Magnesium vs manganese cofactors for metallonuclease enzymes. A critical evaluation of thermodynamic binding parameters and stoichiometry

An experimental analysis of Mg2+ vs, Mn2+ binding to Escherichia coli ribonuclease H and exonuclease III enzymes by isothermal titration calorimetry clearly demonstrates a 1:1 stoichiometry for metal binding to ribonuclease H, hut distinct metal-dependent behaviour for exonuclease III, suggesting caution in the interpretation and generalization of results concerning the location and stoichiometry of Mg2+ binding sites from crystallographic and mechanistic experiments with Mn2+.