DNA molecules within cells are long and constantly undergo processes like replication and transcription, which can lead to tangles and knots. To manage these topological challenges, cells rely on topoisomerases. These enzymes temporarily break and rejoin DNA strands to relieve strain. Type 2 Topoisomerase is a specific enzyme that uniquely cuts both strands of the DNA double helix simultaneously.
This enzyme is ATP-dependent, meaning it requires energy from ATP hydrolysis to perform its function. Unlike Type 1 topoisomerases, which make single-strand breaks, Type 2 Topoisomerase creates a double-strand break, allowing for more complex DNA rearrangements. Its ability to manage DNA tangles and supercoils is fundamental for maintaining genetic integrity and function in all living organisms.
The Mechanism of DNA Unraveling
Type 2 Topoisomerase operates through a “two-gate” mechanism to untangle DNA. The enzyme binds to a “gate” (G-segment) of DNA and captures a “transport” (T-segment).
Upon binding of two ATP molecules, the enzyme undergoes a conformational change, leading to the cleavage of the G-segment. This creates a transient double-strand break, where catalytic tyrosine residues form covalent bonds with the 5′ ends of the broken DNA strands. This transient break is protected by the enzyme.
The T-segment is then passed through the newly created break in the G-segment. This passage is driven by the energy released from ATP hydrolysis. Once the T-segment has successfully passed through, the enzyme reseals the double-strand break in the G-segment, restoring the DNA’s integrity.
This unique mechanism allows Type 2 Topoisomerase to perform several specific actions on DNA topology. It can relax both positive and negative supercoils, which are over- or under-winding of the DNA helix, by changing the linking number of circular DNA by increments of two. The enzyme also plays a role in decatenation, the process of separating interlinked circular DNA molecules. Furthermore, it can unknot DNA, resolving entanglements within a single DNA molecule.
Vital Roles in Cellular Life
The actions of Type 2 Topoisomerase are indispensable for numerous cellular processes, supporting genetic stability and propagation. During DNA replication, as the double helix unwinds, positive supercoils accumulate ahead of the replication fork. Type 2 Topoisomerase actively removes these supercoils, ensuring DNA strands separate and are accurately duplicated. Without this activity, replication would cease, leading to incomplete DNA synthesis.
Beyond replication, Type 2 Topoisomerase also contributes to efficient DNA transcription. The unwinding of DNA during transcription generates torsional stress, which the enzyme helps alleviate, allowing RNA polymerase to move smoothly along the DNA template. This regulation of DNA topology ensures that genes can be accessed and expressed.
A widely recognized role is in chromosome segregation during cell division. After DNA replication, sister chromatids remain intertwined, forming catenated structures. Type 2 Topoisomerase is the primary enzyme responsible for decatenating these interlinked chromosomes, ensuring their proper separation and distribution into daughter cells. Failure of this decatenation leads to chromosome bridges and breaks, causing genetic instability and cell death.
Topoisomerase II as a Therapeutic Target
The importance of Type 2 Topoisomerase in cellular processes makes it an attractive target for therapeutic agents, particularly in cancer chemotherapy. Many anti-cancer drugs, known as topoisomerase II poisons, interfere with the enzyme’s function. These drugs, such as etoposide and doxorubicin, stabilize the transient DNA-enzyme complex after the DNA has been cut but before it is resealed.
By trapping the enzyme in this DNA-cleaved state, these drugs convert Type 2 Topoisomerase into a DNA-damaging agent. This leads to an accumulation of double-strand breaks in the DNA, which are damaging to the cell. Rapidly dividing cells, such as cancer cells, are particularly susceptible to this induced DNA damage because they rely heavily on Type 2 Topoisomerase activity for proliferation. The extensive DNA damage then triggers cell death in these cancerous cells.
In addition to cancer therapy, some antibiotics also target bacterial Type 2 Topoisomerases, specifically DNA gyrase and topoisomerase IV. For instance, fluoroquinolone antibiotics inhibit bacterial gyrase, preventing it from introducing negative supercoils necessary for bacterial DNA replication and transcription. This disruption of bacterial DNA management ultimately leads to bacterial cell death, making Type 2 Topoisomerase a broad-spectrum target in medicine.