Topoisomerases bacterial

Gyrase is another term for bacterial topoisomerase II. The enzyme consists of two A and two B subunits and is responsible for the negative supercoiling of the bacterial DNA. Negative supercoiling makes the bacterial DNA more compact and also more readily accessible to enzymes that cause duplication and transcription of the DNA to RNA. 

Quinolones. Table 1 Quinolones - Classification of bacterial topoisomerases... 

Bacterial as well as eukaryotic chromosomes contain too much DNA to fit easily into a cell. Therefore, the DNA must be condensed (compacted) to fit into the cell or nucleus. This is accomplished by supercoiling the DNA into a highly condensed form. When relaxed circular DNA is twisted in the direction that the helix turns, the DNA becomes positively supercoiled, if it is twisted in the opposite direction, it is called negatively supercoiled. Bacterial DNA is normally found in a negatively supercoiled state. Supercoiling reactions are catalyzed by topoisomerases. 

II cleaves the two complementary strands of DNA four base pairs apart and the resulting 5 -phosphoryl groups become covalently linked to a pair of tyrosine groups, one in each half of the dimeric topoisomerase II enzyme. Several groups of drugs are known that selectively inhibit topoisomerases in bacteria (quino-lones) or mammalian cells (etoposide, tenoposide). Quinolones are used to treat bacterial infections inhibitors of mammalian topoisomerases are cytostatic drugs used for the treatment of cancer. 

Conserved catalytic (adenylation) domain in NAD-dependent (bacterial) and ATP-dependent (archaeal-eukaryotic) DNA ligases (Aravind and Koonin, 1999) Conserved nucleotide joining-cleaving domain in type I and II topoisomerases, DnaG-type primases, OLD nucleases, and RecR (Toprim domain) (Aravind et al, 1998b)... 

The quinolones are good general antibiotics for systemic infections, and they are particularly useful for urinary tract infections because high concentrations are excreted into the urine. The mode of action involves interference with DNA replication by inhibiting DNA gyrase, a bacterial enzyme related to mammalian topoisomerases that breaks and reseals double-stranded DNA during replication. 

An extensive database has demonstrated that many chemicals that are positive in this test also exhibit mutagenic activity in other tests. There are, however, examples of mutagenic substances, which are not detected by this test reasons for these shortcomings can be ascribed to the specific nature of the endpoint detected, differences in metabolic activation, or differences in bioavailability. On the other hand, factors which enhance the sensitivity of the bacterial reverse mutation test can lead to an overestimation of mutagenic activity. The bacterial reverse mutation test may not be appropriate for the evaluation of certain classes of chemicals for example, highly bactericidal compounds (e.g., certain antibiotics) and those which are thought (or known) to interfere specifically with the mammalian cell replication system (e.g., some topoisomerase inhibitors and some nucleoside analogues). In such cases, mammalian mutation tests may be more appropriate. 

Substances known as intercalators, such as rifamycin and actinomycin D (bottom) are deposited in the DNA double helix and thereby interfere with replication and transcription (B). As DNA is the same in all cells, intercalating antibiotics are also toxic for eukaryotes, however. They are therefore only used as cytostatic agents (see p. 402). Synthetic inhibitors of DNA topoisomerase II (see p. 240), known as gyrase inhibitors (center), restrict replication and thus bacterial reproduction. 

Pharmacology The fluoroquinolones are synthetic, broad-spectrum antibacterial agents that inhibit DNA gyrase and topoisomerase IV. DNA gyrase is an essential enzyme that is involved in the replication, transcription, and repair of bacterial DNA. Topoisomerase IV is an enzyme known to play a key role in the partitioning of the chromosomal DNA during bacterial cell division. 

Bacterial topoisomerases (called DNA gyrases) are inhibited by several Important classes of antibiotics, including the coumarins, such as novobiocin quinolones, such as nalidixic acid and fluoroquinolones, such as ciprofloxacin. 

The quinolones are inhibitors of bacterial DNA gyrase (topoisomerase II) and topoisome-rase IV. These two enzymes are essential for the uncoiling of DNA. More specifically, DNA gyrase exists as an A2B2 tetramer encoded by gyrA and gyrB genes, and... 

Fluoroquinolones are inhibitors of bacterial topoisomerase 11 (DNA gyrase). This enzyme reduces the supercoiling of DNA in order to allow separation of the two strands of DNA that are required for replication and transcription. In the first stage, topoisomerase II cleaves the two strands of DNA, after repairing it, and restores the supercoiling of DNA. ... 

Mecfianism of Action A fluoroquinolone that inhibits two enzymes, topoisomerase II and IV, in susceptible microorganisms. Therapeutic Effect Interferes with bacterial DNA replication. Prevents or delays resistance emergence. Bactericidal. Pharmacokinetics Well absorbed from the GI tract after PO administration. Protein binding 20%. Widely distributed. Metabolized in liver. Primarily excreted in urine. Half-life 7-14 hr. 

MECHANISM FIGURE 24-21 Bacterial type I topoisomerases alter linking number. A proposed reaction sequence for the bacterial topoi-somerase I is illustrated. The enzyme has closed and open conformations. (a) A DNA molecule binds to the closed conformation and one... 

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