Peptidoglycan
Peptidoglycan[edit]
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[edit]
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.
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[edit]
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[edit]
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).
Role in Antibiotic Action[edit]
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[edit]
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[edit]
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.
