The binding efficiency of three different receptorsR1, R2 and R3 were tested against a ligand using
equilibrium dialysis, with a constant concentration of receptor and varying concentrations of ligand. The
Scatchard plot of receptor titration with different concentration of ligand is given below
(ris moles of bound ligand per moles of receptor and c is concentration of free ligand)
Q.50 The number of ligand binding sites present on receptors R1 and R3, respectively are
(A)1 and 4 (B)1 and 1 (C)4 and 1 (D)2 and 2
📘 Question Overview
The Scatchard plot represents r/c (bound ligand/free ligand) versus r (bound ligand per receptor).
In such a plot:
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X-intercept = number of binding sites (n)
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Slope = −1/Kd (affinity)
✅ Correct Answer
(D) 2 and 2
🧠 Explanation
From the given Scatchard plot:
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All receptor lines (R1, R2, R3, R4) intersect the x-axis at r = 2
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This means maximum binding (Bmax) = 2
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Therefore, each receptor has 2 ligand binding sites
For Question 50:
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R1 binding sites = 2
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R3 binding sites = 2
❌ Option-wise Explanation
| Option | Explanation |
|---|---|
| A) 1 and 4 | Incorrect. No receptor shows different x-intercepts |
| B) 1 and 1 | Incorrect. X-intercept is not at r = 1 |
| C) 4 and 1 | Incorrect. No receptor shows 4 binding sites |
| D) 2 and 2 | ✅ Correct. X-intercept for both R1 and R3 is 2 |
🔍 Final Conclusion
The number of ligand binding sites for both R1 and R3 is 2, making Option D the correct answer.
