DNA topoisomerases are highly exploited targets for antimicrobial drugs. mechanisms have been reported for all those known classes of antibiotics with some strains exhibiting multiple resistance phenotypes, which is a consequence of natural selection and human mismanagement . The danger that these strains pose is demonstrated by the increased mortality and morbidity rates for infected patients when compared to those infected with susceptible strains , . Unfortunately this increase in resistance has not been met with an increase in the development of new antibiotics, with the total number of new drugs being brought to market actually decreasing . Clearly there is an urgent need for the development of new antibiotics and management strategies. Extensive attempts to validate new target enzymes for antimicrobials have met with little success , with the majority of successful drugs inhibiting a handful of cellular processes. One of the most successfully exploited drug targets is the DNA topoisomerase (topo) class of enzymes C. DNA topoisomerases are essential and ubiquitous enzymes responsible for controlling the topological state of DNA . This is accomplished by the reaction of an active-site tyrosine with the phosphate backbone of the DNA to generate a covalent intermediate (the so-called cleavage complex), followed by either strand passage of another segment of DNA or free rotation of the broken strand C. DNA topoisomerases are classified as either type I or type II based on whether they cleave one or both strands of the LGX 818 supplier DNA , and further subdivided into IA, IB, IC, IIA or IIB based on structural and mechanistic differences . The essential nature of these enzymes and the vulnerability of the cleavage complex, which, if stabilised, rapidly results in cell death, make them ideal drug targets. The type IIA topoisomerases have been the most LGX 818 supplier exploited class, acting as targets for many anticancer and antibacterial drugs. DNA gyrase is usually a type IIA topoisomerase of particular importance due to it being a target for numerous antibacterial drugs and its distinct mechanism. All type IIA topoisomerases are capable of removing supercoils from DNA (relaxation) in an ATP-dependent manner ; gyrase introduces unfavorable supercoils into DNA RAB21 LGX 818 supplier in the presence of ATP, but relaxes DNA when ATP is usually absent . Whereas eukaryotic type IIA topoisomerases are dimeric in nature, gyrase forms a heterotetramer of two GyrB subunits, which contain the ATPase domains, and two GyrA subunits, which contain the active-site tyrosines . During the reaction cycle, the segment of DNA to be LGX 818 supplier cleaved (the gate or G segment) binds to the DNA-binding saddle in GyrA. ATP binding causes the GyrB subunits to dimerise and capture a second segment of DNA (the transported or T segment) . The G segment is then cleaved and the break pried open by conformational changes, allowing the T segment to pass through. The G segment can then be religated. The differences in mechanism and structure between gyrase and eukaryotic topoisomerases, as well as its low homology to human type IIA topoisomerases, have allowed the development of bactericidal drugs that target bacterial topoisomerases with a high degree of specificity. The mechanism of action for DNA gyrase inhibitors is usually highly varied, with different chemical families inhibiting different actions in the reaction cycle LGX 818 supplier . The most successful class of gyrase inhibitors is the billion-dollar quinolone family of drugs. Quinolones have the potent ability to stabilise the cleavage complex of DNA gyrase, resulting in double-strand breaks and cell death , . The exact mechanism by which this occurs remains unclear, but several crystal structures of quinolones bound to gyrase or its sister enzyme topo IV have been published C. These structures suggest that quinolones bind in pockets near the active-site tyrosines while simultaneously intercalating with the cleaved DNA, presumably distorting it in such a way as to prevent religation. In contrast, the aminocoumarin class of inhibitors target the ATPase activity of the enzyme in a competitive manner, binding in a pocket that overlaps with the ATP-binding site and sterically hindering.