Enzymes

How enzymes work

Enzymes are globular proteins that lower the activation energy of reactions. The substrate binds to the complementary active site, forming an enzyme–substrate complex.

• Lock and key model: rigid active site.
• Induced fit model: the active site changes shape slightly to fit the substrate closely, putting strain on bonds.

Factors affecting rate

• Temperature: rate rises to an optimum; above it, bonds holding the tertiary structure break and the enzyme denatures.
• pH: changes ionisation of R groups, altering the active site.
• Substrate concentration: rate increases until all active sites are occupied (Vmax).
• Enzyme concentration: rate increases if substrate is in excess.

Inhibition

• Competitive: inhibitor resembles the substrate and binds to the active site; overcome by increasing substrate concentration.
• Non-competitive: inhibitor binds elsewhere (allosteric site), changing the active site's shape; not overcome by more substrate.
• End-product inhibition regulates metabolic pathways.

Michaelis–Menten constant (Km)

The substrate concentration at which the rate is half of Vmax. A low Km means high affinity of the enzyme for its substrate.

Immobilised enzymes

Enzymes fixed in alginate beads or on surfaces: reusable, product not contaminated, often more stable (e.g. lactase for lactose-free milk).

Check yourself

1. Explain the induced fit model.
2. How can the effect of a competitive inhibitor be reduced?
3. What does a low Km value show?