Transcript Inhibitors of Protein Synthesis

Inhibitors of Protein Synthesis
• Bacterial cells are 50% protein by dry weight
– Inhibition of protein synthesis leads to cessation of
growth or cell death
– Bacterial 70S ribosomes differ sufficiently from
eukaryotic ribosomes to allow selective toxicity
• Other properties of the antibiotics still produce
side effects.
• Ribosomes are cytoplasmic
– Drug must successfully enter (and stay) in the
cytoplasm and exert effect.
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The antibiotics
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• Families
– Aminoglycosides, macrolides, tetracyclines, etc.
• Individual drugs
– Chloramphenicol, clindamycin, mupirocin
• Variety of structures
– Mostly multi-ring nuclei with side chains
• Vary in effectiveness against different bacteria
– Variation not in site of attack but in successful entry
into cells and variations in bacterial defense
Review of Protein synthesis
• 30S subunit, Initiation factors, and mRNA come
together.
• fMet-tRNA binds to mRNA
• 50S subunit binds to form initiation complex
• 2nd aminoacyl tRNA arrives at A site
• Amino acid transferred from 1st tRNA to aa of new
tRNA (peptide bond formed)
• Uncharged tRNA moves to E site, then leaves
• Translocation of ribosome puts tRNA with growing
chain in P site; elongation continues.
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4
www.emunix.emich.edu/.../ genetics/transl4.htm
Polysomes
http://bass.bio.uci.edu/~hudel/bs99a/lecture21/polysome.gif
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Aminoglycosides
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As name implies, molecules
comprised of amino sugars.
Includes streptomycin,
gentamycin, kanamycin, etc.
Highly polar molecules, do
not distribute well into body
compartments.
Administration iv and im only
Narrow therapeutic index.
Streptomycin
http://www.bmb.leeds.ac.uk/mbiology/u
g/ugteach/icu8/images/antibiotics/genta
micin.gif
Mechanism of action for aminoglycosides
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• Bactericidal, rare among protein synthesis
inhibitors
– Transport through the wall, through PG of G+,
through porins or through OM directly in G-,
disrupting OM.
– Transport though cell membrane by carrier, using
electrochemical gradient (uses energy).
– Binds to ribosomes, keeping [free drug] low
– Combination of membrane damage and inhibition of
protein synthesis is bactericidal
Aminoglycosides cont.
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• Aminoglycosides bind various sites on both
ribosomal subunits
– Freeze translation after initiation step, prevent
polysome formation
– Interfere with codon recognition, resulting in
misreading
• Sometimes “cured” by ribosomal protein mutation
• Concentration dependent killing and postantibiotic effect
– Related to membrane damage and ribosome
binding
Aminoglycoside resistance
• Altered ribosomes
– Mutations first observed with streptomycin
• Inadequate transport of drug
– Seen mostly in strict anaerobes
• Enzymatic modification of drug
– Acetylation, adenylation, or phosphorylation
– Decrease transport and ribosome binding
– Info on plasmids and transposons (e.g. Tn5)
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Tetracylcines
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Note “tetra”
Various family members
created by modifying the left 3
rings: chlortetracycline,
oxytetracycline, doxycycline…
animal feed, aquaculture
http://www.sp.uconn.edu/~terry/image
s/micro/tetracycline.gif
Vary in lipid solubility, but enter cell by membrane
carrier; through OM of G- via porins.
Bind to ribosomes, block binding of next aa-tRNA,
preventing further protein synthesis.
Resistance
• Influx/efflux
– Mutations in OM proteins retards entry
– More significant, plasmid encoded protein
responsible for efflux of drug: pumped out
• Change in ribosome binding site
– Plasmid encoded protein that binds to ribosome
and blocks binding site (presumably without
preventing tRNA binding itself)
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Macrolides
• Binds to 50S subunit
• Binding is reversible
• Either prevents
transfer of peptide or
access by next
tRNA, preventing
elongation.
• Other family
members:
azithromycin,
clarithromycin
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http://www.elmhurst.edu/~chm/vchembook/654a
ntibiotic.html
Inhibition of 50S subunit, resistance
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• Macrolides, chloramphenicol, ketolides,
clindamycin, and streptogramins are somewhat
related, all bind to 50S subunit.
• Two common forms of resistance
– Alteration of ribosome binding site
• Methylation of 23S rRNA at target
• Plasmid encoded enzyme
– Efflux of drug
– Many Gram – also resistant due to OM
Streptogramins & Mupirocin
• Streptogramins- from Streptomyces
– Used as mixture of two related drugs
– Low activity against most Gram –
– Especially useful against MRSA and most VRE
• Mupirocin
– Topical
– Binds to isoleucyl tRNA synthetase
– Used to eliminate carriage of nasal MRSA
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Pharmacodynamics
• Aminoglycosides
– Iv or im, poor absorption, renal excretion
• Tetracyclines, macrolides, others
– Oral administration generally possible, but food
often interferes, e.g. mineral complexing
– Generally pass into most body compartments
– Renal, fecal excretion, many metabolized first
– Some drugs concentrated in phagocytic cells
• Should be useful for treating some intracellular
bacterial infections.
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Toxicity
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• Aminoglycosides
– Renal toxicity most important and most common
• Inhibition of phospholipases and other enzymes
result in inhibition of prostaglandin synthesis
– Ototoxicity
• Damage to hair cells, tinnitus, loss of hearing
• Vestibular damage, headache, nausea, dizzy
• Chloramphenicol
– Damage to mitochondrial membranes
– Hematological effects, e.g anemia
Toxicity-2
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• Tetracyclines
– Photosensitization
– Discoloration of early dentition
• Macrolides
– Good therapeutic index, but still various side effects
– Major side effect is GI disturbance (yeah, I know)
• Clindamycin
– Broad spectrum problem: pseudomembranous
colitis