Roles of exogenous divalent metals in the nucleolytic activity of Cu,Zn superoxide dismutase

It is well known that the wild type Cu,Zn superoxide dismutase (holo SOD) catalyzes the conversion of superoxide anion to peroxide hydrogen and dioxygen. However, a new function of holo SOD, i.e., nucleolytic activity has been found [W. Jiang, T. Shen, Y. Han, Q. Pan, C. Liu, J. Biol. Inorg. Chem. 11 (2006) 835-848], which is linked to the incorporation of exogenous divalent metals into the enzyme-DNA complex. In this study, the roles of exogenous divalent metals in the nucleolytic activity were explored in detail by a series of biochemical experiments. Based on a non-equivalent multi-site binding model, affinity of a divalent metal for the enzyme-DNA complex was determined by absorption titration, indicating that the complex can provide at least a high and a low affinity site for the metal ion. These mean that the holo SOD may use a “two exogenous metal ion pathway” as a mechanism in which both metal ions are directly involved in the catalytic process of DNA cleavage. In addition, the pH versus DNA cleavage rate profiles can be fitted to two ionizing-group models, indicating the presence of a general acid and a general base in catalysis. A model that requires histidine residues, metal-bound water molecules and two hydrated metal ions to operate in concert could be used to interpret the catalysis of DNA hydrolysis, supported by the dependences of loss of the nucleolytic activity on time and on the concentration of the specific chemical modifier to the histidine residues on the enzyme.