Peptidoglycan and the gram positive cell wall

What are the two basic units that compose peptidoglycan?

1. N-acetyl-D-glucosamine (NAG)

2. N-acetyl-D-muramic acid (NAM)

How are NAG and NAM bonded together?

Through a beta-1,4 glycosidic bond

What is beta-1,4 bond sensitive to?

Lysozyme

What are 3 natural things that have lysozyme?

1. Tears

2. Saliva

3. Egg white

What happens if the beta-1,4 bond is broken?

Then the cell will become destabilized

Why is it a good thing that lysozyme is present in our tears and saliva?

This is because it creates a defence against bacteria as the lysozyme will break the glycosidic bonds to destabilize the cell

How is NAG and NAM linked?

Through a tetra peptide linkage

What is the difference in the tetra peptide structure between gram positive and gram negative bacterium?

Gram negative: Have mesoDap (D,L-diaminopimelic acid) as the third peptide

Gram positive: Have L-lysine as the third peptide

List the tetra peptide structure

L-alanine, D-glutamic acid, mesoDap or L-lysine, D-alanine

What connects NAG and NAM?

Cross linking of tetra peptides

Where does the cross linking occur? What type of bond is used?

At the amino group of DAP or lysine and the C-terminal D-ala carboxyl group. peptide bond

Is it a direct or indirect linkage in gram negatives?

Direct link between DAP and D-ala

Is it a direct or indirect linkage in gram positive bacteria?

Indirect linkage

Describe the indirect linkage in gram positive bacteria

L-lys is attached to a pentaglycine (cross-bridge) which is attached to the D-ala

What is the difference between D and L amino acids?

L is the natural form while D is the modified form

Are the cross bridges conserved in gram positive bacteria?

No it varies among bacteria (can have different types of cross bridges)

What is the function of the cross bridge? Are they covalent linkages or ionic?

Provide rigid stability

Covalent linkages

What are the 3 steps of peptidoglycan biosynthesis?

1. Cytoplasmic

2. Membrane-associated

3. Cell wall- associated

What are two challenges associated with the membrane?

1. How to get hydrophilic molecules across the hydrophobic membrane

2. Getting soluble drugs to the target

What does GlcNAc stand for?

N-acetyl glucosamine (NAG)

What does MurNAc stand for?

N-acetyl muramic acid (NAM)

Describe the four cytoplasmic steps of peptidoglycan biosynthesis

1. UDP is added to NAG (GlcNAc)

2. UDP-NAG is converted to UDP-NAM

3. 5 amino acids are added enzymatically with the last addition being a D-ala dipeptide (D-ala-D-ala)

4. The UDP-NAM pentapeptide is transferred to C55PP resulting in the formation of lipid I.

What is the UDP-NAM pentapeptide known as?

Parks nucleotide

What is C55PP? What is it commonly known as?

It is a membrane bound carrier lipid called undecaprenyl phosphate.

Where does the D-ala dipeptide come from?

By racemazing L-ala using alanine racemase (an enzyme)

What is UDP?

It is uridine diphosphate and is a nucleotide precursor

What is the conversion of UDP-NAG to UDP-NAM inhibited by?

Phosphomycin

What inhibits the synthesis and addition of D-ala dipeptide?

D-cycloserine

Why can D-cycloserine get through the cytoplasm?

It is a small enough drug

What are 3 properties of undecaprenyl phosphate?

1. Is hydrophobic

2. Is large

3. Has a heavy carbon chain

What is undecaprenyl phosphate? What do they do?

A C55 isoprenoid phosphate that carriers precursors to peptidoglycan, LPS, and techie acids through the membrane