Topoisomerases are enzymes that maintain the structure of DNA within cells. Imagine DNA as a long, delicate string that needs organization. These enzymes act like “DNA detanglers,” preventing and resolving knots and twists that arise in genetic material. Without their continuous action, fundamental DNA processes would grind to a halt.
The DNA Topology Problem
The double-helical structure of DNA presents physical challenges during cellular activities. When DNA strands unwind, like untwisting a phone cord, tension builds up elsewhere along the molecule. This tension manifests as supercoiling, where the DNA helix becomes excessively twisted, either overwound (positive supercoiling) or underwound (negative supercoiling).
This torsional stress, particularly positive supercoiling, accumulates ahead of processes requiring DNA unwinding, such as replication and transcription. If this strain is not relieved, the DNA helix can become so tangled that enzymes like DNA and RNA polymerases cannot continue their work, blocking access to genetic information. Topoisomerases are therefore necessary for managing these topological changes, ensuring DNA remains accessible and functional.
Classification and Mechanisms
Topoisomerases are categorized into two main types based on their mechanisms of action. These enzymes temporarily break and rejoin DNA strands, allowing the DNA structure to relax or unwind. Each type employs a unique approach to accomplish these changes.
Type I Topoisomerases
Type I topoisomerases introduce a transient break in one strand of the DNA double helix. After creating this break, the enzyme allows the DNA strand to rotate around the intact strand, relieving torsional stress or supercoiling. Once tension is alleviated, the enzyme reseals the broken DNA strand, restoring its integrity. This process does not require ATP, making it an energy-efficient method for managing DNA topology.
Type II Topoisomerases
Type II topoisomerases involve the simultaneous breakage of both strands of the DNA double helix. These enzymes capture a DNA segment, create a double-strand break, and pass a second, intact segment through the temporary gap. After passage, the broken DNA strands are rejoined, changing the DNA’s topological state by two turns per cycle. This process is powered by ATP hydrolysis, providing energy for the enzyme’s conformational changes and DNA transport. A key example is DNA gyrase, found in bacteria, which introduces negative supercoils into DNA and can also relax positive supercoils.
Key Biological Processes Requiring Topoisomerases
Topoisomerases are important for several biological processes, ensuring DNA-dependent cellular functions. Their ability to manage DNA topology is important during extensive DNA manipulation.
During DNA replication, topoisomerases work ahead of the replication fork, where the DNA double helix unwinds. As helicase separates the two DNA strands, positive supercoiling accumulates, creating torsional strain. Topoisomerases relieve this supercoiling, preventing DNA from becoming tangled and allowing replication machinery to synthesize new DNA strands.
Transcription, the process of copying genetic information from DNA into RNA, generates torsional stress. As RNA polymerase moves along the DNA template, it causes localized unwinding and overwinding, similar to replication. Topoisomerases mitigate this strain, ensuring RNA polymerase can efficiently transcribe genes without topological barriers.
Chromosome segregation, the separation of duplicated chromosomes during cell division, relies on topoisomerases. After DNA replication, daughter DNA molecules can become intertwined, a state known as catenation. Type II topoisomerases, through their double-strand breaking and rejoining activity, are responsible for unlinking these intertwined chromosomes (decatenation), allowing them to be pulled apart into daughter cells during mitosis.
Medical Relevance of Topoisomerase
The functions of topoisomerases make them attractive targets for therapeutic interventions, particularly in treating diseases characterized by rapid cell division. Modulating their activity can selectively harm undesirable cells while sparing healthy ones.
In cancer chemotherapy, “topoisomerase poisons” exploit these enzymes. These drugs, such as etoposide and doxorubicin, bind to the topoisomerase-DNA complex after the enzyme cuts the DNA strands. Instead of allowing the enzyme to reseal breaks, these drugs stabilize the complex, trapping the enzyme on DNA and preventing religation. This leads to an accumulation of permanent single- or double-strand breaks in DNA, which triggers programmed cell death (apoptosis) in rapidly dividing cancer cells.
Topoisomerases are targeted by antibiotics to combat bacterial infections. Fluoroquinolones, for example, inhibit bacterial topoisomerases, primarily DNA gyrase and topoisomerase IV. Bacterial DNA gyrase is unique in its ability to introduce negative supercoils, necessary for bacterial DNA replication and transcription. By inhibiting these bacterial enzymes, fluoroquinolones prevent bacteria from replicating their DNA and dividing, killing bacterial cells without affecting human cells due to structural differences.