Bacterial Cell Wall Flashcards
Give an overview of cell walls?
Cell walls common to many organisms: plants, bacteria, fungi
Cell walls of different biological kingdoms are all distinct, but all comprised of polymers of repeating subunits:
In plants – cellulose (a polymer of glucose)
In bacteria – peptidoglycan - a polymer of two related amino sugars cross-linked by peptides
Primary function is structural: protection from osmotic pressure and other physical forces
What is the importance of the bacterial cell wall?
A scaffold providing:
Supporting and protective mesh surrounding and protecting the cytoplasmic membrane from physical forces such as osmotic pressure
An anchoring point for those components of the bacterium that interact with its environment:
Extracellular proteins (including MSCRAMMs – microbial surface components recognizing adhesive matrix molecules)
Mycolylarabinogalactan (capsule of Mycobacterium tuberculosis)
What is the importance of the bacterial cell wall to humans?
Role in virulence (shock)
Immune system recognition
Major target for antibiotics
Fundamental biological interest
What is the bacterial cell wall made out of?
Peptidoglycan (murein)
This is a mesh like structure - but not a diffusion barrier as small/large proteins can pass through - including exotoxins
Monomer subunits:
NAM - N-acetylmuramic acid (MurNAc)
NAG - N-acetylglucosamine (GlcNAc)
NAM + NAG = basic subunit that is polymerised to form the cell wall structure
The sugars are very closely related and only differ at one position
N-acetylmuramic acid is the lactyl-ether of N-acetylglucosamine
Linked by beta 1,4-glycosidic bonds
There are small peptide bridges between the two sugars that undergo cross-linking = mechanical rigidity
It is essentially a crystal lattice of amino sugars
Describe the peptidoglycan structure in E.coli?
Contain some D-enantiomer amino acids
This has evolved to reduce the likelihood of proteolytic attack on the cell wall - due to protease evolving to attack L-amino acids
Meso-diaminopimelate - (an amino acid) forms links with an L-amino acid and a D-amino acid on an adjacent chain
Peptides in mature peptidoglycan are comprised of 4 amino acids but when synthesised initially they contain 5
This is to do with energy required for polymerisation synthesis on the outside of the membrane
Describe peptidoglycan in S.aureus?
Uses L-Lys and D-Ala for crossing linking - uses a penta-glycine bridge between the two peptide chains
Variations are in the oligopeptide stem - some sides are mostly invariant - position 4 is always alanine
What is the source of peptidoglycan in biosynthesis?
The basic subunit is N-acetylglucosamine - used as the basis for both sugars
N-acetylglucosamine is produced by the hexosamine pathway from fructose
The product is linked to UDP
D- alanine and D-Glutamate are formed from racemase enzymes to produce the D stereochemistry of the amino acids
What are the three stages of peptidoglycan synthesis?
Cytoplasm - synthesis of UDP-sugar pentapeptide
Membrane-bound (intracellular) - attachment of lipid carrier, addition of crosslinking amino acids and an extra carbohydrate
Extracellular - polymerisation and crosslinking of pentapeptide and carbohydrate to yield final polymer
Describe stage one of peptidoglycan synthesis?
Occurs in the cytoplasm
UDP-MurNAc pentapeptide is formed
UDP - allows coupling of other things by breaking this bond
Describe stage two of peptidoglycan synthesis?
At the membrane
MraY -> MurG
The lipid tail acts to tether the precursor in the membrane
Then there is enzyme catalysed flipping to the other side of the membrane by flippases
Describe stage three of peptidoglycan synthesis?
Transglycosylation - polymerisation of glycan strands
Transpeptidation - cross-linking between glycan strands
There is cleavage of the terminal D-alanine residue
They are tightly linked and need to occur sequentially
This requires energy - the oligopeptide is synthesised with an extra amino acid on the end, so the cleavage of this bond provides the energy to drive this
Can’t use ATP as it is being synthesised on the other side of the membrane
Both reactions catalysed by Penicillin-binding proteins (PBPs)
Bi-functional enzymes, with two separate domains
Membrane-associated enzymes
What is gram-positive and gram-negative bacteria?
Gram positive - cytoplasmic membrane with a surrounding thick peptidoglycan wall
Gram negative -cytoplasmic membrane with a thin peptidoglycan wall (less comprehensively cross-linked), with another additional outer membrane
Describe a gram-negative cell wall?
Contains other useful constituents such as:
Lipoprotein - called Braun’s
Very abundant with an Mr - 7 kDa
C-terminal end has a lys residue that is covalently linked to meso-diaminopimelic acid is peptidoglycan
Hydrophobic head that buries into the outer membrane
This co-ordinates the cell wall with the outer membrane providing structural rigidity
Describe a gram-positive cell wall?
Contains:
Teichoic acid - polymers of glycerol-phosphate
Diverse in structure and multiple types in many given species
Can be anchored/attached in many different places e.g.
○ N-acetylmuramic portion
○ Terminal D-alanine in the tetra-peptide cross linkages
○ To the cytoplasmic membrane - therefore called lipoteichoic acid
Also providing co-ordination between the cell wall and the membrane = additional rigidity
What are some prokaryotes with unusual walls?
Archaea - S-layer, a thick mat of glycoprotein, tethered to the membrane, larger pores
Chlamydiae - bacterial pathogen that cause disease with no peptidoglycan wall
Large cysteine rich proteins that make up the p-layer - equivalent to the cell wall
Tethered to the outer membrane by small cysteine rich proteins - comparable to the Braun lipoprotein
