ROLE OF WATER IN THE SPECIFIC BINDING OF MANNOSE AND MANNOOLIGOSACCHARIDES TO CONCANAVALIN-A

We report here, the first solution state evidence for the role of wate r molecules in the specific interaction of carbohydrates with a legume lectin, concanavalin A. Concanavalin A from Canavalia ensiformis is a protein containing 237 amino acid residues with each monomer possessi ng one sugar binding site as well as sites for transition-metal ions, Mn2+ and Ca2+. The lectin binds specifically to alpha-anomers of monos accharides, D-glucopyranoside and D-mannopyranoside, and recognizes th e trimannosidic core of N-linked glycoproteins, O-(alpha-D-mannopyrano syl)-alpha-D-mannopyranoside with high specificity, which constitutes the minimum carbohydrate epitope that completely fills the sugar bindi ng site. Sensitive isothermal titration microcalorimetry coupled with osmotic stress strategy on concanavalin A was used to dissect out the differential involvement of water molecules in the recognition of the branched trimannoside (alpha-D-mannopyranosyl)-alpha-D-mannopyranoside ), the individual dimannosidic arms O-(alpha-D-mannopyranosyl)-alpha-D -mannopyranoside and -(alpha-D-mannopyranosyl)-alpha-D-mannopyranoside ) as well as the monomer unit, D-mannopyranoside, The specific binding of concanavalin A to different sugars, is accompanied by differential uptake of water molecules during the binding process. These results n ot only complement the X-ray crystallographic studies of legume lectin -sugar complexes displaying structurally conserved water molecules med iating the specific ligation of the sugars with the corresponding site s in the binding pocket but also provide a rationale for the observed compensatory behavior of enthalpies with entropies in lectin-sugar int eractions.