Lecture 36: Membrane proteins and membrane fluidity Flashcards
Can glycans recognise non-cell cells?
Yes
Do both lipids and proteins diffuse through a membrane?
Yes, and lipids diffuse laterally
How is the rate of lateral diffusion in a membrane measured?
Using FRAP
What does FRAP stand for?
Fluorescence Recovery After Photobleaching
What is FRAP?
- Where fluorophores on membrane lipids are bleached with intense light.
- Rate of fluorescence recovery indicates lateral diffusion.
Which ways can substances in the membrane move?
- Rotation
- flexion
- Lateral diffusion
- Transverse diffusion
Describe the mobement of lipids within a membrane
Rotation and Flexion occur at a high rate, much quicker than lateral diffusion, and signficantly quicker than transverse diffusion (which is so slow that it has to be catalysed by enzymes)
Descibe the movement of proteins within a mebrane
Proteins move more slowly, primarily through lateral diffusion, and their movement is often constrained by their size, interactions, and specific roles (e.g., signaling or anchoring).
Do proeins flip-flop/transversely diffuse at all?
No
What happesn if an increased temperature casuses a membrane to be too fluid?
Fluidity is increased as the temperature increases
Lipid molecules move faster
The membrane becomes more permeable
WHat happens if a decrease in temperature causes a membrane to be too solid?
Fluidity is decreased as the temperature decreases
Lipid molecules move more slowly: gel-like
The membrane becomes less permeable
Is it true that there’s faster movement in membranes at higher temperatures?
Yes
What type of lipids increase membrane fluidity?
- Unsaturated lipids gives kinks and they don’t pack well together
- Short chains allow fewer interactions between lipids
- Lower melting points
What types of lipids decrease membrane fluidity?
Saturated chains
Long chains
Higher melting points
Is it true that changing lipid composition can help regulate the temperature in organisms that can’t regulate their temperature intrinsically?
Yes
How may plants regulate their temperature?
Plants have sensors in the plasma membrane that detect changes in fluidity
Fluidity increases indicates temperature is increasing
Allows the plant to prepare for heat stress
Descibe the structure of cholesterol
- Steroid 4 hydrocarbon ring structure, that is stiff
- Non-polar hydrocarbon tail, that is floppy
- Polar head group
Is it true that cholesterol can increase fluidity in the middle of the membrane (flexible tail) and decrease fluidity at the edge of the membrane (steroid tail)?
Yes
What does ethanol do to the membrane fluidity?
It increases it
Alcoholics and membrane fluidity
- One of the toxic effects of alcohol is due to an
increase in membrane fluidity - Chronic alcoholics compensate by increasing cholesterol
content of membrane - So when they finally sober up….
- Stabilisizing effect of cholesterol remains, and This creates a state of reduced membrane fluidity, as the elevated cholesterol levels are not immediately reversed.
- Membranes will become too rigid, leading to withdrawal symptoms
How often does transverse diffusion of lipids happen?
Transverse diffusion (flip-flop) once every 3 days (rare)
What catalyses the ‘flip-flop’ of lipids?
Proteins known as phospholipid translocators (flippases) catalyze the flip-flop event to maintain phospholipids in the correct monolayer.
Proteins destined for membranes are synthesized on one membrane of the endoplasmic reticulum (not in the ER)
Proteins destined for membranes are synthesized on one membrane of the endoplasmic reticulum (not in the ER)
What are the many functions of membrane proteins?
- Transporters
- Receptors
- Enzymes
- Anchors
What do integal membrane proteins do?
(a & b) Integral membrane proteins traverse (cross) the membrane
What do peripheral membrane proteins do ?
(c) Peripheral membrane proteins associate with a membrane face
Is it true that some proteins bind to the surface of integral proteins?
Yes
Are some membrane proteins covalently anchored to the membrane?
Yes
describe the types of integral membrane proteins
- Single span hydrophobic α-helix
Either C- or N-terminal can be intra-cellular - Multi-spanning containing α-helices
7 transmembrane helix protein a big family
But can have more or fewer helices - β-barrel protein forming a pore
Do integram membrane proteins flip flop?
No
What is the membrane topolgy maintained by?
Topology is maintained by hydrophobic and electrostatic interactions
Can the loops of integral proteins form binding sites?
Yes
What is ICAM?
ICAM is a crucial adhesion molecule in cell-cell communication, especially during immune responses. Its ability to mediate interactions between cells makes it essential for inflammation, immune surveillance, and maintaining tissue integrity.
On which cells is ICAM expressed?
Expressed on cells of the immune system and endothelial cells
Describe the structure of ICAM
- It has 5 extracellular immunoglobulin domains
- Single transmembrane spanning helix
- Short cytoplasmic tail
What is Bacteriorhodopsin?
Bacteriorhodopsin is a light-driven proton pump found in the cell membranes of certain archaea, particularly Halobacterium salinarum, which thrives in saline environments. It is a type of protein that captures light energy and uses it to move protons (H⁺ ions) across the membrane, creating a proton gradient that can then be used to generate ATP through chemiosmosis.
Are porins a type of outer membrane protein?
Yes
Describe porins
- Forms a barrel shaped structure with a pore in the centre.
- Outside of structure is hydrophobic, whilst the inside is hydrophilic
Describe peripheral membrane proteins
- Do not interact with the hydrophobic core of the membrane
- Can be cytoplasmic OR exoplasmic (and does not change)
- Interact with lipid head groups and integral membrane proteins
- Interactions are non-covalent
- Electrostatic interactions, H-bonds, van der Waals bonds
Describe anchored proteins
Proteins anchored to the membrane through hydrocarbon groups
The protein is covalently attached to a hydrocarbon group
The hydrophobic hydrocarbon group inserts into the lipid bilayer
Describe Ankyrin and Spectrin
- Spectrin cytoskeleton protein creating a scaffold on the intra-cellular side of membrane
- Ankyrin binds to several integral membrane proteins AND to spectrin (attaches spectrin).
- Maintain plasma membrane integrity via the spectrin-actin based cytoskeletal structure
Cells are covered in cybohyrdates….
- Cells are covered in carbohydrates on the exoplasmic side of membranes
- The carbohydrates can be attached to both lipids (glycolipids) and proteins (glycoproteins)
- the glycocalyx is a network of glycoproteins with mucus like consistency
- The function is: Physical barrier (protects against viruses and bacteria), Mechanosensing, Possible roles in cell shape
Is it true that most membrane proteins have at least one carbohydrate unit?
Yes
( Exist as either oligosaccharide chains or single sugar residues
; Glycoproteins usually have oligosaccharide chains
; Glycolipids usually have single sugar residues
)
IS it true that few membrane lipids have carbohydrate units?
Yes
What is the function of membrane carbohydrates?
- Cell-cell recognition, communication and adhesion
- This is especially important in immune responses, when cells need to distinguish between self and non-self cells
Blood groups and carbohydrates
Different carbohydrates are present on different blood group antigens
Our genes determine which enzymes we have and hence which blood group
Key messages
Membrane fluidity is crucial for correct functioning of cells
Membrane proteins have a variety of different structures and can be integral or peripheral
Glycans are on the outside of membranes and attached to either proteins or lipids
