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Type II topoisomerase
Type II topoisomerases cut both strands of the DNA helix simultaneously in order to manage DNA tangles and supercoils. They use the hydrolysis of ATP, unlike Type I topoisomerase. In this process, these enzymes change the linking number of circular DNA by ±2.
==Function==
Once cut, the ends of the DNA are separated, and a second DNA duplex is passed through the break. Following passage, the cut DNA is religated. This reaction allows type II topoisomerases to increase or decrease the linking number of a DNA loop by 2 units, and it promotes chromosome disentanglement. Reactions involving the increase in supercoiling require two molecules of ATP. For example, DNA gyrase, a type II topoisomerase observed in ''E. coli'' and most other prokaryotes, introduces negative supercoils and decreases the linking number by 2. Gyrase is also able to remove knots from the bacterial chromosome. Along with gyrase, most prokaryotes also contain a second type IIA topoisomerase, termed topoisomerase IV. Gyrase and topoisomerase IV differ by their C-terminal domains, which is believed to dictate substrate specificity and functionality for these two enzymes. Footprinting indicates that gyrase, which forms a 140-base-pair footprint and wraps DNA, introduces negative supercoils, while topoisomerase IV, which forms a 28-base-pair footprint, does not wrap DNA.
Eukaryotic type II topoisomerase cannot introduce supercoils; it can only relax them.
The roles of type IIB topoisomerases are less understood. Unlike type IIA topoisomerases, type IIB topoisomerases cannot simplify DNA topology (see below), but they share several structural features with type IIA topoisomerases.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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