2. From studies of chemistry, we know that in a straight chain hydrocarbon, such

Question

2. From studies of chemistry, we know that in a straight chain hydrocarbon, such as a fatty acid, each methylene group lengthens the chain by about 0.13 nm. From studies of protein structure, we know that one turn of an -helix takes 3.6 amino acid residues and lengthens the axis of the helix by about 0.56 nm. Using this information, answer the following questions: (Answer parts a-c to one decimal place in units of nm, e.g. 9.0 nm.)

a. (1 pt) Using this information, how thick is the non-polar core of the bilayer containing the GLUT1 transporter (i.e. what is the length of each helix)?

b. (1 pt) How many fatty acid methylene groups would be needed to span this distance? (Round to closest whole number.)

c. (1 pt) Since fatty acids in membrane lipids always have an even number of carbons in their chains, what chain lengths might be common in this membrane? (Write the number of carbons. More than one number may be correct.)

d. (2 pts) Most membrane lipids have 16 or 18 carbons, how can the length of these fatty acids be accommodated in the membrane?

Explanation / Answer

ALL ANSWERS ARE CALCULATED ON THE BASIS OF DATA PROVIDED IN THE QUESTION AND MAY OR MAY NOT BE ACTUAL DATA OF THE LIVING SYSTEM.

a.

According to the available literature on GLUT1 glucose transporter contains 12 membrane crossing helices with 20 amino acids each.
So from the data provided in question:

3.6 amino acids make one turn, therefore 20 amino acids will make 20/3.6 = 5.56 turns
And as each turn lengthens the axis of the helix by about 0.56 nm, so 5.56 turns would result in an alpha helix of length = 5.56 x 0.56nm = 3.1nm

b.

As the question states that with addition of each methyl group in fatty acid lengthens the chain by about 0.13 nm. And from the above question we understood the width of bilayer to be somewhat near 3 nm (as helices of GLUT1 transporter are membrane panning).

So, if 0.13nm of length is added by 1 methyl group.
Then, for 3 nm the number of 3/0.13 = 23 methyl groups

c.

As membrane is a bilayer structure, and considering the data derived from answer in part a), its average thickness is around 3.0 nm.
Therefore, each layer of bilayer (assuming to be tightly packed) has around 1.5 nm length to accommodate.

So the maximum number of fatty acid it can have is 1.5/0.13 = 11.5 number of carbons.
But as membrane lipids have even numbered carbon chains, it can be either 8 or or in some cases may exceed up to 12 carbons in chain.

d.

Most membrane lipids that have 16-18 carbons generally have 0-3 double bonds which generally decrease the length of the chain and also, creates a bend in the chain to accommodate it in the given space of bilayer.

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