The data in the table in the table below is collected for an enzyme-catalyzed
reaction.
The Km for this enzyme is approximately:
8 x 10-6 mM
2.0 x 10-5 mM
8.0 x 10-5 mM
2 x 10-2 mM
Correct Answer and Explanation
The correct answer is option B: 2.0×10−5 mM, because from the velocity data the reaction’s
Vmax is about 280 μmol·min⁻¹, so
Vmax/2 ≈ 140 μmol·min⁻¹, which occurs at
[S] = 2.0×10−5 mM; therefore this substrate concentration equals
Km for this enzyme.
Understanding Km from the Table
For a simple Michaelis–Menten enzyme, the Michaelis constant (Km) is defined as
the substrate concentration at which the initial velocity
(V0) is half of the maximum velocity (Vmax).
Looking at the table, the velocities level off near 277–280 μmol·min⁻¹, so
Vmax ≈ 280 μmol·min⁻¹. Half of this value is about
140 μmol·min⁻¹, and the table shows that when
[S] = 2×10−5 mM, the velocity is exactly
140 μmol·min⁻¹, matching Vmax/2.
Thus, the substrate concentration at this point is
Km = 2.0×10−5 mM.
Option-wise Explanation
At this substrate concentration, V₀ = 80 μmol·min⁻¹, which is far below
Vmax/2 (140 μmol·min⁻¹), so it cannot represent Km.
Here V₀ = 140 μmol·min⁻¹, which equals half of the maximal rate (~280 μmol·min⁻¹),
satisfying the definition of Km; therefore this is the correct choice.
At this higher substrate concentration, the velocity is already 224 μmol·min⁻¹,
much greater than Vmax/2 and lies in the near-saturation region,
so it does not correspond to Km.
This concentration gives a velocity of about 280 μmol·min⁻¹, essentially
Vmax, indicating enzyme saturation; therefore it is far above
Km and incorrect.
SEO‑oriented introduction:
Km calculation from enzyme catalyzed reaction data is a core skill in enzyme kinetics and frequently asked in competitive exams such as CSIR NET Life Sciences and GATE Biotechnology. By identifying Vmax from a substrate–velocity table and then locating the substrate concentration that gives half‑maximal velocity, students can quickly determine Km and evaluate every multiple‑choice option with confidence.
