THERMODYNAMICS OF LECTIN-CARBOHYDRATE INTERACTIONS

The trisaccharide 3,6-di-O-(alpha-D-mannopyranosyl)-D-mannose, which i s present in all asparagine-linked carbohydrates, was previously shown by titration microcalorimetry to bind to the lectin concanavalin A (C onA) with nearly -6 kcal mol(-1) greater enthalpy change and 60-fold h igher affinity than methyl-alpha-D-mannopyranoside (Mandal, D. K., Kis hore, N., and Brewer, C. F. (1994) Biochemistry 33, 1149-1156). Simila r studies of the binding of a series of monodeoxy derivatives of the a lpha(1-3) residue of the trimannoside showed that this arm was require d for high affinity binding (Mandal, D. K., Bhattacharyya, L., Koenig, S. H., Brown, R., D., III, Oscarson, S., and Brewer, C. F. (1994) Bio chemistry 33, 1157-1162). In the present paper, a series of monodeoxy derivatives of the alpha(1-6) arm and ''core'' Man residue of the trim annoside as well as dideoxy and trideoxy analogs were synthesized. Iso thermal titration microcalorimetry experiments establish that the 3-, 4-, and 6-hydroxyl groups of the alpha(1-6)Man residue of the trimanno side binds to the lectin, along with the 2- and 4-hydroxyl groups of t he core Man residue and the 3- and 4-hydroxyl groups of the alpha(1-3) Man residue. Dideoxy analogs and trideoxy analogs showed losses of aff inities and enthalpy values consistent with losses in binding of speci fic hydroxyl groups of the trimannoside. The free energy and enthalpy contributions to binding of individual hydroxyl groups of the trimanno side determined from the corresponding monodeoxy analogs are observed to be nonlinear, indicating differential contributions of the solvent and protein to the thermodynamics of binding of the analogs. The therm odynamic solution data agree well with the recent x-ray crystal struct ure of ConA complexed with the trimannoside (Naismith, J. H., and Fiel d, R. A. (1996) J. Biol. Chem. 271, 972-976).