Thermodynamic binding studies of lectins from the diocleinae subtribe to deoxy analogs of the core trimannoside of asparagine-linked oligosaccharides

Lectins from seven different species of the Diocleinae subtribe have been r ecently isolated and characterized in terms of their carbohydrate binding s pecificities (Dam, T. K., Cavada, B. S., Grangeiro, T. B., Santos, C. F., d e Sousa, F. k M., Oscarson, S., and Brewer, C. F. (1998) J. Biol Chem. 273, 12082-12088). The lectins included those from Canavalia brasiliensis, Crat ylia floribunda, Dioclea rostrata, Dioclea virgata, Dioclea violacea, and D ioclea guianensis. All of the lectins exhibited specificity for Man and Glc residues, but much higher affinities for the branched chain trimannoside, 3,6-di-O-(alpha-D-mannopyranosyl)-D-mannose, which is found in the core reg ion of all asparagine-linked carbohydrates. In the present study, isotherma l titration microcalorimetry is used to determine the binding thermodynamic s of the above lectins, including a new lectin from Canavalia grandiflora, to a complete series of monodeoxy analogs of the core trimannoside. From lo sses in the affinity constants and enthalpies of binding of certain deoxy a nalogs, assignments are made of the hydroxyl epitopes on the trimannoside t hat are involved in binding to the lectins. The pattern of binding of the d eoxy analogs is similar for all seven lectins, and similar to that of conca navalin A which is also a member of the Diocleinae subtribe. However, diffe rences in the magnitude of the thermodynamic binding parameters of the lect ins are observed, even though the lectins possess conserved contact residue s in many cases, and highly conserved primary sequences. The results indica te that non-contact residues in the lectins, even those distant from the bi nding sites, modulate their thermodynamic binding parameters.