DNA topoisomerase I is a specialized enzyme found in all forms of life, from bacteria to humans, that manages the complex structure of DNA. It resolves structural issues in the DNA molecule, ensuring the stability and proper functioning of genetic material. Its discovery in the 1970s marked a significant step in understanding how cells maintain their DNA.
Understanding DNA Supercoiling
DNA is an incredibly long molecule packed tightly within a cell or nucleus. To fit, it undergoes supercoiling, where the DNA helix twists upon itself, much like a tangled telephone cord. This coiling can be either positive (overwound) or negative (underwound), and both states create tension within the DNA molecule.
Supercoiling is important for compacting DNA, but it also creates topological challenges during processes that require DNA strands to separate or move past each other. During DNA replication or transcription, the unwinding of the DNA double helix generates additional twists and tangles ahead of the moving molecular machinery. Without a mechanism to relieve this torsional stress, these processes would halt, making DNA topoisomerase I an important player in cellular operations.
The Specific Role of Topoisomerase I
DNA topoisomerase I specializes in relieving torsional stress caused by DNA supercoiling. It achieves this by creating a transient single-strand break in the DNA backbone. This action allows one DNA strand to rotate around the other, unwinding supercoils and relaxing the DNA molecule.
The ability to make a single-strand break distinguishes DNA topoisomerase I from DNA topoisomerase II, which creates a transient double-strand break. This difference in mechanism means Topoisomerase I can change the linking number of circular DNA by single units, while Topoisomerase II typically changes it by multiples of two. Eukaryotic topoisomerase I can relax both positive and negative supercoils, playing a role in separating daughter chromosomes after DNA replication.
How Topoisomerase I Works
The mechanism of DNA topoisomerase I involves a series of coordinated steps. First, the enzyme binds to a supercoiled DNA segment. A specific tyrosine amino acid within the enzyme then attacks a phosphodiester bond in one of the DNA strands, creating a transient single-strand break. This creates a covalent bond between the enzyme and the 5′ phosphoryl end of the broken DNA strand, while the 3′ end remains non-covalently associated.
Following cleavage, the enzyme facilitates the controlled rotation of the intact DNA strand through the gap created by the break. This rotation removes the supercoils from the DNA. Once torsional stress is relieved, the enzyme re-ligates the broken DNA strand by reversing the phosphotyrosyl bond, restoring the DNA’s integrity. The enzyme then dissociates from the DNA, ready to act on another supercoiled region.
Biological Importance of Topoisomerase I
The proper functioning of DNA topoisomerase I is fundamental for numerous biological processes that rely on accessible and untangled DNA. During DNA replication, as the double helix unwinds, positive supercoils accumulate ahead of the replication fork. DNA topoisomerase I helps relax these supercoils, allowing the replication machinery to proceed smoothly.
Similarly, in gene transcription, where specific DNA segments unwind to create RNA copies, topoisomerase I alleviates torsional stress, enabling RNA polymerase to move along the DNA. It also contributes to DNA repair mechanisms and accurate segregation of chromosomes during cell division by disentangling intertwined DNA molecules. Without the activity of DNA topoisomerase I, cells would face severe impediments in these processes, potentially leading to DNA damage, genomic instability, and ultimately, cell death.
Topoisomerase I as a Therapeutic Target
The involvement of DNA topoisomerase I in fundamental cellular processes makes it an attractive target for therapeutic interventions, particularly in cancer treatment. Cancer cells are characterized by rapid, uncontrolled division, which places immense stress on their DNA replication and transcription machinery. Drugs known as topoisomerase I inhibitors exploit this vulnerability by interfering with the enzyme’s function.
A prominent class of these inhibitors is the camptothecins, which includes drugs like irinotecan and topotecan. These compounds work by stabilizing the transient covalent complex formed between DNA topoisomerase I and DNA after it has cleaved a single strand. By preventing re-ligation of the DNA strand, these drugs “trap” the enzyme on the DNA, leading to persistent single-strand breaks.
When replication forks collide with these trapped complexes, they can convert single-strand breaks into more lethal double-strand breaks, triggering cell death in rapidly dividing cancer cells. These inhibitors are used in the treatment of various cancers, including colorectal, ovarian, and lung cancers.