Abstract
The conformational features of sucrose in the combining site of lentil lectin have been characterized through elucidation of a crystalline complex at 1.9-A resolution, transferred nuclear Overhauser effect experiments performed at 600 Mhz, and molecular modeling. In the crystal, the lentil lectin dimer binds one sucrose molecule per monomer. The locations of 229 water molecules have been identified. NMR experiments have provided 11 transferred NOEs. In parallel, the docking study and conformational analysis of sucrose in the combining site of lentil lectin indicate that three different conformations can be accommodated. Of these, the orientation with lowest energy is identical with the one observed in the crystalline complex and provides good agreement with the observed transferred NOEs. These structural investigations indicate that the bound sucrose has a unique conformation for the glycosidic linkage, close to the one observed in crystalline sucrose, whereas the fructofuranose ring remains relatively flexible and does not exhibit any strong interaction with the protein. Major differences in the hydrogen bonding network of sucrose are found. None of the two inter-residue hydrogen bonds in crystalline sucrose are conserved in the complex with the lectin. Instead, a water molecule bridges hydroxyl groups O2-g and O3-f of sucrose.
Highlights
Specific recognition of carbohydrates by protein receptors is of growing interest in biology
The conformational features of sucrose in the combining site of lentil lectin have been characterized through elucidation of a crystalline complex at 1.9-Å resolution, transferred nuclear Overhauser effect experiments performed at 600 Mhz, and molecular modeling
There are no large differences with the structures of the uncomplexed lentil lectin (Loris et al, 1993, 1994a), or with other legume lectins, especially the pea or L. ochrus lectins
Summary
Specific recognition of carbohydrates by protein receptors is of growing interest in biology. In the special case of weakly bound ligands, with a high rate of exchange between the bound and the free states, transferred nuclear Overhauser effects (TRNOEs) can be observed, giving information about the conformation of the carbohydrate in the binding site (Clore and Gronenborn, 1982; Ni, 1994) This method allowed the determination of the conformational changes undergone by oligosaccharides upon binding to ricin (Bevilacqua et al, 1990, 1992), to antibodies (Glaudemans et al, 1990, Bundle et al, 1994, Arepalli et al, 1995), and to a human lectin (Cooke et al, 1994). Several methods of docking monosaccharides to Interaction of Sucrose with Lentil Lectin protein receptors have been developed using molecular mechanics Such methods allow for a systematic exploration of all possible positions and orientations of the ligand in the binding site (Rao et al, 1989; Imberty et al, 1991). It is possible, using computer simulations, to evaluate the conformational space accessible for flexible ligands such as oligosaccharides (Reddy and Rao, 1992; Imberty and Perez, 1994)
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