DNA Targeting Agents Flashcards
Replication of the Bacterial Chromosome
Genetic information in bacterial cell is stored in form of double stranded, covalently closed circle of DNA.
Replication begins at specific site called origin.
The origin replicated and then the DNA replication proceeds in two directions. The Two original strands serve as templates for the synthesis of new strands. This is called semi conservative replication.
Bacterial Topoisomerases: Type I, II and special topoisomerases.
Type 1 topoisomerases break single strands of duplex DNA, pass another sinle DNA strand through the break and then reseal the break.
Type 2 break both strands of duplex DNA, pass another duplex DNA strand through the break and reseals both breaks.
Specialised topoisomerases catalyze transposition or intergration/excision of bacteriophage DNA from bacterial chromosome.
Topoisomerase II (gyrase) and IV
Topoisomerases maintain cellular DNA in an appropriate state of supercoiling in both replicating and non-replicating regions of the chromosome
Type II topoisomerases (TOP) resolve topological problems in DNA. TOP II (gyrase) introduces negative supercoils. TOP IV decatanates circular chromosome.
All topoisomerases relax supercoiled DNA; only DNA gyrase can introduce negative supercoils (this is energy-dependant.) Gyrase removes excess +ve supercoiling which could inhibit DNA replication. After DNA replication, topoisomerase IV helps to separate daughter DNA molecules. DNA gyrase involved in regulation of gene transcription (sensitive to temperature, 02 and salt)
Supercoiling of the bacterial chromosome:
Relaxed circular DNA is circular and supercoiled DNA can be positive or negatiely supercoiled. Positive results from overwinding of the strand. Negative results from unwinding of the DNA then releasing the tension.
DNA catenation:
Catenation is the linkage of two circular DNA strands like chain links. This occurs after DNA replication.
This achieved after the action of DNA gyrase which has cut both strand of the DNA so that a second DNA duplex can be passed through the break. Then the DNA is resealed. This is to relieve strain on the DNA and promote disentanglement by decreasing the linking number by 2.
DNA gyrase:
4 subunits: 2 A monomers + 2 B monomers
It binds DNA and wraps the DNA around itself
DNA cleaved in both strands, forming a 4-base stagger.
One region of duplex DNA is passed through double-stranded break. Resealing the break introduces 2 new negative supercoils. The process is driven by ATP hydrolysis.
Topoisomerase IV:
2 ParC + 2 ParE gene products
Doesnt wrap DNA round itself, has superior decatenating ability & inferior relaxing ability. Its major role is decatenation of replicated daughter chromosomes. It can also unknot DNA. Mutation of ParC can result in accumulation of catenanes, which prevents chromosome separation.
Allows for the 2 new interlinked chromosomes to separate.
Quinolones Inhibiting DNA synthesis:
Quinolones target DNA gyrase and topoisomerase IV:
Bind to bacterial DNA gyrase and topoisomerase IV after strand breakage preventing resealing of DNA. Disrupt DNA replication + repair. Bacericidal.
Quinolones mode of action;
Quinolones self-assemble to form a supermolecule inside the DNA gyrase-induced DNA bubble.
Quinolones binds to the unpaired bases via H-bonds.
Quinolones trap DNA-gyrase complex after DNA cleavage at open-gate stage. This forms a quinolone-enzyme-DNA complex. This complex formation is reversible, gyrase can close the DNA gate if quinolone is washed out. Lethal concs of quinolones required to block DNA synthesis is less than that required to kill cells. Cell death could be due to DNA ends in the complexes being released from constraint by gyrase, creating equivalent of double-strand DNA breaks
Targets of quinolones:
DNA gyrase is main target in G -ves
Topoisomerase IV is main target in G +ves
Pharmacological properties of quinolones:
Good bioavailability and penetrates tissues well.
Excellent oral absorption- cirpofloxacin, ofloxacin & levofloxacin can be blocked by action of antacids.
Prolonged post-antiobiotic effect against most G-ve pathogens. Different quinolones eliminated by different routes.
spectrum of activity of quinolones:
G+ve aerobes: moderate activity against S. aureus, streptococci G -ve aerobes: excellent activity vs enterobacteriaceae (cipro is best), moderate activity vs P. aeruginosa (cipro best); all active vs N. meningitidis + H. influenzae. Trovafloxacin good against anaerobes such as B. fragilis. Used for gastroenteritis, travellers diarrhoea, UTIs, STDs osteomyelitis.
Adverse effects of quinolones:
GI upset in 5% Headache/dizziness Allergic reaction in 1-2% avoid if preggers Arthropathy
Metronidazole: Background MOA Pharmacology spectrum of activity Clinical use Adverse effects
Class of AA called 5-nitroimidazoles
synthetic
originally seen to be effective vs protozoa like Trichomonas & Giardia.
Used for serious anaerobic + polymicrobial infections.
Enters cell by passive diffusion, activated by reductive process. Short-lived reduction metabolites produced at the low redox values attainable by anerobes. MND is an efficient electron acceptor
Protonated nitro radical anions cause damage to bacterial DNA –> strand breakage –> cell death
Also effective vs nongrowing organisms.
Rapidly and almost completely absorbed orally; reaches all tissues and bodily fluids.
metabolised by liver into several compounds of which one had antibacterial activity. Drug and metabolites mainly excreted in urine.
administered orally, as topical gel/cream, or IV
Extremely active vs anaerobic G-ves. drug of choice vs Bacteriodes. fragilis.
C. perfringens + C. difficile are susceptible
Variabley active vs anaerobic G +ve cocci.
Other aerobic and facultative anaerobes generally resistant.
Used vs most anaerobic infections.
excellent penetration –> Brain, bone, joint, skin & soft tissue. Drug of choice vs C. difficile pseudomembranous colitis.
Combination therapy againt H. pylori with clarithromycin, amoxycillin & omeprazole
trochomoniasis, giardiasis
Generally well tolerated
Most serious effects involve CNS, but are rare.
Minor GI effects –> nausea, metallic taste, dry mouth
vaginal or urethral burning
5-nitrofurans
Nitrofurantoin: Use restricted to treatment of lower UTIs
Active vs most UT pathogens, proteus spp. and P. aeruginosa are resistant.
May work by the nitro groups being reduced intracellularly and may result in a reduction product reacting with DNA
