Cell Walls of Prokaryotes

Question 1

What are the major categories of bacteria and how do they differ?

Answer 1

1. Gram-positive
2. Gram-negative
   - The difference in staining is due to their cell wall
    Gram-positive: thick peptidoglycan layer
    Gram-negative: thin multilayer structure
   - Additional layer: periplasm (thick layer between the outer membrane and the cytoplasmic membrane)

Question 2

What is peptidoglycan?

Answer 2

 Rigid layer primarily responsible for the strength of the wall
 Compound unique to bacteria
 Confer shape and rigidity on the cell

Question 3

Why is peptidoglycan important?

Answer 3

 Withstand osmotic pressure and prevent bursting (cell lysis)
 Confer shape and rigidity on the cell

Question 4

What are the components of peptidoglycan?

Answer 4

*	Glycan: long chains of sugars
	2 sugars alternate along the chain
-	N-acetylglucosamine (NAG)
-	N-acetylmuramic acid (NAM); the peptide portion is attached to NAM
	Very similar sugars
	β(1,4) glycoside bond
-	Same bond as cellulose
-	Rigid bond

• Peptide portion:
   Tetrapeptide (4 amino acids)
   All gram-negative bacteria have the same 4 amino acids: L-Alanine, D-Glutamic acid, DAP (meso-diaminopimelic acid), D-Alanine
• Apart from L-alanine, these amino acids are very unique
   Gram-positive peptide: L-alanine, D-glutamic acid, L-lysine, D-alanine
• Structurally, DAP and L-lys are very similar (and part of the same metabolic pathways)

Question 5

What amino acids are found in peptidoglycan?

Answer 5

 All gram-negative bacteria have the same 4 amino acids: L-Alanine, D-Glutamic acid, DAP (meso-diaminopimelic acid), D-Alanine

 Gram-positive peptide: L-alanine, D-glutamic acid, L-lysine, D-alanine
 Structurally, DAP and L-lys are very similar (and part of the same metabolic pathways)

Question 6

What is transpeptidation? Where is it found?

Answer 6

• Transpeptidation: link between 2 adjacent tetrapeptides, which is by a covalent bond
   Gram-negative: peptide bond between DAP and D-alanine
   Gram-positive:L-lysine – interbridge (short peptide) – D-alanine
  • The interbridge varies between species

Question 7

Describe 2-3 antimicrobial agents. Where do they act?

Answer 7

• Lysozyme
   Enzyme found in all invertebrate secretions (tears, saliva, etc.)
   Targets and breaks the NAG-NAM β(1,4) bond
   Cell wall can’t counteract the osmotic pressure, causing lysis
• Penicillin and Cephalosporin
   Disrupt transpeptidation
   Wall no longer counteracts osmotic pressure

Question 8

What are the components of gram-positive cell wall? In what proportion?

Answer 8

 90% peptidoglycan
- Often several sheets of peptidoglycan stacked one upon another
 10% teichoic acid

Question 9

What is teichoic acid?

Answer 9

• Component of gram-positive cell wall
• Long polysaccharide substituted with sugars or amino acids
• Contain phosphorys so carry negative charge

Question 10

Describe a subcategory of teichoic acid and its roles.

Answer 10

• Lipoteichoic acid
   Teichoic acid covalently bound to membrane lipids (embedded into lipid bilayer)
   Anchors peptidoglycan of cell wall to membrane

Question 11

What is the outer membrane? What proportion of the cell wall does it occupy?

Answer 11

In gram-negative bacteria, peptidoglycan is thin (about 10% of cell wall)
Most of the cell wall is the outer membrane

Question 12

What are the components of the outer membrane?

Answer 12

1. Lypopolysaccharide (LPS) layer which contains lipid A
    Lipid A: glucosamine-phosphate disaccharide backbone (instead of glycerol)
   o 4 fatty acids attached by ester bond
    Core polysaccharide: C5, C6, C7 monosaccharides
    Outer polysaccharide: C5 monosaccharides only
2. Phospholipid inner layer
    Lipoproteins: anchors peptidoglycan to outer layer
    Porins:
   o Channels for large hydrophilic molecules (proteins, peptides, glycans)
   o These molecules reach the periplasm (periplasmic space)
   o They then need to be broken down to enter cell

Question 13

What is the periplasmic space?

Answer 13

Located between outer surface of the cytoplasmic membrane and the inner surface of the outer membrane

Site of polymeric hydrolytic enzymes and binding proteins

Question 14

Do all archaea have cell walls?

Answer 14

No.

Question 15

What are the different types of archaeal cell wall

Answer 15

1. S-Layer

2. Pseudomurein cell wall

Question 16

Describe the structure of the S-layer.

Answer 16

 Paracrystalline structure

 Outer protein layer (not lipid or peptidoglycan)

Question 17

What organisms possess an S-layer?

Answer 17

 Most common archaeal cell wall

 Associated with some bacteria, gram-positive and gram-negative (located outside of cell wall)

Question 18

What are the functions of the S-layer?

Answer 18

 Structural reinforcement (2nd reinforcement for bacteria)
 Selective permeability barrier
-Especially against viruses in bacterial cells

Question 19

Describe the pseudomurein cell wall.

Answer 19

 Similar to peptidoglycan (in structure)
- Long chains of alternating sugars: NAG and N-acetyltalosaminuronic acid
 Linked by β(1,3) bonds: peptide cross-links