Question

A D amino acid would interrupt an alpha-helix made of L-amino acids. Another naturally occurring hindrance to the formation of an alpha-helix is the presence of:

a. a nonpolar residue near the carboxyl terminus
b. a negatively charged Arg residue
c. a positively charged Lys residue
d. two Ala residues side by side
e. a Pro residue

Answer 1

A proline residue (Pro) presents a naturally occurring hindrance in an alpha-helix structure due to its unique, rigid ring-like structure that disrupts the helix's conformation.

Step-by-step explanation:

In the structure of proteins, the alpha-helix and beta-pleated sheet represent key features of the secondary structure, which are held together by hydrogen bonding in the protein's backbone. Specifically for the alpha-helix, this involves the oxygen atom of a carbonyl group forming a bond with the hydrogen atom of an amide group four residues away, contributing to the characteristic helical twist.

When concerning factors that may interrupt or hinder the formation of an alpha-helix in a protein composed mostly of L-amino acids, one naturally occurring hindrance is the presence of a Pro residue. The unique ring-like structure of proline, with its side chain bonded to the amino group, introduces a kink due to its rigid cyclic structure. This constraint makes it incompatible with the regular helical structure, thus disrupting the formation or continuation of an alpha-helix.