Human DNA is a long molecule that must be organized to fit inside the microscopic nucleus. To manage this packaging, cells use specialized enzymes like Human Topoisomerase II (Topo II). Topo II manages DNA’s structure, altering its shape to ensure cellular processes can occur without the genetic material becoming tangled.
This enzyme is part of the topoisomerase family, which resolves topological challenges from DNA’s double-helix structure. These enzymes are found in all living organisms. In humans, Topo II prepares DNA for cell division, a process requiring the duplication and separation of the genome.
The DNA Supercoiling Challenge
A cell’s DNA, if stretched out, would be about two meters long, yet it must be packed into a nucleus only a few micrometers in diameter. This compaction is achieved by coiling the DNA strand upon itself, similar to twisting a rubber band. This coiling is necessary for storage but creates physical challenges for the cell.
One challenge is supercoiling, where the DNA helix becomes over-twisted or under-twisted. This torsional stress is a consequence of processes like DNA replication and transcription, where the two strands must be separated. As cellular machinery moves along the DNA, it creates knots and tangles that can halt these processes.
Another problem is catenation, which occurs after DNA replication. When a chromosome is duplicated, the two new copies can become interlinked like rings in a chain. These interlocked molecules, called catenanes, must be separated before the cell can divide, or it can lead to genetic instability and cell death.
Mechanism of Action
To resolve supercoiling and catenation, Topoisomerase II uses a multi-step mechanism. The enzyme functions as a dimer, with two identical protein subunits working together. This dimer binds to one DNA segment, the G-segment or “gate” segment, using energy from the hydrolysis of adenosine triphosphate (ATP).
Once bound to the G-segment, the enzyme captures a second DNA segment, the T-segment or “transport” segment. Topo II then creates a temporary, double-stranded break in the G-segment. The enzyme remains covalently attached to the 5′ ends of the broken DNA, preventing the loose ends from causing damage.
With the gate open, the enzyme passes the T-segment through the break. The enzyme then reseals the break in the G-segment, restoring the DNA’s integrity. This process untangles the DNA without creating lasting damage. The cycle concludes when the T-segment is released and ATP is hydrolyzed, which resets the enzyme.
Humans express two forms of Topoisomerase II: alpha (Topo IIα) and beta (Topo IIβ). While sharing a similar mechanism, they have different roles. Topo IIα is found mainly in actively dividing cells as a component of mitotic chromosomes. Topo IIβ is present in most cells regardless of division state and is involved in gene transcription in non-dividing cells.
Role in Cellular Processes
One of Topoisomerase II’s primary roles is during DNA replication. As replication machinery unwinds the double helix, it generates torsional stress and supercoils ahead of the replication fork. Topo II travels ahead of this machinery, cutting and resealing the DNA to relieve this strain, allowing replication to proceed.
Without this activity, the buildup of supercoils would halt replication. As replication finishes, the two new daughter DNA molecules are often intertwined. Topo II is responsible for decatenating, or unlinking, these sister chromatids for their proper separation.
This role is important during cell division, or mitosis. After chromosomes are duplicated, they must be segregated into two new daughter cells. Topo IIα is needed for this final separation, resolving remaining catenations between sister chromatids so they can be pulled apart during anaphase. Without functional Topo II, chromosomes remain tangled, leading to breakages and an unequal distribution of genetic material, which is lethal for the cell.
Clinical Significance and Therapeutic Targeting
The reliance of rapidly dividing cells on Topoisomerase IIα makes this enzyme a target for cancer therapy. Cancer involves uncontrolled cell proliferation, so cancer cells are constantly replicating their DNA and undergoing mitosis. They have a greater need for Topo IIα activity compared to most normal, non-dividing cells, creating a therapeutic window to selectively target them.
Chemotherapy drugs known as “Topo II poisons,” such as etoposide, doxorubicin, and teniposide, exploit this vulnerability. These drugs interfere with the final step of the enzyme’s mechanical cycle. They allow Topo II to bind to DNA and make the double-stranded cut but stabilize the “cleavage complex,” trapping the enzyme while it is bound to the broken DNA ends.
This action prevents the resealing of the DNA break. The trapped complex becomes a form of DNA damage, and when replication machinery collides with these breaks, it triggers programmed cell death, or apoptosis. The effectiveness of these therapies can be compromised if cancer cells develop resistance through mutations in the gene that codes for Topo IIα (TOP2A), which can alter the enzyme’s structure.