, and O6. In addition, the carbohydrate makes van der Wa als contacts with the protein, involving Phe-123 in particular. These interactions are very similar to those found in the monosaccharide com plexes with concanavalin A and isolectin 1 of Lathyrus ochrus, confirm ing the structural relatedness of this family of proteins. Comparison of the pea lectin complex with the unliganded pea lectin and concanava lin A structures indicates differences in the conformation and water s tructure of the unliganded binding sites of these two proteins. Furthe rmore, a correlation between the position of the carbohydrate oxygen a toms in the complex and the bound water molecules in the unliganded bi nding sites is found. Binding of the trimannose core through a single terminal monosaccharide residue strongly argues that an additional fuc ose-binding site is responsible for the high affinity pea lectin-oligo saccharide interactions.