Enzymes and Biological Reactions

15min Part 5 / Ch9 / Lesson 2

Prerequisites: 5-9-1

Objectives

Enzymes: proteins that catalyze chemical reactions in living organisms.

Key features:

Substrate specificity: act only on specific substrates (lock-and-key model)
Optimal temperature: maximum activity around 35–40°C (body temperature)
Optimal pH: varies by enzyme (pepsin: pH 2, trypsin: pH 8)
Denaturation: high temperature or pH extremes destroy the protein structure → loss of catalytic activity

Enzyme	Substrate	Reaction	Location
Amylase	Starch	→ Maltose	Saliva
Pepsin	Protein	→ Peptides	Stomach (pH 2)
Trypsin	Protein	→ Peptides	Pancreas (pH 8)
Lipase	Fats	→ Fatty acids + glycerol	Pancreas
Catalase	H₂O₂	→ H₂O + O₂	Liver

Major enzymes and their functions

Temperature vs enzyme activity

Rising temperature increases reaction rate, but above the optimum, the protein denatures and loses activity
Inorganic catalysts (MnO₂, etc.) are not denatured by heat → a key difference from enzymes

Q1 What is the property of enzymes acting on only specific substrates called?

Q2 What is the optimal pH for pepsin?

Q3 Why do enzymes lose activity at high temperatures?

Q1. Compare adding MnO₂ vs catalase (liver extract) to hydrogen peroxide. Name two differences between enzymes and inorganic catalysts.

Solution

Both catalyze $\mathrm{2H_2O_2 \rightarrow 2H_2O + O_2}$ .

Differences:

Substrate specificity: Catalase acts only on H₂O₂, while MnO₂ can catalyze other reactions as well
Heat stability: Catalase denatures and loses activity at high temperatures, while MnO₂ retains its catalytic function