Thermodynamic analysis of chitooligosaccharide binding to Urtica dioica agglutinin by isothermal titration calorimetry

UDA (Urtica dioica agglutinin) contains two hevein like domains with two no n-identical interacting sites and is specific for chitooligosaccharides. Th e binding of chitooligosaccharides to UDA was studied by Isothermal Titrati on Calorimetry. Each site is composed of three subsites, each binding to a sugar residue. Thermodynamic parameters obtained show that while chitobiose has two independent non-interacting sites, chitotriose, chitotetrose and c hitopentose have two interacting sites on each monomer of UDA. Values of bi nding enthalpy (Delta H) increase almost by a factor of 7 in going from chi tobiose to chitotriose indicating the existence of three subsites in the co mbining site of UDA. The binding constant for chitotetrose and chitopentose increase without any further enhancement in the values of Delta H indicati ng that for oligomers larger than chitotriose interaction is favoured entro pically.