Lecture 4 - Outer membrane Flashcards
Describe the general structure of the OM
- Asymmetric hydrophobic lipid bilayer
- Outer leaflet = LPS
- Inner leaflet = phospholipid
Describe the composition of the lipid A component of LPS
Glucosamine phosphate disaccharide with fatty acid side chains embedded in outer leaflet
Compare and contrast the structure of lipid A of LPS to a phospholipid
- Glycerol for phospholipid not glucosamine
- Both have phosphates and fatty acid side chains
Describe the composition of the core polysaccharide component of LPS
<10 sugars joined to lipid A eg heptulose, NAG, KDO
Describe the composition of the O antigen/ O side chain component of LPS and how it affects colony appearance
- <40 repeat units of 3-5 sugars
- No O chain = rough
- 1 repeat unit = semi-rough
- 2+ repeat units = smooth
List the 7 functions of LPS
- -ve charge from phosphates and charged sugars
- Stabilise OM bc lipid A in outer leaflet
- Permeability barrier bc tight LPS = restricts and side chains protecting from immune
- O antigen = antigenic but changes quickly
- Lipid A can be endotoxin
- Mediate adhesion to host
- O antigen serotype = identification and typing
What are porins, their size limit, and the 2 types?
- Transmembrane trimers for hydrophilic molecules <600
- General = passive, hydrophilic, water filled, non-specific
- Specific = solute specific
What are the OM’s transport proteins, their size limit, and describe the features of the receptor involved
- Energy coupled transporters
- Molecules <5000
- Receptor = specialised beta-barrel structure, specific, high affinity
Describe the process of transport by the OM’s transport proteins
- Substrate bound by OM receptor
- Receptor-substrate complex interacts with TonB in periplasm
- TonB’s proton motive force energy given to receptor so substrate released into periplasm
- ABC transporter in PM moves substrate to cytoplasm
What is the location and function of Braun’s lipoprotein?
- 30% bound to PG, hydrophobic end in OM
- Maintain correct distance between PG and OM for envelope integrity
Describe the structure of the periplasm
- 30-70nm wide
- Enclosed by OM and PM
- 20-40% cell vol
- Gel-like
What 6 things does the periplasm contain and give examples of each
- Binding proteins = transport
- Degradation and hydrolytic enzymes eg proteases, nucleases
- Detoxification enzymes eg beta-lactamases
- PG synthesis enzymes eg murein hydrolase, transpeptidase
- Chemoreceptor proteins
- Glucans/glucose oligosaccharides
What are the suggested roles of glucans/glucose oligosaccharides?
Osmoprotectants, virulence factor, mostly unsure
Do G+ve bacteria have a periplasm? If not, how do they carry out the functions of periplasmic proteins?
- Sometimes have narrow one but unclear
- Lipoteichoic acid = possible G+ve version of glucans
- G+ve secrete equivalent proteins
