Enzymes

What Are Enzymes?

Enzymes are biological catalysts — they speed up chemical reactions in living organisms without being used up themselves. They are proteins, made of amino acid chains folded into a specific 3D shape.

Why enzymes are important: Without enzymes, metabolic reactions would be too slow to sustain life. Enzymes can increase reaction rates by millions of times.

The Lock-and-Key Model

Each enzyme has an active site — a specific region on the enzyme where the substrate (the molecule it acts on) binds. The active site has a precise shape, complementary to the shape of the substrate.

The lock-and-key model:

• The enzyme is the "lock"; the substrate is the "key"
• Only the correctly shaped substrate can fit the active site
• This explains why enzymes are specific — each enzyme only works on one (or a few related) substrate(s)

Mechanism:

1. Substrate collides with and binds to the active site → enzyme-substrate complex formed
2. The reaction occurs (the enzyme catalyses the reaction)
3. Products are released; the enzyme's active site is unchanged and free to work again

Note: The induced fit model (more advanced) suggests the active site changes shape slightly to accommodate the substrate — not required for GCSE but useful to mention.

Factors Affecting Enzyme Activity

1. Temperature

• At low temperatures: particles have low kinetic energy → fewer collisions → slower reaction
• As temperature increases: more kinetic energy → more collisions → faster reaction
• Optimum temperature: The temperature at which the enzyme works fastest (for most human enzymes, ~37°C — body temperature)
• Above the optimum: Heat causes the enzyme to denature — the bonds holding the protein in shape break, the active site changes shape, the substrate can no longer fit → the enzyme permanently loses its function
• Denaturation: Permanent change to the enzyme's shape — the enzyme cannot work again

2. pH

• Each enzyme has an optimum pH at which it works fastest
• Changes in pH alter the ionisation of amino acids → change the shape of the active site → substrate fits less well → slower reaction
• Extreme pH: Denatures the enzyme — permanently changes the active site shape
• Examples:
  • Pepsin (stomach protease): optimum pH ~2 (acidic — works in stomach acid)
  • Amylase (salivary): optimum pH ~7 (neutral — works in the mouth)
  • Arginase (liver): optimum pH ~9.5 (slightly alkaline)

3. Substrate Concentration

• Higher substrate concentration → more collisions with enzyme active sites → faster reaction
• But: there is a maximum rate — when all active sites are occupied (saturated), increasing substrate concentration has no further effect
• Enzyme concentration: Similarly, more enzyme = faster reaction (at constant substrate concentration) up to a point

Enzyme Specificity — Why It Matters

Each enzyme is specific because its active site has a unique shape that matches its substrate. This specificity is crucial for metabolism:

• Amylase digests only starch (not proteins or fats)
• Proteases digest proteins (not starch or fats)
• Lipases digest lipids/fats (not starch or proteins)

Common exam mistake: Students often say enzymes are "destroyed" by high temperature. The correct term is denatured — permanently altered shape. The enzyme itself (as chemical mass) is not destroyed, but it can no longer function.