An enzyme follows Michaelis-Menten kinetics with Km = 0.4 mM. What substrate concentration is required to achieve a velocity equal to 75% of Vmax?

• A 0.4 mM
• B 0.8 mM
• C 1.2 mM ✓
• D 3.2 mM

Explanation

Setting v = 0.75 Vmax in the Michaelis-Menten equation gives 0.75 = [S]/(Km + [S]), so 0.75Km + 0.75[S] = [S], hence 0.25[S] = 0.75Km and [S] = 3Km. With Km = 0.4 mM, three times that value is 1.2 mM. Students who forget the factor of three often pick 0.4 mM, which yields only half of Vmax, not three quarters.

Reference: Lippincott's Illustrated Reviews: Biochemistry, 8th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.