Man alpha 1-2 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 alp ha 1-2 Man in complex with the carbohydrate binding legume lectin concanava lin A (Con A). Man alpha 1-2 Man alpha-OMe binds more tightly to concanaval in A than do its alpha 1-3 and alpha 1-6 linked counterparts. There has bee n much speculation as to why this is so, including a suggestion of the pres ence of multiple binding sites for the alpha 1-2 linked disaccharide, Cryst als of the Man alpha 1-2 Man alpha-OMe-Con A complex form in the space grou p 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 presen t in the asymmetric unit. In three of the four subunits, electron density f or the disaccharide is visible, In the fourth only a monosaccharide is seen . In one subunit the reducing terminal sugar is recognized by the monosacch aride site; the nonreducing terminal sugar occupies a new site and the majo r 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 su gar adopts a different conformation about its inter-sugar glycosidic linkag e in order for the methyl group to access a hydrophobic pocket. In the thir d subunit, electron density for both binding modes is observed. We demonstr ate that an extended carbohydrate binding site is capable of binding the di saccharide in two distinct ways. These results provide an insight in to the balance of forces controlling protein carbohydrate interactions.