G+ cell wall Flashcards by Brynn Walker

Sugar/Glycan component of Peptidoglycan

Basic units are NAG and NAM

-joined by ß-1,4 glycosidic linkages into long backbone chains

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NAG = ?

N-acetyl-D-glucosamine

*also: GlcNAc

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NAM = ?

N-acetyl-D-muramic acid

*also: MurNAC

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ß-1,4 glycosidic linkages are susceptible to what?

Lysozyme

-positively charged protein found in many human secretions

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What kind of peptide is in peptidoglycan?

A tetra peptide

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The main difference between tetra peptides of gram + and gram - is?

The third amino acid

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4 amino acids in the gram + tetrapeptide

L-ala-D-glu-L-lys-D-ala.

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The tetra peptide is linked to which glycan in the backbone?

NAM

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What is the 3rd amino acid for gram positive bacteria (like S. aureus)

L-lys

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What is the 3rd amino acid for gram negative bacteria (like E. coli)

mesoDap = D,L-diaminopimelic acid

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Direct cross linking of glycan chains by the tetrapeptide occurs in

Gram negative cells

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Cross linking via a cross bridge occurs in?

Gram positive cells

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What is the typical link between the tetrapeptides in direct linking

via a peptide bond between the amino group of dap or lys and the D-ala C-terminus.

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What are the 3 stages of peptidoglycan synthesis

Cytoplasmic steps
Membrane associated steps
Cell wall steps

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5 parts in the cytoplasmic step of peptidoglycan formation

NAG is activated by combining it with UDP
Conversion of UDP-NAG to UDP-NAM (inhibited by phosphomycin)
- 1st committed step?
Enzymatic addition of 5 amino acids of which the last dipeptide is D-ala-D-ala
- addition of D-ala dipeptide is inhibited by cycloserine
Transfer of the UDP-NAM pentapeptide (aka. Park’s nucleotide) to a membrane bound carrier lipid
* *this forms Lipid I

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Why is phosphomycin a bad antibiotic ?

Toxic to humans and bacteria can easily develop resistance to it

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how is the D-ala dipeptide formed?

By an alanine racemase which turns the natural L-ala into the unusual D-ala
-makes it a great target for antibiotics

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3 parts in the membrane step of peptidoglycan synthesis

Lipid I from the cytoplasmic step is turned into Lipid II by the formation of a ß-1,4 link between NAG and NAM (still on the carrier lipid)
The pentapeptide is then modified (occurs in a species specific way)
The disaccharide unit is transferred from the lipid carrier to the backbone chain by transglycosylation

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Which 2 drugs can inhibit transglycosylation

Vancomycin and Ristoceitin

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What is C55-P

A hydrophobic lipid carrier used to attach the pentapeptide to the membrane and helps transport the peptidoglycan precursor to the outer part of the membrane

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2 cell wall steps in peptidoclycan synthesis

Formation of peptide crosslinks by transpeptidation

2. Removal of the terminal D-alanine on the pentapeptide by a carboxy-peptidase

Which class of antibiotics inhibits transpeptidation (formation of peptide cross links)

Penicillins

What is transglycosylation

transfer of a sugar residue from one glycoside to another

Describe the method of vancomycin activity

Vancomycin binds to the terminal D-ala-D-ala of the pentapeptide. The binding prevents transglycosylation (and to a certain extent transpeptidation) to occur.

*this is because vancomycin is a very bulky antibiotic and can effectively block the bacterial enzyme

Name one way in which bacteria can become resistant to vancomycin

By using a different terminal dipeptide | -vancomycin will only recognize D-ala-D-ala

Bacitracin - effect

Blocks release of Pi from lipid carrier [undecaprenyl-PP ---> undecaprenyl-P + Pi] -prevents the lipid carrier from reforming *toxic so it is only used topically in things like polysporin

Phosphomycin action

Inhibits MurA --> formation of UDP-NAM is prevented.

Cycloserine action

Competitive inhibitor of the D-alanine racemase and D-ala-D-ala synthetase. Blocks synthesis of D-ala-D-ala.

Penicillins action

aka. ß-lactams Inhibit penicillin-binding proteins, prevent transpeptidation

Bacteria may contain up to __ different penicillin binding proteins

What are the 3 high MW PBPs

PBP1, 2, and 3

what is the role of the high MW PBPs

catalyze transglycosylation and transpeptidation reactions in peptidoglycan synthesis

PBP3 is specifically required for?

Peptidoglycan synthesis in the septum

What are the 3 low MW PBPs?

PBP 4, 5, and 6

What is the role of the low MW PBPs?

appear to be dispensible

Alterations in the the PBPs can lead to what kind of resistance ?

Non β-lactamase β-lactam antibiotic resistance | -resistance to penicillins without having the β-lactamase enzyme

β-lactamase

Enzyme which degrades the ß-lactam ring on penicillins

3 types of cell wall hydrolases

Also called autolysins: N-acetylglucosaminidase, N-acetylmuramidase, endopeptidases

N-acetylglucosaminidase and N-acetylmuramidase work by....?

Cutting the glycan backbone of the peptidoglycan via lysozome action

Endopeptidases work by?

Cutting the peptide links (in the crossbridges?)

3 roles of autolysins

1. Essential for cell division 2. Required for morphogenesis and turnover - inserting new peptidoglycan into the cell wall 3. DNA release and biofilm formation - those things can only be released if the cell wall is cut

similarities in the structure of LTAs and TAs

Both are polymers of glyerol or ribitol phosphates | -common to have substitution groups like NAG or D-ala

TA linking

TAs are covalently linked to the peptidoglycan | -linkage is via a unit made of sugar residues

LTA linking

LTAs have glycolipids which allows them to anchor to the cell membrane -anchored to the outer leaflet

6 biological functions of LTA and TA

1. Facillitate adhesion - helps in colonization 2. Capture divalent cations 3. Binding site for some enzymes that can cleave peptidoglycan 4. Activate complement 5. Confer resistance to cationic antimicrobial peptides 6. Activate innate immune response - induce cytokine production

D-ala substitutions plan an important role in which functions of LTA and TA?

Resistance to antimicrobial peptides and activation of the innate immune response

Explain the role of D-ala substitutions in resistance to antimicrobial peptides

D-ala substitutions make the LTA more positively charged This increased positive charge repels cationic antimicrobial peptides -CAMPS insert into the cell membrane and cause cell death

Explain the role of D-ala substitutions in immune recognition

TLR 2 is involved? WHAT IS IMPORTANT ABOUT THIS?

Surface proteins can be associated with the cell wall by

Non covalent or covalent bonds

4 examples pf LPXTG proteins

Collagen adhesion protein Iron binding proteins -Critical for survival where iron is limited Cell surface pilins (in the pili) Protein A -Binds to Fc terminus of human immunoglobulins in a non-immune fashion

LPXTG proteins have a C-terminal.....

Sorting sequence in the protein It is a ~35aa stretch encompassing a conserved LPXTG motif (X is any amino acid) Proteins are amide-bonded to the peptide cross-bridge of peptidoglycan by an enzyme called sortase. Sortase is a transpeptidase and cleaves between the T and the G