Energetics of 5-bromo-4-chloro-3-indolyl-alpha-D-mannose binding to the Parkia platycephala seed lectin and its use for MAD phasing.

Parkia platycephala belongs to the most primitive group of Leguminosae plants. Its seed lectin is made up of three homologous beta-prism repeats and exhibits binding specificity for mannose/glucose. The properties of the association between the lectin from P. platycephala seeds and monosaccharide ligands were analysed by isothermal titration calorimetry and surface plasmon resonance. The results are consistent with the lectin bearing three thermodynamically identical binding sites for mannose/glucose per monomer with dissociation constants in the millimolar range. Binding of each ligand by the lectin is enthalpically driven. Crystals have been obtained of the lectin in complex with a brominated derivative of mannose (5-bromo-4-chloro-3-indolyl-alpha-D-mannose), which were suitable for deriving an electron-density map by MAD phasing. In agreement with the thermodynamic data, six Br atoms were found in the asymmetric unit of the monoclinic P2(1) crystals, which contained two P. platycephala lectin molecules. The availability of other Br derivatives of monosaccharides (glucose, galactose, fucose) may make this strategy widely useful for structure elucidation of novel lectins or when (as in the case of the P. platycephala lectin) molecular-replacement methods fail.