Peptidoglycan and the gram positive cell wall II

Describe the five membrane steps

1. Formation of a b-1,4 linked NAG-NAM disaccharide on the C55PP (carrier) generates lipid II

2. Modification of the cross-bridge depending on the type of species

3. Flipping of the structure to the external membrane

4. The disaccharide unit is transferred from the lipid carrier to the backbone peptidoglycan chain by transglycosylation

5. Undecaprenyl pyrophosphate is recycled

What flips the structure to the external face? Is it energy dependent?

Flippase and no

What is transglycosylation?

The transfer of a sugar residue from one glycoside to another

Where is the disaccharide being joined to on the peptidoglycan backbone?

At the L-ala

What inhibits the formation of a B-1,4 linked NAG-NAM disaccharide on the carrier lipid?

Lysozyme

What inhibits the transfer of the disaccharide unit from the lipid carrier to the backbone chain?

Vancomycin and Ristocetin

What is the recycling of undecaprenyl phosphate inhibited by?

Bacitracin

What is vancomycin?

It is a last resort antibiotic and is a large molecule that cannot enter the cell. Therefore it acts externally.

What is bacitracin?

It is a small antimicrobial peptide that can complex with the lipid

Describe the 2 steps in cell wall peptidoglycan biosynthesis

1. Formation of peptide crosslinks by transpeptidation

2. The terminal D-ala is removed from the pentapeptide by carboxypeptidase

What is peptidase?

It is an enzyme that breaks peptide bonds

What inhibits the formation of peptide crosslinks by transpeptidation?

Penicillins

What inhibits the terminal D-ala being removed from the pentapeptide?

Vancomycin

What does vancomycin bind to?

D-ala, D-ala

What is transpeptidation?

The formation of crosslinks between two peptide chains

What mediates the transpeptidation and transglycosylation?

Penicillin binding proteins

What are penicillin binding proteins?

They are bacterial proteins that are targets for beta-lactam antibiotics

What do penicillins inhibit?

Transpeptidation

How many different PBPs with varying molecular weights and function can bacteria have?

8

What are the 3 high molecular weight PBPs? What do they do?

1,2,3. They catalyze transglycosylation and transpeptidation reactions during peptidoglycan synthesis

What is PBP3 required for?

Peptidoglycan synthesis in the septum

What are the 3 types of low molecular weight PBPs? Do we need them?

4,5,6. No they are dispensable

What is beta-lactamase? How did it arise?

It is an enzyme that will degrade the beta-lactam ring on pencillins. Arose through mutations to their proteins

What is the beta-lactamas gene commonly used for?

An ampicillin-resistant marker on cloning plasmids

Due to the beta-lactamase gene, what is added to antibiotics to fight it?

Clavulanic acid

Are teichoic acids found in gram positive or gram negative?

Gram positive

What are TAs linked to?

Covalently linked to peptidoglycan

What is the LTA?

These are TAs that contain glycolipids which allow the molecules to anchor to the cell membrane (cytoplasmic membrane)

What is the role of TAs and LTAs?

They provide structural integrity in addition to the peptidoglycan

What are two polymers of techoic acids?

1. Glycerol teichoic acid

2. Ribitol techoic acid

What happens to these polymers once they are synthesized?

They are incorporated into the cell wall

In the polymers of TAs, what is NAG often substituted for?

D-ala but it is species specific

How is TA covalently linked to peptidoglycan?

Via a linkage unit made up of sugar residues

How is LTA anchored to the outer leaflet of the cytoplasmic membrane?

Via a glycolipid anchor

Is LTA hydrophobic, hydrophilic, or amphipathic?

Amphipathic