Studies on carbohydrate binding to a lectin purified from Streptomyces sp

The anti-B specific lectin produced by Streptomyces sp. was shown to have two carbohydrate-binding sites with binding constants of 8.3 . 10(3) M-1 (15 degrees C) and 2.2 . 10(3) M-1 (4 degrees C) for L-rhamnose and D-galactose, respectively, calculated according to Scatchard plots. The binding of specific sugars to the lectin not only induced a peculiar ultraviolet difference spectrum showing a blue shift of tryptophan absorption, but also caused crystallization of the lectin at a concentration of 1 mg per ml or more. The solvent-perturbation studies on the lectin showed that the number of solvent-exposed tryptophan (or average extent of exposure) was two in the absence of L-rhamnose, and three in the presence of the sugar. This suggests that one tryptophan residue appears outside as the result of sugar-binding to the lectin, which is reflected by the difference spectra. Oxidation of two tryptophan residues with N-bromosuccinimide led to complete loss of carbohydrate-binding activity of the lectin, indicating that these residues are important for retaining the activity.