DNA Ligase/Topoisomerase Study

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1. A. E.coli DNA ligase can act as a DNA gyrase in the presence of high concentrations of AMP. Explain.
B. Would you expect the E.coli ligase/AMP combination to resemble type I or II topoisomerase, or neither. Justify.
C. Would you expect human DNA ligase to act as a topoisomerase in the presence of high concentrations of AMP?


I know that human ligase uses ATP where ecoli ligase uses NAD+. For this reason, I feel that human ligase would be similar to a type II topoisomerase since it also uses ATP. For question A, I don't understand how AMP could be involved, since it is normally forms NMP. My guess is that it simply can't use ATP, but if AMP is available, it can still use that as an energy source.
For question B, I would also expect a resemblance to a type II topoisomerase because of the AMP involvement.
For question C, I wouldn't expect human DNA ligase to act as a topoisomerase, since it is involved in filling in nucleotides, where topoisomerases break nucleotides.

Thanks for your help!
 

Answers and Replies

  • #2
epenguin
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OK you have to look at the mechanism of the ligase (in both directions). I actually had only ever heard or remembered the ATP one, but it goes through adenylylation of the protein and the AMP is then transferred to a 5'P of the DNA forming a pyrophposphate bond and then... . If a ligase uses NAD+ instead of ATP can you not imagine a mechanism with a lot in common?
 
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Are you referring to the mechanisms of topoisomerases and ligases being in common?
 
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epenguin
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I am not offhand very familiar with facts of the mechanisms of topoisomerases. I think the question can be answered just knowing the mechanism of ligase (which you can find e.g. on http://en.wikipedia.org/wiki/DNA_ligase) and thinking about the mechamism.

If the NMN is a problem for you, just think about enzymes using ATP and work out an answer for those.

In second place then a mechanism using NMN is fairly predictable and not very different.
 
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