M&R 1 - Membrane Bilayer Flashcards
What is the composition of the membrane?
40% lipid
60% protein
1-10% carbohydrate
List the 5 general function of the membrane bilayer
Highly selective permeability barrier
Allows control of enclosed chemical environment
Communication - allows info travel between cells and their environment
Recognition - to signal molecules, adhesion proteins
Signal generation in response to stimuli
What type of molecule is a phospholipid?
Amphipathic
Describe some features of fatty acid chains?
C14 —> C24
Hydrophobic hydrocarbon chains
How does a cis double bond affect membrane stability?
A cis double bond introduces a kink in the fatty acid chain.
This reduces the phospholipid packaging, increasing membrane fluidity
How is a glycolipid formed?
The phosphocholine moiety from sphingomyelin is replaced with a sugar.
What are the two types of glycolipid?
Cerebroside - single sugar as head group
Ganglioside - complex oligosaccharide as head group
How are bilayers formed?
Spontaneous in water driven by VdW forces between hydrophobic tails.
Cooperative structure is stabilised by electrostatic and H bonds between hydrophilic moieties.
What is sphingomyelin?
It is the only phospholipid not based on glycerol
What are the 3 modes of motility of proteins?
Change in conformation
Rotational
Lateral diffusion
What are the restraints on protein motility
Lipid mediated effects - separate out into fluid phase
Membrane protein associations
Associations with extra membranous proteins (eg cytoskeleton)
Describe peripheral proteins
These are proteins that are attached to the surface of membranes by electrostatic and hydrogen bonds. These can be removed by changes in pH or ionic strength.
Describe integral proteins
These interact with the hydrophobic region of the lipid bilayer.
Can’t be removed by changes in ionic strength or pH but require agents (detergents) these compete for the non polar interactions in the bilayer.
Why is orientation of membrane proteins important?
Receptors for hydrophilic molecules must face extra cellular space.
How is the erythrocyte membrane prepared for analysis of protein?
It undergoes osmotic haemolysis to release cytoplasmic content. Then undergo gel electrophoresis to separate.
What does it mean when most if the proteins are released with high ionic strength medium?
This means the proteins are peripheral in the cytoplasmic face.
What does it mean for the proteins that are dissociated from the RBCs membrane by detergents?
This means these are integral proteins.
How do glycoproteins lock orientation of lipid bilayer?
These contain a covalently binded carbohydrate. This is highly hydrophilic which prevents flip flop rotation.
The carbohydrates can also be useful for cellular recognition to allow cells to form an immune response.
What is the erythrocyte cytoskeleton made of?
A network of spectrin and actin molecules.
What is spectrin?
A long floppy rod like molecule. a and B subunits wrap round each other forming an antiparallel heterodimer. Then two of these combine head to head to form a heterotetramer.
How does the spectrin attach to the lipid bilayer?
The rods of spectrin are cross linked into networks by short actin profilaments. Adducin and band 4.1 molecule attaches to the end of the spectrin networks. The spectrin actin network is attached to the lipid bilayer via Ankyrin. This way the integral proteins attach to the cytoskeleton - preventing lateral movement of membrane proteins.
Describe the pathophysiology of hereditary spherocytosis
Spectrin levels are depleted by 40-50%
The cells become round, meaning less resistance to lysis
More haemolysis - cleared by spleen
Haemolytic anaemia.
Describe the pathophysiology of hereditary elliptocytosis
Defect in spectrin molecule meaning unable to form heterotetramers resulting in fragile elliptiod cells.
If the positive residues are located on the c terminal
Which side of the membrane is the n terminal targeted for?
The lumen side of the membrane
Explain what the stop transfer signal is
The region of highly hydrophobic primary peptide sequence followed by a charged amino acid. This allows the protein to span the entire hydrophobic section of bilayer.
