Mode of action of penicillin is to inhibit Transpeptidation reaction

Penicillin is a widely used antibiotic that targets and inhibits bacterial cell wall synthesis. The primary mode of action of penicillin is to inhibit the transpeptidation reaction, which is an essential step in the formation of the bacterial cell wall.

Transpeptidation reaction and cell wall synthesis

The bacterial cell wall is composed of peptidoglycan, a mesh-like structure that provides strength and rigidity to the bacterial cell. The peptidoglycan is made up of long chains of alternating N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) residues, which are cross-linked by short peptide chains.

The transpeptidation reaction is a critical step in the synthesis of peptidoglycan. Transpeptidases, also known as penicillin-binding proteins (PBPs), catalyze the cross-linking of the peptide chains by forming covalent bonds between the amino acid residues. These cross-links provide stability to the cell wall.

Inhibition of transpeptidation by penicillin

Penicillin exerts its antibacterial effect by inhibiting the transpeptidation reaction. It does so by binding irreversibly to the active site of transpeptidases, thereby preventing the formation of the covalent bonds between the peptide chains. This inhibition weakens the cell wall structure and leads to cell lysis and death.

Selective action on bacterial cells

One of the reasons penicillin is highly effective as an antibiotic is its selective action on bacterial cells. The transpeptidases targeted by penicillin are unique to bacterial cells and are not present in human cells. This selective inhibition allows penicillin to specifically target and kill bacterial cells while sparing host cells.

Resistance to penicillin

Despite its effectiveness, bacteria can develop resistance to penicillin through various mechanisms. One common mechanism is the production of β-lactamase enzymes, which can hydrolyze the β-lactam ring of penicillin, rendering it inactive. Additionally, bacteria can alter their transpeptidases, making them less susceptible to penicillin binding.

In conclusion, the mode of action of penicillin is to inhibit the transpeptidation reaction in bacterial cells. By binding irreversibly to the active site of transpeptidases, penicillin prevents the formation of covalent bonds between peptide chains, leading to weakened cell walls and ultimately cell death. This selective action on bacterial cells makes penicillin a highly effective antibiotic.