Topoisomerase

Topoisomerases are enzymes that participate in the overwinding or underwinding of DNA. The winding problem of DNA arises due to the intertwined nature of its double-helical structure. During DNA replication and transcription, DNA becomes overwound ahead of a replication fork. If left unabated, this torsion would eventually stop the ability of DNA or RNA polymerases involved in these processes to continue down the DNA strand.

In order to prevent and correct these types of topological problems caused by the double helix, topoisomerases bind to double-stranded DNA and cut the phosphate backbone of either one or both the DNA strands. This intermediate break allows the DNA to be untangled or unwound, and, at the end of these processes, the DNA backbone is resealed again. Since the overall chemical composition and connectivity of the DNA do not change, the tangled and untangled DNAs are chemical isomers, differing only in their global topology, thus the name for these enzymes. Topoisomerases are isomerase enzymes that act on the topology of DNA.[1]

Bacterial topoisomerase and human topoisomerase proceed via the same mechanism for replication and transcription.

Discovery

James C. Wang was the first to discover a topoisomerase when he identified E. coli topoisomerase I. Topo EC-codes are as follows: type I, EC 5.99.1.2; type II: EC 5.99.1.3. His discovery was made in the 1970s.

Function

The double-helical configuration that DNA strands naturally reside, makes them difficult to separate and yet they must be separated by helicase enzymes, if other enzymes are to transcribe the sequences that encode proteins, or if chromosomes are to be replicated. In so-called circular DNA, in which double-helical DNA is bent around and joined in a circle, the two strands are topologically linked, or knotted. Otherwise identical loops of DNA, having different numbers of twists, are topoisomers, and cannot be interconverted by any process that does not involve the breaking of DNA strands. Topoisomerases catalyze and guide the unknotting or unkinking of DNA[2] by creating transient breaks in the DNA using a conserved tyrosine as the catalytic residue.[1]

The insertion of (viral) DNA into chromosomes and other forms of recombination can also require the action of topoisomerases.

Clinical significance

Many drugs operate through interference with the topoisomerases . The broad-spectrum fluoroquinolone antibiotics act by disrupting the function of bacterial type II topoisomerases. These small molecule inhibitors act as efficient anti-bacterial agents by hijacking the natural ability of topoisomerase to create breaks in chromosomal DNA.

Some chemotherapy drugs called topoisomerase inhibitors work by interfering with mammalian-type eukaryotic topoisomerases in cancer cells. This induces breaks in the DNA that ultimately lead to programmed cell death (apoptosis). This DNA-damaging effect, outside of its potential curative properties, may lead to secondary neoplasms in the patient.

Topoisomerase I is the antigen recognized by Anti Scl-70 antibodies in scleroderma.

Topological problems

There are three main types of topology:

Outside of the essential processes of replication or transcription, DNA must be kept as compact as possible, and these three states help this cause. However, when transcription or replication occurs, DNA must be free, and these states seriously hinder the processes. In addition, during replication, the newly replicated duplex of DNA and the original duplex of DNA become intertwined and must be completely separated in order to ensure genomic integrity as a cell divides. As a transcription bubble proceeds, DNA ahead of the transcription fork becomes overwound, or positively supercoiled, while DNA behind the transcription bubble becomes underwound, or negatively supercoiled. As replication occurs, DNA ahead of the replication bubble becomes positively supercoiled, while DNA behind the replication fork becomes entangled forming precatenanes. One of the most essential topological problems occurs at the very end of replication, when daughter chromosomes must be fully disentangled before mitosis occurs. Topoisomerase IIA plays an essential role in resolving these topological problems.

Classes

Topoisomerases can fix these topological problems and are separated into two types depending on the number of strands cut in one round of action:[3] Both these classes of enzyme utilize a conserved tyrosine. However these enzymes are structurally and mechanistically different. For a video of this process click here.

Topoisomerase[5] Subfamily Type Function Multimericity Metal Dependence ATP Dependence Single-or Double-Stranded Cleavage? Cleavage Polarity Change In Link Number (L)
Topoisomerase I

(E. coli)

 Type IA Removes (-), but not (+) supercoils Monomer Yes (Mg2+) No SS 5' ±1
Topoisomerase III

(E. coli)

 Removes (-), but not (+) supercoils; Overlaping function with Topoisomerase IV
Topoisomerase IIIα

(H. sapiens)

 Removes (-), but not (+) supercoils; Assists in the unlinking of precatenanes in cellular DNA replication; Can catalyze the knotting, unknotting, and interlinking of single-stranded circles as well as the knotting, unknotting, catenation, and decatenation of gapped or nicked duplex DNA circles.
Topoisomerase IIIβ

(H. sapiens)

 Unknown function
Reverse DNA Gyrase

(E. coli)

 Removes (-), but not (+) supercoils Heterodimer
Reverse DNA Gyrase

(Archaea)

 Removes (-), but not (+) supercoils
Topoisomerase I

(H. sapiens)

 Type IB Remove (+) and (-) supercoils; Relaxes compensatory (-) supercoils; Generates right-handed solenoidal supercoils; Supports fork movement during replication; Thought to be similar in structure to tyrosine recombinases. Monomer No No SS 3' ±1
Topoisomerase V

(Archaea)[6]

 Type IC Relaxes (+) and (-) supercoils. Involved in DNA repair Monomer No No SS 3' ±1
Topoisomerase II / DNA Gyrase

(E. coli)

 Type IIA Generates (-) supercoils (the only topoisomerase known to do this) Heterotetramer Yes (Mg2+) Yes DS 5' ±2
Topoisomerase IV

(E. coli)

 Relaxes (-) supercoils; Role in decatenation Heterotetramer
Topoisomerase IIα

(H. sapiens)

 Essential; Unlinks interwined daughter duplexes in replication; Contributes to DNA relaxation during transcription Heterodimer
Topoisomerase IIβ

(H. sapiens)

 Role in suppressing recombination or supporting transcription in neurons Heterodimer
Topoisomerase VI

(Archaea)

 Type IIB Relaxes (+) and (-) supercoils; Responsible for decatenating replication intermediates; May be exclusive to the archaea. Heterotetramer Yes (Mg2+) Yes DS 5' ±2

Both type I and type II topoisomerases change the linking number (L) of DNA. Type IA topoisomerases change the linking number by one, type IB and type IC topoisomerases change the linking number by any integer, whereas type IIA and type IIB topoisomerases change the linking number by two.

See also

References

  1. 1 2 Champoux JJ (2001). "DNA topoisomerases: structure, function, and mechanism". Annu. Rev. Biochem. 70: 369–413. doi:10.1146/annurev.biochem.70.1.369. PMID 11395412.
  2. ↑ C.Michael Hogan. 2010. Deoxyribonucleic acid. Encyclopedia of Earth. National Council for Science and the Environment. eds. S.Draggan and C.Cleveland. Washington DC
  3. ↑ Wang JC (April 1991). "DNA topoisomerases: why so many?". J. Biol. Chem. 266 (11): 6659–62. PMID 1849888.
  4. ↑ "Structural studies of type I topoisomerases". Nucleic Acids Researcj. 37(3): 693–701. 2009.
  5. ↑ Champoux, James (2001). "DNA Topoisomerases: Structure, Function, and Mechanism". Annual Review of Biochemistry. 70: 369-413.
  6. ↑ "Structural studies of type I topoisomerases". Nucleic Acids Research. 37(3): 693–701. 2009.

Further reading

External links

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