Download Pharmacology 34: Bacterial and Mycobacterial Infections (Cell Wall

Pharmacology 34: Bacterial and Mycobacterial Infections (Cell Wall Synthesis)
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Penicillin = inhibits biosynthesis of peptidoglycan (bacterial cell wall)
o Peptidoglycan -> peptide-cross-linked sugar polymers; aka murein; essential for
bacterial survival
 Mycoplasma pneumoniae and intracellular form of Chlamydia trachomatis do
not have murein
β-lactam antibiotics -> cell wall synthesis inhibitors (inhibit transpeptidase enzyme)
Gram-positive bacteria -> retain stain and appear purple
o Very thick murein layer
o Lipoteichoic acids and wall teichoic acids -> covalently attached to uncross-linked
peptides by sortases
Gram negative bacteria -> lose stain, appear pink
o Have outer membrane (limits penetration)
o Thin murein layer
o Have porins in outer membrane to allow molecules to pass in and out
o Lipopolysaccharide on outer leaflet of outer membrane -> protect from toxic
hydrophilic host molecules (bile salts) and allow adherence and evasion of host
 Polymyxin = topical antibiotic binding to LPS (too toxic for systemic use)
Peptidoglycan Biosynthesis
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Stage 1 intracellulary (synthesis of murein monomers) -> Stage 2/3 export of monomers to
surgace and polymerization into linear peptidoglycan polymers, and cross-linking
Synthesis of murein monomers:
o Murein monomer = disaccharide connected via beta linkage (N-acetylglucosamine
[NAG] and N-acetyl muramic acid [NAM])
o Cytoplasmic conversion of UDP-NAG to UDP-NAM-peptide (Park nucleotide)
 Mur A transfers enolpyruvate from PEP to UDP-NAG -> UDP-NAG pyruvate enol
ether
 MurB reduces it to produce UDP-NAM
 MurC, MurD, MurE add AA L-alanin, D-glutamate, and either L-lysine or DAP to
UDP-NAM
 Gram-positive use L-lysine and Gram-negative use DAP
 MurF adds D-Ala-D-Ala (from L-alanine catalyzed by alanine racemase and DAla-D-Ala ligase B)
 UDP-NAM-L-Ala-D-Glu-L-Lys/mDAP-D-Ala-D-Ala = Park nucleotide
o Inner surface of cytoplasmic membrane transfer of UDP-NAM-peptide to bactoprenyl
phosphate
 BP = “carrier” cuz murein monomers assembled on it, deliverd to plasma
membrane, and rleased
 MraY achnors UDP-NAM-peptide to BP (readily reversible)
 MurG transfers NAG to NAM to produce NAM-NAG = Lipid II
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In Gram-positive bacteria, linker peptide (5 gly residues) added to DAP
 In S. aureus, FemA, FemB, and FemX assemble gly-pentapeptide branch
o FemX attaches first glycine(essential for survival)
o FemA and FemB not essential for survival
In Gram-negative bacteria, murein monors cross-linked directly without
branching peptide
Murein monomer transferred to outer cytoplasmic membrane
Polymerization:
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Murein monomers on external cytoplasmic membrane -> polymerization to long glycan
catalyzed by peptidoglycan glycosyltransferases (PGT)
o BP released in each glycosylation reaction catalyzed by dephosphorylase
o PGTs usually bifunctional proteins but can be monofunctional PGTs (MGTs)
Cross-linking:
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Murein chains cross-linked by transpeptidases (TPs) aka penicillin-binding proteins (PBPs)
Activation step -> TP attacks D-Ala-D-Ala amide bond on glycan polymer (releasing alanine)
Coupling step -> free amino group (Gram-positive) or DAP (Gram-negative) attacks intermediate
= new amide bond cross-link
o Penicillin mimics D-Ala-D-Ala binding TP active site forming enzyme-penicillin complex
(inactivates TP)
Differences in # and type of cross-links and glycan chain length -> each bacterial species its
characteristic shape and size and cell-wall thickness
Multiple TPs are present in each bacteria for different parts of the wall
In MRSA, strains have resistant TP that cross-links peptidoglycan when exposed to methicillin
Mycobacterial Cell Wall Synthesis
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Corynebacteriae -> Mycobacterium tuberculosis and Mycobacterium leprae
o High G+C Gram positives (cell envelopes have characteristics of both Gram + and Gram –
bacteria)
o Have outer membrane (with extractable lipids)
o NAM sugars have NAG-arabinogalactan polymers attached to mycolic acids
 Mycolic acids essential for outer membrane
o Synthesis of NAG-arabinogalactan -> NAG phosphate transferred from UCO-NAG to
mycobacterial bactoprenyl phosphate -> rhamnose added -> galactos/arabinose added
(catalyzed by arabinosyl transferase)
o Synthesis of mycolic acid -> FAS1 catalyzes formation of saturated hydrocarbon chains > FAS2 links chains -> mycolic acid
o Mycolic acids attach to NAG-arabinogalactan -> attach to NAM
Mycobacterial cell envelope thick, asymmetric, and highly impermeable to
hydrophilic/hydrophobic substances
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Treatment regimens have long-term therapy with combo antibiotics to cure TB
Autolysins and cell wall degradation
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For bacteria growth, cell wall must expand with new murein incorporated; for division, cell wall
must be broken
o Use autolysins -> allow for remodeling and expansion (preferences for different bonds
in murein)
New murein synthesis must balance autolysin-mediated destruction for bacterial survival
o Blocking murein synthesis (penicillin) -> autolysis
Bactericidal effect of cephalexin inhibits transpeptidation step of cell wall synthesis