Thermodynamic studies of saccharide binding to artocarpin, a B-cell mitogen, reveals the extended nature of its interaction with mannotriose [3,6-di-O-(alpha-D-mannopyranosyl)-D-mannose]

The thermodynamics of binding of various saccharides to artocarpin, from Ar tocarpus integrifolia seeds, a homotetrameric lectin (M-r 65,000) with one binding site per subunit, was determined by isothermal titration calorimetr y measurements at 280 and 293 K, The binding enthalpies, Delta H-b, are the same at both temperatures, and the values range from -10.94 to -47.11 kJ m ol(-1). The affinities of artocarpin as obtained from isothermal titration calorimetry are in reasonable agreement with the results obtained by enzyme -linked lectin absorbent essay, which is based on the minimum amount of lig and required to inhibit horseradish peroxidase binding to artocarpin in enz yme-linked lectin absorbent essay (Misquith, S,, Rani, P. G., and Surolia, A. (1994) J. Biol. Chem. 269, 30393-30401). The interactions are mainly ent halpically driven and exhibit enthalpy-entropy compensation. The order of b inding affinity of artocarpin is as follows: mannotriose >Man alpha 3Man > GlcNAc(2)Man(3)> Me alpha Man >Man >Man alpha 6Man >Man alpha 2Man >Me alph a Glc >Glc, i.e. 7>4>2>1.4>1>0.4>0.3>0.24>0.11. The Delta H for the interac tion of Man alpha 3Man, Man alpha 6Man, and Me alpha Man are similar and 20 kJ mol(-1) lower than that of mannotriose, This indicates that, while Man alpha 3Man and Man alpha 6Man interact with the lectin exclusively through their non-reducing end monosaccharide with the subsites specific for the al pha 1,3 and alpha 1,6 arms, the mannotriose interacts with the lectin simul taneously through all three of its mannopyranosyl residues. This study thus underscores the distinction in the recognition of this common oligosacchar ide motif in comparison with that displayed by other lectins with related s pecificity.