Concanavalin A distorts the beta-GlcNAc-(1-->2)-Man linkage of beta- GlcNAc-(1-->2)-alpha-Man-(1-->3)-[beta-GlcNAc-(1-->2)-alpha-Man- (1-- >6)]-Man upon binding

Carbohydrate recognition by proteins is a key event in many biological processes. Concanavalin A is known to specifically recognize the pentasaccharide core (beta-GlcNAc-(1-->2)-alpha- Man-(1-->3)-[beta- GlcNAc-(1-->2)-alpha-Man-(1-->6)]-Man) of N-linked oligosaccharides with a Ka of 1.41 x 10(6 )M-1. We have determined the structure of concanavalin A bound to beta-GlcNAc-(1-->2)-alpha-Man-(1-->3)-[beta- GlcNAc-(1-->2)-alpha-Man- (1-->6)]-Man to 2.7A. In six of eight subunits there is clear density for all five sugar residues and a well ordered binding site. The pentasaccharide adopts the same conformation in all eight subunits. The binding site is a continuous extended cleft on the surface of the protein. Van der Waals interactions and hydrogen bonds anchor the carbohydrate to the protein. Both GlcNAc residues contact the protein. The GlcNAc on the 1-->6 arm of the pentasaccharide makes particularly extensive contacts and including two hydrogen bonds. The binding site of the 1-->3 arm GlcNAc is much less extensive. Oligosaccharide recognition by Con A occurs through specific protein carbohydrate interactions and does not require recruitment of adventitious water molecules. The beta-GlcNAc-(1-->2)-Man glycosidic linkage PSI torsion angle on the 1-->6 arm is rotated by over 50 degrees from that observed in solution. This rotation is coupled to disruption of interactions at the monosaccharide site. We suggest destabilization of the monosaccharide site and the conformational strain reduces the free energy liberated by additional interactions at the 1-->6 arm GlcNAc site.      

Man alpha1-2 Man alpha-OMe-concanavalin A complex reveals a balance of forces involved in carbohydrate recognition.

We have determined the crystal structure of the methyl glycoside of Man alpha1-2 Man in complex with the carbohydrate binding legume lectin concanavalin A (Con A). Man alpha1-2 Man alpha-OMe binds more tightly to concanavalin A than do its alpha1-3 and alpha1-6 linked counterparts. There has been much speculation as to why this is so, including a suggestion of the presence of multiple binding sites for the alpha1-2 linked disaccharide. Crystals of the Man alpha1-2 Man alpha-OMe-Con A complex form in the space group P2(1)2(1)2(1) with cell dimensions a = 119.7 A, b = 119.7 A, c = 68.9 A and diffract to 2. 75A. The final model has good geometry and an R factor of 19.6% (Rfree= 22.8%). One tetramer is present in the asymmetric unit. In three of the four subunits, electron density for the disaccharide is visible. In the fourth only a monosaccharide is seen. In one subunit the reducing terminal sugar is recognized by the monosaccharide site; the nonreducing terminal sugar occupies a new site and the major solution conformation of the inter-sugar glycosidic linkage conformation is adopted. In contrast, in another subunit the non reducing terminal sugar sits in the so called monosaccharide binding site; the reducing terminal sugar adopts a different conformation about its inter-sugar glycosidic linkage in order for the methyl group to access a hydrophobic pocket. In the third subunit, electron density for both binding modes is observed. We demonstrate that an extended carbohydrate binding site is capable of binding the disaccharide in two distinct ways. These results provide an insight in to the balance of forces controlling protein carbohydrate interactions.