Cell Wall and Cell Membrane Drugs

Main role of peptidoglycan in cell wall

Provide shape and rigidity

Difference between cell wall in gram positive and gram negative

More peptidoglycan in gram positive, no outer membrane

Gram negative has inner membrane, smaller peptidoglycan, outer membrane and then LPS

Stages of peptidoglycan synthesis (overview)

1. Precursor formation in cytoplasm

2. Transport and peptidoglycan formation at cell membrane

3. Crosslinking peptidoglycan chains outside cell

What is lipid II in E. coli?

The unit of peptidoglycan that is flipped across cytoplasmic membrane to build cell wall

Structure: Undecaprenol linked by pyrophosphate to PG building block composed of a NAG-NAM disaccharide and a pentapeptide stem

Steps of lipid II formation and flipping

Precursors (UDP-NAG and UDP-NAM-Pentapeptide) made in cytoplasm

Latter linked to Undecaprenol-P to form lipid I

MurG uses lipid I and UDP-NAG to make lipid II

Lipid II flipase translocates lipid II across inner membrane

Name of lipid II flippase protein (in many bacteria)

MurJ

How is lipid II flipped by MurJ?

Lipid II enters and binds to C-lobe

Sodium binds C-lobe, closing entry (lipid II trapped)

Opens on other side, chloride ion binds releasing lipid II on other side

MurJ returns to usual shape

Flippase in S. aureus

FtsW

What antibiotics inhibit precursor synthesis? How do they work?

Cycloserine: Alanine analogue—inhibits conversion of L-ala to D-ala meaning lipid II can’t be built

Fosfomycin: Phosphoenol Pyruvate analogue—binds to pyruvate transferase meaning NAG can’t convert to NAM

What antibiotic inhibits transport? How does it work?

Bacitracin: inhibits dephosphorylation of bactoprenol/C55, meaning it can’t be reused to flip more lipid II across, stopping synthesis

Main uses of bacitracin

Topical treatment of gram positives

Often used with other topical abx for skin infections, tattoo aftercare etc

Some features of non-ribosomal peptide synthesis plus example antibiotic

Unique structural features eg D-amino acids

Have hundreds of different building blocks compared to proteins made by ribosomal synthesis

Made of modules which can be further broken down into domains

Bacitracin

Antibiotics inhibiting crosslinking peptidoglycan chains

Penicillins

Cephalosporins

Glycopeptides

How can glycopeptides etc potentially reach gram negative cell walls?

Via porins, but many not able to

Examples of glycopeptides

Vancomycin

Teicoplanin

Vancomycin overview

Non-ribosomal peptide synthesis

Binds D-Ala-D-Ala, thus inhibiting transglycolase: no cross-linking

Used for MRSA

IV only

How do beta-lactam antibiotics work?

Bind to penicillin binding protein (PBPs) as they resemble D-Ala-D-Ala

This inhibits transpeptidation, preventing cross-linking

Leads to cell lysis

Functions of PBPs

Transpeptidases involved in peptidoglycan synthesis

Maintenance of rod shape

Septum formation

Groups of beta-lactam antibiotics

Penicillins

Cephalosporins

Carbapenems

Monobactams `q1111111111111111111111111111111111

First gen cephalosporins

Gram-positive only

Second generation cephalosporins

Gram negative

Some activity against G+ cocci

More resistant to beta-lactamase than gen 1

Third generation cephalosporins

Broader spec

Can penetrate CNS

Ceftazidime can treat pseudomonas

Fourth generation cephalosporins

Greater spectrum again

Greater resistance to beta-lactamase

Can cross BBB

Anti-pseudomonas

What are monobactams?

Beta-lactams but only have one beta-lactam ring

Only work on gram negatives

Aztreonam only drug in class

Carbapenem overview

Broad spec beta-lactams; very resistant to beta-lactamases

Meropenem ad Imipenem examples

Broadest spectrum of all beta-lactams

Examples of antibiotic classes that bind cell membrane

Polymyxins and colistin

Destroys membranes

Active against G-

Serious side-effects

Mostly derm/eye infections

Polymyxin overview

Only used if less toxic antibiotics ineffective

Eg Pseudomonas, other highly resistant bacteria

Cyclic peptides, bind phospholipids like detergent

Neuro- and nephrotoxic

Daptomycin overview

Relatively new cell membrane antibiotic

G+ infections

Get rapid depolarization: loss of membrane potential, inhibiting synthesis of DNA, RNA, protein

Non-ribosomal peptide synthesis