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

Answer)

(a) We know that alpha helix rise per amino acid is 1.5 (A) Angstroms. And here 19 Amino acids span in the helix will be 19* 1.5 = 28.5 A.
Therefore, the width of the bilayer that could be spanned by this helix will be 28.5 A.

(b) one alpha helix turn is around 5.4 A.
Therefore, the complete turns of the helix it take to cross the membrane will be 28.5/5.4 = 5.28 helix.

(C) In the portion of the polypeptide chain the amino acids that crosses the bilayer will be non polar because the environment of lipid bilayer is hydrophobic. So the amino acids will be valine, alanine, Methionine, proline, glycine, isoleucine, leucine and phenylalanine and tryptophan.

(d) Yes, we expect this membrane-spanning helix to be amphipathic. Because amphipathic means having both hydrophilic and hydrophobic parts because as glycophorin is a single pass molecule so it have both hydrophobic tail and hydrophilic head.

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