Man a1-2 Mana-OMe concanavalin A complex reveals balance of forces involved in carbohydrate recognition

Abstract

We have determined the crystal structure of the methyl glycoside of Man alpha 1-2 Man in complex with the carbohydrate binding legume lectin concanavalin A (Con A). Man alpha 1-2 Man alpha-OMe binds more tightly to concanavalin A than do its alpha 1-3 and alpha 1-6 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 alpha 1-2 linked disaccharide, Crystals of the Man alpha 1-2 Man alpha-OMe-Con A complex form in the space group P2(1)2(1)2(1) with cell dimensions a = 119.7 Angstrom, b = 119.7 Angstrom, c = 68.9 Angstrom and diffract to 2.75 Angstrom. The final model has good geometry and an R factor of 19.6% (R-free = 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.</p