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Lecture 7 PMB
Fichas del conjunto (16)
- 1G- vs G+G(-): IM < Peptidoglycan < OM (cell envelope)
--> PG thinner and one continuous layer, have an OM
G(+): IM < Peptidoglycan
--> PH thicker and have multiple layers - 2Gram Positive Cell WallPeptidoglycan surface: Teichoic acids (bind Ca++ and Mg++), lipoteichoic acids (modify shape and division), other transmembrane proteins
- 3Gram Negative Cell EnvelopeOM attached to PG by lipoproteins
Periplasm in between OM and IM
OM has porins for permeability, surface has lipo-polysccharides & proteins - 4lipopolysaccharides (LPS)Lipid A (ENDOTOXIN): attached to OM; fatty acids linked to sugar phosphates, which bind Ca++ to exclude hydrophobic comp.s
Core polysaccharide: attached to Lipid A, contains sugars and KDO
O-specific polysaccharide (O-antigen): species/strain-specific; made of repeating branched hexoses; binds plant/animal host tissues; helps evade immune response - 5What does the hydrophyllic nature of LPS help the cell to do?--> protects the cell surface from bile salts, hydrophobic antibiotics, and complement activation
- 6peptidoglycan sacculusone large molecule...
determines the shape of the cell
glycan chains run around circumference; peptide crosslinks connect glycan chains
NOT a static structure, is constantly recycled - 7What is the PG wall useful for?protects against lysis in hypotonic environments!!
- 8disaccharide pentapeptidea monomer of PG
for each: NAG and NAM connected in glycan chain (contains L- and D-amino acids); DAP helps form peptide crosslinks - 9Mur pathwayPG monomer attached to bactoprenol in cytoplasmic membrane --> forms Lipid II complex, which is flipped by MurJ into periplasm --> transglycosylation adds monomer to growing glycan strand, bactoprenol recycled --> transpeptidation adds crosslinks between peptide chains attached to NAM molecules
- 10Penicillin binding proteins (PBPs)--> located in cytoplasmic membrane with soluble domains facing the periplasm
High-MW: do transglycosylation and/or transpeptidation outside the cytoplasmic membrane (1 does BOTH, while 2 & 3 are only transpeptidases)
Low-MW: peptidases; make peptide crosslinks, remove amino acids, or break peptide crosslinks in mature peptidoglycan - 11Lytic transglycosylasesbreak the β(1,4) linkages btwn NAG & NAM
Ex: Lysozyme - 12SEDS proteins--> shape, elongation, division, and sporulation
can transglycosylate --> can substitute for PBP1 proteins
RodA is the SEDS protein that functions in lateral walls; FtsW functions at division site - 13Normal transpeptidation mechanism1) serine on PBP forms covalent intermediate with the penultimate D-Ala,
releasing the terminal D-ala
2) PBP-D-ala intermediate attacked by NH2 group of DAP on a neighboring peptide
--> forms crosslink, releases PBP enzyme - 14Penicillin Gβ-lactam antibiotic that mimics the D-ala-D-ala residues at the end of PG
peptides --> accidentally attacked by DAP on peptide during transpeptidation --> β-lactam ring broken, irreversible intermediate formed --> PG cross links stop forming + continues to be lysed --> cells burst due to osmotic pressure - 15β-lactamasesenzymes that break the β-lactam ring of penicillin, inactivating the antibiotic
- 16How do PBPs, LTGs, and SEDS proteins influence cell shape?--> work at specific positions at specific times
PBP2/RodA depletion (rodA ts): circular cell, no lateral cell walls
PBP3 depletion (ftsI ts; ftsW ts): cell elongated, no septa
PBP1 depletion: no shape at all