Lecture 6: Membrane Proteins Flashcards
Most membrane proteins consist of
50% lipid and 50% protein by weight.
Compare the MW of one protein and lipid molecule
- proteins are much larger than lipids.
- 1 protein molecule per 50-100 lipid molecules.
The fundamental structural elements of membranes are
lipids.
Which is responsible for carrying out specific membrane functions
proteins.
Name four functions of membrane proteins
1-transporters.
2-linkers and anchors.
3-receptors.
4-enzymes.
Define linkers and anchors
- linkers: link proteins on the cytosolic face of the membrane to proteins on the non-cytosolic face.
- anchors: anchor proteins on either face to the membrane.
Define receptors
- detect chemical signals on one face of the membrane and relay the message on the other.
- interact with receptor–>change in configuration–>signal.
Some membrane proteins function as enzymes to
catalyze specific reactions on either side of the membrane.
Define the fluid mosaic model
- fluid because lipids are in constant movement.
- mosaic because proteins are inserted in “random” places throughout the membrane.
Two classes of membrane-associated proteins
1-integral membrane proteins are directly attached to the membrane, physical contact with hydrophobic tails.
2-peripheral membrane proteins are bound to the membrane indirectly, only by electrostatic bonds between integral membrane proteins.
Orientations in which transmembrane proteins span the lipid bilayer
- “in-out”: one-orientation (odd number of membrane spanning regions).
- “in-in”: two orientations (even number of membrane spanning regions).
Describe the protein structure
- hydrophilic region consisting of amino acids with charged and polar side chains, two aqueous terminals.
- hydrophobic region containing amino acids with uncharged non-polar side chains.
The hydrophobic region of the transmembrane protein is located
in the interior of the bilayer, in physical contact with tails.
How can we dissolve transmembrane proteins
they can be solubilized, only by treatments that disrupt the lipid bilayer, thus hydrophobic interactions.
Most commonly used reagent for solubilization are
detergents.
Detergents are
small, amphipathic, lipid like molecules.
Name two commonly used detergents and describe their abilities
SDS (sodium dodecyl sulfate): strong and ionic, extracts 100% of transmembrane proteins, however destroys conformation.
Triton X-100: mild non-ionic, as it has a non-ionized but polar groups at its hydrophobic ends, extracts 50% of transmembrane proteins but keeps 3D conformation.
How do detergents differ from lipids
they only have ONE hydrophobic tail and are much smaller than lipid molecules.
Detergents cluster into
micelles due to their cone shape.
Describe the process of solubilization
1- mixed in great excess.
2- hydrophobic ends of detergents bind to hydrophobic region of transmembrane proteins and tails of lipids, thereby separating the proteins from lipids and createing protein-detergent complexes.
Type of bond linking amino acids
peptide bond.
Three types of amino acids found in protein molecules
- phenylalanine= hydrophobic (uncharged non-polar side chain).
- serine=uncharged polar side chain.
- glutamate=negatively charged side chain.
Peptide bonds in a polypeptide chain are
-polar and form hydrogen bonds with each other.
1st folding patter of membrane-spanning segments
a-helix, where the N-H group of every peptide bond is linked by a hydrogen bond to the C=O of a neighbouring peptide bond located 4 peptide bonds away *in the same chain.
The a-helix is generated when
a single polypeptide chain turns around itself forming an inelastic cylinder.
1st type of membrane-spanning a-helix
- all amino acid aside chains are uncharged and non-polar (hydrophobic).
- hydrophobic amino acid side chain exposed on the outside of the a-helix, in contact with tails of lipids.
- partial or complete charges will destabilize the bilayer.
Receptors are
- transmembrane proteins with a single, hydrophobic a-helix.
- responsible for extracellular signals.
The extracellular part of receptor proteins binds the _______ while there ___________ signals to the cell’s interior.
ligand, cytosolic part.
2nd type of membrane-spanning a-helix
- hydrophobic amino acid side chains lie on the outside of the helix.
- hydrophilic amino acid side chains lie on the inside forming water-filled pores.
How are water-filled pores created
by packing five 2nd type of membrane-spanning a-helices side by side in a ring within the hydrophobic region of the lipid bilayer.
Water-filled pores function in
selective transport of large polar molecules (amino acids, nucleotides, sugars) and small charged molecules (ions).
2nd folding patter of membrane-spanning segments
b-sheets, where individual polypeptide chains are held together by hydrogen-bonding.
Adjacent polypeptide chains in a b-sheet can run either in
1- the same direction (parallel).
2- opposite direction (anti-parallel).
A b-sheet can be curved into a
cylinder, forming an open-ended b-barrel.
The water-filled pores inside the b-barrel function in
the selective transport of large polar molecules and small charged molecules, with weights up to 600 daltons.
In contrast to transmembrane proteins some integral membrane proteins are
anchored by covalently attaching to lipids or glycolipids.
How do we dissociate peripheral membrane proteins
by treatments with polar reagents, such as solution in high pH or high concentration of NaCl, that do not disrupt the lipid bilayer.
