Flashcards in Membranes And Receptors 1 Deck (91):
What specialised function does a mitochondrial membrane have?
Energy conservation by oxidative phosphorylation
What approximately is a membrane's dry weight composition?
What is the normal percentage composition of water in a hydrated membrane bilayer?
Define an Amphipathic molecule?
A molecule (especially a protein) containing both hydrophilic and hydrophobic moieties
Describe the general STRUCTURE of a phospholipid
1. Two Fatty acid chains, attached to a...
2. Glycerol back-bone
3. Phosphate + head group (changeable) attached to the glycerol
A phospholipid is described as Amphipathic. Explain why in terms of its structure.
Phospholipids contain both hydrophilic and hydrophobic moieties:
1. Hydrophilic - polar head group and attached phosphate
2. Hydrophobic - fatty acid tails
A range of polar head groups are found in phospholipids. List four:
There are an enormous range of fatty acids found in phospholipids. What chain lengths are most commonly found?
What are the 5 general functions of a biological membrane (cell and organelle membrane)?
1. Highly selective permeability barrier
2. Controls enclosed chemical environment
3. Controls communication - flow of information between cells and their environment
4. Recognition - signalling molecules, adhesion proteins, immune surveillance
5. Signal generation in response to stimuli - electrical or chemical
What is the effect of cis double bonds in unsaturated fatty acid chains of phospholipids on phospholipid packing?
It introduces a kink in the fatty acid chain which decreases phospholipid packing
Which type of phopsholipid is not based on glycerol? What is it based on instead?
Shingolipids - they are based on aliphatic amino acids e.g. sphingosine
If you replace a phosphocholine moiety with sugar what type of molecule do you get?
Describe the structure of a glycolipid
Lipid residue -glycerol or sphingosine backbone with FA chains
Carbohydrate residue - lipid residue bound to a monosaccharide or oliogosaccharide by a glycosidic bond
What do you call glycolipids which have head groups consisting of sugar monomers?
What do you call glycolipids which have head groups consisting of oligosaccharides?
What proportion of total membrane lipid content is cholesterol?
What is the conformation of sphingomyelin like in relation to other phospholipids in the membrane?
It resembles other phospholipids - often has a phosphocholine head group
What is the favoured structure of phospholipids and glycolipids in aqueous media?
How is a biliayer formed by phospholipids and glycolipids in aqueous media?
Spontaneously - driven by Van der Waals forces between their hydrophobic tails
How are lipid bilayers stabilised?
By non-covalent forces- electrostatic and hydrogen bonding between hydrophilic moieties and between hydrophilic groups and water
What do pure lipid bilayers have a very low permeability to?
Ions and most polar molecules
What are the four modes of mobility in a lipid bilayer?
1. Intra-chain motion
2. Fast axial rotation
3. Fast lateral diffusion within the plane of the bilayer
Describe 'intra-chain motion' of a lipid in a bilayer
The formation of kinks in the fatty acyl chains
Describe 'axial rotation' of a lipid in a bilayer
Rotation of a phospholipid around its own axis
Describe 'flip-flop' movement of a lipid in a bilayer
The movement of lipid molecules from one half of the bilayer to the other, on a one-for-one exchange basis
What is the effect of unsaturated double bonds in the fatty acid side chains of phospholipids on the dynamics of the lipid bilayer?
They disrupt the hexagonal packing of phospholipds and therefore increase membrane fluidity
List some of the roles of membrane proteins
enzymes, transporters, pumps, ion channels, receptors and energy transducers
What is the normal dry weight of protein in membranes and why can this vary?
Normally it is approximately 60%. This can vary depending on the function of the membrane e.g. there is only 18% in myelin (causing increased electrical resistance) and 75% in mitochondria
What are the three modes of motion permitted for proteins in bilayers?
Why does flip-flop movement not occur with membrane proteins?
It is energetically unfavourable...
What are the restrains on the motility of proteins in membranes?
1. Lipid mediated effects
2. Membrane protein associations
3. Association with peripheral proteins e.g. cytoskeleton
What is meant by 'lipid-mediated effects' on protein motility?
Proteins have a tendency to separate out from gel-phase into liquid phase or cholesterol poor regions
What are the two broad categories of membrane proteins?
peripheral and integral
What are peripheral membrane proteins?
Proteins bound to the surface of membranes by electrostatic and hydrogen bond interactions
What are integral membrane proteins?
Proteins that interact extensively with the hydrophobic regions of the lipid bilayer
How can you experimentally differentiate peripheral and integral membrane proteins?
Peripheral proteins can be removed from the membrane by changes in pH or ionic strength which alters their POLAR interactions. Integral proteins are not removed by pH or changes in ionic strength - they require agents (detergents, organic solvents) that compete for NON-POLAR interactions in the bilayer
Why is the asymmetric orientation of membrane proteins important?
It is important for function e.g. a receptor for a hydrophilic extracellular messenger (such as insulin) must have its recognition site directed towards the extracellular space in order to function
What are erythrocyte ghosts?
Erythrocytes (RBCs) without their cellular components - only their membranes. These are prepared by osmotic haemolysis
The membrane proteins band 3 and band 7 (from erythrocytes) only dissociate from the red cell membrane when detergents are used. What type of membrane protein are they?
Integral membrane proteins
The majority of erythrocyte membrane proteins dissociate from the red cell membrane when they are treated with high ionic strength medium or the pH is changed. What type of membrane protein are they?
Peripheral membrane proteins
Band 3 and Band 7 proteins contain covalently attached carbohydrate units. What type of integral membrane protein are they?
Describe an important function of carbohydrate groups on membrane proteins?
Cellular recognition - e.g. to allow tissues to form or in immune recognition
What two molecules create a network that makes up the erythrocyte cytoskeleton?
actin and spectrin
On what face of the erythrocyte membrane is the cytoskeleton?
Describe the structure of spectrin
Spectrin is a long, floppy, rod-like molecule. Its alpha and beta subunits wind together to form an anti-parallel heterodimer, that then associates head-to-head with another heterodimer to form a heterotetramer (alpha2,beta2)
How to actin and spectrin form networks together?
The tetramer rods of spectrin are cross-linked into networks by short actin protofilaments (~14 actin monomers) and band 4.1 and Adducin which form interactions towards the ends of the spectrin rods
How is the cytoskeleton of actin and spectrin attached to the plasma membrane?
The adaptor proteins Ankyrin (band 4.9) and band 4.1 link Spectrin to the integral membrane proteins (Band 3 and Glycophorin A)
What effect does the attachment of integral membrane proteins to the skeleton have on the membrane dynamics?
It restricts the lateral mobility of the membrane protein
In the common dominant form of Hereditary Spherocytosis, what is the cause of the disease? What effect does this have?
Spectrin may be depleted by 40-50%. The reduced cytoskeleton causes RBCs to become rounder and more prone to lysis during their passage through capillary beds. Therefore there is increased clearance of RBCs by the spleen and if the erythrocyte production by the bone marrow cannot compensate, haemolytic anaemia occurs.
What is a common defect in Hereditary Elliptocytosis? What effect does this have?
Spectrin molecules are unable to form heterotetramers resulting in fragile elliptoid cells and therefore increased clearance of damaged cells by the spleen and haemolytic anaemia.
What causes ribosome synthesis of membrane proteins to stop until the ribosome has been transferred to the rough ER?
Binding of a large protein/RNA complex (the signal recognition particle (SRP)) to the signal/leader sequence of the nascent polypeptide. Binding of the SRP to the polypeptide and ribosome locks the ribosome complex and prevents further protein synthesis while the ribosome is in the cytoplasm
What is the signal/leader sequence?
A characteristic hydrophobic amino acid sequence of 18-30 amino acids with a number of basic amino acids at the N-terminus of the nascent polypeptide. This is the region of the nascent polypeptide where the SRP binds
How is the inhibitory constraint on further translation, caused by the SRP, removed?
Upon binding of the SRP to the SRP receptor (or docking protein) on the ER membrane, the SRP is released from the signal sequence of the nascent polypeptide and translation can continue
On release of the signal peptide by SRP at the ER membrane, how is the synthesis of the nascent polypeptide directed through the ER membrane?
The signal sequence interacts with the signal sequence receptor (SSR) within a protein translocator complex (Sec61) in the ER membrane. This complex directs the polypeptide through the ER membrane and the ribosome becomes bound to the complex
How is transfer of the polypeptide through the ER membrane arrested?
Once the stop transfer signal spans the hydrophobic core of the lipid bilayer, a lateral gating mechanism releases the membrane protein from the protein translocator in the lipid bilayer. The ribosome then is thought to detach from the ER and protein biosynthesis continues in the cytoplasm
What is the 'stop transfer signal'?
A region of highly hydrophobic primary sequence between 18-22 amino acids long followed directly by charged amino acids which, in a alpha-helical form, is long enough to span the hydrophobic core of the bilayer. This sequence forms the transmembranous region of the protein
What happens to the signal sequence of nascent secretory or membrane polypeptides?
It is cleaved by signal peptidases even before protein synthesis is completed
Which direction does the membrane protein get inserted into the ER membrane in the above mentioned mechanism?
N terminus is in the ER lumen and the C terminus is in the cytoplasm
The nascent chain undergoes further processing as it passes from the ER and through the cis to trans Golgi. What direction would a protein be orientated upon fusion of a vesicle with the cell membrane if its C-terminus has been located in the cytosol at the ER membrane when it was being synthesised?
It would remain in the cytosol
List some functions that may be specific to different regions of the plasma membrane
Interaction with the basement membrane
Interaction with adjacent cells
Absorption of bodily fluids
Synapses - nerve junctions
Electrical signalling conduction
Changing shape - may change properties of a particular region
Why is the thickness of the lipid bilayer always approximately the same thickness?
The fatty acid chains are roughly the same length (C14-C22 with C16 and C18 most prevalent)
Why is a glycolipid not a phospholipid?
It doesn't contain phosphate! Just a lipid residue attached to a carbohydrate residue
Why is flip flip of phospholipids between lamellae rare?
It is thermodynamically unfavourable to go through the hydrophobic inner membrane
Describe the structure of cholesterol
1. Polar head group
2. Rigid (can't twist) planar steroid ring structure
3. Non-polar hydrocarbon tail
What is the endothermic phase transition of phospholipid bilayers?
It is the endothermic heat transfer required for a membrane to go from crystalline to fluid phase
What affect does cholesterol have on the endothermic phase transition of phospholipid bilayers?
It abolishes the endothermic phase transition. It stabilises the environment of the membrane by buffering its fluidity as it has properties which increase and decrease membrane fluidity
What ratio of cholesterol to phospholipids tends to be seen in plasma membranes?
How can cholesterol increase and decrease membrane fluidity at the same time?
Increase - reduced phospholipid packing
Decrease - reduced phospholipid chain motion due to the rigid planar steroid ring structure
What is the effect of cholesterol on the plasma membrane at high temperatures?
It decreases membrane fluidity
What is the effect of cholesterol on the plasma membrane at low temperatures?
It increases membrane fluidity
Why do we need to digest cholesterol?
Our bodies (liver) can make only half the cholesterol that we need
What is the evidence that there are proteins in membranes?
1. Functional: specificity in biology is determined by proteins e.g. facilitated diffusion, ion gradients, specificity of cell responses
2. Biochemical:membrane fractionation and gel electrophoresis or freeze fracture
Why can't proteins flip flop between lamellae of the plasma membrane?
It is too thermodynamically unfavourable for their large hydrophilic structures to pass through the hydrophobic inner membrane to get to the other side
What can restrict protein mobility through a membrane?
1. Aggregation of proteins
2. Tethering - e.g. to basement membrane or cytoskeleton
3. Interaction of proteins with other cells
4. Lipid mediated effects
Membranes are capable of 'phase separation'. What is phase separation?
They have the ability to be part liquid and part in gel phase at the same time, in spatially separated populations
How many amino acids does a protein need to span a membrane?
What is a common secondary structure of transmembrane portions of proteins?
What enzyme catalyses the formation of disulphide bonds in the ER?
protein disulphide isomerase
What post-translation mechanism targets newly synthesised proteins to the lysosome?
Protein glycosylation. Glycosylation is required to prevent auto-digestion of the lysosomal membrane by the lysosome's hydrolytic enzymes
Where are phospholipids synthesised?
Where on glycerol are the fatty acid side chains attached in a phospholipid?
C1 and C2
What is the backbone of phospholipids?
What is the natural conformation of double bonds in fatty acid chains of phospholipids?
Where does the hydroxyl group in cholesterol's head group hydrogen bond to on a phospholipid?
The double-bonded oxygen in the ester bond of the phospholipid
Name three bonds that peripheral proteins use to interact with the surface of membranes
Name the peripheral proteins of the erythrocyte cytoskelton
Ankyrin (band 4.9)
Name the integral proteins of teh erythrocyte cytoskeleton
Band 3 - anion exchanger
Where are mitochondrial membrane proteins synthesised before being targeted to membrane destinations for post-translational incorporation?
How are the ribosomes synthesising membrane proteins attached to the endoplasmic reticulum?
What is often found on the C-terminal side of the stop transfer sequence in the membrane protein?
One or two basic amino acids