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Catalytic cycle of TDP1. (A) Nucleophilic attack on the tyrosyl–DNA 3′-phosphate by the imidazole N2 atom of H263. H493 donates a proton to the outgoing Top I peptide. (B) Formation of the TDP1-DNA covalent intermediate. (C) H493 activates a water molecule which attacks the 3′-P breaking the N-P bond and hydrolyzing the phosphoenzyme intermediate. (D) Release of DNA 3′-phosphate from TDP1.
