J Biol Chem, Vol. 274, Issue 42, 29694-29698, October 15, 1999

Thermodynamic Studies of Saccharide Binding to Artocarpin, a B-Cell Mitogen, Reveals the Extended Nature of Its Interaction with Mannotriose [3,6-Di-O-(-D-mannopyranosyl)-D-mannose]

P. Geetha Rani, Kiran Bachhawat, Sandra Misquith, and Avadhesha Surolia^¶

From the  Molecular Biophysics Unit, Indian Institute of Science, and the ^¶ Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore 560 012, India

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, H_b, are the same at both temperatures, and the values range from 10.94 to 47.11 kJ mol¹. 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>Man3Man>GlcNAc₂Man₃>MeMan>Man>Man6Man>Man2Man>MeGlc>Glc, i.e. 7>4>2>1.4>1>0.4>0.3>0.24>0.11. The H for the interaction of Man3Man, Man6Man, and MeMan are similar and 20 kJ mol¹ lower than that of mannotriose. This indicates that, while Man3Man and Man6Man interact with the lectin exclusively through their nonreducing end monosaccharide with the subsites specific for the 1,3 and 1,6 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.