Q.35 Given below is the hydropathy plot of a monomeric transmembrane protein.
How many transmembrane α–helices are present in the protein?
(A) 1 (B) 2 (C) 4 (D)5

What is Given

• A hydropathy plot of a monomeric transmembrane protein

• X-axis: Amino acid position

• Y-axis: Hydropathy index (kJ/mol)

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Key Concept: Hydropathy Plot & Transmembrane Helices

• Hydrophobic amino acids show positive hydropathy values

• Transmembrane α-helices:

  • Are ~20–25 amino acids long

  • Appear as continuous peaks above the threshold line

• Only long, continuous hydrophobic regions are counted

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Analysis of the Given Hydropathy Plot

Step 1: Identify Hydrophobic Peaks

From the plot:

• One strong, broad peak between approximately residues 20–40

• This region:

  • Lies well above the hydropathy threshold

  • Has sufficient length (~20 amino acids)

Step 2: Examine Remaining Regions

• Other peaks:

  • Are short

  • Cross the threshold only briefly

  • Represent surface or internal hydrophobic patches, not membrane-spanning helices

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Final Interpretation

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Correct Answer

1\boxed{1}1

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Why Other Options Are Incorrect

Option (B): 2

• Only one region meets length + hydrophobicity criteria

Option (C): 4

• Would require four long hydrophobic stretches (not present)

Option (D): 5

• Impossible for a monomeric protein with this plot

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SEO-Friendly Introduction

Understanding the hydropathy plot of a transmembrane protein is a fundamental concept in biochemistry and molecular biology, frequently tested in IIT-JEE, GATE, JAM, and CSIR-NET exams. A hydropathy plot helps identify membrane-spanning α-helices by highlighting long stretches of hydrophobic amino acids.

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How Transmembrane α-Helices Appear in Hydropathy Plots

• Continuous positive hydropathy index

• Length of 20–25 amino acids

• Represents lipid-bilayer spanning regions

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Why Only One Helix Is Present Here

• Only one region shows:

  • Strong hydrophobicity

  • Sufficient length

• Other peaks are too short to span the membrane

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Final Conclusion

✔ The hydropathy plot shows only one transmembrane α-helix
✔ Correct Answer: Option (A)