Type II topoisomerases

I topoisomerase of mammals is a 100 kD monomeric protein whose activity is ATP-independent. This enzyme binds to double-stranded DNA and cleaves one of the DNA strands of the duplex, simultaneously forming an enzyme-DNA covalent bond between a tyrosine residue and the 3 -phosphate of the cleaved DNA. The type II topoisomerases are dimeric enzymes, which are ATP-dependant. Two isoforms of topoisomerase II exist, topoisomerase a and (3, with apparent molecular weights of 170 and 180 kD. Topoisomerase... 

Goni-Urriza M, Arpin C, Capdepuy M et al (2002) Type II topoisomerase quinolone resistance-determining regions otAeromonas caviae,A. hydrophila, and A. sobria complexes and mutations associated with quinolone resistance. Antimicrob Agents Chemother 46(2) 350-359... 

A. Maxwell, D. M. Lawson (2003). The ATP-binding site of type II topoisomerases as a target for antibacterial drugs. Curr. Top. Med. Chem. 3 283-303. 

Eukaryotic cells also have type I and type II topoisomerases. The type I enzymes are topoisomerases I and III the type II enzymes are topoisomerases Ha and II/3. The eukaryotic type II topoisomerases cannot underwind DNA (introduce negative supercoils), but they can relax both positive and negative supercoils. We consider one probable origin of negative supercoils in eukaryotic cells in our discussion of chromatin in Section 24.3. The process catalyzed by eukaryotic type II topoisomerases is illustrated in Figure 24-22. 

So, when either replication fork encounters a functional Tus-Ter complex, it halts the other fork halts when it meets the first (arrested) fork. The final few hundred base pairs of DNA between these large protein complexes are then replicated (by an as yet unknown mechanism), completing two topologically interlinked (catenated) circular chromosomes (Fig. 25-17b). DNA circles linked in this way are known as catenanes. Separation of the catenated circles in E. coli requires topoi-somerase IV (a type II topoisomerase). The separated chromosomes then segregate into daughter cells at cell... 

As the two strands of the double helix are separated, positive supercoils are produced in the region of DNA ahead of the replication fork. These interfere with further unwinding of the double helix. DNA topoisomerases Types I and II remove supercoils. Human topoiso-merase II is targeted by anticancer agents, such as etoposide, and DNA gyrase (a Type II topoisomerase found in E. coli that can introduce negative supercoils) is targeted by the antimicrobial quinolones. 

Type II topoisomerases are essential and function in replication, DNA repair, transcription, and chromosome segregation at mitosis.345,349 Yeast with a top2 mutation dies during mitosis with hopelessly entangled daughter chromosomes.353 A fluorescent antibody to eukaryotic topoisomerase II binds to chromosomes, probably at the bases of the radial loops... 

Gyrase gyrA Type II topoisomerase replication swivel, DNA... 

Type I and type II topoisomerases relax negatively supercoiled DNA in steps of one and steps of two, respectively. Type II topoisomerases can also add additional negative supercoils (as indicated by the double arrow). The latter reaction requires energy input, which is encoded by ATP cleavage. 

Catenation by topoisomerases. (a) Two circular DNAs can be catenated by type I topoisomerase only if one of the DNAs is nicked. This is not necessary when using a type II topoisomerase. (b)... 

Gyrase Type II topoisomerase that catalyzes conversion of relaxed DNA to a superhelical form requires ATP ... 

Topoisomerases are fundamental enzymes that ensure DNA replication, transcription, or recombination [66]. Both type I and type II topoisomerases act by changing... 

Fig. 11.1 Examples of initial crystallization hits from a single on-chip screening experiment of a type II topoisomerase ATPase do-main/ADP, 12mgmL b (A) Irregular spherulite. (B) Phase separation and spherulites with nucleating microcrystals. (C) Thin plate clusters. (D) Thick plate stacks. (E) Well-formed microcrystals. (F) Large single crystals. All scale bars are 100 pm.

Type I topoisomerases catalyze the relaxation of supercoiled DNA, a thermodynamically favorable process. Type II topoisomerases utilize free energy from ATP hydrolysis to add negative supercoils to DNA. The two types of enzymes have several common features, including the use of key tyrosine residues to form covalent links to the polynucleotide backbone that is transiently broken. 

Type II Topoisomerases Can Introduce Negative Supercoils Through Coupling to ATP Hydrolysis... 

Supercoiling is catalyzed by type II topoisomerases. These elegant molecular machines couple the binding and hydrolysis of ATP to the directed passage of one DNA double helix through another that has been temporarily cleaved. These enzymes have several mechanistic features in common with the type I topoisomerases. 

Fig. 3. The reactions of type II topoisomerases. The figure shows the major DNA transformations that can be carried out by type II topoisomerases.

Of the type II topoisomerases, the best studied with respect to the formation of a protein-DNA complex is DNA gyrase. The properties of the gyrase-DNA complex have been examined by many methods, including nuclease protection, filter binding, sedimentation, and electron microscopy. 

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