Figure 1.

Elementary topoisomerase reactions and rationale for suicide substrates. (A) DNA strand cleavage results from attack of an active-site tyrosine residue forming a covalent 3′-phosphotyrosine–DNA intermediate. DNA strand ligation results from attack of the displaced 5′-OH and expulsion of the active site tyrosine. (B) Pre-activated 3′-pNP DNA is illustrated on the right. Enzyme-mediated transesterification of the 5′-OH will expel para-nitrophenol. If a mutant enzyme lacking the tyrosine nucleophile is used, then competing cleavage reactions cannot occur.