Crystallographic, thermodynamic, and molecular modeling studies of the mode of binding of oligosaccharides to the potent antiviral protein griffithsin†‡

The mode of binding of oligosac- charides to griffithsin, an antiviral lectin from the red alga Griffithsia sp., was investigated by a combi- nation of X-ray crystallography, isothermal titration calorimetry, and molecular modeling. The struc- tures of complexes of griffithsin with 1?6a-manno- biose and with maltose were solved and refined at the resolution of 2.0 and 1.5 A u , respectively. The thermodynamic parameters of binding of 1?6a- mannobiose, maltose, and mannose to griffithsin were determined. Binding profiles of 1?6a-manno- biose and mannose were similar with Kd values of 83.3 lM and 102 lM, respectively. The binding of maltose to griffithsin was significantly weaker, with a fourfold lower affinity (Kd = 394 lM). In all cases the binding at 308C was entropically rather than enthalpically driven. On the basis of the experimen- tal crystal structures, as well as on previously deter- mined structures of complexes with monosaccha- rides, it was possible to create a model of a triden- tate complex of griffithsin with Man9GlcNAc2 ,a high mannose oligosaccharide commonly found on the surface of viral glycoproteins. All shorter oligo- mannoses could be modeled only as bidentate or monodentate complexes with griffithsin. The ability to mediate tight multivalent and multisite interac- tions with high-mannose oligosaccharides helps to explain the potent antiviral activity of griffithsin. Proteins 2007;67:661-670. V C 2007 Wiley-Liss, Inc.*