A small generic section of the primary structure of an
helix is given by -amino acid
1-amino acid
-2amino acid
-3amino acid
-4amino acid
-5amino acid
-6amino acid
- Which amino acid residue’s backbone forms a hydrogen bond with the backbone of the first (1\textsuperscript{st}) residue
The Correct Answer and Explanation is:
Correct Answer: The fifth (5ᵗʰ) amino acid residue’s backbone forms a hydrogen bond with the backbone of the first (1ˢᵗ) residue in an α-helix.
Explanation
In an α-helix, a common secondary structure in proteins, the polypeptide chain coils into a right-handed spiral stabilized by intramolecular hydrogen bonds. These hydrogen bonds occur between the carbonyl oxygen (C=O) of one amino acid and the amide hydrogen (N-H) of another.
Specifically, the hydrogen bond forms between the carbonyl oxygen of the amino acid at position i and the amide hydrogen of the amino acid at position i+4. This is often referred to as the i → i+4 hydrogen bonding pattern. Therefore, in this pattern:
- The 1ˢᵗ residue (i) forms a hydrogen bond with the 5ᵗʰ residue (i+4).
- The 2ⁿᵈ residue forms a hydrogen bond with the 6ᵗʰ, and so on.
This pattern helps stabilize the helical structure and is crucial to the α-helix’s integrity.
The side chains (R-groups) of the amino acids in an α-helix project outward from the helical backbone, minimizing steric hindrance and maximizing hydrogen bonding. The result is a compact, stable structure commonly found in transmembrane proteins, DNA-binding proteins, and structural motifs like coiled-coils.
The α-helix typically has:
- 3.6 residues per turn,
- A rise of 1.5 Å per residue,
- A pitch (height per full turn) of about 5.4 Å.
Knowing that hydrogen bonding occurs between every fourth residue in the backbone allows us to deduce that in a small segment of seven amino acids, the first residue’s backbone will form a hydrogen bond with the fifth.
This characteristic hydrogen bonding pattern is fundamental to the α-helix’s stability and function in biological systems.
