ESR studies on the effect of ionic radii on displacement of Mn2+ bound to a soluble beta-galactoside binding hepatic lectin.

Binding of divalent metal ions to hepatic soluble beta-galactoside binding lectin was studied using electron spin resonance (ESR) spectroscopy. The Mn2+ bound to hepatic lectin could be displaced by Mg2+, Cu2+, Ni2+ and Ca2+ but not by Sr2+. As the ionic radii of Mg2+ (0.65 A), Cu2+ (0.73 A) and Ni2+ (0.72 A) are appreciably smaller than Ca2+ (0.99 A), it appears that the Mn2+ binding site is more accessible to Mg2+, Cu2+, and Ni2+ as compared to Ca2+, the ionic radius of Mn2+ being 0.80 A. Sr2+ with an ionic radius of 1.13 is thus unable to displace bound Mn2+. Surprisingly, the presence of specific sugars like alpha-lactose, or alpha-D-galactose facilitated the displacement of bound Mn2+ by metal ions whereas non-specific sugars, i.e. alpha-D-glucose, beta-D-fructose and alpha-D-ribose had no effect. It appears that minor perturbations in the saccharide binding site significantly affect the ability of the metal binding site to ligate bivalent metals.