The overall catalytic efficiency of both Type IA and Type II topoisomerases can be greatly enhanced by the addition of metal ions, typically Mg 2+ or Mn 2+ ions 21, 22, 23.Īlthough sharing a highly conserved topoisomerase core domain, Type IA topoisomerase subfamily members such as topoisomerase 1 (bacteria) and topoisomerase 3 employ slightly different amino acids for catalysis 9, 12, 14, 24, and exhibit distinct kinetic features in DNA relaxation and decatenation 25, 26, 27, 28. In comparison, Type II topoisomerases, which share a comparable TOPRIM catalytic center with Type IA topoisomerases, are believed to use divalent cations for DNA cleavage 14, 20. For Type IA topoisomerase enzymes, divalent cations are required for DNA rejoining, whereas their participation in DNA cleavage remains controversial 12, 14, 18, 19. Employing divalent cations at the catalytic site to trigger a phosphodiesterase activity is a general feature of many nucleic acid enzymes 13, 14, 15, 16, 17. A divalent metal ion is commonly observed at the enzyme reaction center, coordinated by three or four conserved acidic residues 10, 11, 12. A reverse phosphoryl transfer initiated by the nucleophilic 3’-OH rejoins the DNA 5, 8, 9. To cleave DNA, the tyrosine forms a covalent bond with the 5’-cleaved DNA end (tyrosyl-phosphate), leaving a 3’-OH group at the other DNA end. A conserved tyrosine residue acts as nucleophile to break the DNA phosphodiester backbone. Type IA topoisomerases employ general acid-base catalysis to cleave DNA. They use a three-step strategy-DNA cleavage, strand passage, DNA rejoining-to alter DNA topology, including supercoil relaxation and decatenation 1, 5, 6, 7. Type IA topoisomerases share a padlock-shaped topoisomerase-core at their N-terminus while their C-terminal tail size and structure is highly diversified across species 3, 4. All living organisms have at least one copy of a type IA DNA topoisomerase 1, 2, 3.
0 kommentar(er)
