Metal-binding and folding thermodynamics of Escherichia coli ribonuclease HI related to its catalytic function.

Escherichia coli ribonuclease HI (RNH) hydrolyzes the RNA strands of RNA/DNA hybrids in the presence of Mg at the highest level, relative to other metal ions. The Mg binding affinity was 8.39 × 10 M, which was lower than those of other metal ions. The low-affinity binder can express the maximum catalytic activity of RNH. The stability of RNH increased with increasing metal ion concentration, except for Zn. The thermodynamic origin for enhancing the stability of RNH with Mg was more favorable entropy compared to those with other metal ions, indicating that Mg binding changes the RNH structure while maintaining flexibility. Upon H124A mutation, the metal ion binding affinities decreased for Mn and Zn to a relatively large extent. The present thermodynamic analyses provide information on the structural dynamics of RNH with metal ion exchangeable binding, which can reasonably explain the metal-ion-dependent catalytic activity.