Physicochemical and saccharide-binding studies on the galactose-specific seed lectin from Trichosanthes cucumerina

Physicochemical and saccharide-binding studies have been performed on Trichosanthes cucumerina seed lectin (TCSL). The agglutination activity of TCSL is highest in the pH range 8.0–11.0, whereas below pH 7.0 it decreases quite rapidly, which is consistent with the involvement of imidazole side chains of His residues, which titrate in this pH range, in the sugar-binding activity of the lectin. The lectin activity is unaffected between 0 and 60 °C, but a sharp decline occurs at higher temperatures. Isoelectric focusing studies show that TCSL has three isoforms with pI values of 5.3, 6.2, and 7.1, with the isoform of pI 6.2 being the most abundant. Circular dichroism spectroscopic studies reveal that TCSL contains about 28.4% β-sheet, 10.6% β-turns, 7% polyproline type 2 structure, with the remainder comprising unordered structure; the α-helix content is negligible. Binding of 4-methylumbelliferyl-β-d-galactopyranoside (MeUmbβGal) to TCSL results in a significant increase in the fluorescence intensity of the ligand, and this change has been used to obtain the association constant for the interaction. At 25 °C, the association constant, Ka, for the TCSL–MeUmbβGal interaction was determined as 6.9×104 M−1. Binding of nonfluorescent, inhibitory sugars was studied by monitoring their ability to reverse the fluorescence changes observed when MeUmbβGal was titrated with TCSL.