A gyrase ( v. AltGr. Γῦρος, gyros = circle, rounding ) is an enzyme that alters the spatial orientation of closed DNA molecules. It belongs to the type II topoisomerases, which create a double-strand break in the DNA - dependent ATP in prokaryotic cells. The gyrase provides negative supercoiling - a " unwinding " of the DNA in contrast to the normal structure (B- DNA) with about ten base pairs per turn.
As gyrase = topoisomerase II in this form occurs ( this protein structure ) only in bacteria, gyrase inhibitors can also be used as antibiotics. The affinity of the gyrase inhibitors of bacterial gyrase to higher than to human topoisomerases. However, the mechanism of action is not 100% selective and as gyrase inhibitors have cytostatic properties.
The binding of ATP to gyrase is blocked by the antibiotic novobiocin.