# DBT JRF 2020 — Question 49

Exam: DBT JRF 2020 · Section: A · Topic: Biochemistry · Difficulty: Medium

## Question

Ligands 'A' and 'B' bind to protein 'P' with dissociation constants of 1 nM and 100 nM, respectively. Which of the following is true?

## Options

- **A.** Both 'A' and 'B' bind 'P' with equal affinity
- **B.** 'B' binds 'P' with more affinity
- **C.** Dissociation constant is not related to affinity
- **D.** 'A' binds 'P' with more affinity

## Answer

Option C

## Explanation

Dissociation constant $K_d = \dfrac{[\text{protein}][\text{ligand}]}{[\text{complex}]}$ . A lower $K_d$ indicates tighter binding (higher affinity). Ligand A: $K_d = 1 \text{ nM}$ (very tight binding). Ligand B: $K_d = 100 \text{ nM}$ (100× weaker binding). Affinity is inversely proportional to $K_d$ : $\text{Affinity} \propto \dfrac{1}{K_d}$ . Therefore A binds protein P with 100× greater affinity than B. This is the basis for therapeutic drug selection — compounds with lower $K_d$ values are typically more potent.

Source: https://mypyqbuddy.com/questions/ligands-a-and-b-bind-to-protein-p-6a05a8a02adafec470293217