Enzymes: their role as biological catalysts; the lock-and-key and induced-fit models; the formation of enzyme-substrate complexes; the effects of temperature, pH, substrate concentration and enzyme concentration; and competitive and non-competitive inhibition.

What this dot point is asking

Eduqas wants you to explain how enzymes act as catalysts, describe the lock-and-key and induced-fit models, explain the four factors affecting rate, explain denaturation, and distinguish competitive from non-competitive inhibition. This is a Core Concepts statement, so enzyme questions appear on every paper and underpin respiration, photosynthesis, digestion and the kidney.

Enzymes as catalysts

A reaction needs a minimum energy to start, the activation energy. Enzymes provide an alternative route with a lower activation energy, so reactions proceed quickly at body temperature. They are specific (each catalyses one reaction or a small group), not used up, and work in tiny amounts.

The lock-and-key model treats the active site as a rigid, exactly complementary shape. The induced-fit model refines this: the active site is only approximately complementary until the substrate binds, when it moulds around the substrate, distorting the substrate's bonds and lowering the activation energy further. Eduqas favours induced fit because it explains both specificity and catalysis.

Factors affecting the rate

• Temperature. Rate rises as molecules gain kinetic energy and collide more often with enough energy, up to the optimum. Above it, the enzyme denatures: bonds holding the tertiary structure break, the active site changes shape and the substrate no longer fits.
• pH. Each enzyme has an optimum pH. Away from it, changes in charge alter ionic and hydrogen bonds, distorting the active site and reducing the rate; extreme pH denatures the enzyme.
• Substrate concentration. Rate rises as more enzyme-substrate complexes form, then plateaus when all active sites are occupied (the enzyme is saturated, giving the maximum rate, $V_{max}$).
• Enzyme concentration. With excess substrate, rate is proportional to enzyme concentration, because more active sites are available.

Inhibition

Some inhibition is part of normal control (for example end-product inhibition, where the product of a pathway inhibits an earlier enzyme to switch the pathway off when enough product is made).

Examples in context

Example 1. Why a fever is dangerous. Human enzymes have an optimum near $37^{\circ}\text{C}$; a high fever pushes the temperature towards the point where vital enzymes begin to denature, which is why very high body temperatures are life-threatening.

Example 2. Statins and competitive inhibition. Statins competitively inhibit an enzyme in the cholesterol-synthesis pathway, slowing cholesterol production. This is a real medical use of competitive inhibition that Eduqas may set as an applied context.

Try this

Q1. State what is meant by the activation energy of a reaction. [1 mark]

• Cue. The minimum energy needed for a reaction to occur.

Q2. Explain why an enzyme is specific to its substrate. [2 marks]

• Cue. The active site has a shape (set by the tertiary structure) complementary to the substrate, so only the correct substrate fits to form an enzyme-substrate complex.

Q3. Explain why increasing substrate concentration eventually has no further effect on the rate. [2 marks]

• Cue. At high substrate concentration all the active sites are occupied (the enzyme is saturated), so adding more substrate cannot increase the rate; enzyme concentration becomes limiting.