2.1.2Topoisomerase（拓扑异构酶）

★—Topoisomerase: Topo，cleave and reseal DNAstrands (即能水解又能连接磷酸二酯键)。

       Release the supercoil constraint ahead of replication forks.

Topoisomerase functions in a three-step process: 

★1.Cleavage of one or both strands of   DNA(断开)

★2. Passage of a segment of DNA（理顺）

★3. Resealing of the DNA break（连接）

Classification: Topo I and TopoⅡ 

★Topo I: Also called ω-protein(ω蛋白) in prokaryotes. 

★It cuts one DNA strand, allows the broken DNA strand to rotate around the other strand to relax the constraint, and then reseals the cut. 

★Without consuming ATP(不需要ATP). 

 

· Anticancer drug:

 · Camptothecin(喜树碱)：

 · topotecan(托泊替康)

 · Irinotecan(依立替康)

★TopoⅡ: It is namedgyrase(旋转酶)in prokaryotes. 

 · It cuts both strands of dsDNA, releases the supercoil constraint, and reforms the phosphoester bonds.  

 · It can change dsDNA into the negative supercoil（负超螺旋） state with consumption of ATP.