β-lactam Antibiotics: Penicillin

Question 1

Classify antibiotic based on their mechanism of action.

Answer 1

1. Cell wall synthesis inhibitors e.g. Cephalosporins, Penicillin
2. Protein synthesis inhibitors e.g. Macrolides, Aminoglycosides, Tetracyclines
3. Nucleic acid inhibitors e.g. Proflavine, fluoroquinolones
4. Metabolism inhibitors e.g. Sulphonamides
5. Compromising cell membrane e.g. Polymyxin, tyrothricin

Question 2

Draw and describe the structure of Penicillin.

Answer 2

It is a 4-membered β-lactam ring fused to a 5-membered thiazole ring
………….

Draw structure.

Question 3

Using appropriate equations, discuss penicillin instability.

Answer 3

• In a typical amide, there is resonance stabilisation due to the delocalisation of electrons between O and N, as represented by the structure below:
• Amide resonance structures *

The typical bond angle in amides is 120°, which is ideal for resonance.

However, in the β-lactam ring, the bond angle is 90°. This introduces a ring strain of 30°, which affects resonance stability.
As a result, the resonances structures that are typical in a normal amide, do not occur in a β-lactam ring.

• equation showing the resonance structure that doesn’t occur in β-lactam ring *

This makes penicillin unstable in the presence of acids, bases and nucleophiles.

• Draw equations of reaction between penicillin and acids, bases and nucleophiles *

Question 4

Discuss the mechanism of action of penicillin.

Answer 4

• Bacterial cell wall consists of a series of cross-linked peptidoglycan units, which are polysaccharides made of N-acetyl muramic acid (NAM) and N-acetyl glucosamine (NAG)
• Each NAM unit ha a short peptide chain
• The enzyme transpeptidase crosslinks these peptides by replacing the terminal D-alanine reside of one chain with the terminal D-glycine residue of another to complete the cell wall
• Penicillin resembles the D-alanine residue of the peptide chain, and this causes transpeptidase to erroneously bind to it instead of the D-alanine residue
• Once bound, penicillin is attacked by the serine-OH group at the active site of the enzyme, opening the beta-lactam ring and causing penicillin to covalently bind to it.
• This renders the enzyme inactive and unable to crosslink the peptidoglycan chains
• This leads to the formation of a weak and incomplete cell wall, which easily bursts, killing the bacterial cell.

Question 5

Highlight the SAR of penicillin.

Answer 5

The following features are essential for activity:
1. A β-lactam-containing bicyclic fused ring system
2. A free 3-carboxylate group
3. A cis relationship between 5- and 6- hydrogen
4. A 6- amido group
5. A trans relationship between the 3-carboxylate group and the 6-amido group

Question 6

With the aid of a suitable equation, describe the synthesis of penicillin.

Answer 6

The total synthesis of penicillin G has a very low yield of about 1%.

Thus the partial synthesis approach in which 6-aminopenicilanic acid (APA) is produced in a fermentation process and the side chain is added by the reaction of APA with an acyl halide is adopted.

Eqn of APA + Acyl halide

Question 7

What are the major drawbacks of penicillin G?

Answer 7

1. Acid instability of Penicillin G which makes it orally non-bioavailable.
2. Emergence of Penicillin-resistant bacterial strains with the enzyme β- lactamase which splits open the β-lactam ring rendering the penicillin molecule inactive.

Question 8

What approach was taken to produce acid-stable penicillin?

Answer 8

Electron withdrawing groups are added to prevent intramolecular nucleophilic attack by the carbonyl oxygen near the β-lactam ring, thus making the penicillin able to withstand the acidic environment of the stomach.

This attack is facilitated by N’s lone pair, which donate electron density via mesomeric effect.