​Topoisomerase​

Topoisomerases are nucleases that break the phosphodiester bonds in DNA reversibly. Topoisomerases function to relieve the positive supercoiling that arises from DNA unwinding mediated by helicases during replication. The first topoisomerase was discovered by James Wang in 1971 from Escherichia coli. Understanding the Topoisomerase types and structure helps us learn the complicated mechanisms underlying DNA dynamics. In this article, we will cover Topoisomerase - structure, function, types, and more.

What is Topoisomerase?

Topoisomerases are nuclear enzyme that are responsible for relieving the supercoiling of DNA and maintaining DNA topology during several cellular processes like transcription and chromosome segregation. It does so by breaking the phosphodiester bonds of DNA reversibly. There are several types of DNA Topoisomerases in eukaryotes and prokaryotes.

Some of them could cleave only one strand of the DNA and some of them could cleave both the strands. Depending on the number of strands they can cleave they are classified as Type I Topoisomerase and Type II Topoisomerase. Type I can cleave only one strand whereas Type II can cleave both strands.

Types of Topoisomerase

Primarily there are two types of Topoisomerases: Type I Topoisomerases and Type II Topoisomerases. Both these types are differentiated based on their function, structure, and the mechanism by which they cleave DNA. Both these types are discussed below.

👁 Topoisomerase

Type I Topoisomerase

Type I Topoisomerases are involved in relieving torsional stress and managing the supercoiling of DNA by introducing single-strand breaks and allowing the DNA to pass through the gap or rotate around the intact strand.

• There are two main subtypes within Type I Topoisomerases: Type IA and Type IB.
• Type IA Topoisomerases covalently bind 5'-phosphate during DNA hydrolysis and effect Topological changes in DNA through a 'strand pass' mechanism in which it cleaves one strand of the dsDNA and allows the other strand to pass through the gap followed by resealing.
• On the other hand, Type IB Topoisomerase binds to 3'-phosphate and relieves supercoiling by nicking a single strand of DNA and allowing other strand end to rotate with respect to other around the intact phosphodiester bond on the oppposite strand.
• Type IA Topoisomerases can be grouped into three classes- eubacterial Topo IA, eubacterial and eukaryotic Topo III and eubacterial and archaeal reverse gyrase whereas Type IB Topoisomerase is represented by Topo IB only.

Topoisomerase I Function

The functions include:

• Relieves torsional stress in DNA and help relax DNA.
• Introduces single-strand breaks to allow DNA rotation or passage through.
• Maintains DNA topology during cellular processes like replication and transcription.
• Helps in breaking down DNA strands during recombination.

Type II Topoisomerase

Type II Topoisomerases regulate the DNA topology by introducing double-strand breaks in DNA molecules.

• There are two main subtypes within Type II Topoisomerases: Type IIA and Type IIB.
• Both Type IIA and Type IIB Topoisomerase uses duplex strand passage mechanism and have almost same core but there overall tertiary structure differs.
• Type IIA Topoisomerases are found in all cellular organisms, as well as in viruses. They are divided into three classes eukaryotic Topo II, bacterial topo IV, and bacterial and archaeal DNA gyrase.
• Type IIB Topoisomerases include topo IV from plant and homologues of Spo 11 from yeast.

Topoisomerase II Function

The functions include:

• Topoisomerase II facilitates the relaxation of supercoiled DNA.
• It plays a crucial role in DNA replication, transcription, and recombination.
• It resolves DNA knots and tangles to maintain genomic stability.
• This enzyme is essential for chromosome segregation during cell division.
• DNA gyrase promotes the negative supercoils of DNA.

Functions of Topoisomerase

The ability of Topoisomerases to break phosphodiester bonds reversible makes them crucial components in many cellular processes like replication, transcription, seggregation, etc. Primary functions of Topoisomerases are as follows:

• Relieving Torsional Stress: DNA Topoisomerases help alleviate torsional stress and strain in DNA that arises during various cellular processes, such as DNA replication, transcription, and recombination. Thereby prevent the overwinding (positive supercoiling) or underwinding (negative supercoiling) of DNA strands.
• Unwinding and Untangling of DNA: During processes like DNA replication, the DNA double helix needs to be unwound to expose the template strands for copying. Topoisomerases, especially Type II, untangle and unwind the DNA by introducing transient double-strand breaks, allowing the passage of another DNA segment through the break.
• Facilitating DNA replication, transcription, and seggregation: Topisomerases relieve over winding of DNA thus help in smooth execution of any of the process that involve DNA unwinding by creating a smooth path for enzymes involved. During segregation they uncoil the intertwined DNA and jelps in proper segregation.
• Therapeutic targets: The Topisomerases serve as targets of some drugs that are use in differen types of cancer treatment. These drugs interrupt with Topoisomerase activity and kill rapidly dividing cells.

Topoisomerase in DNA Replication

During DNA replication, topoisomerases play a vital role in ensuring the accurate and efficient duplication of genetic material. These enzymes help relieve the tension that builds up ahead of the replication fork as DNA strands unwind and separate. By temporarily breaking the DNA strands topoisomerases alleviate this tension by allowing the replication machinery to proceed smoothly. This process is crucial for preventing DNA damage and maintaining genomic integrity during cell division.

Also, topoisomerases help in the removal of supercoils that arise as a result of DNA unwinding, ensuring that the replicated DNA molecules are correctly packaged and functional. Thus, topoisomerases are important players in the complicated process of DNA replication thereby ensuring the correct transmission of genetic information from one generation to the next.

Topoisomerase Inhibitors

Topoisomerase inhibitors are drugs that interfere with the activity of Topoisomerases, crucial enzymes involved in DNA topology regulation. They are widely used in cancer chemotherapy due to their ability to disrupt DNA replication and repair, leading to cell death. There are two main types: Topoisomerase I inhibitors and Topoisomerase II inhibitors. However, some drugs, like the fluoroquinolone antibiotics (e.g., ciprofloxacin), act as dual inhibitors of both Topoisomerase I and Topoisomerase II.

Topoisomerase I inhibitors

• Topoisomerase I inhibitors are together called Campothecins.
• This class of drugs includes camptothecin, topotecan, and irinotecan.
• Camptothecins stabilize the covalent complex formed between Topoisomerase I and DNA, preventing religation of the cleaved DNA strand. This results in the accumulation of DNA breaks and inhibition of DNA replication.

Topoisomerase II inhibitors

• Anthrocyanines, Anthracycline, Podophyllotoxin Derivatives, and Amsacrine are common classes of drugs that function as Topoisomerase II inhibitors.
• All these drugs stabilize the cleavable complex formed by Topoisomerase II with DNA, preventing the enzyme from completing the religation step.
• This leads to the accumulation of double-strand breaks and inhibition of DNA replication and transcription finally resulting in cell death.

Although these inhibitors are effective in targeting rapidly dividing cancer cells, they can also affect normal cells, leading to side effects. Researchers continue to explore novel Topoisomerase inhibitors with improved selectivity and reduced toxicity for enhanced cancer therapy.

Topoisomerase vs Helicase

Topoisomerase and helicase are important nucleases that are involved in uncoiling of the DNA helix however their functions are quite different. The differences between topoisomerase and helicase are listed below.

Difference Between Topoisomerase and Gyrase

Gyrase is a type of Topoisomerase but it has some typical characters that distinguishes it from other Topisomerases in the group.

Conclusion - Topoisomerase

In conclusion, topoisomerases play a role in maintaining the integrity and stability of DNA during various cellular processes. They relieve torsional stress, unwind and untangle DNA strands and facilitate DNA replication, transcription, and segregation. Understanding the different types and functions of topoisomerases provides insights into their therapeutic potential, particularly as targets for cancer treatment. Topoisomerase inhibitors, such as camptothecins and anthracyclines, are widely used in chemotherapy to disrupt DNA replication and induce cell death in rapidly dividing cancer cells. However, further research is needed to develop more selective and less toxic inhibitors for improved cancer therapy.

Also Read:

• Difference between Eukaryotic and Prokaryotic Replication
• Molecular Basis of Inheritance Notes Class 12
• Diagram of DNA Replication
• Diagram of DNA Structure
• What are the Three Main Parts of a Nucleotide?
• Difference Between Replication And Transcription