Dna girasa

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DNA gyrase
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The current mechanochemical model of Gyrase activity.
DNA gyrase, often referred tosimply as gyrase, is an enzyme that unwinds double-stranded DNA. This causes supercoiling of the DNA. Many antibiotics work by attacking bacterial DNA gyrase.
DNA gyrase is a type II topoisomerase (EC5.99.1.3) that introduces negative supercoils (or relaxes positive supercoils) into DNA by looping the template so as to form a crossing, then cutting one of the double helices and passing the otherthrough it before releasing the break, changing the linking number by two in each enzymatic step. This process occurs in prokaryotes (in particular, in bacteria), whose single circular DNA is cut by DNAgyrase and the two ends are then twisted around each other to form supercoils. Very recently, gyrase has been reported from the apicoplast of malarial parasite Plasmodium falciparum (unicellulareukaryote).
The unique ability of gyrase to introduce negative supercoils into DNA is what allows bacterial DNA to have free negative supercoils. The ability of gyrase to relax positive supercoils comesinto play during DNA replication. The right-handed nature of the DNA double helix causes positive supercoils to accumulate ahead of a translocating enzyme, in the case of DNA replication, a DNApolymerase. The ability of gyrase (and topoisomerase IV) to relax positive supercoils allows superhelical tension ahead of the polymerase to be released so that replication can continue.
[edit]Mechanochemical model of gyrase activity
A single molecule study[1] that has characterized gyrase activity as a function of DNA tension (applied force) and ATP has proposed the mechanochemical model shown inthe figure. Upon binding to DNA (the "Gyrase-DNA" state), there is a competition between DNA wrapping and dissociation, where increasing DNA tension increases the probability of dissociation. Upon...
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