What are the major classes of membrane proteins?
- Peripheral
- Integral
- Lipid-Anchored
What is a requirement of membrane spanning helices?
Main chain C=O and N=H must be hydrogen bonded to neutralise their charges.
On the Kyte and Doolittle Scale (from Arg = -4.5 - Ile =+4.5), what is the minimum number that predicts a transmembrane helix?
+1.6
How many residues are used in the window on a hydropathy plot, and why?
19, as this is the number of residues in a typical transmembrane helix.
On a hydropathy plot, what value must peaks be above to be a transmembrane helix?
+1.6
Can Kyte Doolittle analysis be performed on transmembrane ß-barrels?
No - as the residues alternate between hydrophobic-hydrophilic
What are three kinds of hydrophobic protein anchors?
- Acylated
- Prenylated
- GPI Anchor
What is myristoylation?
An amide bond to an N-terminal glycine
Where is myristoylation typically found?
The inner leaflet of eukaryotes
What is prenylation?
A thioester link to C-terminal cysteine
When does prenylation occur?
Post-translationally
What is a thioesterase?
An enzyme that reversibly cleaves thioester lipid anchors.
What is GPI?
Glycosyl phophatidylinositol
Where do GPI anchors face?
Exoplasm
What is the general structure Aquaporin?
- 6 transmembrane helices + 2 short helices per subunit
- Aquaporin is a tetramer
- Each subunit has a pore
How wide is the aquaporin pore?
2.8Å
Which two highly conserved residues form the gate?
Arg195 and His180
Is the pore lined with hydrophobic or hydrophilic residues?
Hydrophobic
What happens in the aquaporin channel if an uninterrupted chain of water molecules forms?
The chain becomes a ‘proton conducting wire’, co-translating protons from one side of the membrane to another, forming hydronium ions
What mechanism stops proton co-translation in aquaporin?
Water is stabilised by two highly conserved Asn residues.
They hydrogen bond the lone pairs on the O on water.
What is the general structure of the Potassium Channel?
- Four identical subunits
- Each subunit has two membrane spanning helices
- One channel per tetramer
- P segment (selectivity filter)
What the are key features of the potassium channel p segment?
- Selectivity filter to allow only potassium
- Homologous in all potassium channels
- Mutations stop selectivity
- Functional even if bacterial segment replaced with mammalian segment
What is the rate of transport of the potassium channel?
108 ions/second
What is the diameter of the potassium channel selectivity filter?
3Å
