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CASE FILES: BIOCHEMISTRY
covalent structure. There are two classes of topoisomerases. Type I topoiso-
merases relax DNA from negative supercoils formed by the action of type II
topoisomerase by creating transient single-strand breaks in DNA without any
expense of ATP. Type II topoisomerases (also called DNA gyrases) change
DNA topology by making transient double-strand breaks in DNA and
require ATP consumption (Figure 10-3).
During DNA replication, type II topoisomerase, or TOPO II, plays an
important role in the fork progression by continuous removal of the excessive
positive supercoils that stem from the unwinding of the DNA strands. TOPO II
has the ability to cut both strands of a double-stranded DNA molecule, pass
another portion of the duplex through the cut, and reseal the cut in a process
that uses ATP. Hydrolysis of ATP by TOPO IIs inherent ATPase activity pow-
ers the conformational changes that are critical for the enzyme’s operation.
Based on the DNA substrate, TOPO II can change a positive supercoil into a
negative supercoil or increase the number of negative supercoils by two.
The DNA gyrase of
E. coli
is composed of two 105,000-dalton A subunits
and two 95,000-dalton B subunits encoded by
gyrA
and
gyrB
genes, respec-
tively. The A subunits, which carry out the strand-cutting function of the
gyrase, are the site of action of the quinolones. DNA gyrase inhibition dis-
rupts DNA replication and repair, transcription, bacterial chromosome
separation during division, and other cell processes involving DNA. The
drugs inhibit gyrase-mediated DNA supercoiling at similar concentrations
that are required to inhibit bacterial growth (0.1 to 10 pg/mL). Mutations in
gyrA
gene that encodes the A subunit of the polypeptide can confer resistance
to these drugs. Eukaryotic cells lack DNA gyrase but have a similar type of
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