Peptidoglycan

Structure of peptidoglycan.

Peptidoglycan, also known as murein, is a polymer that forms a protective structural layer in the cell walls of bacteria. It is a crucial component that provides mechanical strength and shape to the bacterial cell, preventing it from bursting due to osmotic pressure.

Structure

Peptidoglycan is composed of glycan chains cross-linked by short peptides. The glycan chains consist of alternating units of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM). These sugars are linked by β-(1,4) glycosidic bonds.

Chemical structure of murein.

The peptide component typically consists of a short chain of amino acids attached to the NAM residues. The cross-linking of these peptide chains provides the rigidity and strength necessary for the bacterial cell wall.

Function

Peptidoglycan serves several critical functions in bacterial cells:

• Structural support: It maintains the shape of the cell and protects against mechanical damage.
• Osmotic protection: It prevents the cell from lysing in hypotonic environments by counteracting the internal osmotic pressure.
• Cell division: It plays a role in the process of binary fission, where new peptidoglycan is synthesized to form the septum that divides the cell.

Synthesis

The synthesis of peptidoglycan involves several steps:

1. Precursor formation: NAG and NAM are synthesized in the cytoplasm and linked to a lipid carrier. 2. Polymerization: The glycan chains are polymerized at the cell membrane. 3. Cross-linking: The peptide chains are cross-linked by penicillin-binding proteins (PBPs).

Catalytic mechanism of penicillin-binding proteins.

Role in Antibiotic Action

Peptidoglycan is the target of several antibiotics, such as penicillin and vancomycin. These antibiotics inhibit the synthesis of peptidoglycan, leading to cell lysis and death.

• Penicillin: Inhibits the transpeptidation reaction by binding to PBPs, preventing cross-linking of the peptidoglycan.
• Vancomycin: Binds to the D-Ala-D-Ala terminus of the peptide chain, blocking the transglycosylation and transpeptidation steps.

Gram-Positive vs. Gram-Negative Bacteria

Schematic of a Gram-positive bacterial cell wall.

Peptidoglycan is a major component of the cell wall in both Gram-positive bacteria and Gram-negative bacteria, but its structure and thickness differ:

• Gram-positive bacteria: Have a thick peptidoglycan layer, which retains the crystal violet stain used in the Gram staining procedure.
• Gram-negative bacteria: Have a thin peptidoglycan layer located between the inner and outer membranes.

Peptidoglycan Chain Extension

During bacterial growth, peptidoglycan chains are extended by the addition of new disaccharide-pentapeptide units. This process is tightly regulated to ensure the integrity of the cell wall during cell division.

Related Pages

• Bacterial cell wall
• Antibiotic resistance
• Gram staining
• Penicillin