Membranes - Biological Function - Membrane Proteins Flashcards
What is the functional evidence of a membrane protein?
Facilitated diffusion
Ion gradients
Specificity of cell response
What is the biochemical evidence of a membrane protein?
Membrane fractionation + gel electrophoresis (Protein gel electrophoresis)
Freeze fracture
These are ways to see if there are receptors on the membrane of a cell.
What are the allowed motions of a protein in the phospholipid bilayer?
Three modes are permitted: Conformational change Rotational movement Lateral movement What is not allowed is flip-flop! Only phospholipids are allowed to flip-flop.
Give examples of how membrane proteins can be restricted in their mobility.
By aggregation where it is bound to another protein on the membrane. This makes the whole collective more difficult to move.
Tethering where the membrane protein is bound to a protein either on the outside of the cell to a substrate or inside the cell to the cytoskeleton.
By binding to another cell’s proteins in interaction with other cells.
What are peripheral membrane proteins?
Proteins bound to the surface.
They are electrostatic and hydrogen bond interactions
They are removed by changes in pH or ionic strength
What are integral membrane proteins?
These interact extensively with the hydrophobic domains of the lipid bilayer which means it has parts of its protein inside the lipid bilayer.
They cannot be removed by changes in pH or ionic strength.
They can be removed however by agents that compete for non-polar interactions like detergents and organic solvents.
What is the special feature of the protein (polypeptide chain) of an integral membrane protein that is found on the inside of the lipid bilayer.
The amino acids R-groups are usually small and hydrophobic in order to stick there.
Look at page 13 for hydropathy plots. Explain them.
Yup.
How can proteins be associated with the lipid bilayer in order to adhere?
By a hydrophobic chain for an integral membrane.
By a post translational lipid modification (fatty acid can be added)
If peripheral and integral proteins of the RBC membrane are fractionated and put into Gel electrophoresis and a salt wash is added. How can you see a difference?
The integral membrane protein is still stuck on the membrane. The peripheral proteins have been ‘washed off’ due to the salt.
What does the erythrocyte cytoskeleton consist of?
Spectrin which looks like a chain. Ankyrin which is an attachment protein attaches the spectrin to Band 3 on the plasma membrane.
Spectrin is also anchored to glycophorin A on the plasma membrane via band 4.1 which is also an attachment protein. The anchors on the plasma membrane are transmembrane proteins. The spectre lattice adhere through attachment proteins (ankyrin and band 4.1).
Why is the cytoskeleton important?
To maintain the structure of the cell.
What happens if the cytoskeleton does not work as normal in a red blood cell?
Haemolytic anaemias can occur.
What is hereditary spherocytosis?
A haemolytic anaemia where the spectrin is depleted by 40-50%
This means that the erythrocytes round up and become less resistant to lysis. They are then cleared in an excess in the spleen.
What is hereditary ellipticcytosis?
A haemolytic anaemia where the is a defect in the spectrin molecule. This makes it unable to form heterotetramers and because of this a fragile ellipsoid cell is formed.
