31. Which of the following gaphs represents uncompetitive inhibition?

Introduction

Questions based on enzyme inhibition are very common in biochemistry and competitive exams
like NEET, CSIR-NET, GATE, and university exams. One such important question is identifying
the graph representing uncompetitive inhibition. In this article, we analyze
each graph using Lineweaver–Burk plots and explain why the correct option represents
uncompetitive inhibition.

Question

Which one of the following graphs represents uncompetitive inhibition?

(All graphs are Lineweaver–Burk plots of
1/V₀ vs 1/[S])

Basic Concept: Uncompetitive Inhibition

Before analyzing the graphs, recall the key properties of uncompetitive inhibition:

Key Characteristics

• Inhibitor binds only to the enzyme–substrate (ES) complex
• Both V_max decreases and K_m decreases
• The ratio K_m/V_max remains constant

Effect on Lineweaver–Burk Plot

• Parallel straight lines
• Same slope
• Different intercepts

 Parallel lines = Uncompetitive inhibition

Understanding the Axes

• X-axis: 1/[S]
• Y-axis: 1/V₀

Slope of Lineweaver–Burk Plot

Slope = K_m / V_max

Option-Wise Explanation

Option (A)

• Lines intersect on the Y-axis
• Same Y-intercept → V_max unchanged
• Different slopes → K_m changed

This pattern represents competitive inhibition, not uncompetitive inhibition.

Option (B)

• Lines intersect left of the Y-axis
• Both slope and intercept change
• Neither parallel nor same intercept

This graph is characteristic of mixed inhibition.

Option (C)  Correct Answer

• Lines are perfectly parallel
• Slope remains the same
• Y-intercept increases with inhibitor
• X-intercept shifts left

This matches uncompetitive inhibition where:

• V_max ↓
• K_m ↓
• K_m/V_max remains constant

✔ Correct representation of uncompetitive inhibition

Option (D)

• Lines intersect on the X-axis
• Same X-intercept → K_m unchanged
• Different Y-intercepts → V_max decreased

This represents non-competitive inhibition, not uncompetitive inhibition.

Final Answer

Option (C)

Quick Comparison Table

Inhibition Type	Vmax	Km	Line Pattern
Competitive	Same	Increases	Intersect Y-axis
Non-competitive	Decreases	Same	Intersect X-axis
Mixed	Decreases	Changes	Intersect left of Y
Uncompetitive	Decreases	Decreases	Parallel lines

Important Exam Tip

Always remember:
Parallel Lineweaver–Burk lines = Uncompetitive inhibition

Conclusion

The graph representing uncompetitive inhibition is Option (C)
because it shows parallel Lineweaver–Burk plots, indicating that both V_max and
K_m decrease proportionally. Mastering these graphical patterns is essential for
scoring high in enzyme kinetics questions.