The DNA dependence of the ATPase activity of human DNA topoisomerase IIα

We have purified human topoisomerase II alpha from HeLa cells and studied its ATPase reaction. The ATPase activity is stimulated by DNA and shows apparent Michaelis-Menten kinetics. Although the ATPase activity of human topoisomerase II alpha is lower than that of Saccharomyces cerevisiae, it is more active in decatenation, implying more efficient coupling of the ATPase to DNA strand passage under these conditions. Using plasmid pBR322 as the DNA cofactor, the reaction shows hyperstimulation by DNA at a base pair to enzyme dimer ratio of 100-200:1. When DNA fragments are used as the cofactor, the reaction requires >similar to 100 base pairs to stimulate the activity and fragments of similar to 300 base pairs show hyperstimulation. This behavior can be rationalized in terms of the enzyme requiring fragments that can bind to both the DNA gate and the ATP-operated clamp in order for the ATPase reaction to be stimulated. Hyperstimulation is a consequence of the saturation of DNA with enzyme. The mechanistic implications of these results are discussed.