Question

An alpha-helix has the sequence: Ser-Glu-Gly-Asp-Trp-Gln-Leu-His-Val-Phe-Ala-Lys-Val-Glu. The amide proton (HN) of the leucine residue is hydrogen bonded to the carbonyl (CO) of: A) Asp B) Ala C) Gln D) Gly E) His

Answer 1

In the α-helix, the amide proton of Leucine is hydrogen bonded to the carbonyl group of Glu (Glutamic Acid), which is four positions ahead in the sequence, making the answer Asp.

Step-by-step explanation:

In an α-helix, every helical turn includes 3.6 amino acid residues, with hydrogen bonds stabilizing the structure between the amide proton (HN) and the carbonyl (CO) of residues four positions apart.

Given the sequence Ser-Glu-Gly-Asp-Trp-Gln-Leu-His-Val-Phe-Ala-Lys-Val-Glu and the question about the hydrogen bonding partner for the amide proton of the Leucine residue, we look four positions prior to find the carbonyl group bonded to it. In this sequence, the hydrogen bond would be formed between the amide proton of Leucine and the carbonyl group of Glu (Glutamic Acid), which is four positions ahead of it in the primary structure of the polypeptide. Therefore, the correct answer is A) Asp.