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 Dioclea grandiflora (DGL) to deoxy analogs of the 'core' trimannoside, 3,6- di-O-(α-D-mannopyranosyl)-α-D-mannopyranoside was determined by isothermal titration microcalorimetry (ITC) in the first paper of this series (Dam, T. K., Oscarson, S., and Brewer, C. F. (1998) J. Biol. Chem. 273, 32812-32817). The data showed binding of specific hydroxyl groups on all three residues of the trimannoside, similar to that observed for ConA (Gupta, D., Dam, T. K., Oscarson, S., and Brewer, C. F. (1997) J. Biol. Chem. 272, 6388-6392). However, differences exist in tile thermodynamics of binding of monodeoxy analogs of the α(1-6) Man residue of the trimannoside to the two lectins. The x-ray crystal structure of DGL complexed to the core trimannoside, 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. However, differences were noted in the arrangement of ordered water molecules in the binding sites of the two lectins. The present study presents ITC measurements of DGL and ConA binding to the monodeoxy analogs of the trimannoside in hydrogen oxide (H₂O) and deuterium oxide (D₂O). The solvent isotope effects present in the thermodynamic binding data provide evidence for altered solvation of the parent trimannoside complexes at sites consistent with the x-ray crystal structures of both lectins. The results indicate that the differences in the thermodynamics of DGL and ConA binding to α(1-6) monodeoxy analogs of the trimannoside do not correlate with solvation differences of the parent trimannoside complexes.