topoisomeraasi
Topoisomerase, sometimes referred to in Finnish as topoisomeraasi, is a class of enzymes that regulate the topological state of DNA by transiently breaking and rejoining strands. They relieve or introduce DNA supercoiling and resolve entanglements that arise during processes such as replication, transcription, recombination, and chromatin organization. Topoisomerases are divided into two main types: type I and type II. Type I enzymes cleave a single DNA strand and alter the linking number by increments of one without requiring ATP in many cases. Type II enzymes cut both DNA strands, pass another duplex through the break to change the linking number by two, and typically require ATP for activity. Type II enzymes can also decatenate intertwined circular DNA molecules.
The catalytic mechanism involves a transient covalent enzyme–DNA intermediate formed by a catalytic tyrosine that forms
In cells, topoisomerases participate in replication fork progression, transcription, chromosome condensation and segregation, and genome maintenance.
Clinical and pharmacological relevance is notable. Bacterial topoisomerases are targeted by fluoroquinolone antibiotics. In humans, topo