Lec 2- Target cell wall Flashcards
Sites of action of antibacterial agents
- Ribosome (protein synthesis)- aminoglycosides (gentamicin); tetracyclines; chloramphenicol; macrolides; azalides; lincosamides; oxazolidinones; mupirocin; fudisic acid; pleuromutilins
- Metabolism (DNA synthesis)- Anti-folates; sulphonamides; trimethoprim
- Cell wall- beta-lactams; glycopeptides
- Chromosome (DNA replication)- Quinolones; Rifampicin; Nitroimidazole; Nitrofurans
- Cell membrane disruption- Polymyxins; Daptomycin
Main anti-bacterial agents- acting on the cell wall
- Beta-Lactams: Penicillin; cephalosporins; carbapenems; monobactams; clavulanic acid; sulbactam; tazobactam
- Glycopeptides: Vancomycin; Teicoplanin
- Others that act as anti-mycobacterial (TB) agents
- Cycloserine (Petidoglyca)
- Isoniazid (Mycolic acid)
- Ethambutol (Arabinogalactan)
Others that act as anti-mycobacterial (TB) agents and there targets
- Cycloserine
- Target- Peptidoglycan
- Isoniazid
- Target- Mycolic acid
- Ethambutol
- Target- Arabinogalactan
General bacterial cell wall structure
-Gram-negative
- Inner membrane
- Periplasmic space- this is where the peptidoglycan lies to improve wall strength
- Outer membrane- extra cell membrane for drugs to cross (drugs can struggle to do this)
- Phospholipid
- Peptidoglycan- the presence of this in the membrane that makes a wall structure either gram-positive or negative
- lipoprotein
- protein
- Lipopolysaccharide
- Porins
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General bacterial cell wall structure
-Gram-Positive
- Cytoplasmic membrane
- Peptidoglycan- much larger than -ve bacteria
- Phospholipid
- Protein
- Lipoteichoic acid
NB- no extra membrane- one of the main difference
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General bacterial cell wall structure
- Beta-lactam and glycopeptide anti-biotics inhibit synthesis of the peptidoglycan component of the bacterial cell wall
- Peptidoglycan is essential to nearly all bacteria
- Peptidoglycan is unique to bacterial cell walls, no related polymer is found in mammalian cells
Detail Gram-negative peptidoglycan
- M + G combine to make long strands- however, these individual strands are weak
- Cross-link with peptides between the sugar chains give the strength
- Transglycosylase- extends the glycan chains
- Transpeptidase- removes terminal D-alanine and cross-links the peptides
- D-amino acids do not occur in mammalian (another target that is different)
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Detail gram-positive cell wall structure
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Formation of cross-link (Gram-negative)
- Cross-linking shows how beta-lactamases work
- Nucleophilic attack of Transpeptidase (OH group) on D-alanine breaking the peptide bond
- This leaves a 4 amino acid chain attached to transpeptidase as well as a free D-alanine
- Diaminopimelic acid (3rd) on a different chain has a NH2 group which then attacks the ester bond between D-alanine and transpeptidase
- This attack generates a new peptide (covalent) bond between the two chains
- This is UNIQUE to bacteria, hence no harm to the host
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Structures of penicillins G and V: Beta-lactam antibiotics
- Beta-lactam best-known agent to interfere with cross-link formation
- Beta-lactam is an amide bond in a cyclic formation, this is the main component
- Beta-lactam is very reactive because the normal bond angle for Carbon is 108’ but the angle of a square is 90’, this puts a lot of strain into the system which will be released when it reacts
- Penicillin G couldn’t be taken orally because the phenylacetic acid section of the molecule would be broken down in acid, this would then lose it effectivness-had to be given IV
- Penicillin V was far more acid stable
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Formation of cross-link (Gram-negative)
- Penicillins take the place of the D-alanie-D-alaine
- The transpeptidase then attacks the carbonyl of the penicillin
- This enzyme intermediate (Trans-penicillin) is stable, therefore
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Penicillin inhibition of transpeptidase
- Very similar structures
- 4 membered cyclic ring of the beta-lactam opens up into a 5 membered ring hence the increasing instability
- The 5 membered rings of the penicillin hinder the diaminopimelic acids ability to reach the ester bond to break and form an amide bond hence no cross-link
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Development of semi-synthetic penicillins
- While PenV and PenG were the first effective penicillins, they have limitations- better penicillins were needed but hard to synthesize
- When starved of phenylacetic acid, Penicillium chrysogenum produces the penicillin nucleus, 6-amino penicillanic acid (6-APA)- the hard bit
- 6-APA has little intrinsic activity
- 6-APA can be converted to active penicillin by reaction with an activated acid (e.g. acyl chloride)
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Structures of semi-synthetic penicillins
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Hydrolysis of penicillin by Beta-lactamase
-Penicillin Resistance
- One of the challenges is resistant to anti-microbial agents
- Bacterial mechanism against Beta-lactams is that the transpeptidase isn’t a transpeptidase but it would attack the beta-lactam with water, which isn’t stable in water hence hydrolysing the enzyme, inactivating it
- This is now known as the beta-lactamase
- Hydrolysis is not a new reaction (hence why we have penicillin for reconstitution), the beta-lactamase acts as a catalyst and speeds up the reaction by 107 fold
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Beta-lactamase Inhibitors
- Still, contain a beta-lactam ring
- In the 5 membered ring instead of sulphur (penicillins) they have Oxygen or sulphone
- Sulbactam- a natural product
- Not anti-biotic just prevent hydrolysis of the beta-lactam
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Mechanism of Beta-lactamase inhibitors
- Beta-lactamase has an OH group which undergoes a nucleophilic attack of the carbonyl group of the 4 membered ring
- This opens the beta-lactam ring
- Because of the structure of clavulanic acid, this opening of the ring produces a reactive intermediate- causing stably cross-linked acyl-enzyme which is not rapidly hydrolysed by the enzyme
- Hence the beta-lactamase is not available to hydrolyse the penicillins
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Cephalosporins
- Cephalosporins (Cephems) are another class of beta-lactam antibiotic
- Cephalosporins C is a naturally produced antibiotic (by the fungus Acremonium)
- Ceph C can be chemically modified to a range of semi-synthetic cephalosporins- to give different properties, have 2 groups that can be modified
- Has beta-lactam ring, the difference between this and penicillins is that it contains 6 membered rings with sulphur and not 5 membered ring
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Semisynthetic cephalosporins
- Cephalosporin C limited clinical stability
- Semisynthetic cephalosporins(E.g. cephalothin, 1964) were produced to improve stability
- 1st generation cephalosporins
- 2nd generation broader spectrum- more gram-negative activity
- Now on 5th generation (e.g. Ceftobiprole, Ceftaroline)
- 4th and 5th generation very broad spectrum
Carbapenems
- Has Beta-lactam and COOH (mimic D-alanine)
- No sulphur in the 5 membered ring system (Its carbon) as well as a double bond
- Thienamycin is a naturally produced carbapenem antibiotic (Streptomyces)- most potent to date
- Broad-spectrum (G+ and G-) and resistance to Beta-lactamase
- Can be chemically modified to a range of semi-synthetic carbapenems:
- Imipenem (sensitive to renal peptidase)
- Meropenem (resistant to the renal peptidase
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Monobactams
- Nocardicin A is a naturally produced monobactam beta-lactam antibiotic
- Aztreonam is a synthetic monobactam, resistant to beta-lactamases and is the only clinically used monobactam
- Often tolerated by patients who have hypersensitivity to penicillins
- Doesn’t have the 5 or 6 membered rings hence good for hypersensitivity
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Adverse drug reaction
- “Mild” adverse drug reaction (ADR) (Hypersensitivity) to penicillins occur in about 1% of patients (diarrhoea, nausea, rash)
- 10% tend to report some of these effects but these are often not related to the penicillin
- This is a type I response (IgE)
- Believed to be due to “haptenization” of proteins
- Due to the reactivity of beta-lactam ring, it reacts with nucleophiles on proteins, mainly albumin (HSA), Abs can then react with the modified albumin to create a response
- Cross-sensitivity is low between classes
- Severe anaphylaxis occurs in around 0.01%, mechanism unclear
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