A reaction mixture contains an enzyme with Km = 0.2 mM at a substrate concentration of 20 mM. If the substrate concentration is now doubled, the expected change in initial velocity is:
- A It doubles, following first-order kinetics
- B It shows almost no increase, because the reaction is near zero-order ✓
- C It falls, because excess substrate inhibits the enzyme
- D It increases fourfold
Correct answer: B. It shows almost no increase, because the reaction is near zero-order
Explanation
At 20 mM the substrate is 100 times the Km, so the enzyme is essentially fully saturated and velocity approximates Vmax. Under saturation the rate depends only on enzyme concentration, not on substrate concentration, so further increases in substrate produce negligible change: this is zero-order kinetics. Doubling would apply only when substrate is far below Km, where v is proportional to substrate concentration. Substrate inhibition exists for some enzymes but is not predicted by simple Michaelis-Menten behavior and is not implied here.
Reference: Harper's Illustrated Biochemistry, 32nd ed.
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