FP - Inhibitors of Cell Wall Synthesis

Question 1

What was the first discovered inhibitors of cell wall synthesis?

Answer 1

Penicillin (Alexander Fleming 1928)

Question 2

What are features of Gram -ve bacteria and Gram +ve bacteria?

Answer 2

• Gram-negative bacteria contain outer and inner membranes with a THIN layer of peptidoglycan
• Gram-positive bacteria contain only inner membranes with a THICK layer of peptidoglycans

Question 3

What do cell wall inhibtors target and which bacteria strain?

Answer 3

Cell wall inhibitors target peptidoglycan synthesis

• Cell wall inhibitors are largely targeting/working on growing Gram-positive organisms

Question 4

What is peptidoglycan?

Answer 4

Peptido:

• peptide mostly alanine and isoglutamine

some variations

Glycan:

• N-acetylglucosamine (NAG or GlcNAc)
• N-acetylmuramic acid (NAM or MurNAc)
  = ‘glycans’ (sugars)
  β-1→4 bonds

Question 5

1) Synthesis of N-acetyl-muramic acid

Answer 5

UDP-NAcGln → UDP-NAcetyl muramic acid

• Enzyme: Phosphoenolpyruvate

Blocked by Phosphomycin

Question 6

2) Synthesis of D-Ala-D-Ala

Answer 6

1) L-Ala → D-Ala

• Enzyme: Racemase

Alanine mimicked by Alaphosphin

2) D-Ala → D-Ala-D-Ala

• Enzyme: Synthase

Inhibited by Cycolserine (Oxamycin)

Question 7

3) Transfer of the disaccharide unit to the cell wall

Answer 7

Releases (C55-P-P) – to be recycled

Blocked by:

• Vancomycin
• Nisin

These bind to the D-Ala-D-Ala end and prevent this substrate from occupying its site on the enzyme

Question 8

4) Recycling of C55-P-P

Answer 8

C55-P-P → C55-P + P

• Blocked by Bacitracin

Interferes with sterol synthesis, so only used in topical applications

Question 9

5) Cross-linking by Penicillin Binding Proteins (PBPs)

Answer 9

Transpeptidases or Penicillin-binding proteins (PBPs) are responsible for the cross-linking of peptide chains

Question 10

Different penicillin-binding proteins (6)

Answer 10

• PBP-1: transpeptidase
• PBP-2: shape maintenance
• PBP-3: septum formation
• PBP-4, PBP-5, PBP-6: control the amount of cross-linking and cleave the terminal D-Ala

Question 11

How do Beta-lactam antibiotics work and what is the consequence ?

Answer 11

Occupy the active site of the enzyme (PBP)

• COVALENT IRREVERSIBLE BINDING

PBP cannot cross-link peptide chains and cell wall loses rigidity

Question 12

What is autolysis, and how does penicillin contribute to it?

Answer 12

Autolysis is the process of bacterial cell lysis

• Penicillin activates autolytic enzymes, leading to the lysis of the bacterium

Question 13

What are new β-lactam derivatives required for? (3)

Answer 13

1. Increased acid stability
2. Resistance to -lactamases
3. Increased activity against Gram –ve bacteria

Question 14

What are the 4 types of beta-lactam antibiotics?

Answer 14

1. Penicillins
2. Cephalosporins
3. Monobactams
4. Carbapenems

All beta-lactam antibiotics contain the same core 4-member “beta-lactam” ring

Question 15

Four groups of beta-lactamases

Answer 15

1) Penicillinases

2) Cephalosporinases

• Inactivate cephalosporins and aminopenicillins

3) Extended-spectrum β-lactamases (ESBL)

• Inactivate all β-lactams except carbapenems.

4) Carbapenemases (several different classes)

• Inactivate carbapenems and, depending on the enzyme, other β-lactams as well

Question 16

What are 2 Inhibitors of β-lactamases and how do they function?

Answer 16

1. Clavulanic acid

2. Avibactam
Co-administered with ceftazidime (Avycaz)

• A covalent, slowly reversible inhibitor
• Acylates an OH in the enzyme’s active site