Peptidoglycan

Structure and Composition

Peptidoglycan is a major structural polysaccharide found in bacterial cell walls. It provides rigidity and shape to the cell and protects against osmotic pressure.

• Type: Heteropolysaccharide

• Repeating Sugars: N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM)

• Glycosidic Linkage: β(1→4) linkage between NAG and NAM

• Function: Structural support in bacteria

• Organism: Bacteria

• Branched?: No

Short peptide chains cross-link these polysaccharides, forming a strong, mesh-like structure. The peptide cross-links are essential for the integrity of the bacterial cell wall.

Peptidoglycan Structure

• N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) alternate to form linear chains.

• Peptide side chains attached to NAM residues are cross-linked by short peptides, creating a lattice structure.

Example: The peptide cross-links are formed by proteins (amino acid chains), not by carbohydrates or lipids.

Gram-Positive vs. Gram-Negative Bacteria

Differences in Cell Wall Structure

• Gram-Positive Bacteria:

  • Thick peptidoglycan layer

  • Stains purple with Gram stain due to retention of crystal violet dye

  • No outer membrane

• Gram-Negative Bacteria:

  • Thin peptidoglycan layer

  • Has an outer membrane outside the peptidoglycan

  • Stains pink/red with Gram stain (crystal violet is washed out, safranin counterstain is retained)

Example: The Gram-negative cell wall consists of a thin peptidoglycan layer with an outer membrane on top.

Summary Table: Peptidoglycan in Bacterial Cell Walls

Practice Questions (with Answers)

• What type of molecule provides the 'cross-links' between the peptidoglycan polysaccharides? Answer: Protein (short peptide chains)

• Which of the following best describes the structure of the Gram-negative cell wall? Answer: A thin layer of peptidoglycan with an outer membrane on top.

Additional info: Peptidoglycan is a target for antibiotics such as penicillin, which inhibit the enzymes responsible for cross-linking the peptide chains, weakening the cell wall and leading to bacterial cell death.