The alpha-helical secondary structure found in protein a. is disrupted by the pr

Question

The alpha-helical secondary structure found in protein

a. is disrupted by the presence of a proline residue

b. is stabilized by intrachain hydrogen bonding involving amide hydrogen atoms and carbonyl oxygen atoms

c. sequesters amino acid R-groups within the helix center

d. involved three intertwined protein strands

e. two of the above are correct

I thought that the answer to this question was e, meaning that a and b were both correct. In class, my professor said that the correct answer to this question is b only. Any help understanding this would be appreciated! Thanks.

Explanation / Answer

Answer : The correct answer is B (is stabilized by intrachain hydrogen bonding involving amide hydrogen atoms and carbonyl oxygen atoms).

Explanation : A constraint on the formation of the alpha helix is the presence of Prolineresidues. In proline, the nitrogen atom is part of a rigid ring and rotation about the N -- Calpha bond is not possible. Thus, a Proline residue introduces a destabilizing kink in an alpha helix. In addition, the nitrogen atom of a Proline residue in peptide linkage has no substituent hydrogen to participate in hydrogen bonds with other residues. For these reasons,proline is only rarely found within an alpha helix.

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