M&R - Lipid Bilayer and Lipids/Proteins Flashcards
Do all regions of plasma membrane have the same function?
No
Dynamic environment, constantly changing
Functions include - secretion, transport, interaction with adjacent cells, interaction with basement membrane
What is the composition of a membrane?
40% lipid
60% protein
1-10% carbohydrate
20% of total weight = water
Membranes are described as amphipathic molecules, what does this mean?
Contain both hydrophobic and hydrophillic moiety
What are the three predominant lipids found in a cell membrane?
Phospholipid (Small head groups - choline, amine, )
Sphingomyelin - doesn’t have glycerol backbone
Glycolipid - two types = cerebrosides and ganglioside
What is the length of a fatty acid chain?
Which is more prevalent?
C14-C24
C16-C18
Name some of the roles of membrane proteins
Functional = Enzymes Transporters Pumps Ion channels Receptors Energy transducers Facilitated diffusion
Biochemical = membrane fractionation, freeze fracture
What are the four ways lipids can mobilise in a cell membrane?
Flexion
Rotation
Lateral diffusion
Flip Flop - flip head group through hydrophobic domain to the other side
What effect does the presence of a double bond in a fatty acid side chain have?
Introduces a kink into the chain that reduces the ability of the phospholipid to pack in crystalline array and so contributes to increased membrane fluidity
What is the role of cholesterol in the lipid bilayer?
What effect does cholesterol have on the fluidity of the membrane bilayer?
Stabilises the structure
Cholesterol reduces phospholipid packing and maintains membrane in fluid phase = increasing fluidity
Rigid cholesterol ring structure held closely to fatty acid chains reduces intrachain vibrations = reducing fluidity
What ways can a protein mobilise in a cell membrane?
Conformational change
Rotational
Lateral
NO FLIP FLOP - energetically unfavourable
How are membrane proteins restricted in movement?
Lipid mediated effects - proteins tend to seperate into fluid phase or cholesterol poor region
Membrane protein associations
Association with extra membranous proteins e.g. Cytoskeleton
What are the two types of membrane proteins and describe?
Peripheral proteins - bound to surface.
Electrostatic or hydrogen bond interactions.
Removed by changes in pH or ionic strength
Integral proteins
Interact with hydrophobic domains of bilayer
Removed by detergents/organic solvents which dissolves bilayer to release protein
What is the length of amino acids that tend to span bilayer?
18-22 amino acids
What arrangement are the transmembraneous domains?
Alpha helical
What is the name of the plots used to determine if a membrane is membrane spanning?
Hydropathy plots
High hydrophobic score = membrane spanning
What are the functions of a biological membrane? (5)
Highly selective permeability barrier
Control of enclosed chemical environment
Communication
Recognition - signalling molecule, adhesion proteins, immune surveillance
Signal generation in response to stimuli - electrical/chemical
How many hydrophobic domains are there in bacteriorhodopsin?
Why is this molecule important?
7 domains
Bacteria that traps light. Can be grown in a dish to make lots of protein. Has the same structure as a lot of the receptors.
What terminal of the protein faces in and which faces out?
Why is orientation important?
C terminal out
N terminal in cytosol
Important for function
What are the integral proteins of the erythrocyte skeleton?
Glycophorin A
Band 3
What are the peripheral membranes of the erythrocyte skeleton?
Actin
Ankyrin
Adducin
Band 4.1
What are the two haemolytic anaemias?
Hereditary spherocytosis - Spectrin depleted by 40-50%, erythrocytes round up, less resistant to lysis, cleared by spleen = anaemia
Heridtary elliptocytosis - defect in spectrin molecule, unable to form stable heterotetramers, fragile elliptoid cells
Describe the secreted protein biosynthesis
Signal sequence recognised by SRP
Binding of SRP prevents further protein synthesis
SRP recognised by receptor - directs SRP ribosome complex to protein translocator complex
SRP released
Protein synthesis continue
Translocates through until stop sequence reached
Protein released
Ribosome detaches from ER
Signal sequence cleaved by signal peptidase
What anchors proteins in the bilayer?
Stop sequence in the middle of the protein
Ribosome continues to make protein, cannot force through bilayer so rest of protein made in cytoplasm
What direction would the protein enter bilayer if there are more positive residues at N terminus?
C terminal section will pass into lumen
What happens in the presence of N terminal signal sequence in absence of signal peptidase cleavage?
C terminal to ER lumen - C terminus flops in
What happens when there are more positive residues at the C terminal end of the start transfer sequence?
N terminal translocated and in ER lumen
Protein orientation reversed
No signal peptidase
