What is Enzyme Digestion?
Enzymes are biological catalysts that speed up the chemical reactions of digestion — breaking carbohydrates, proteins, and fats into absorbable nutrients — without being consumed themselves. Their activity follows Michaelis-Menten kinetics, which relates reaction rate to substrate concentration.
Digestive enzymes (like amylase, pepsin, and lipase) lower the activation energy needed to break down food molecules; their reaction rate v follows the Michaelis-Menten equation v = (Vmax·[S])/(Km+[S]).
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Step-by-step worked examples
An enzyme has Vmax = 100 µmol/min and Km = 5 mM. Find the reaction rate at [S] = 5 mM.
v = (Vmax×S)/(Km+S) v = (100×5)/(5+5) = 500/10 v = 50 µmol/min (exactly half of Vmax, as expected when [S] = Km)
An enzyme has Vmax = 60 µmol/min and Km = 20 mM. Find v at [S] = 100 mM.
v = (60×100)/(20+100) = 6000/120 v = 50 µmol/min
Pepsin has Vmax = 40 µmol/min and Km = 8 mM. Find v when substrate is low, [S] = 2 mM.
v = (40×2)/(8+2) = 80/10 v = 8 µmol/min (far below Vmax because [S] << Km)
Flashcards
Quick quiz
Q1.Using v=(Vmax×S)/(Km+S), find v when Vmax=80 µmol/min, Km=10 mM, and S=10 mM.
Q2.Which enzyme begins protein digestion in the stomach?
Q3.What does a lower Km value indicate?
Q4.What happens to reaction rate as substrate concentration increases far beyond Km?
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Common mistakes
Enzymes are used up in the reactions they catalyze. — Correct: Enzymes are not consumed — they lower activation energy and can be reused repeatedly.
All digestive enzymes work best at the same pH. — Correct: Enzymes have different pH optima — pepsin works best in acidic stomach pH (~2), while pancreatic enzymes prefer the alkaline small intestine (~8).
Reaction rate increases forever as substrate concentration rises. — Correct: Rate plateaus at Vmax once all enzyme active sites are saturated with substrate.
Enzymes can work on any substrate. — Correct: Enzymes are highly specific — their active site shape matches only one or a few substrates.
FAQ
What is the formula for enzyme reaction rate?
The Michaelis-Menten equation: v = (Vmax·[S])/(Km+[S]), relating rate v to substrate concentration [S].
What is enzyme digestion?
The process by which digestive enzymes like amylase, pepsin, and lipase chemically break food into absorbable molecules.
How to calculate enzyme reaction rate?
Plug the substrate concentration, Vmax, and Km into v = (Vmax·[S])/(Km+[S]).
What are examples of digestive enzymes?
Salivary amylase breaks down starch, pepsin breaks down protein in the stomach, and pancreatic lipase breaks down fat in the small intestine.