Gyrase

Encyclopedia : G : GY : GYR : Gyrase

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DNA gyrase, often referred to simply as gyrase, is a type II topoisomerase (EC [5.99.1.3]) that introduces negative supercoils (or relaxes positive supercoils) into DNA by generating a loop in the template, cutting one of the strands and passing one over the other, changing the linking number by two in each enzymatic step

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 comes into 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 DNA polymerase. 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.

Mechanochemical Model of Gyrase Activity

A single molecule study Gore J, Bryant Z, Stone MD, Nollmann M, Cozzarelli NR, Bustamante C, [Analysis of DNA Gyrase Using Rotor Bead Tracking"], Nature 2006 Jan 5 (Vol. 439): 100-104. that has characterized gyrase activity as a function of DNA tension (applied force) and [ATP] has proposed the mechanochemical model shown in the 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 wrapping and ATP hydrolysis, two negative supercoils are introduced into the template, providing opportunities for subsequent wrapping and supercoiling events.

Inhibition by Antibiotics

Gyrase is only found in bacteria; it is not found in eukaryotes. This property makes gyrase a good target for antibiotics. Two classes of antibiotics that inhibit gyrase are the coumarins (including novobiocin), and the quinolones (including nalidixic acid and ciprofloxacin).

DNA gyrase is a member of a class of enzymes called topoisomerases. Quinolones bind these enzymes and prevent them from decatenating replicating DNA. Quinolone resistant bacteria frequently harbor mutated topoisomerases which resist quinolone binding.

See also

• Topoisomerase

 

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