A small generic section of the primary structure of a helix is given by amino acid – amino acid, amino acid, amino acid – amino acid; amino acid, amino acid

A small generic section of the primary structure of a helix is given by amino acid – amino acid, amino acid, amino acid – amino acid; amino acid, amino acid. Which amino acid residue’s backbone forms a hydrogen bond with the backbone of the seventh residue?

The Correct Answer and Explanation is:

Correct Answer: The backbone of the third amino acid residue forms a hydrogen bond with the backbone of the seventh residue in an α-helix.

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Explanation

In an α-helix, a common secondary structure in proteins, the backbone atoms of the polypeptide chain form a regular pattern of hydrogen bonds. These hydrogen bonds occur between the carbonyl oxygen (C=O) of one amino acid and the amide hydrogen (N–H) of another amino acid located four residues ahead in the sequence.

This pattern can be described as i → i+4 hydrogen bonding, where the carbonyl oxygen of residue “i” forms a hydrogen bond with the amide hydrogen of residue “i+4”. This stabilizes the helical structure by aligning the polypeptide chain into a right-handed coil, with 3.6 amino acids per turn.

In the given sequence format:

nginxCopyEditamino acid - amino acid, amino acid, amino acid - amino acid; amino acid, amino acid

Let’s identify the positions. Removing the formatting characters and counting the residues, we get a sequence of 7 amino acids:

1. amino acid
2. amino acid
3. amino acid
4. amino acid
5. amino acid
6. amino acid
7. amino acid

To find which residue forms a hydrogen bond with the 7th residue, we use the i → i+4 rule in reverse:
Let the 7th residue be “i”. Then the residue that formed a hydrogen bond with it is residue i-4 = residue 3.

Therefore, the third residue‘s carbonyl oxygen would have formed a hydrogen bond with the amide hydrogen of the seventh residue, helping stabilize the α-helix.

This hydrogen bonding pattern is crucial for the structural integrity and function of many proteins, contributing to the compact, rod-like shape characteristic of α-helices. It also allows side chains to extend outward from the helix, facilitating interactions with other molecules.