Q. 27 The substrate saturation profile of an enzyme that follows Michaelis-Menten kinetics is depicted in
the figure. What is the order of the reaction in the concentration range between 0.8 to 1.4 M?
(A) Zero
(B) Fraction
(C) First
(D) Second

Question Overview

An enzyme follows Michaelis–Menten kinetics, and its substrate saturation profile is shown in the figure.
The graph plots initial velocity (v) versus substrate concentration [S].

Question:
What is the order of the reaction in the substrate concentration range 0.8 M to 1.4 M?

Options

• (A) Zero
• (B) Fraction
• (C) First
• (D) Second

Understanding the Michaelis–Menten Curve

The Michaelis–Menten equation is:

v = ^V_max[S] / _{Km + [S]}

This equation explains how the enzyme reaction rate depends on substrate concentration.

Key Regions of the Curve

Low [S] (≪ K_m)

• Rate is directly proportional to [S]
• First-order reaction

Intermediate [S] (≈ K_m)

• Rate increases non-linearly
• Fractional order reaction

High [S] (≫ K_m)

• Enzyme becomes saturated
• Rate becomes constant and equals V_max
• Zero-order reaction

Analysis of the Given Range (0.8 M – 1.4 M)

• The curve levels off between 0.8 M and 1.4 M
• The initial velocity remains nearly constant
• All enzyme active sites are fully occupied

This indicates that increasing substrate concentration does not increase the reaction rate.

Correct Answer

✅ (A) Zero

At high substrate concentration, the reaction rate becomes independent of substrate concentration,
which is the defining feature of a zero-order reaction.

Explanation of All Options

(A) Zero — Correct ✅

• Enzyme is saturated
• Rate = V_max
• Independent of substrate concentration

(B) Fraction — Incorrect ❌

• Occurs near K_m, not at saturation
• The curve would not be flat

(C) First — Incorrect ❌

• Applies only at low substrate concentration
• Rate must increase linearly with [S]

(D) Second — Incorrect ❌

• Not applicable to Michaelis–Menten enzyme kinetics

Final Conclusion

In the substrate concentration range 0.8 M to 1.4 M, the enzyme operates under
substrate-saturated conditions. Therefore, the reaction follows zero-order kinetics.

✔ Final Answer: Zero Order Reaction