Structure - Peptidoglycans Flashcards
What is the function of peptidoglycans?
Maintenance of cell shape and preservation of cell integrity, resisting turgor.
What is the general structure of peptidoglycans?
Linear glycan strands with peptide stems cross0linked by short peptides.
What are glycan strands composed of ?
N-acetylglucosamine and N-Acetylmuramic acid linked by 1-4 bonds.
Where are the cross linkages found?
On the amino stems that substitue on MurNAc residues.
Where do the variations in PG exist?
Peptide stem, glycan strands and composition of interpeptide bridge.
What is an example of PG differentiation?
In Staphyloccus Aureus where the reducing end of the glycan contain either MurNAc or GlcNAc.
Why is coupling of PG cell wall with essential functions important in medicine?
It makes it a good target for antibiotics.
Where is the PG wall found in GNB?
The periplasm between OM and IM.
What is the structural differences of the PG cell wall in GNB and GPB?
In GNB it is a monolayer, whilst GPB it has multiple.
Why does GPB have a thicker PG?
As it lacks an OM thus is exposed to external environment.
What are the two additional structural constituents of GPB?
Teichoic Acids and Mycolic Acids
Teichoic Acids
These contribute to surface charge and hydrophobicity, affecting binding of extracellular molecules.
Mycolic Acids
Long chain FA important in virulence and viability.
What is the function of myolic acids?
Impede chemical entry, causing slower growth in organisms with more resistance to chemical agents.
What drives shape generation?
Asymmetric PG synthesis driven by cytoskeltal elements.
What is the first step of PG synthesis?
Cytoplasmic stage where the UDP-N-AcetylMurNAc precursor and UDP-N-Acetylglucosamine precursor are synthesised.
What is the second stage of PG synthesis?
Cytoplasmic membrane stage where precursors form lipid intermediates; Phosphate MurNAc transfers to bactoprenol yielding Lipid 1, with addition of UDP-N-Acetylglucosamine forming Lipid II.
Bactoprenol
This cycles PG monomers through the PM, inserting them at points of growth in the cell wall in GPB.
Why is bactoprenol lipophilic property important?
Enables cell to transport hydrophilic precursors from aqueos environment of the cytoplasm through the hydrophobic membrane.
What is the third stage of PG synthesis?
Outer side of cytoplasmic membrane where LipidII is polymerised, incorporating into growing PG, facilitated by PBP.
Pencillin Binding Proteins
These catalyse transglycolysation and transpeptidations reactions responsible for formation of glycosidic and peptide bonds on the pg.
Teichoic Acids
These are anionic polymers within GPB cell walls, composed of repeating glycerol or ribitol phosphate polymers.
What is the function of Teichoic acids?
Structural integrity with negative charged phosphates.
Mycolic Acids
These are long-chain fatty acids found in lipid-rich cell walls of mycobacteria.
What are the functions of mycolic acids?
Immunoreactions
Form wax-like layer to protect against nvironmental stressors.
What are the functions of the PG CELL WALL?
Support/rigidity, osmotic pressure resistance and host immune system interaction.
What is the first step in PG synthesis?
The precursor UDP-bound NAG and NAM are synthesised in the cytoplasm from glucose.
What is the second step in PG synthesis?
Transport into the periplasm, assembled into a PG chain by glycosyltransferases, transpeptidases and carboxypeptidases
What are the functions of the three PG enzymes in the periplasm?
GT adds precursors to growing PG chain
TP crosslinks through peptide bond formation
CP remove D-alanine to facilitate cross linkage.
What is the function of bactoprenol?
Covalent attachment of the finished cross-linked molecule to the cell membrane.
How can PG wall influence shape of the cell?
3D mesh like structure of repeating sugars and short peptide chains.
What are examples of PG cell wall determining shape?
Helical arragnement forms cylindircal rod-shaped bacteria.
Penicillin Binding Proteins
These are membrane associated proteins involved in the biosynthesis of PG.
