Structure of basic winged-bean lectin and a comparison with its saccharide-bound form

The crystal structure of the saccharide-free form of the basic form of wing ed-bean agglutinin (WBAI) has been solved by the molecular-replacement meth od and refined at 2.3 Angstrom resolution The final R factor is 19.74b for all data in the resolution range 8.0-2.3 Angstrom. The asymmetric unit cont ains two half-dimers, each located on a crystallographic twofold axis. The structure of the saccharide-free form is compared with that of the complex of WBAI wi th methyl-alpha-D-galactoside. The complex is composed of two di mers in the asymmetric unit. The intersubunit interactions in the dimer are nearly identical in the two structures The binding site of the saccharide- free structure contains three ordered water molecules at positions similar to those of the hydroxyl groups of the carbohydrate which an hydrogen bonde d to the protein. Superposition of the saccharide-binding sites of the two structures shows that the major changes involve expulsion of these ordered water molecules and a shift of about 0.6 Angstrom of the main-chain atoms o f the variable loop.