3 - The Cell Wall Flashcards
Functions of cell wall
- Provides shape
- Helps anchor flagella
- Prevents cell bursting
- May contribute to pathogenicity
Cell wall structure
- Composed of peptidoglycan (murein)
- Mesh like polymer enclosing the cell wall (peptidoglycan sacculus)
- Repeating NAG and NAM molecules in a chain
What does NAG stand for
N-acetylglucosamine
What does NAM stand for
N-acetylmuramic acid
What bond are sugars linked by in cell wall
- Beta 1, 4, glycosidic bonds
- Sugar chains are cross linked by tetrapeptides (4 alternating D- and L- amino acids, connected to NAM)
Peptidoglycan cross links
- The presence of D-amino acids protects against most peptidases which recognise only L-amino acids in proteins
- Three of the amino acids are not found in proteins: D-glu, D-ala, mesodiaminopimelic acid (DAP)
Peptidoglycan sacculus
- Strong but elastic
- Able to stretch and contract in response to osmotic pressure
Gram positive cell wall
- Comprises many peptidoglycan layers forming a dense network
- Cell wall is negatively charged
- In gram stain, ethanol shrinks the pores of thick peptidoglycan, thus crystal violet is retained and stains bacteria purple
Why is cell wall negatively charged
due to peptidoglycan amino
acids and phosphate groups on teichoic acids
Lipoteichoic acids
Covalently bound to plasma membrane lipids (anchor wall to PM)
Functions of teichoic acids
- Taxonomically useful
- Maintain cell envelope structure by anchoring wall to PM
- Some involved in binding pathogenic bacteria to host tissues
- Help protect cell from harmful chemicals (e.g. antibiotics)
- Function in ion uptake and cell division processes
Gram negative cell wall
- Does not contain teichoic acids
- Located in periplasmic space (periplasm)
- Has one or a few layers of peptidoglycan
- Not as highly cross linked as gram +’ve PG (larger pores, crystal violet removed and counterstain pink)
Where does cell wall synthesis occur
The cytoplasm, plasma membrane and periplasm
Steps of cell wall synthesis in the cytoplasm
- UDP derivatives of NAM and NAG are made
- Sequential addition of amino acids to UDP-NAM to form NAM-pentapeptide (has additional D-alanine)
Steps of cell wall synthesis in plasma membrane
- NAM-pentapeptide is transferred to BP phosphate via pyrophosphate bond
- UDP transfers NAG to BP-P-P-NAM-Pentapeptide
- Bactoprenol carrier transports completed NAG-NAM-pentapeptide unit across the membrane
How does NAG-NAM pentapeptide cross the membrane
- Flipped across by flippase enzyme
- BP-P-P remains embedded in the membrane
Steps of cell wall synthesis in the periplasm
- NAG-NAM-pentapeptide is attached to the peptidoglycan
chain, increasing its length by one repeat unit (transglycosylation) - Bactoprenol carrier moves back across membrane, losing a phosphate, ready to begin a new cycle (BP-P-P –> BP-P)
- Peptide cross-links between glycan chains are formed by transpeptidation, mediated by transpeptidases
Transpeptidation
Terminal alanine removed to
give a tetrapeptide (TP): this
provides energy to link the
peptides
Antibiotics which inhibit cell wall synthesis
Penicillins (e.g. penicillin, ampicillin, methicillin) and cephalosporins (e.g. cephalothin)
how do antibiotics which inhibit cell wall synthesis work
- Block murein cross-linking by binding to transpeptidases
- β-lactam ring resembles D-alanine-D-alanine in murein pentapeptide
- Transpeptidase binds to penicillin instead of its true substrate
Why do β-lactams cause cell lysis
- Penicillins (PC) act only on growing bacteria
- Penicillin binding proteins (PBPs) first noted capacity to bind PC
- PBPs include transpeptidases, and some autolysins
- Penicillins bind to PBPs and may thus destroy bacteria by disregulating the autolysins
What are autolysins
- Enzymes that hydrolyse bonds in existing PG to make room for new units e.g. endopeptidases
- Autolysins normally function in well-regulated way to allow cell growth and division
- Autolysin activity is tightly controlled as unregulated activity would weaken cell wall
Where is lysozyme found
Sweat glands, mucous membranes (eyes, respiratory, digestive and urogenital tracts)
How does lysozyme attack gram positive cell wall structure
- Breaks β-1,4 glycosidic bonds (NAM-NAG)
- Lysozyme-treated Gram positive cells form protoplasts
