Question

Glycophorin A is a glycoprotein that extends across the erythrocyte membrane. The portion of the polypeptide that extends across the membrane bilayer contains 19 amino acid residues that are folded into an alpha-helix. (

a) What is the width of the bilayer that could be spanned by this helix?

(b) How many complete turns of the helix does it take to cross the membrane?

(c) Given that the bilayer interior contains non-polar fatty acyl chains, which of the 20 amino acids would you expect to find among those in the portion of the polypeptide chain that crosses the bilayer?

(d) Would you expect this membrane-spanning helix to be amphipathic? Explain, describing what is meant by the term.

Answer 1

a) One amino acid occuppies 0.15nm length in an alpha helix. Then 19 aminoacids will occupy = 19nm/0.15nm = 2.85nm. Therefore, the width of the lipid bilayer occupied by the 19 aminoacids is 2.85nm.

b) One helix turn occupies 0.54 width of the lipid bilayer. Then how any helix turn will occupy 2.85nm width?

= 2.85nm/ 0.54nm = 5.27 helices.

c) Valine, glycine, alanine, leucine, isoleucine, phenylalanine, Methionine and tryptophan are the aminoacids found in the bilayer.

d) Yes. Amphipathic means having both hydrophilic and hydrophobic. Lipid bilayer made up of hydrophobic tail and hydrophilic head. Therefore, hydrophobic aminoacids will occupy the tail part and hydrophilic aminoacids will occupy the head part.