Proteins and enzymes, Section 6, proteins in membranes and lipids (Dr. Taylorson)

Question 1

What is a membrane protein’s usual affinity for water ?

Answer 1

Neither high nor low, it depends on the region of the protein because these proteins are often amphiphilic.

Question 2

Are saturated or unsaturated fatty acids more flexible ?

Answer 2

Unsaturated, because the double-bond introduces a kink in the molecule.
Double bonds are normally cis-.

Question 3

What are the three types of membrane proteins ?

Answer 3

Integral, peripheral and lipid anchored.

Question 4

How thick is the usual membrane ?

How much of this is hydrophobic ?

Answer 4

5nm.

3nm are hydrophobic.

Question 5

To be in a membrane, an alpha helix must have hydrophobic sides chains.
However, how does the polar amide bond cope w/ the non polar environment ?

Answer 5

There is a v strong driving force impelling the carbonyl and amide groups to H-bond to each other as they do in the hydrophobic interior of a soluble protein. Indeed, H-bonds are stronger in a non-polar environment.

Question 6

Is it possible for a single helix to exist in a membrane ?

Answer 6

Yes, but it is also possible to get many amphipathic helices that form a pore (like the bovine cytochrome bc1 complex).

Question 7

What is the usually distance between 2 residues in an alpha helix ?
Work out how many AAs usually make up membrane alpha helices.

Answer 7

The distance between 2 residues in an alpha helix is 0.15nm
Thus, in a membrane w/ a 3nm hydropobic region, we can fit 20AAs (and the most common membrane helices are 20-22AAs long).

Question 8

Are membrane proteins more alpha-helices or beta sheets ?

Answer 8

Generally alpha-helices in eukaryotes and beta sheets in prokaryotes.