Enzyme | Best 11th Class Biology Notes | Federal Board
Chapter 3 :
Enzymes
Enzymes
- Metabolism = Sum of all chemical rxns in a cell.
- Enzymes = Biological catalysts → ↑ rxn speed w/o consumption.
Properties:
✅ ↑ Speed of chemical rxns.
✅ Small qty required.
✅ pH & Temp sensitive.
✅ Specificity: Highly or slightly specific.
✅ Work in vivo & vitro.
✅ Some need cofactors for activity.
✅ ↓ Activation energy.
✅ No effect on equilibrium.
- Payen & Persoz (19th century) → Discovered diastase, an enzyme digesting starch.
Enzyme Structure
- Globular proteins (except ribozymes).
- Ribozymes = RNA-based enzymes (e.g., peptidyl transferase).
- Active Site = 3D cavity where rxn occurs.
- Binding site = Holds substrate.
- Catalytic site = Converts substrate → product.
Cofactors:
- Holoenzyme = Enzyme + cofactor (active).
- Apoenzyme = Enzyme w/o cofactor (inactive).
- Pepsinogen → Pepsin (Activated in HCl).
Types of Cofactors
🔹 Inorganic (Activators) → Fe²⁺, Mg²⁺, Cu²⁺, Zn²⁺ (Detach after rxn).
🔹 Organic:
- Coenzymes (Vitamin derivatives) → ATP, NAD⁺, FAD⁺.
- Prosthetic groups (Permanently attached) → Heme in cytochromes.
Mechanism of Enzyme Action
- E + S → ES → EP → E + P
- Enzyme alters pH, charge, bond stress → Easier rxn.
Models of Action
- Lock & Key (Fischer, 1894) → Fixed shape active site, highly specific.
- Induced Fit (Koshland, 1959) → Flexible active site, molds to substrate.
Activation Energy (Ea)
- Enzymes ↓ Ea, making rxns possible w/o excessive heat.
- Rxn Pathway:
A + E → AE Complex + B → AB + E
Factors Affecting Enzymes
Temp:
- Optimum: Max activity (~37°C humans, ~70°C bacteria).
- Too high → Denaturation.
- Too low → Inactive but reversible.
pH:
- Optimum pH varies (e.g., Pepsin = 2, Trypsin = 8).
- Extreme pH → Denaturation.
[Enzyme] ↑ → Rxn rate ↑ (until substrate saturation).
[Substrate] ↑ → Rxn rate ↑, but plateaus at Vmax.
Enzyme Inhibition
❌ Competitive (Reversible)
- Inhibitor competes for active site.
- Example: Malonate inhibits succinate dehydrogenase.
❌ Non-Competitive (Reversible or Irreversible)
- Binds allosteric site, alters active site shape.
- Example: Cyanide inhibits cytochrome oxidase.
🔄 Feedback Inhibition
- End product inhibits 1st enzyme in pathway.
- Example: Threonine inhibits 1st enzyme in its synthesis pathway.
Enzyme Classification (IUB 1961)
1️⃣ Oxidoreductases (Redox rxns) → Cytochrome oxidase.
2️⃣ Transferases (Transfer groups) → Hexokinase.
3️⃣ Hydrolases (Hydrolysis) → Pepsin, trypsin.
4️⃣ Lyases (Break bonds w/o H₂O) → Histidine decarboxylase.
5️⃣ Isomerases (Rearrange molecules) → Phosphohexose isomerase.
6️⃣ Ligases (Join molecules using ATP) → DNA ligase.
Enzyme Uses
🏭 Industry:
- Food → Amylase (syrups), Protease (meat tenderizing).
- Detergents → Lipase & Protease (stain removal).
- Paper/Leather → Ligninase (bleaching), Protease (hide processing).
🏥 Medicine:
- Diagnosis → ALT, AST (Liver), Troponin (Heart).
- Treatment → Streptokinase (Clot dissolving), Asparaginase (Cancer).
🧬 Genetic Engineering:
- DNA cutting → Restriction enzymes.
- DNA joining → Ligases.
- DNA copying → Polymerases (PCR).
