🧬 Introduction to Enzymes
What are Enzymes?
- Biological catalysts that speed up biochemical reactions
- Derived from Greek ‘en’ (inside) and ‘zyme’ (yeast)
- Essential for metabolism – life impossible without them
- Remain unchanged after reaction completion
- Required in very small quantities
- Most are globular proteins (except ribozymes – RNA based)
🌟 Critical Concept: The turnover number is the maximal number of substrate molecules converted to product per active site per unit time. This measures enzyme efficiency!
Animation showing enzyme-substrate interaction
🔬 Enzyme Structure & Cofactors
Active Site Structure
Active site = Binding site + Catalytic site
Consists of 3-12 amino acids brought together by protein folding
Cofactors
Non-protein parts required for enzyme function
Can be inorganic (activators) or organic (coenzymes/prosthetic groups)
Enzyme Components
| Component | Description | Examples |
|---|---|---|
| Holoenzyme | Active enzyme with cofactor | Complete functional enzyme |
| Apoenzyme | Protein part without cofactor | Inactive enzyme |
| Coenzyme | Organic, loosely attached cofactor | ATP, NAD⁺, FAD⁺ |
| Prosthetic Group | Organic, covalently bound cofactor | Heme in cytochromes |
💡 Vitamin Connection: Vitamins are raw materials for coenzymes! B₂ → FAD, B₃ → NAD⁺, Biotin → carboxylation reactions
⚙️ Mechanism of Enzyme Action
Reaction Pathway
E + S → ES Complex → EP Complex → E + P
Enzyme (E) binds Substrate (S) forming Enzyme-Substrate complex (ES), which transforms to Enzyme-Product complex (EP) before releasing Product (P)
Lock & Key Model
• Active site has definite rigid shape
• Substrate fits perfectly like key in lock
• Emil Fischer (1894)
• Example: Sucrase, Maltase
Induced Fit Model
• Active site is flexible
• Modifies shape upon substrate binding
• D. Koshland (1959)
• Example: RuBisCO (regulatory enzyme)
Induced fit model – enzyme changes shape upon substrate binding
📊 Factors Affecting Enzyme Activity
Temperature Effects
- Q₁₀ Rule: Rate doubles with 10°C increase (up to optimum)
- Optimum temperature: 25-42°C for most enzymes
- Thermophilic enzymes: Work at 70°C+ (used in detergents)
- Low temp: Inactivation (reversible)
- High temp: Denaturation (irreversible)
pH Effects
| Enzyme | Optimum pH | Site of Action |
|---|---|---|
| Pepsin | 2.0 | Stomach |
| Salivary Amylase | 6.8 | Mouth |
| Trypsin | 7.5-8.5 | Small intestine |
| Pancreatic Lipase | 9.0 | Small intestine |
Concentration Effects
Enzyme Concentration
Rate ∝ [Enzyme] (with excess substrate)
Substrate Concentration
Increases until saturation (Vmax)
🚫 Enzyme Inhibition
Competitive
• Binds active site
• Structurally similar to substrate
• Reversible by ↑[substrate]
• Example: Sulfa drugs
Non-Competitive
• Binds allosteric site
• Changes enzyme shape
• Not reversed by ↑[substrate]
• Example: Cyanide, heavy metals
Feedback Inhibition
- End product inhibition – regulates metabolic pathways
- Example: High ATP inhibits phosphofructokinase in glycolysis
- Negative feedback maintains homeostasis
- Positive feedback amplifies processes (less common)
⚠️ Real-world Application: Enzyme inhibitors are used in drug design! Many antibiotics and medications work by inhibiting specific enzymes in pathogens.