Topoisomerases are enzymes that manage the complex structure of DNA. DNA is a long, double-helical molecule that can become tangled or overwound, similar to a twisted phone cord. These enzymes resolve such topological issues by temporarily breaking and rejoining DNA strands, ensuring the proper functioning of cellular processes. They maintain the integrity and accessibility of our genetic material.
The Role of Topoisomerases in DNA Management
DNA, with its double-helical structure, is prone to tangling and supercoiling. This can be positive (overwinding) or negative (underwinding). Such topological stress impedes essential cellular activities like DNA replication, where strands must unwind to be copied, and transcription, where genetic information is read. Without a mechanism to relieve this tension, enzymes like DNA and RNA polymerases would be unable to move along the DNA helix.
Topoisomerases address these challenges by acting as molecular managers of DNA topology. They bind to DNA and introduce transient breaks in the sugar-phosphate backbone of one or both DNA strands. This temporary break allows the DNA to untangle or unwind, releasing torsional strain. Once the issue is resolved, the enzyme reseals the DNA backbone, restoring the molecule’s integrity. This “cut and reseal” mechanism ensures DNA remains accessible for all necessary cellular functions.
Topoisomerase I: Mechanism and Specific Actions
Topoisomerase I enzymes alter DNA topology by introducing temporary single-strand breaks in the DNA double helix. These enzymes do not require ATP for their catalytic activity. They function by forming a transient covalent bond between a tyrosine residue in the enzyme and a 3′-phosphate in the DNA. This break allows the cleaved DNA strand to rotate around the intact strand, relaxing supercoiling.
This mechanism effectively relaxes negative supercoils, which can accumulate behind the replication fork during DNA unwinding. Topoisomerase I also resolves DNA tangles that arise during processes like DNA replication, transcription, and chromosome condensation. By temporarily relieving torsional strain, these enzymes ensure the progression of genetic processes.
Topoisomerase II: Mechanism and Specific Actions
Topoisomerase II enzymes manage DNA by simultaneously cutting both strands of the DNA helix. This mechanism involves passing an intact segment of DNA through the transient double-strand break before resealing it. Unlike Topoisomerase I, this process depends on ATP hydrolysis, which provides energy for the conformational changes required for strand passage. This action changes the linking number of circular DNA by increments of two.
A primary role of Topoisomerase II is in decatenation, the unlinking of tangled DNA circles that form after DNA replication. Without this enzyme, daughter chromosomes would remain interlinked, impeding cell division. Topoisomerase II also unknots DNA and manages both positive and negative supercoils, particularly during chromosome segregation. This enzyme ensures chromosomes are properly separated and distributed to daughter cells during cell division.
Topoisomerases in Medicine
Topoisomerases are targets for anti-cancer drugs due to their involvement in DNA replication and cell division. Cancer cells divide rapidly, and their reliance on these enzymes for DNA management makes them vulnerable to inhibitors. Inhibiting topoisomerases can lead to an accumulation of DNA breaks, which triggers cell death in these fast-growing cells.
Topoisomerase I inhibitors, such as camptothecins (e.g., irinotecan and topotecan), stabilize the transient DNA-enzyme complex after a single-strand break, preventing DNA resealing. This increases DNA strand breaks, interfering with replication and transcription, and inducing cell death. Topoisomerase II inhibitors, including etoposide and doxorubicin, similarly trap the enzyme on the DNA after a double-strand break, causing persistent DNA damage. These “topoisomerase poisons” are used in chemotherapy to induce replication fork arrest and double-strand break formation, inhibiting cancer cell proliferation.