Pg subunits are held in the inner membrane by a

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PG subunits are held in the inner membrane by a hydrophobic carrier (bactoprenol) until they are added into the existing PG scaffold Added monomer is NAG-NAM- pentapeptide (or Lipid II) NAM of new subunit is added to NAG of existing chain Bactoprenol-PPi is recycled This step is transglycosylation
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After subunit is added to glycan chain, crosslinks are made between the peptide chains attached to each NAM
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Peptidoglycan synthesis 1) Lipid I = prenylated NAM-pentapeptide, synthesized by Mur pathway in the cytoplasm and inner membrane 2) Lipid II = NAG added to Lipid I by MurG enzyme in cytoplasmic membrane 3) Translocation of Lipid II to periplasmic (outer) face of the cytoplasmic membrane (enzymes unknown) 4) Transglycosylation = linking NAG of new monomer onto NAM of existing chain, by PBP1 (and unknown enzymes) 5) Transpeptidation = crosslinking new peptidoglycan strand into a sheet via D-ala-DAP linkage in G cells or L-lys-D-ala linkage in G + cells, by PBP1, PBP2, and PBP3
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Penicillin-binding proteins Identified in 1970s as proteins that bind radiolabeled penicillin Located in cytoplasmic membrane with soluble portion facing OUT or entirely in periplasmic space. High-MW PBPs (1,2,3) perform transglycosylation and transpeptidation outside the cytoplasmic membrane PBP1 has both activities, while PBP2 and 3 are only transpeptidases Low-MW PBPs are peptidases that remove amino acids or
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