INF1 - F. CHROMOSOME AND RIBOSOME INHIBITORS-COVERED Flashcards
what are stages of DNA replication
- origin: strand of DNA
- helicase: splits 2 DNA strands
- replication fork
- DNA polymerase: joins 2 strands through covalent bonds
differences between genome of bacteria and humans
- bacteria has single copies of one of more chromosomes (haploid)
humans have 2 or more (diploid) - bacteria has plasmids
humans don’t - bacteria have circular or linear dsDNA
humans have linear dsDNA in nucleus and chloroplasts and circular dsDNA in mitochondria and plasmids - bacteria has DNA in nucleoid of cytoplasm and in plasmids
humans have DNA in nucleus and mitochondria, chloroplasts and plasmids in cytosol - bacteria don’t have histones
humans have histones in nuclear chromosomes - bacteria genome is is smaller than humans
what is transcription
mRNA synthesis
what steps are involved in transcription
- initiation
- elongation of RNA transcript
- termination
what do RNA polymerases do
synthesise RNA by binding to promoters locater near the beginning of a gene and initiate transcription
*target for antibiotics as structure is different to eukaryotic RNA polymerase
what do topoisomerases do
unravels coil of DNA
humans have 6
bacteria have 4
how do quinolones (ie - fluoroquinolones - only used in UK) work
- selectively inhibit topoisomerases not found in mammalian cells
- broad spectrum and bactericidal against gram-negative bacteria (E. coli) and gram-positive cocci (P. aeruginosa)
- gram +ve exhibit higher rates of resistance than gram -ve
Ciprofloxacin
Moxifloxacin
Ofloxacin
Levofloxacin
what do quinolones treat
- UTI, GI, genital, resp tract infections
- Ciprofloxacin: only good oral anti-pseudomonal drug available
- increased risk of C. difficile infection and MRSA colonisation
- good for those that can’t take beta-lactams as similar activity
what is the mode of action of fluoroquinolones
- selectively inhibit topoisomerases 2 and 4 in bacterial cells
gram -ve: DNA girase (2) is key target
uncoils the supercoils and recoils for packaging
gram +ve: topoisomerase 4 is key target
removes links and knots generated behind replication fork
structure of DNA gyrase and topoisomerase 4
enzyme tetramers - 2 A and B subunits
A subunits - cut both DNA strands on one chain, other chain passes through break and resealed
B subunits - derive energy for action of A subunits from hydrolysis of ATP
Fluoroquinolones bind to A subunit, trapping the broken DNA in a complex with DNA gyrase
DNA gyrase can’t reseal the DNA, chromosomes fragmented, cell death
what does rifampicin treat
- bactericidal against Mycobacterium tuberculosis
- treatment and prophylaxis
- well absorbed orally
- good penetration into bone, CNS, chest
- used in combination as resistance emerges rapidly if used as mono therapy
what is the mode of action of rifampicin
- acts upon B-subunit of bacterial RNA polymerase
- inhibition of initiation stage of RNA transcription as prevents first nucleotide being incorporated into RNA chain
- rifampicin resembles 2 adenosine nucleotides in RNA which form the basis of binding to B-subunit
- doesn’t inhibit transcription once initiated
how do nitroimidazoles and nitrofurans work
- causes strand breakage of DNA by a direct chemical method not through inhibiting an enzyme
what is the mode of action of metronidazole
- nitroimidazole antibiotic
- broad-sprectrum against aerobic protozoa, bacteria, helminths
- pro-drug: active after reduction of nitro group
- non-enzymatic reduction by reacting with reduced ferredoxin
- metabolites are unstable and react with DNA causing irreversible damage
- alternative to penicillin for mouth infections in combination (amoxicillin or clarithromycin plus omeprazole) with debridement
what is the mode of action of nitrofurantoin
- nitrofuran antibiotic
- active against range of gram+ve and gram-ve organisms (esp. E. coli)
- forms reactive intermediates by reduction which react with DNA causing strand breakage
- most active in acidic urine
- first line for UTIs
differences in protein synthesis in bacteria and eukaryotes
- 70S ribosome in bacteria, 80S in eukaryotes
- active uptake of antibiotics into bacterial cell, limited penetration in mammalian cells
why does resistance occur to these antibiotics which affect protein synthesis
- alteration/protection of ribosome from antibiotic
- active uptake and efflux of antibiotic issues
where is the site of protein synthesis
ribosomes
what is the structure of ribosomes
eukaryotes - 80S
prokaryotes - 70S
- rRNA and ribosomal proteins
- rRNAs fold an interact to give the ribosome 3D structure
- proteins located more peripherally
- translation works from 5’ to 3’ end
what is the A site on ribosomes
- binds incoming tRNA carrying its amino acid (aminoacyl tRNA site)
- but the first tRNA binds directly to P site where codon AUG is and then the rest start at A site
what is the P site on ribosomes
- peptide bond is formed between the amino acid linked to the tRNA in P site and amino acid linked to tRNA in A site
- catalysed by peptidyl transferase (ribozyme in large subunit)
what is the E site on ribosomes
- where the tRNA moves and is ejected from ribosome having delivered its amino acid and added it to growing polypeptide
what are tetracyclines
- 4 cyclic rings
- broad-spectrum
- bind to 30S ribosomal subunit and block tRNA binding at A site, protein synthesis stops
- bacteriostatic
what are tetracyclines used for
- chlamydia
- acne
- resp infections
- rickettsia
