Enzymes

Enzymes are biological catalysts — they speed up chemical reactions without being used up or permanently altered. All enzymes are globular proteins with a unique 3D shape determined by their amino acid sequence.

Why Enzymes Are Essential

Without enzymes, metabolic reactions would be too slow to sustain life. Enzymes lower the activation energy needed for a reaction to proceed. They are involved in both:

• Anabolic reactions — building complex molecules (e.g. photosynthesis builds glucose)
• Catabolic reactions — breaking down molecules (e.g. digestion breaks down starch)

The Induced Fit Model

The Induced Fit Model (Daniel Koshland, 1958) explains how enzymes work:

1. The substrate approaches the enzyme's active site
2. The active site changes shape slightly to wrap around the substrate more closely — like a glove moulding to a hand
3. An enzyme–substrate complex forms
4. The reaction occurs — bonds are broken or formed
5. Products are released and the enzyme returns to its original shape, ready to catalyse again

Note: The old Lock-and-Key model (Emil Fischer, 1894) suggested a rigid fit. The Induced Fit model is now accepted because it explains why enzymes can act on slightly different substrates and why the active site is flexible.

Enzyme Specificity

Each enzyme only catalyses one type of reaction because the active site has a specific shape complementary to its substrate. This is called enzyme specificity. The 3D shape of the active site is determined by the sequence of amino acids in the protein and the way the protein folds (hydrogen bonds, disulfide bridges, ionic bonds).

Factors Affecting Enzyme Activity

Temperature: As temperature increases, molecules move faster and collide more often with the active site → reaction rate increases. At the optimum temperature (~37°C for human enzymes), the rate is maximum. Above this, the enzyme denatures — the active site permanently loses its shape because the bonds holding the 3D structure break.

pH: Each enzyme has an optimum pH. Extreme pH disrupts the ionic and hydrogen bonds, causing denaturation. Examples: pepsin (stomach) = pH 2, salivary amylase = pH 7, trypsin (small intestine) = pH 8.

Substrate concentration: Rate increases as more substrate molecules collide with active sites, until saturation — all active sites are occupied and the rate plateaus (Vmax).

Enzyme concentration: More enzyme molecules = more active sites available → faster rate (provided substrate is not limiting).

Immobilised Enzymes

Immobilised enzymes are enzymes attached to an insoluble material (e.g. alginate beads, glass beads, or a membrane). Advantages:

• Can be reused many times (reduces cost)
• Easy to separate from products
• More thermostable (stable at higher temperatures)
• Can be used in continuous flow processes

Industrial uses of enzymes:

• Food & beverages: lactase in lactose-free milk, pectinase to clarify fruit juice, amylase in baking
• Biofuels: cellulase breaks down plant material for bioethanol production
• Medicine: enzymes in biosensors (e.g. glucose oxidase in diabetic test strips)
• Pharmaceuticals: enzymes used to synthesise drugs, e.g. penicillin acylase