Question

The most common secondary structure of protein molecule is-

• A. Beta helix
• B. Gamma helix
• C. Alpha helix
• D. Delta helix

Hint:

There are 3.6 amino acid residues per turn of a standard alpha helix.
Proline is often called a "helix breaker" because its structure is incompatible with the geometry of a standard alpha helix.

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
The question is related to protein biochemistry, specifically looking for the most frequently occurring secondary structural motif in proteins.

Step 2: Detailed Explanation:

• Protein Levels of Organization: Proteins have four levels of structure: primary (sequence), secondary (local folding), tertiary (3D shape), and quaternary (multi-subunit).

• Secondary Structure: This level refers to the spatial arrangement of the polypeptide backbone, stabilized primarily by hydrogen bonds.

• The Alpha Helix ($\alpha$-helix): This structure was first proposed by Linus Pauling.

• In an $\alpha$-helix, the polypeptide chain is wound tightly around an imaginary axis, with the R-groups of the amino acids protruding outward.

• Stability is provided by hydrogen bonds between the carbonyl oxygen (C=O) of one amino acid and the amide hydrogen (N-H) of the amino acid located four residues ahead in the sequence.

• Prevalence: The $\alpha$-helix is the most common and thermodynamically stable secondary structure found in many globular and fibrous proteins (like keratin).

• While the beta-pleated sheet is also common, the $\alpha$-helix is generally considered the "default" or most frequent local folding pattern due to its efficient hydrogen bonding network.
Final Answer:
The alpha helix is the most common and fundamental secondary structure found in proteins across various organisms.