Enzymes
Biological catalysts — proteins that dramatically speed up chemical reactions in living cells without being permanently changed or consumed in the process.
A. Properties of Enzymes
Key Properties
- Biological catalysts: Speed up reactions by lowering activation energy — the enzyme itself is not consumed
- Protein in nature: Made of amino acids folded into a precise 3D shape (except ribozymes, which are RNA-based)
- Highly specific: Each enzyme acts on only one substrate — determined by the unique shape of its active site
- Reusable: Released unchanged after catalysis — can act on many substrate molecules
- Temperature sensitive: Activity rises to an optimum (~37°C in humans) then falls sharply as the enzyme denatures
- pH sensitive: Each enzyme has an optimum pH; extreme pH causes denaturation
- Denaturation is permanent: The active site's shape is irreversibly destroyed — the enzyme cannot recover
⚡ Key distinction Inhibition is reversible; denaturation is permanent. Above ~40°C human enzymes denature. Pepsin (pH 2) denatures in alkaline conditions. Amylase (pH 7) denatures in strong acid.
B. Models of Enzyme Action
Lock & Key vs Induced Fit
| Model | Proposed By | Description | Accepted? |
|---|---|---|---|
| Lock and Key | Emil Fischer (1894) | Active site is a rigid, fixed shape — substrate fits exactly like a key into a lock. Explains specificity but not flexibility. | Partially — explains specificity |
| Induced Fit | Daniel Koshland (1958) | Active site is flexible — it changes shape slightly to mould around the substrate when it binds, like a glove around a hand. More accurate. | ✅ Currently accepted model |
⚡ MCQ Tip Fischer (1894) = Lock & Key = rigid active site. Koshland (1958) = Induced Fit = flexible active site. Induced Fit is the currently accepted model.
C. Factors Affecting Enzyme Activity
Factor Summary
| Factor | Effect | Extreme effect |
|---|---|---|
| Temperature | ↑ temp → ↑ activity (more collisions) up to optimum ~37°C | Above optimum → denaturation (permanent) |
| pH | Each enzyme has its own optimum pH | Extreme pH → denaturation |
| Substrate concentration | ↑ substrate → ↑ rate until all active sites are occupied (saturation) | Rate plateaus at saturation — adding more substrate has no effect |
| Enzyme concentration | ↑ enzyme → ↑ rate (more active sites) | Limited by substrate availability |
| Competitive inhibitor | Similar shape to substrate — blocks active site temporarily | Reversible — overcome by adding more substrate |
| Non-competitive inhibitor | Binds elsewhere (allosteric site) → distorts active site shape | Often irreversible — cannot be overcome by adding substrate |
Temperature
Effect↑ temp → ↑ activity (more collisions) up to optimum ~37°C
Extreme effectAbove optimum → denaturation (permanent)
pH
EffectEach enzyme has its own optimum pH
Extreme effectExtreme pH → denaturation
Substrate concentration
Effect↑ substrate → ↑ rate until all active sites are occupied (saturation)
Extreme effectRate plateaus at saturation — adding more substrate has no effect
Enzyme concentration
Effect↑ enzyme → ↑ rate (more active sites)
Extreme effectLimited by substrate availability
Competitive inhibitor
EffectSimilar shape to substrate — blocks active site temporarily
Extreme effectReversible — overcome by adding more substrate
Non-competitive inhibitor
EffectBinds elsewhere (allosteric site) → distorts active site shape
Extreme effectOften irreversible — cannot be overcome by adding substrate
D. Important Enzymes to Know
Key Enzymes, Locations & Optimum pH
| Enzyme | Location | Reaction | Optimum pH |
|---|---|---|---|
| Amylase | Saliva & Pancreas | Starch → Maltose | pH 7 (neutral) |
| Pepsin | Stomach | Proteins → Peptides | pH 2 (strongly acidic) |
| Lipase | Pancreas, Small intestine | Fats → Fatty acids + Glycerol | pH 7–8 |
| Trypsin | Pancreas (acts in small intestine) | Proteins → Smaller peptides | pH 8 (slightly alkaline) |
| Lactase | Small intestine | Lactose → Glucose + Galactose | pH 6–7 |
| Catalase | Most cells (especially liver) | Hydrogen peroxide → Water + Oxygen | pH 7 |
| DNA Polymerase | Nucleus | Synthesises new DNA strands during replication | pH 7–8 |
Amylase
LocationSaliva & Pancreas
ReactionStarch → Maltose
Optimum pHpH 7 (neutral)
Pepsin
LocationStomach
ReactionProteins → Peptides
Optimum pHpH 2 (strongly acidic)
Lipase
LocationPancreas, Small intestine
ReactionFats → Fatty acids + Glycerol
Optimum pHpH 7–8
Trypsin
LocationPancreas (acts in small intestine)
ReactionProteins → Smaller peptides
Optimum pHpH 8 (slightly alkaline)
Lactase
LocationSmall intestine
ReactionLactose → Glucose + Galactose
Optimum pHpH 6–7
Catalase
LocationMost cells (especially liver)
ReactionHydrogen peroxide → Water + Oxygen
Optimum pHpH 7
DNA Polymerase
LocationNucleus
ReactionSynthesises new DNA strands during replication
Optimum pHpH 7–8
⚡ MCQ Tip Pepsin = pH 2 (most acidic — stomach). Trypsin = pH 8 (alkaline — pancreas/small intestine). Amylase = pH 7 (saliva). Catalase in LIVER cells breaks down toxic H₂O₂.
Quick MCQ Revision
| Fact | Answer |
|---|---|
| Optimum temp for human enzymes | 37°C (body temperature) |
| Optimum pH for pepsin | pH 2 (stomach — strongly acidic) |
| Optimum pH for amylase | pH 7 (saliva — neutral) |
| Optimum pH for trypsin | pH 8 (pancreas — slightly alkaline) |
| Lock & Key model | Emil Fischer, 1894 — rigid active site |
| Induced Fit model | Daniel Koshland, 1958 — flexible active site (accepted) |
| Denaturation | Permanent loss of enzyme shape — caused by high temp or extreme pH |
| Competitive inhibitor | Blocks active site — reversible, overcome by more substrate |
| Catalase substrate | Hydrogen peroxide → Water + Oxygen (found in liver) |
| Amylase substrate | Starch → Maltose (saliva & pancreas) |