Differential solvation of "core" trimannoside complexes of the Dioclea grandiflora lectin and concanavalin A detected by primary solvent isotope effects in isothermal titration microcalorimetry

The thermodynamics of binding of the Man/Glc-specific seed lectin from Dioc lea grandiflora (DGL) to deoxy analogs of the "core" trimannoside, 3,6-di-O -(alpha-D-mannopyranosyl)-alpha-D-mannopyranoside was determined by isother mal titration microcalorimetry (ITC) in the first paper of this series (Dam , T, K., Oscarson, S., and Brewer, C. F. (1998) J. Biol. Chem. 273, 32812-3 2817). The data showed binding of specific hydroxyl groups on all three res idues of the trimannoside, similar to that observed for ConA (Gupta, D., Da m, T. R., Oscarson, S., and Brewer, C. F. (1997) J. Biol. Chem. 272, 6388-6 392), However, differences exist in the thermodynamics of binding of monode oxy analogs of the alpha(1-6) Man residue of the trimannoside to the two le ctins. The x-ray crystal structure of DGL complexed to the core trimannosid e, presented in the second paper in this series (Rozwarski, D. A, Swami, B. M., Brewer, C. F., and Sacchettini, J. C. (1998) J, Biol. Chem, 273, 32818 -32825), showed the overall structure of the complex to be similar to that of the ConA-trimannoside complex. Furthermore, the trimannoside is involved in nearly identical hydrogen bonding interactions in both complexes. Howev er, differences were noted in the arrangement of ordered water molecules in the binding sites of the two lectins, The present study presents ITC measu rements of DGL and ConA binding to the monodeoxy analogs of the trimannosid e in hydrogen oxide (H2O) and deuterium oxide (D2O). The solvent isotope ef fects present in the thermodynamic binding data provide evidence for altere d solvation of the parent trimannoside complexes at sites consistent with t he x-ray crystal structures of both lectins, The results indicate that the differences in the thermodynamics of DGL and ConA binding to alpha(1-6) mon odeoxy analogs of the trimannoside do not correlate with solvation differen ces of the parent trimannoside complexes.