Q.37 The following figures show the plot of
reaction rate (v) versus
substrate concentration (mM) for an enzyme-catalyzed reaction
in the presence and absence of an inhibitor.
Match the possible reaction types with the plots.
(P) Competitive inhibition
(Q) Substrate inhibition
(R) Michaelis–Menten
(S) Non-competitive inhibition
Enzyme kinetics is a core topic in biochemistry and frequently tested in exams such as
CSIR-NET, GATE, IIT-JAM, NEET-PG, and university-level assessments.
This problem focuses on understanding reaction rate (v) versus substrate concentration ([S]) plots
in the presence and absence of enzyme inhibitors.
Given Reaction Types
- (P) Competitive inhibition
- (Q) Substrate inhibition
- (R) Michaelis–Menten kinetics
- (S) Non-competitive inhibition
Analysis of Individual Plots
Plot (i): Michaelis–Menten Kinetics
This plot shows a classical rectangular hyperbola where the reaction rate (v)
increases with increasing substrate concentration and finally reaches a maximum velocity (Vmax).
The substrate concentration at Vmax/2 is 1 mM, which corresponds to the Michaelis constant (Km).
There is no inhibitor effect visible.
Correct Match: (R) Michaelis–Menten kinetics
Plot (ii): Substrate Inhibition
In this plot, the reaction rate initially increases with substrate concentration,
but after reaching an optimum point, the rate decreases at high substrate levels.
This behavior is characteristic of substrate inhibition,
where excess substrate binds to inhibitory sites on the enzyme.
Correct Match: (Q) Substrate inhibition
Plot (iii): Competitive Inhibition
This curve shows that the maximum velocity (Vmax) remains unchanged,
but the substrate concentration required to reach Vmax/2 increases from 1 mM to 2 mM.
This indicates an increase in Km with no change in Vmax,
which is a hallmark of competitive inhibition.
Correct Match: (P) Competitive inhibition
Plot (iv): Non-Competitive Inhibition
This plot shows a decrease in maximum velocity from Vmax to
2Vmax/3, while the substrate concentration at Vmax/2 remains unchanged.
This behavior indicates non-competitive inhibition, where the inhibitor reduces enzyme activity
regardless of substrate concentration.
Correct Match: (S) Non-competitive inhibition
Final Matching Summary
| Plot | Reaction Type |
|---|---|
| (i) | Michaelis–Menten (R) |
| (ii) | Substrate inhibition (Q) |
| (iii) | Competitive inhibition (P) |
| (iv) | Non-competitive inhibition (S) |
Correct Answer
Option (B): P – iii, Q – ii, R – i, S – iv
Exam Tip
- Competitive inhibition: Vmax unchanged, Km increases
- Non-competitive inhibition: Vmax decreases, Km unchanged
- Substrate inhibition: Reaction rate decreases at very high substrate concentration
