Calreticulin is a mol. chaperone found in the endoplasmic reticulum in eukaryotes, and its interaction with N-glycosylated polypeptides is mediated by the glycan Glc1Man7-9GlcNAc2 present on the target glycoproteins. Here, we report the thermodn. parameters of its interaction with di-, tri-, and tetrasaccharide, which are truncated versions of the glucosylated arm of Glc1Man7-9GlcNAc2, detd. by the quant. technique of isothermal titrn. calorimetry. This method provides a direct est. of the binding consts. (Kb) and changes in enthalpy of binding (DHb Deg) as well as the stoichiometry of the reaction. Unlike past speculations, these studies demonstrate unambiguously that calreticulin has only one site per mol. for binding its complementary glucosylated ligands. Although the binding of glucose by itself is not detectable, a binding const. of 4.19*104 M-1 at 279 K is obtained when glucose occurs in a-1,3 linkage to ManaMe as in Glca1-3ManaMe. The binding const. increases by 25-fold from di- to trisaccharide and doubles from tri- to tetrasaccharide, demonstrating that the entire Glca1-3Mana1-2Mana1-2ManaMe structure of the oligosaccharide is recognized by calreticulin. The thermodn. parameters thus obtained were supported by modeling studies, which showed that increased no. of hydrogen bonds and van der Waals interactions occur as the size of the oligosaccharide is increased. Also, several novel findings about the recognition of saccharide ligands by calreticulin vis a vis legume lectins, which have the same fold as this chaperone, are discussed. [on SciFinder (R)]