Lecture 4 - Bacterial Outer Membrane.pdf

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University of Toronto St. George
Cell and Systems Biology
William Navarre

MGY377H © Lisa | Page 13 L E C T U R E 4 : B A C T E R I A L O U T E R M E M B R A N E DIFFERENCES BETWEEN Gm+ & Gm- CELL WALL Characteristics Gm- Gm+ Peptidoglycan yes (thin layer) yes (thick layer) Outer membrane yes no Periplasm yes no LPS yes no Lipoteichoic acid no yes Teichoic acid no yes Lipid content high low INNER (CYTOPLASMIC) MEMBRANE  present in Gm- & Gm+  made of a symmetric phospholipid bilayer (the phospholipids vary in composition & relative ratios in diff bacteria) – phospholipids consist of a glycerol backbone w head groups (derived from diff molecules) & fatty acid chains attached to hydroxyl group of glycerol  polar  membranes serve many functions 1. permeability barrier: prevents leakage & functions as a gateway for transport of nutrients into & out of the cell, regulates passage of charged molecules 2. protein anchor: site of many proteins involved in transport, bioenergetics, & chemotaxis 3. energy conservation: site of generation & use of proton motive force (higher [H] conc outside) PERIPLASM (PERIPLASMIC SPACE)  present only in Gm- bacteria, not in Gm+  located in the space bw the outer membrane & cytoplasmic membrane (includes spaces bw strands of PG) – outside of CM  in Gm+ bacteria, PG is outside of CM, but PG isn’t a layer, it is very permeable (space bw PG & BM isn’t considered a chemical barrier)  is a dynamic, changeable matrix of materials  is iso-osmotic w cytoplasm  both periplasm & cytoplasm have the same # of molecules & ions per unit volume  the periplasm contains: 1. PG is the main constituent & is highly hydrated & expanded (gel-like) 2. proteins associated w the PG 3. detoxifying enzymes –-lactamase & other enzymes that inactivate antibodies 4. scavenging hydrolytic enzymes – degrade compounds that are too large or too highly charged to pass through the CM ex. alkaline phosphatase, asparaginase 5. nucleases – ribonucleases, deoxynucleases 6. periplasmic binding proteins – soluble proteins that binds specifically to solutes (ex. AAs, sugars, vitamins) 7. lipoproteins (Braun’s) & structural proteins (OmpA) – stabilizes the outer membrane  there are a variety of diff classes of proteins in the periplasmic space w a variety of fxs MGY377H © Lisa | Page 23  primarily as a defense mechanism or serves as a way to take up nutrients from the envmt OUTER MEMBRANE  a structure unique to Gm- bacteria  an asymmetric lipid bilayer consists of phospholipids (inner leaflet), lipopolysaccharides (LPS; outer leaflet), & protein  ~8 nm thick  a strong permeability barrier to hydrophobic compounds & large hydrophilic agents (small hydrophilic agents can be transported through porins)  thus, is responsible for resistance to various antibiotics & chemotherapeutic agents  the outer membrane contains: 1. phospholipids 2. lipopolysaccharides (LPS)  a unique constituent of bacterial OM  composed of 3 parts 1) the proximal, hydrophobic lipid A region 2) the distal, hydrophilic O-antigen polysaccharide region the protrudes into the medium 3) the core oligosaccharide region that connects the two other regions LIPID A  contains the hydrophobic, membrane-anchoring region of LPS  consists of a phosphorylated N-acetylglucosamine (NAG) dimer (phosphate group provides a negative charge) w 4-7 fatty acids attached  all FA in lipid A are saturated  lipid A fatty acid chains are associated w phospholipids of the inner leaflet & form the outer leaflet of the OM (hydrophobic region)  are attached to core oligosaccharides through KDO (2-keto-3-deoxyoctonic acid), which is -vely charged 2+ 2+  -vely charged lipid A allows LPS to bind +vely charged molecules, divalent cations Mg & Ca  allows outer membrane to be stabilized MGY377H © Lisa | Page 33  there are some variations in lipid A in terms of the sugar dimer, phosphorylation of the sugars,& # of fatty acids  amount of FAs determines the hydrophobicity CORE OLIGOSACCHARIDE OF LPS  consists of a short chain of sugars ex. KDO-Hep-Hep-Glu-Gal-Glu-GluNAc  2 unusual sugars are usu present in the core polysaccharide: 1. heptose 2. KDO (is unique & invariably present in LPS, serves as an indicator in assays for LPS)  w minor variations, the core polysaccharide is common to all members of a bacterial genus but is structurally distinct in other genra of Gm- bacteria  Salmonella, Shigella, & Escherichia have similar but not identical cores O-POLYSACCHARIDE OF LPS  consists of repeating oligosaccharide subunits made up of 3-5 sugars  length ranges up to 40 repeat units (much longer than the core polysaccharide)  it maintains the hydrophilic domain of the LPS  great variation occurs in the composition of the sugars in the O-polysaccharide sidechain bw species & even strains of Gm- bacteria  at least 20 diff sugars are known – many are characteristically unique dideoxyhexoses which occur in nature only in Gm- cell walls  variation in sugar content contribu
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