Inhibitors of Protein Synthesis

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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

• 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

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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.

4
www.emunix.emich.edu/.../ genetics/transl4.htm
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Polysomes
http//bass.bio.uci.edu/hudel/bs99a/lecture21/pol
ysome.gif
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Aminoglycosides
As name implies, molecules comprised of amino
sugars. Includes streptomycin, gentamycin,
kanamycin, etc. Highly polar molecules, do not
distribute well into body compartments. Administra
tion iv and im only Narrow therapeutic index.
Streptomycin
http//www.bmb.leeds.ac.uk/mbiology/ug/ugteach/icu
8/images/antibiotics/gentamicin.gif
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Mechanism of action for aminoglycosides

• 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

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Aminoglycosides cont.

• 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
  post-antibiotic effect
• Related to membrane damage and ribosome binding

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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
Note tetra Various family members created by
modifying the left 3 rings chlortetracycline,
oxytetracycline, doxycycline animal feed,
aquaculture
http//www.sp.uconn.edu/terry/images/micro/tetrac
ycline.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.
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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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(No Transcript)
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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

http//www.elmhurst.edu/chm/vchembook/654antibiot
ic.html
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Inhibition of 50S subunit, resistance

• 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

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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

• 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

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Toxicity-2

• 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