An organism’s DNA is a long, double-helical molecule whose structure creates logistical challenges. During processes like replication and gene expression, unwinding the DNA can lead to tangles and supercoils elsewhere in the molecule. To manage these topological problems, cells use enzymes known as topoisomerases. These enzymes prevent and correct DNA entanglements that would otherwise halt cellular operations. Among them, Topoisomerase 2 is notable for its unique mechanism and broad impact on DNA metabolism.
Defining Topoisomerase 2
Topoisomerase 2 is a member of the Type II family of topoisomerase enzymes. Its defining characteristic is the ability to create a temporary, double-stranded break in a segment of DNA. This action distinguishes it from Type I topoisomerases, which only cut a single DNA strand. By cutting both strands, Topoisomerase 2 can pass another segment of the DNA duplex directly through the opening to resolve complex entanglements.
Eukaryotic organisms, including humans, have two primary isoforms of this enzyme: Topoisomerase IIα (alpha) and Topoisomerase IIβ (beta). The alpha isoform is found mainly in actively dividing cells, where it aids DNA replication and chromosome separation. The beta isoform is expressed in non-dividing, differentiated cells, suggesting a role in maintaining gene expression.
Topoisomerase 2 functions as a homodimer, composed of two identical protein subunits. This structure allows it to bind and cleave both strands of the DNA helix simultaneously. Its catalytic cycle depends on adenosine triphosphate (ATP) for energy. The enzyme uses energy from ATP hydrolysis to drive the conformational changes needed to pass one DNA segment through another, not for the DNA breaking or rejoining itself.
The Mechanism of Topoisomerase 2 Action
The action of Topoisomerase 2 is a process described as a “two-gate” or “strand-passage” mechanism, which alters DNA topology without permanent damage. The cycle begins when the enzyme binds to a “gate” segment of DNA, called the G-segment. This G-segment is positioned within a groove in the enzyme complex.
Once the G-segment is secured, the enzyme captures a second “transported” DNA segment, the T-segment. The binding of two ATP molecules closes the enzyme’s N-terminal “gate,” trapping the T-segment inside and initiating the G-segment’s cleavage. The enzyme uses tyrosine amino acids to break both strands of the G-segment, forming a temporary covalent bond between the protein and the broken DNA ends.
With the G-segment opened, the T-segment is passed through the break. The enzyme then rejoins, or ligates, the G-segment ends. ATP hydrolysis powers the enzyme’s reset, which opens an “exit gate” to release the T-segment and return the enzyme to its initial state. This sequence changes the DNA’s linking number, a measure of its coiling, in steps of two.
Essential Biological Functions of Topoisomerase 2
Topoisomerase 2 performs several functions. During DNA replication, as the double helix unwinds, positive supercoils accumulate ahead of the replication fork. This creates torsional stress that could impede the process. Topoisomerase 2 relieves this stress by introducing negative supercoils, allowing replication to proceed.
At the conclusion of replication, the two new daughter DNA molecules are often physically intertwined, a state called catenation. Topoisomerase 2 performs decatenation, cutting one duplex to allow the other to pass through. This action is necessary to separate sister chromatids before cell division, ensuring each daughter cell receives a complete set of chromosomes.
The enzyme also helps condense DNA into compact chromosomes before mitosis. In transcription, the process of reading genes, Topoisomerase 2 relaxes supercoiling generated by RNA polymerase. This relaxation ensures that genes, particularly long ones, can be expressed efficiently.
Topoisomerase 2 in Human Health and Disease
Topoisomerase 2 is a target in medicine, particularly for cancer treatment. Cancer cells are characterized by rapid proliferation and are highly dependent on the enzyme for DNA replication and cell division. This dependency makes Topoisomerase 2 an effective target for chemotherapy drugs known as inhibitors, which interfere with its catalytic cycle.
A major class of these drugs, known as Topoisomerase 2 poisons, includes etoposide, doxorubicin, and mitoxantrone. These agents do not block the enzyme from cutting DNA. Instead, they trap it in an intermediate state where it is covalently bound to the broken DNA strands. This stable “cleavage complex” creates permanent double-strand breaks, which trigger programmed cell death in the cancer cell.
Cancer cells can develop resistance to these drugs through mutations in the Topoisomerase 2 gene or by reducing the enzyme’s presence. The mechanism that makes these drugs effective also causes side effects, as they affect healthy dividing cells. This damage to normal tissues can sometimes lead to treatment-related secondary malignancies years after the initial cancer is treated.