Proteins that span a membrane as an alphahelix have a charac

Proteins that span a membrane as an alpha-helix have a characteristic structure in the region of the lipid bilayer. Which of the three 20-amino acid sequences listed below is the most likely candidate for such a transmembrane segment. Explain your answer.

A. ITLIYFGVMAGVIGTILLIS

B. ITPIYFGPMAGVIGTPLLIS

C. ITEIYFGRMAGVIGTDLLIS

Solution

Answer: A. ITLIYFGVMAGVIGTILLIS (IleThrLeuIleTyrPheGlyValMetAlaGlyValIleGlyThrIleLeuLeuIleSer)

Reason:

Protein B has ITPIYFGPMAGVIGTPLLIS, in which three proline units are present (IleThrProIleTyrPheGlyProMetAlaGlyValIleGlyThrProLeuLeuIleSer) so that it is not an alpha-helix have a characteristic structure in the region of the lipid bilayer because proline residues may damage the alpha-helical structure. Protein C. has glutamic acid (E), arginine (R), and aspartic acid in the given peptide, ITEIYFGRMAGVIGTDLLIS (IleThrProIleTyrPheGlyProMetAlaGlyValIleGlyThrProLeuLeuIleSer) so that these amino acids are incompatible to \"transmembrane hydrophobic environment with alpha -helical secondary structure\". However, protein A has higher number of mostly nonpolar residues to make a membrane as an alpha-helix have a characteristic structure in the region of the hydrophobic lipid bilayer.