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 Artocarpus integrifolia seeds, a homotetrameric lectin (M(r) 65, 000) with one binding site per subunit, was determined by isothermal titration calorimetry measurements at 280 and 293 K. The binding enthalpies, DeltaH(b), are the same at both temperatures, and the values range from -10.94 to -47.11 kJ mol(-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 ligand required to inhibit horseradish peroxidase binding to artocarpin in enzyme-linked lectin absorbent essay (Misquith, S., Rani, P. G., and Surolia, A. (1994) J. Biol. Chem. 269, 30393-30401). The interactions are mainly enthalpically driven and exhibit enthalpy-entropy compensation. The order of binding affinity of artocarpin is as follows: mannotriose>Manalpha3Man>GlcNAc(2)Man(3)>MealphaMan>Man>M analpha6Man> Manalpha2Man>MealphaGlc>Glc, i.e. 7>4>2>1.4>1>0.4>0.3>0.24>0.11. The DeltaH for the interaction of Manalpha3Man, Manalpha6Man, and MealphaMan are similar and 20 kJ mol(-1) lower than that of mannotriose. This indicates that, while Manalpha3Man and Manalpha6Man interact with the lectin exclusively through their nonreducing end monosaccharide with the subsites specific for the alpha1,3 and alpha1,6 arms, the mannotriose interacts with the lectin simultaneously through all three of its mannopyranosyl residues. This study thus underscores the distinction in the recognition of this common oligosaccharide motif in comparison with that displayed by other lectins with related specificity.