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
why is there resistance to tetracyclines
- failure of active uptake system
- ribosomal protection/modification
efflux mechanisms
what are aminoglycoside-aminocyclitol antibiotics
- bactericidal
- take up by active transport under aerobic conditions
- active against some gram+ve and many gram-ve
- used in combo with beta-lactams
- interfere with 30S subunits and block initiating factors and tRNA binding
- causes mRNA codon to be misread
- injection for sys infs as poorly absorbed from gut
why does resistance to aminoglycosides occur
- inactivation by amino glycoside-modifying enzymes
- ribosomal alterations
- loss of permeability
- efflux pumps
what is streptomycin used for
M. tuberculosis
*ototoxicity causing deafness is a side effect
what is gentamicin used for
Septicaemia and endocarditis
what are macrolides
- similar to penicillins so used as an alternative
- clarithromycin, azithromycin, clindamycin, telithromycin, erythromycin etc
- GI side effects
what are macrolides used for
- resp, skin, soft tissue infections
- Helicobacter pylori
what is the mode of action of macrolides
- inhibit protein synthesis by binding to 50S subunit, blocking translocation by preventing movement of peptide-tRNA from A to P site
what bacteria does macrolides affect
- gram +ve
- gram -ve intrinsically resistant due to permeability of OM
why is there resistance to macrolides
- target modification - adenine methylation in 50s subunit
- ATP-binding cassette efflux pump
- ribosomal mutation
what is chloramphenicol
- broad-spectrum
- gram -ve and gram +ve bacteria, mycoplasmas, rickettsia spp. and chlamydia spp.
- inhibit protein synthesis by binding to 50S subunit at A site and inhibiting peptidyl transferase and inhibits peptide bond formation
- mammalian subunit sensitive to chloramphenicol: inhibition can occur in rapidly growing cells (aplastic anaemia)
- use is limited to serious infections
- safe for ocular use - bacterial conjunctivitis
why is there resistance to chloramphenicol
- enzymatic modification
- reduced uptake
- target modification of 50S subunit
what is used to treat conjunctivitis
chloramphenicol drops or ointment
what is used to treat otitis media
macrolides
what is used to treat sinusitis (inflam of nasal sinuses)
macrolides and tetracyclines
