Stages of Cell Wall Synthesis

3 Stages of Cell Wall Synthesis (CME/SEC)

1. Cytoplasmic Stage (Synthesis of Precursors)

  • Precursor Formation:

    • Begins with the synthesis of NAG (N-Acetyl Glucosamine) from glucose, which requires UTP (Uridine Triphosphate).

    • The reaction leads to the formation of UDP-NAG (Uridine Diphosphate-N-Acetyl Glucosamine), by acquiring 2 phosphate molecules from UTP.

    • UDP-NAG is then converted into UDP-NAM (Uridine Diphosphate-N-Acetylmuramic acid) through the action of Enol Pyruvate Transferase, utilizing Phosphoenolpyruvate (PEP).

    • UDP-NAM is further modified by attaching L-Alanine, D-Glutamic Acid, and L-Lysine, to form UDP-NAM-Tripeptide.

    • The Racemase enzyme catalyzes the conversion of L-Alanine into D-Alanine, resulting in the formation of D-Alanyl-D-Alanine, when 2 molecule combines.

    • Ultimately, this leads to the creation of UDP-NAM-pentapeptide (UDP-NAMpp).

  • Inhibitors (FC):

    • Fosfomycin: It inhibits Enol Pyruvate Transferase, thus blocking the conversion of UDP-NAG to UDP-NAM, impacting peptidoglycan synthesis.

    • Cycloserine: It inhibits the Racemase enzyme, preventing the formation of D-Ala-D-Ala and hindering overall precursor synthesis.

2. Membrane Stage (Elongation and Transfer)

  • Mechanism of Action (MOA):

    • This stage involves the transport of synthesized precursors from the cytosol across the membrane, integrating them into the growing peptidoglycan structure.

    • Key Players (BUP):

      1. Bactoprenol:

        • Acts as a special carrier molecule, facilitating the transfer of building blocks (like UDP-NAMpp) from the cytosol to the membrane.

      2. UDP-NAM-pentapeptide (UDP-NAMpp):

        • The UDP-NAMpp unlinks into 2 separated individual components.

        • NAMpp binds to Bactoprenol, while UDP-NAG is brought separately and then bind to separated NAMpp attached to the Bactoprenol, allowing for the combination of these components.

      3. Peptidoglycan Synthase:

        • This enzyme flips the complex to transfer it outside from the cytosol to the membrane.

        • The complex formed separate from Bactoprenol.

        • The complex then interacts with the growing chain, and the phosphate group attached to Bactoprenol is removed via phosphatase (a dephosphorylation reaction), recycling the carrier back into the cytosol.

  • Inhibitors (VB):

    • Vancomycin: Works by binding to the D-Ala-D-Ala sidechain, preventing its integration into the chain and inhibiting peptidoglycan synthesis without directly inhibiting the peptidoglycan synthase enzyme.

    • Bacitracin: It binds to phosphatase, obstructing the recycling of Bactoprenol, which is crucial for maintaining the supply of lipid carriers.

3. Extracellular Stage (Crosslinking and Final Transpeptidation)

  • Mechanism of Action (MOA):

    • In this final stage, crosslinking occurs between the linear chains of peptidoglycans.

    • This is primarily mediated by membrane-bound transpeptidase, which is essential for providing rigidity to the cell wall.

    • Transpeptidation Catalysis:

      • The transpeptidase enzyme (often referred to as penicillin-binding protein, PBP) catalyzes the final cross-linking of the peptidoglycan chains.

  • Inhibitors (B):

    • Beta-lactams: These antibiotics, which include penicillin, inhibit transpeptidase by mimicking the D-Ala-D-Ala structure.

      • When the beta-lactam ring enters the active site of transpeptidase, it becomes the substrate or target for the enzyme, making the transpeptidase to act on it instead of the side chains and preventing it from cross-linking peptidoglycan chains effectively, thus weakening the cell wall structure.