Question

And-helix is most likely to be held together by: (A) disulfide bonds. (B) hydrophobic effects. (C) hydrogen bonds. (D) ionic attractions between side chains.

Short answer

C) hydrogen bonds.

Step-by-step solution

Understand the Structure of an α-Helix

An α-helix is a common structural element in proteins. It is a right-handed coil or spiral conformation of amino acids, stabilizing the protein structure.

Identify Possible Interactions

Consider the types of interactions listed: disulfide bonds, hydrophobic effects, hydrogen bonds, and ionic attractions between side chains.

Analyze Disulfide Bonds

Disulfide bonds typically form between cysteine residues, which are important for the stabilization of a protein's tertiary structure but not commonly for α-helices specifically.

Analyze Hydrophobic Effects

While hydrophobic effects contribute to the overall stability of proteins, they are not the main force holding the α-helix together.

Analyze Hydrogen Bonds

In an α-helix, hydrogen bonds form between the carbonyl oxygen of one amino acid and the amide hydrogen of another amino acid four residues earlier. These bonds are critical for maintaining the helical structure.

Analyze Ionic Attractions

Ionic attractions between side chains can contribute to the overall protein structure but are not the primary force holding an α-helix together.

Conclusion

Based on the analysis, hydrogen bonds are the primary interactions holding the α-helix together.

Key concepts

α-Helix

An α-helix is a key structural element in proteins. It's a right-handed helix formed by a specific pattern of amino acids. This arrangement allows the protein to be stable and functional. In simpler terms, imagine a spring-like structure made up of protein building blocks called amino acids.
A few characteristics of α-helices include:

• Right-handed coil
• Stabilized by internal hydrogen bonds
• Typically 3.6 amino acids per turn

The repeating pattern in an α-helix provides a strong and stable framework, essential for the protein's overall structure.

Hydrogen Bonds

Hydrogen bonds are crucial for the stability of the α-helix. These weak bonds form between the carbonyl oxygen of one amino acid and the amide hydrogen of another amino acid four residues earlier.
Here’s a simpler way to think about it: imagine how a zipper works. The teeth of the zipper come together to hold the sides in place. In an α-helix, the hydrogen bonds act like the teeth of a zipper. They hold the helical structure, maintaining its shape and stability.

• Hydrogen bonds are weak, but numerous, providing a cumulative strong effect.
• They form at regular intervals, contributing to the helical structure.

This pattern of hydrogen bonding is the main reason why an α-helix remains intact and functional in a protein.

Secondary Protein Structure

Proteins have different levels of structure, and the α-helix is part of what's known as the secondary structure. The secondary structure refers to local folded structures within a protein that form due to interactions between atoms in the backbone of the molecule.
There are two main types of secondary structures:

• α-Helix
• β-Pleated Sheet

While the α-helix is a spiral shape, the β-pleated sheet consists of strands that lie alongside each other, forming a sheet-like arrangement.
Secondary structures are stabilized mainly by hydrogen bonds. They provide the necessary shape and stability needed for a protein to function properly. Understanding these structural elements helps us grasp how proteins behave and perform their biological roles efficiently.