What is the cell wall?

• Fn: Provides rigidity to prokaryotic cell —> prevents water ingress, swelling + subsequent bursting of bacterial cell
  • NB: Eukaryotes rely on sequestering water into vacuoles which are then exocytosed to maintain osmotic status
• Gram + vs Gram -:

• G+ cell wall is basically one big block of peptidoglycan with some other proteins anchored in.

• G- cell wall in detail:

  

• Periplasmic space = from surface of Cytoplasmic membrane to inside of Outer membrane, i.e. includes the peptidoglycan layer.

• Outer membrane: Mostly composed of Lipopolysaccharide (LPS), the structure of which is shown below.

  • Note WBC TLR4 responds strongly to Lipid A, the lipid component of LPS.

The role of Peptidoglycan

• G+ is 90% Peptidoglycan, G- is ~10%.
  • G- cell wall described above: G+ cell wall is basically all
• 2 components, NAM and NAG:
  • NAG: N-acetylglucosamine
  • NAM: N-acetylmuramic acid
    • NAM has a pentapeptide chain, NAG does not. It is this that is cross-linked between peptidoglycan chains.
• These 2 components are laid down alternately in a chain (via transglycosylation);how the chains are cross-linked provides the bacterium with its shape/structure (Cocci, Bacilli, Spirochaete etc).

How does cross linking work?

• Also known as transpeptidation: This is what is catalysed by PBPs.

• The Pentapeptide chain is composed of 5 amino acids: Alanine, Glutamine, Lysine, Alanine & Alanine

  

• Note that D-glutamine and D-alanine are the only D-form amino acids found in nature (the rest are L-form), and are a feature of bacterial prokarya (i.e. they’re NOT found in Archaean prokarya or Eukaryotes)

  • Note only the last 2 Alanine AAs are D-form, the first is L-form.

• Cross linking between G+ and G- cell wall:

G-: Peptide bond between L-Lysine residue and terminal D-Alanine of another chain

G+: An interpeptide ‘bridge’ of 5 Glycine residues links L-lysine and D-Alanine of adjacent chains

• End result: Peptidoglycan chains are cross-linked