Membrane Proteins Flashcards
What are the THREE classes of membrane proteins?
- Integral
- Lipid-Anchored
- Peripheral
How do Integral Proteins attach to the membrane?
Spans the bilayer
How do Lipid-Anchored Proteins attach to the membrane?
Covalently bound to lipids
How do Peripheral Proteins attach to the membrane?
Indirectly associate with membrane via non-covalent interactions with lipid head or other membrane proteins
How many hydrophobic amino acids are associated with the Integral membrane protein membrane spanning region?
20-25
What do Hydropathy plots predict?
Transmembrane helices of Integral Proteins
How long is the α-helix in the membrane spanning region of Integral proteins?
3-4nm
Which class of Integral membrane protein has its amino terminus in the cytosol?
Type II
(all other types have carboxyl terminus in cytosol)
What is the difference between Type I and Type III Integral membrane protein?
Type I has a signal sequence that is cleaved during insertion into the membrane
What is the structural difference of Porins compared to other transmembrane proteins?
Barrel shape comprised of 16 β-stands that span membrane
What is CMC?
Critical Micelle Concentration
What happens to detergents above and below CMC?
- BELOW CMC - detergents dissolve as single molecules in water
- ABOVE CMC - detergent molecules cluster to form micelles
What are Detergents used in purifying membrane proteins?
- Amphipathic molecules
- Solubilise lipids and membrane proteins
What are the characteristics of Ionic detergents?
- Charged
- Disrupts hydrogen and ionic bonds
- Denature proteins at high concentrations
What are the characteristics of Non-Ionic detergents?
- Don’t denature proteins
- Useful in purification of membrane proteins
Which side of the cellular membrane do GPI-linkages occur?
Extracellular side
Which side of the cellular membrane does Acylation and Prenylation occur?
Cytosolic side
What is Acylation?
FA attached to protein
What is the carbon skeleton of Myristic acid?
14:0
What is the carbon skeleton of Palmitic acid?
16:0
Which amino acid does Palmitoylation occur on?
Internal Cys (or Ser)
Which amino acid does Myristolation occur on?
N-terminal Gly
What is an example of a gene that undergoes Acylation by Myristoylation?
v-Src oncogene
What is the structure of Farnesyl?
C15, 3x Isoprenes
What is the structure of Geranylgeranyl?
C20, 4x Isoprenes
Which amino acid does Farnesyl OR Geranylgeranyl attach to?
Cys at/near C-terminus
What is the attachment sequence of Farnesyl OR Geranylgeranyl?
Cys-Φ-Φ-X
(Φ = hydrophobic aa)
What is an example of enzymes that undergoes Prenylation?
- Small GTPases
- Ras, Rab etc
How is GPI linked to a membrane protein?
Linked to C-terminus of protein via Phosphoethanolamine
What is an example of an enzyme that undergoes GPI linkage?
Placental Alkaline Phosphatase (PLAP)
TRUE OR FALSE:
Carbohydrates can only be attached to membrane proteins
False.
Can be attached to both membrane proteins AND lipids
What are carbohydrates attached to lipids called?
Glycolipids
* attached to glycerol or sphingosine backbone
What are large polysaccharide sugars attached to membrane proteins called?
Proteoglycans
What are small oligosaccharides attached to membrane proteins called?
Glycoproteins
On which THREE amino acids does Glycosylation occur on Glycoproteins / Proteoglycans?
- Serine
- Threonine
- Asparagine
Which side of the membrane does Glycosylation occur?
Only occurs on the exoplasmic side of the membrane
