Which amino acid has a secondary amino group that introduces a kink in alpha-helix structures?

• A. Glycine
• B. Proline
• C. Alanine
• D. Leucine

Answer: (B)

Proline has a distinctive backbone feature: its amino group is a secondary amine that is tied into a ring with the side chain. This rings closes around the backbone, making the N–H less available for the hydrogen bonds that stabilize an alpha-helix. Because those backbone hydrogen bonds (between the carbonyl of residue i and the amide hydrogen of residue i+4) hold the helix together, proline’s inability to donate that hydrogen and its rigid ring constrain the backbone. The result is a disruption of the regular helix geometry, causing a bend or kink where proline appears.

Glycine is very flexible and often found in turns, not kinks in helices; alanine and leucine are typical helix-formers and support straight helical regions rather than inducing kinks.