Question

The concentration of ligand is equal to $K_{d}$ when the fraction of the ligand bound to the protein is

• A. 1
• B. $1/4$
• C. $1/2$
• D. $1/8$

Hint:

$K_{d}$ = The concentration for half-saturation ($1/2$).

Answer 1

The Correct Option is C

Step 1: Concept
The dissociation constant ($K_{d}$) is a measure of the affinity between a protein and its ligand.

Step 2: Meaning
The fraction of binding sites occupied ($\theta$) is given by the formula $\theta = [L] / ([L] + K_{d})$.

Step 3: Analysis
If we set the ligand concentration $[L]$ equal to $K_{d}$, the equation becomes $\theta = K_{d} / (K_{d} + K_{d}) = K_{d} / 2K_{d} = 1/2$.

Step 4: Conclusion
Thus, $K_{d}$ is numerically equivalent to the ligand concentration at which 50% (or $1/2$) of the binding sites are saturated. Final Answer: (C)