Topoisomerase I

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Content

 

1.Function

     -How it works

     -Topology

     -Effect on Supercoiling

     -Prokaryotic vs. Eukaryotic

2.Classification

     -Type 1A

     -Type 1B

     -Type 1C

3.Structure

     -Human Topoisomerase I

4.Detailed Mechanism

5.Inhibition

     -Irinotecan

     -Topotecan

     -Camptothecin

6.Effect of Poisons

     -Terbenzimidazole

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Function

     Topoisomerase I is an enzyme that operates by binding to a single strand of DNA in order to relax the DNA (by removing supercoils).  Topoisomerases allow the cell to manage the topology of the DNA by maintaining supercoiling.  The removal of supercoils allows the strand to relax making the DNA more energetically favorable (NCBI).  Topo I functions by nicking one strand of DNA, passing the strand through the break, and then resealing the break.  The action of topoisomerase I changes the linking number by decreasing the number of twists involved in the overall structure of the strand.  The removal of twists cause the DNA to improve its ability to perform transcription and replication (Transcription).  These are two of the few times that DNA wants to get away from its coiled structure.  The mechanism of this event allows the DNA to increase its ability to bind to the holoenzyme therefore increasing transcription.  

 

 

 

This video demonstrates the concept of supercoiling and the effect of topoisomerases on the linking number.

 

Prokaryotic vs. Eukaryotic

     Topoisomerase interacts differently with DNA depending on if it is prokaryotic or eukaryotic.  Eukaryotic topoisomerase I removes both positive and negative supercoils, while prokaryotic topoisomerase I only removes negative supercoils.   Topoisomerase is unique in the fact that it does not require the use of ATP to form a complex with the DNA strand; this is unlike topoisomerase II which requires energy to form a complex (Role of Topoisomerase).

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 Subclasses

      Topoisomerase I can be divided into three separate subclasses:  type IA, type IB, and type IC.  These classes differ in their interaction with the DNA strand as well as their individual effects on the linking number.  Type IA forms a covalent intermediate with the 5' end of the DNA, whereas Type IB binds with the 3'end of the DNA (Classes).  Type IC has just recently been identified, and its mechanism is believed to be similar to Type IB (also referred to as Topo V).  Type IA changes the linking number by one, while both Types IB and IC are able to change the linking number by a multiple of one with each nick of the strand (Classes).

 

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Structure

 

Topoisomerase has been identified as a monomer with four separate subunits.  The main parts of the structure are the N-terminal and the C-terminal.  These two terminals are responsible for attaching to the major and minor grooves, respectively(Sloan-Kettering).  Structure of Human Topoisomerase I (shown above)  (www.Proteopedia.org)

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Mechanism:

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

As shown above, topoisomerase I interacts with a single-strand of DNA by forming a complex between tyrosine and a phosphate on the DNA strand (Interaction).  This is referred to as transesterification.

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Inhibitors

     Inhibition of topoisomerase I can be caused by Irinotecan, Topotecan, or Camptothecin.  Each of these inhibitors are used to treat certain types of cancer.  The treatment keeps topoisomerase from interacting with the DNA strand, which inevitability will lead to cell death due to a lack of unwinding enzymes.  These inhibitors function by interfering with the cleaving and rejoining of the DNA strand by the action of topoisomerases (Interference). 

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Poisons

     Terbenzimidazole is a type of ligand that has been shown to poison topoisomerase I and cause cancer cell death.  It functions by altering the minor groove of the DNA structure causing abnormalities in the binding of topoisomerase I to the DNA strand.  This has been shown to have a damaging effect on the removal of supercoils by topoisomerases (JSTOR).

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References

"Information on Topoisomerase 1"

http://en.wikipedia.org/wiki/Topoisomerase_1

http://www.ncbi.nlm.nih.gov/books/bv.fcgi?highlight=Topoisomerases,Role,DNA%20Replication&rid=mcb.section.3219

http://www.ncbi.nlm.nih.gov/books/bv.fcgi?highlight=27.3&rid=stryer.section.378

http://www.jstor.org.nuncio.cofc.edu/stable/info/43773?seq=1

http://theoncologist.alphamedpress.org/cgi/reprint/2/6/359

http://www.springerlink.com/content/n308p158u567645j/

http://www.callutheran.edu/Academic_Programs/Departments/BioDev/omm/topo1/topo1.htm

http://www.mskcc.org/mskcc/html/10424.cfm

http://www.chemocare.com/whatis/types_of_chemotherapy.asp

 

Image references:

http://proteopedia.org/wiki/index.php/1ej9

http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=stryer.figgrp.3790