Q. 42 The secondary structure topology diagram of 400 amino acid long “Protein-X” is depicted in the
figure. The start and end amino acid residue numbers of each 𝜶-helix are marked. The percentage (correct
to integer number) of residues forming 𝜶-helix is ____ .
Percentage of α-Helix Residues in a Protein Secondary Structure Topology Diagram
Question Overview
A secondary structure topology diagram of a 400 amino acid long protein (Protein-X) is shown.
The start and end residue numbers of each α-helix are marked.
Question:
What is the percentage (correct to integer) of residues forming α-helix in Protein-X?
Understanding the Problem
To solve this question, we must:
- Identify all α-helical regions from the diagram
- Calculate the number of residues in each α-helix
- Find the total number of α-helix residues
- Calculate their percentage out of 400 amino acids
α-Helix Regions from the Diagram
α-Helix 1
Residues: 21 to 70
Number of residues = 70 − 21 + 1 = 50
α-Helix 2
Residues: 225 to 249
Number of residues = 249 − 225 + 1 = 25
α-Helix 3
Residues: 320 to 324
Number of residues = 324 − 320 + 1 = 5
Total α-Helix Residues
50 + 25 + 5 = 80 residues
Percentage of α-Helix Residues
Total amino acids in Protein-X = 400
Percentage = (80 / 400) × 100 = 20%
Correct Answer
✅ 20%
Why This Answer Is Correct
- All α-helix regions were correctly identified
- Residue numbering was counted inclusively
- Percentage was calculated using the total protein length
- Answer was rounded to the nearest integer as instructed
Common Mistakes to Avoid
- ❌ Forgetting to include both start and end residues
- ❌ Ignoring small helices (such as residues 320–324)
- ❌ Dividing by the wrong total protein length
Final Conclusion
Protein-X contains 80 α-helix-forming residues out of a total of 400 amino acids.
Therefore, the percentage of residues forming α-helix is 20%.
✔ Final Answer: 20%
