Studies on metal induced conformation changes in a peripheral blood lymphocyte lectin

A Ca2+ -dependent, glucose-specific lectin was isolated from goat peripheral blood lymphocytes by affinity chromatography on N-acetyl D-glucosamine agarose gel. Since the lectin binding to carbohydrate ligands was metal dependent, it was important to study the divalent metal ion-induced conformational changes in the lectin. The conformational changes were studied by absorption, fluorescence and circular dichroism spectroscopy techniques. Binding of Ca2+, Mn2+ or Mg2+ results in shift in ultraviolet absorption maxima from 281 to 298 nm (red shift). A major increase in absorbance at 245 nm is also exhibited. Binding of Ca2+ and Mn2+ resulted in decrease in intrinsic fluorescence emission maxima with shift from 355.2 nm to 342.4 nm (blue shift). These shifts could be reversed on addition of EDTA. A double reciprocal analysis of fluorescence quenching data suggest that Ca2+ and Mn2+ interact with a single class of binding site with an apparent kd of 1.50 +/- 0.37 microM and 1.25 +/- 0.25 microM, respectively. These data clearly indicate that occupancy of metal binding sites on goat peripheral blood lymphocyte lectin induces a gross conformational change sequestering aromatic amino acids into a hydrophobic environment. These findings were further supported by circular dichroism spectrum which showed a massive alteration in the presence of Ca2+.