Magnesium as a natural substitute for manganese in concanavalin A and other lectins

Addition of magnesium to apo-concanavalin A in the presence of calcium was shown by ultraviolet difference spectroscopy to generate a holoprotein spectroscopically identical to the MnCa-holoprotein. The MgCa- and MnCa-forms bound equally strongly to Sephadex G-75. In kinetic experiments, the binding of Mg2+ was much slower than Mn2+ binding; Kd for Mg2+ was estimated as 7.4 mM. The combined Mg2+ and Mn2+ contents of 10 lectins specific for D-galactose or N-acetyl-D-galactosamine were each close to one atom per subunit, suggesting occupancy of the Mn2+ site by Mg2+ is common in plant lectins.