Membrane Proteins Flashcards
True or False:
Lipids are generally larger than proteins
FALSE
Proteins are much larger than lipids, corresponding to the 1:50-1:100 protein-to-lipid ratio that is found within membranes.
List seven different functional types of membrane proteins
1) Channel transport
2) Carrier transport
3) Receptor
4) Enzyme (catalysis)
5) Anchor (ECM needs anchoring to cell)
6) Cell-cell joining (desmosomes, tight junctions, etc.)
7) Cell-cell recognition
List the two different types of membrane proteins (relating to structure, not function)
1) Peripheral proteins
2) Integral membrane proteins
Which type of membrane protein is easily separated from the membrane? Why is it so easy to separate?
Peripheral membrane proteins are easiest to separate from the membrane. This is because they are not anchored in the membrane, but rather, temporarily associated with the internal or external side
Integral membrane proteins are attached to the cell membrane by ____________
Covalently linked lipid or glycolipid molecules
Peripheral membrane proteins are attached to the cell membrane by ____________
Electrostatic interaction
Peripheral membrane proteins can become disassociated from the membrane following treatment with _____________
Polar reagents (solutions of extremely low pH [H+ high] or salt concentrations that do not disrupt the lipid bilayer)
Integral proteins that span the entire membrane are called _______________
Transmembrane proteins
Integral proteins must have at least one _______________, often an alpha helix
Non-polar region. This region is often coiled in an alpha helix, but can also exist as beta pleated sheets
Transmembrane proteins with an EVEN number of membrane-spanning regions can have _______________ number of orientations.
TWO orientations:
In-In & Out-Out
Transmembrane proteins with an ODD number of membrane-spanning regions can have _______________ number of orientations.
ONE orientation:
In-Out
How are integral membranes studied?
The membrane is solubilized by a reagent, which disrupts the bilayer (hydrophobic interactions)
What is most frequently used as a reagent to study transmembrane proteins?
Detergents are used. These are very small amphipathic molecules, containing both hydrophobic and hydrophilic groups
What are the two most common detergents used in membrane solubilization?
SDS (sodium dodecyl sulfate) is a strong ionic detergent, with an ionized Na+ / O- group at its hydrophilic end.
Triton X-100 is a mild non-ionic detergent, with a non-ionized polar group at its hydrophilic end
How do detergents differ from lipids? Why is this difference important?
They are smaller, and only have a single hydrophobic tail. This makes them shaped more like cones than cylinders, allowing them to tend towards clusters called micelles rather than forming a bilayer.
How to detergents solubilize a membrane?
When mixed in excess, their hydrophobic tails aggregate around the hydrophobic core region of transmembrane proteins, and the hydrophobic tails of lipid molecules. This separates proteins from the lipid bilayer.
Why are type-1 α-helices?
Alpha-helices that are hydrophobic on all sides. This is because the hydrophobic side chain of an amino acid is exposed on the outside of the α-helix.
How do type-1 alpha helices function within the membrane?
They give rise to receptors for extracellular signalling
What are the two domains that make up a type-1 α-helix?
The LIGAND-BINDING domain (ECM, ligand is signal molecule) and the REGULATORY domain
What are type-2 α-helices?
They are helices where the hydrophobic amino acid side chains are concentrated on one side, while the other is hydrophilic (charged).
How do type 2 α-helices function within the membrane?
The type-2 α-helices link together with other type-2, and form hydrophilic pores created by packing many helices side by side
Do water-filled pores function in transporting only water?
No, they function in transporting large polar molecules (amino acids, nucleotides, and sugars) and small charged molecules across the membrane
How to β-sheets interact with the membrane to form pores?
The hydrophobic amino acid side chains interact on the outside with the phospholipid bilayer, while the hydrophilic side chains form the inside of the pore.
Why do both types of pores (alpha and beta) function as selective transporters?
Because they only allow certain HYDROPHILIC molecules across
