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Cell walls Gram-positive cell walls Thick layer of peptidoglycan surrounding the plasma membrane Contain teichoic acids Teichoic acids Polymers of glycerol or ribitol joined by phosphate groups Amino acids or sugars are attached to glycerol or ribitol groups Teichoic acids Can be attached to either peptidoglycan or membrane lipid lipoteichoic acid May contribute to negative charge of cell surface Are not found in gramnegative bacteria Gram-positive periplasmic space Periplasmic space separates membrane from peptidoglycan layer Gram-negative cell walls Thin layer of peptidoglycan surrounded by outer membrane Outer membrane provides addition barrier Porins Channel-forming proteins found in the outer membrane Allow for selective permeability of outer membrane Porins Usually associate in copies of 3 individual monomers Can allow nonspecific or specific passage of molecules < 700 daltons across the membrane Lipopolysaccharide (LPS) Large complex molecule Found in the outer leaflet of the outer membrane Not found in gram-positive bacteria Lipopolysaccharide (LPS) Consists of three parts: 1. Lipid A 2. Core polysaccharide 3. O side chain Lipid A Composed of 2 glucosamines attached to fatty acids and phosphates Is embedded in the outer membrane Is the toxic part of LPS (endotoxin) Core polysaccharide Is joined to Lipid A In Salmonella is composed of 10 sugars (many unusual) e.g. KDO (2-keto-3-deoxyoctonate) O-side chain Most variable region of LPS Composed of a variable number of sugars Rapid changing of LPS can help pathogens avoid immune system Contributes to the negative charge of cell surface O-side chain Stability of LPS is enhanced by presence of divalent cations (Mg 2+) Cations form bridges between O-side chains on different LPS molecules Braun’s lipoprotein Most abundant outer membrane protein Covalently attached to peptidoglycan Hydrophobic end embedded in outer membrane Gram-negative periplasmic space Two theories regarding periplasmic space Thin layer of peptidoglycan surrounded by fluid-filled space Gram-negative periplasmic space Two theories regarding periplasmic space Thin layer of peptidoglycan surrounded by fluid-filled space or Peptidoglycan exists as a gel-like material that fills the space Peptidoglycan Composed of alternating NAG and NAM with short peptide chain attached to the NAM Some amino acids in the D form (not found in proteins) Protects against attack by peptidases Peptidoglycan Peptidoglycan synthesis Basic unit is NAG-NAM-pentapeptide Synthesized in cytoplasm Binding of UDP to sugars activates NAG and NAM Bactoprenol acts as a carrier Bactoprenol Very hydrophobic molecule Allows for transport through the interior of the membrane Peptidoglycan synthesis Bactoprenol transports NAG-NAM-pentapeptide across the membrane NAG-NAM-pentapeptide is transferred to growing peptidoglycan Bactoprenol is transferred back to the cytoplasm Effects of antibiotics on peptidoglycan synthesis Cycloserine blocks conversion of L-Ala to D-Ala Vancomycin blocks peptidoglycan synthesis by inhibiting formation of cross-bridges between strands Effects of antibiotics on peptidoglycan synthesis Bacitracin blocks the dephosphorylation of bactoprenol-PP Peptide cross-bridges Gram-negative: Amino acids directly joined via cross-bridge Peptide cross-bridges Gram-negative: Amino acids directly joined via cross-bridge Gram-positive: Glycine pentapeptide bridge joins amino acids Effect of penicillin on peptidoglycan synthesis Antibiotics belonging to the penicillin family interfere with peptidoglycan synthesis Inhibit formation of peptide cross-bridges Effect of penicillin on peptidoglycan synthesis Bind to proteins involved in peptidoglycan assembly Penicillin binding proteins (PBPs) Effect of lysozyme on peptidoglycan Lysozyme is found in many bodily fluids Cleaves bonds between alternating NAG and NAM subunits