Binding of ATP to Human DNA topoisomerase i resulting in an alteration of conformation and catalytic activity of the enzyme

Wo have studied the molecular mechanism of A IP-inhibitory effect on the DNA relaxation activity of human DNA topoisomerase I. We have demon si rated that ATP directly and specifically binds to human DNA topoisonierasc I. To address the question whether the DNA relaxation activity of human UNA topoisomerase 1 was allosterically regulated by ATP, We examined the confor [national change of human DNA topoisomerase 1 upon ATP binding. Human !)NA topoisomerase I was incubated with various concentrations of ATP prior ID the treatment by various proteinases, such as trypsin. chymotrypsin. t her molysin. \'K protease, and proleinase K. Our results show that in the presence of ATP. human DNA topoisomerase I becomes resistant towards degradation by trypsin and chymotrypsin, suggesting that the conformation of human DXA lopoisoincrase I becomes alterated upon ATP binding. This observation suggests that DNA topoisomerase I exhibits at least two conformations. Onr con formation is in the form of Topo I-ATP complex, which executes protein kinasc activity, and t lie other is in thr form of Topo [-DNA complex, which executes DNA relaxation activity.