PM and Cell wall Flashcards
How is bacterial PM composed?
50% lipid (among that 50% is phospholipids), 50% protein.
Bacteria:
2 Unbranched (Linear) FAs with ester linkage to glycerol and 1 phosphate.
PM not rich in sterols,
How is archaeal PM composed?
Branched isoprenoid chains with ETHER linkage to glycerol + 1 phosphate.
Unsaturated.
= Branched, unsaturated, Fatty alcohols!
How can bacterial PM lipid content differ?
Phosphatidic acid can be conjugated with Serine, Choline or Ethanolamine.
To form:
Cardiolipin (CL)
Phosphatidyl-ethanolamine (PE)
Phosphatidylglycerol (PG)
Gram -ve bacteria have more PE
Gram +ve have more PG and CL.
How can lipid content affect the charge of bacterial PMs?
Gram +ve have more PG and CL lipids
Gram -ve have more PE lipids.
PG is negatively charged, whilst PE is neutral.
Therefore, Gram -ve have more neutrally charged PE than Gram +ve which have more negatively charged PG
= Gram +ve have more negative PM.
How does Archaeal and bacterial PM differ>
Ether linkage of glycerol to 2 branched, isoprenoid, unsaturated Fatty Alcohols.
Some parts of archaeal PM is monolayer, where has very long fatty alcohol chains.
= Confers local lower fluidity and reduced permeability.
The branched, unsaturation of isoprenoid provides resistance to dissociation in hyperthermophilic archaea.
How does PM change with respect to temperature?
Unsaturation?
Lipid movements - rotation, chain rotation, lateral diffusion or flip-flop
Each membrane has transition temp - 50% of PM is fluid-like.
Longer chain FAs confer higher transition temperature.
Unsaturation gives lower transition temperature.
Archaeal PM have unsaturation, but can have v. long chain Fatty aclohols.
Are sterols found in bacterial PM?
Archaea?
MOST bacteria lack sterols.
except Spirochetes, Mycoplasma… = MOLLICUTES
Bacteria can have rafts - containing HOPANOIDS.
Archaea have no sterols/hopanoids.
How can sterols/Hoppanoids affect PM fluidity?
Sterols and hopanoids can act as fluidity buffer molecules - serve to rigidify membrane portions.
Especially important when lacking cell walls.
- MOLLICUTES - contain sterols and lack a cell wall.
What are extrinsic and intrinsic proteins?
Intrinsic proteins can be TM or anchored in PM.
= Strongly bound to PM, with TM proteins extending across 2 faces of envelope.
= NEEDS DETERGENT AND ORGANIC SOLVENTS TO REMOVE
= Lipidation, or transmembrane alpha helices/beta barrels…
Extrinsic proteins are weakly bound to PM, and can be only present on of 2 sides
= can have hydrophobic loops, electrostatic binding and in plane alpha helices to anchor..
What is the function of PM?
Highly regulated barrier of permeability and osmosis.
Transport systems for solutes like nutrients and ions.
Involved in energy generation - as bacteria lack mitochondria - can have respiratory/photosynthetic transport systems - ATP synthases in all, but maybe not always ETC.
Coordinate cell wall synthesis.
What antibiotics target the PM?
Bactericidal and detergent-like antibiotics
Polymixins - effective against both -ve and +ve
Daptomycin - against Gram +ve membranes which are enriched with PG
What are the obligatory structures in Bacteria and ARchaea?
Gram +ve = PM, thick peptidoglycan cell wall.
Gram -ve = PM, thin peptidoglycan cell wall, outer membrane
Mollicutes - PM
Archaea:
Gram +ve = PM, thick pseudomurein cell wall
Gram -ve = PM, and outer membrane.
What facultative structures cna be foundd in bacteria and archaea?
Gram +ve = can have an S layer and Polysaccharide capsule.
S-layer = glycoproteinaceous surface monolayer…
Gram -ve = S layer and capsule
Mollicutes = capsule
Archaea do not have capsules, but can both have S layer.
How does Gram staining work?
Crystal violet enters cytoplasm.
Mordant fixes colour
Alcohol is decoloriser, passing into cell to remove crystal violet.
Thick peptidoglycan cell wall blocks alcohol entry in Gram +ve
= purple
Add Safranin counterstain = pink Gram -ve
What can be found in bacterial cell wall?
Gram -ve:
PM, then lipoproteins can be found in periplasmic space.
Thin peptidoglycan layer
Outer membrane, then LPS with porins.
Gram +ve:
PM, with lipoteichoic acids and teichoic acids, with lipoprotein.
Thick peptidoglycan layer.
What is the composition of peptidoglycan?
N-acetylglucosamine (NAG),
N-acetyl muramic acid (NAM)
Alternating NAG-NAM glycans joined with Beta 1,4-glycosidic bonds.
Glycan chain NAMs are linked with lateral, tetrapeptide chains and peptide interbridges or Interpeptide bonds.
How are glycan chains linked in Gram +ve or Gram -ve chains?
Glycan chain NAMs are linked with lateral, tetrapeptide chains and peptide interbridges or Interpeptide bonds.
Gram -ve bacteria like E.Coli have Interpeptide bonds.
DAP and D-ALA of NAM tetrapeptide chains.
Gram +ve bacteria have INTERPEPTIDE BRDGES
Additional glycine residues on NAM link to Lysine of other NAM.
What is the general outline of the stages of Cell wall synthesis?
Cytoplasmic stage:
Synthesis of precursors, UDP-NAM-Pentapeptide and UDP-NAG.
Membrane stage:
Transport across PM with Lipid transporter
Lipid II-NAG-NAM-pentapeptide.
Parietal stage:
Bind to existing peptidoglycan.
Formation of inter-peptide bridges, with loss of terminal Alanine from NAM-pentapeptide.
How are peptidoglycan chains assembled in the final stage of cell wall synthesis?
Parietal stage:
Cross-linking of peptidoglycan chains occurs via PBPs
Penicillin-binding proteins.
PBPs are ALL transpeptidases.
(some can bind NAG-NAMs are transglycolyases)
During transpeptidation, terminal D-Ala is removedto form tetrapeptide, linking D-ala and DAP to form interpeptide bond (Gram -ve)
What classes of antibiotics target cell wall?
Beta lactams:
Penicillins, cephalosporins, carbapenems, monobactams….
Glycopeptides - Vancomycin, teicoplanin
Fosfomycin
= ALL BACTERICIDAL!!!
What is the MOA of beta-lactams?
FIxation to PBPs - inhibition of transpeptidation.
= Prevents cross-link formation during terminal, parietal stage of cell wall synthesis.
How do glycopeptides work?
Against cell wall synthesis.
= Vancomycin, teicoplanin
Inhibition of peptidoglycan synthesis.
Formation of complex with D-ala-D-ala peptidyl residues of peptidoglycan precurssors after emerging from PM.
What is MOA of Fosfomycin?
Prevent formation of NAG:
Inhibition of Pyruvate-NAG transferase.
= Prevents NAG precursor synthesis and therefore peptidoglycan synthesis,
What is lysozyme?
Lysozyme is an acid hydrolase - hydrolyses Beta-1,4 glycosidic bonds between NAG and NAM in peptidoglycan chains.
Present in secretions like tears, mucus, saliva.
