Peptidoglycan structure and function

Peptidoglycan

Bacterial cell wall is composed of peptidoglycan (also called murein)

– Peptid -> peptide chains

– oglycan -> polysaccharide

Gram Negative Cell wall structure

– Blue: outer membrane

– Pink: Peptidoglycan (safranin – pink after gram staining)

– Big white box: cell (cytoplasmic membrane)

– green: lipopolysaccharide (LPS)

– orange: lipoprotein

– small white box: porin

Note: between cytoplasmic membrane and outer membrane is periplasmic space.

Gram Negative Cell wall structure


– Purple: Peptidoglycan (crystal violet – purple after gram staining)

– Big white box: cell (cytoplasmic membrane)

– green: teichoic acid

– yellow: lipoteichoic acid











Details: Compare and Contrast Cell wall

https://www.youtube.com/watch?v=evmrvBCtpSQ


Peptidoglycan (murein) sacculus

– glycan strands are cross-linked with peptide bonds, hence the name Peptidoglycan.

– peptidoglycan sacculus surrounds cell membrane and provides structural strength and maintains the shape of the cell.

Subunits of Peptidoglycan

N-acetylmuramic acid (NAM)

N-acetylglucosamine (NAG)

– Amino acids that are in D- and L- forms (D-glutamic acid, meso-diaminopimelic acid and D-alanine, L-alanine)

The major function of Peptidoglycan

1) To protect cell from bursting due to osmotic pressure

– Why? Cell is mostly higher in solute concentration than their surrounding.

– Thus, osmosis occurs and water most likely enters the cell, causing cell expansion.

– But, if this rigid structure won’t let bacterial cell expand, bacteria won’t burst to death.

2) To maintain the shape of the cell (rigid)

– Whether cell loses or gains water due to osmosis, cell shape remains the same.

It is important to remember that bacteria are mostly UNICELLULAR.

Which means that they need to have well regulated system to protect them from any damage.

Details: Function of Peptidoglycan

https://www.youtube.com/watch?v=VfOBCC4ClQY




Cross-linking of Peptidoglycan

– NAG-NAM are in helical shape.

– Cross-linking at 3, 4 (most common)

–> Amino end of side chain at position 3 attaches to carboxyl end of D-alanine at position 4

– Cross-linking at 2, 4 (very rare, only in Coryneform bacteria)

–> alpha carboxyl end of D-glutamine at position 2 attaches to carboxyl end of D-alanine at position 4 of another peptide subunit.

     –> Here, diamino acid interpeptide (two different subunits bridging) bridge must be present.

     –> This serves as a virulence factor for pathogenic bacteria. 2, 4 linkage is rare form, so plant’s immune system/peptidase takes longer to act. This allows bacteria to survive longer and colonize before they are killed by immune system.

Details: Increasing PG resistance to peptidase

https://www.youtube.com/watch?v=x-m5fZmc5kg


Details: Modifications in PG to protect microbe from lytic cleavage

https://www.youtube.com/watch?v=7opM8nwQEe8


Sezzart

Reference Vollmer, W. Blanot, D. and De Pedro, M. A. FEMS Microbiol. Rev. (2008) 32(2): p. 149-167.









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