Lipopolysaccharide, periplasm, flagella Flashcards
What are lipopolysaccharides (LPS) and what are its components?
- large complex molecules containing lipid and carbohydrate units
- called endotoxins when free in host
made of: Lipid A, core polysaccharide, O side chain
What are lipopolysaccharides called when they are free in a host?
endotoxins - can induce septic shock/massive immune response
Which parts of an LPS are variable/constant?
o specific = variable
core polysaccharide + lipid A = constant
What is the structure of lipid A?
- 2 glucosamine + 1 phosphate each + linked to 2 fatty acids each: one GlcN has fatty acids with side chains
- lipid A integrated into outer membrane
- remaining LPS projects from the cell surface
What is the structure of the core polysaccharide?
(also called R-antigen or R-polysaccharide)
- 1-4 molecules of KDO
- unusual sugar residues e.g. glucose, galactose, heptulose (x2) in salmonella
- side chains of Glu-Nac, phosphate and ethanolamine++
What is the structure of the O-specific polysaccharide chain?
- highly variable composition (at least 20 diff sugars)
- rough/smooth bacterial variants depends on side chain length
- lipid A + core polysaccharide = straight, O specific = flexible and bent
Why is the O-specific polysaccharide significant for hosts?
They are the variable regions responsible for antigenic makeup of bacteria
- key diagnostic tool
- extend outwards from cell so is the first contact a cell makes with a host
- different O-sp polys linked to specific diseases
What are the functions of the LPS?
- lipid A stabilises the outer membrane
- core polysaccharide charged due to phosphate groups (-ve) = negative charge on surface
- charged, hydrophilic external layer = less permeable to hydrophobic substances e.g. bile salts, antibiotics
- protects against host defences
How does the LPS protect against host defences?
- rough variants more susceptible to phagocytosis
- loss of O-antigen (in E.coli and salmonella) = reduced virulence + less likely to be engulfed in phagocytosis
When are endotoxins released and why are they dangerous?
during cell division + lysis of bacterial cells - cause septic shock syndrome + no direct treatment
immunogenic even when the cell it originated from is dead - fragments of dead cell
LPS of some non-pathogens can also be an endotoxin
Why might endotoxins be useful?
can act to prime immune system against a pathogen as they are constantly in our bodies in small amounts - prep antibodies + immunological memory
Test for endotoxins?
Rabbit Pyrogen test
Limulus amoebocyte lysate (LAL) assay
How does the limulus amoebocyte lysate (LAL) test work?
amaebocytes - RBCs of horseshoe crab
contain a clotting factor that is released when bacterial endotoxin is present - natural immune mechanism against infection
What are the important properties of endotoxins?
- heat stable
- toxic in nanogram amounts
- triggers release of cytokines in a cascade, activates transcription factors such as interferon-β and tumour necrosis factor (TNF) which kills infected/dmged cells
(can result in: inflammation, fever, vasodilation, thrombosis, acute disseminated intravascular coagulation, depletion of platelets/clotting factors -> internal bleeding/hemorrhage, shock sometimes death)
What are porins and where are they located?
- homotrimetric transmembrane protein channels
- permit passage of small molecules up to 600 Da (molecular weight units in size)
- highly conserved structure
- located in the outer membrane -> make it more permeable than the inner membrane
- form water filled channels in OM -> links periplasmic space to the outside -> transfer water soluble molecules
- resistant to protease and detergent degradation: essential for the survival of gram-ve bacteria in harsh environments
How do larger molecules cross the outer membrane?
attached to carriers through ACTIVE TRANSPORT SYSTEMS, not through porins !
In what ways are porins selective?
- mostly non-selective (some in E.coli are selective for example: maltose and maltodextrin selective porin which is also the selective porin for lambda - bacteriophage)
- <600 Da only
- most porins slightly cation selective
What is the structure of a porin?
- 16 stranded antiparallel β
- exceptionally stable
- extra stability from formation of a salt bridge between the N- and C-termini
What is the cross section of a porin monomer like?
- hourglass channel with a central constriction
- hydrophobic band of 25Å that sits in membrane
- charges inside the pore define size of solute that can traverse the channel
What is the periplasmic space and its features?
- space between the outer and cytoplasmic membrane
- compounds diffuse via porins into periplasm
- 1-70nm in size
- can be up to 40% in cell vol
- gel like consistency due to lots of proteins
(removal of cell walls without lysing the cells allows study of the proteins and enzymes present in this space)
Enzyme Activity in Periplasm?
- Nutrient acquisition w/ hydrolytic enzymes
- Energy conservation e.g. electron transport proteins
- some peptidoglycan synthesis enzymes are periplasmic
- periplasmic binding proteins: deliver specific compounds to ABC transporters in cytoplasmic membrane
- chemoreceptors: involved in chemotaxis
Why are proteins exported into the periplasm?
SEC PATHWAY
- most/all proteins made in the cytoplasm
- needs translocating to the periplasmic space to be folded/modified
- ribosome attached to cytoplasmic side of membrane + pushes proteins through a translocase into periplasm
- each translocase is v specific to the protein so proteins have an N-terminal signal peptide
What is the TAT pathway? (twin arginine translocase)
exports fully folded enzymes across cytoplasmic membrane
proteins have twin arginine in N terminal region
What are the 3 classes of membrane transporting systems?
simple transport, group translocation, The ABC system
