🎓 Prepared by students from Boğaziçi University

What is Enzyme Kinetics?

Enzyme kinetics is the study of how fast enzyme-catalyzed reactions proceed and how reaction rate depends on substrate concentration. The Michaelis-Menten model describes this relationship for most enzymes and explains catalytic efficiency.

Short answer

Enzyme kinetics describes reaction rate v as a function of substrate concentration [S]: v = Vmax[S]/(Km+[S]), where Vmax is the maximum rate and Km is the substrate concentration at half-maximal rate.

Michaelis-Menten saturation curve (Km = 10 µM, Vmax = 100 µM/min)

x: [S] (µM) · y: v (µM/min)

Try it: interactive calculator

Reaction rate v

66.67µM/min

= 100*20/(10+20)

Maximum rate Vmax100 µM/minMichaelis constant Km10 µMSubstrate concentration [S]20 µM

Step-by-step worked examples

An enzyme has Vmax = 100 µM/min and Km = 20 µM. Find the reaction rate at [S] = 20 µM.

v = Vmax[S]/(Km+[S])
v = (100×20)/(20+20)
v = 2000/40 = 50 µM/min

The same enzyme reaches v = 80 µM/min with Km = 10 µM. What substrate concentration produced this rate?

80 = 100[S]/(10+[S])
80(10+[S]) = 100[S]
800 + 80[S] = 100[S]
800 = 20[S] → [S] = 40 µM

A competitive inhibitor doubles the apparent Km from 10 to 20 µM while Vmax stays 100 µM/min. Find v at [S] = 20 µM.

v = 100×20/(20+20)
v = 2000/40 = 50 µM/min (half of the uninhibited rate at this [S])

Flashcards

Quick quiz

Q1.In v = Vmax[S]/(Km+[S]), what does Km equal at v = Vmax/2?

Correct answer: A. By definition, Km is the substrate concentration giving half-maximal velocity.

Q2.An enzyme has Vmax = 100 µM/min and Km = 10 µM. What is v at [S] = 10 µM?

Correct answer: B. v = 100×10/(10+10) = 1000/20 = 50 µM/min.

Q3.A low Km value indicates:

Correct answer: B. Low Km means the enzyme reaches half-max rate at low [S] — high affinity.

Q4.At saturating substrate concentration, reaction rate:

Correct answer: C. All active sites are occupied, so rate can't increase further — it plateaus at Vmax.

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Common mistakes

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Km is the same as Vmax. — Correct: Km is a substrate concentration (µM); Vmax is a rate (µM/min) — different units and meanings.

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Doubling [S] always doubles v. — Correct: Only true at low [S] (below Km); near saturation, v barely changes with [S].

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Higher Km always means a better enzyme. — Correct: Higher Km means lower substrate affinity, not necessarily worse catalysis — Vmax and kcat matter too.

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Enzyme kinetics only applies to simple one-substrate reactions. — Correct: The Michaelis-Menten model is a simplification; many enzymes follow more complex kinetics (allosteric, multi-substrate).

FAQ

What is the Michaelis-Menten formula for enzyme kinetics?

v = Vmax[S]/(Km+[S]), relating reaction rate v to substrate concentration [S].

How do you calculate Km in enzyme kinetics?

Km is found experimentally as the [S] at which v = Vmax/2, often via a Lineweaver-Burk plot.

What are examples of enzyme kinetics in biology?

Digestive enzymes like amylase and pepsin, and metabolic enzymes like hexokinase, all follow Michaelis-Menten kinetics.

What is enzyme kinetics used for?

It quantifies how fast reactions occur, how efficient enzymes are, and how drugs and inhibitors affect catalysis.