# DBT JRF 2022 — Question 31

Exam: DBT JRF 2022 · Section: A · Topic: Biochemistry › Enzyme Kinetics - Competitive Inhibition · Difficulty: Medium

## Question

Identify the INCORRECT statement for a competitive reaction.

## Options

- **A.** The $V_{max}$ is unchanged.
- **B.** The dissociation constant ( $K_d$ ) is increased.
- **C.** $K_m$ will increase.
- **D.** The intercept on the x-axis will shift towards the left in the Lineweaver-Burk plot.

## Answer

Option D

## Explanation

Competitive inhibition — identifying the INCORRECT statement: In competitive inhibition, the inhibitor competes with substrate for the SAME active site: • $V_{max}$ is unchanged (with sufficient substrate, inhibitor can be displaced) ✔ CORRECT • Apparent $K_M$ increases (more substrate needed to achieve half-maximal rate) — this effectively means the apparent affinity (not Kd of inhibitor) decreases ✔ • $K_M$ increases ✔ CORRECT • Lineweaver-Burk plot: The y-intercept ( $1/V_{max}$ ) stays the same; x-intercept shifts toward zero (becomes LESS negative, i.e., shifts to the RIGHT from $-1/K_M$ to $-1/K_M^{app}$ ). Saying it shifts LEFT is INCORRECT. In the Lineweaver-Burk double-reciprocal plot: $\frac{1}{v} = \frac{K_M}{V_{max}} \cdot \frac{1}{[S]} + \frac{1}{V_{max}}$ With competitive inhibitor, slope increases but y-intercept stays constant → x-intercept moves RIGHT (toward 0), NOT left. Answer: Option 4 is the INCORRECT statement.

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