fig1

Figure 1. Tdp1 catalytic cycle. Tdp1 utilizes two catalytic histidines to hydrolyze the phosphodiester bond that link adducts to the DNA. Here, we show the example of removal of a Topo1-DNA adduct-3’ phospho-tyrosyl linkage - stabilized by Topotecan. Tdp1 interacts with the Topo1-DNA adduct to initiate Step 1: the nucleophilic attack by His263 - that is, the nucleophilic histidine (in yeast Tdp1 His182) - on the 3’ phospho-tyrosyl linkage forms a 3’ phospho-hystidyl linkage or Tdp1-DNA adduct that releases the tyrosine and by extension Topo1. For Step 2, the general acid/base histidine His493 (His432 in yeast Tdp1) will activate a water molecule to hydrolyze the 3’ phospho-histidyl linkage dissociating Tdp1 from the DNA. However, a single strand nick is left behind by Tdp1 with 5’ hydroxyl and 3’ phosphoryl chemical groups that are processed (reversed) by polynucleotide kinase/phosphatase to facilitate DNA ligase III to regulate the DNA strands. Topo1: topoisomerase I; TPT: topotecan; Tdp1: tyrosyl-DNA phosphodiesterase I