Enzymes as biological catalysts, the induced-fit model of enzyme action, the effects of temperature, pH, substrate and enzyme concentration on rate, and how inhibitors and immobilised enzymes work.

What this dot point is asking

CCEA wants you to explain that enzymes are biological catalysts that lower activation energy, describe enzyme action using the induced-fit model, explain how temperature, pH, substrate concentration and enzyme concentration affect the rate of reaction, and describe how competitive and non-competitive inhibitors and immobilised enzymes work.

How enzymes work

Activation energy is the minimum energy that reacting molecules must have to react. By providing an alternative reaction pathway with a lower activation energy, enzymes let reactions proceed quickly at the relatively low temperatures found in cells (around $37\,^{\circ}\text{C}$ in humans).

In the induced-fit model, the active site is not an exact, rigid match for the substrate at first. When the substrate binds, the active site changes shape slightly to fit closely around it. This puts strain on the bonds in the substrate, which lowers the activation energy and helps the reaction occur. Each enzyme is specific because the shape of its active site is complementary to one particular substrate (or a small group of similar substrates).

Factors affecting rate

A higher temperature increases the kinetic energy of the molecules and so the frequency of successful collisions between enzyme and substrate, which is why rate rises before the optimum. At the optimum pH the charges on the amino acid R-groups in the active site are arranged so the substrate binds best; moving away from this pH changes those charges, weakens binding, and at extremes breaks the bonds maintaining the tertiary structure. When substrate is in excess, all active sites are continuously occupied, so adding more substrate does not increase rate and the graph levels off.

Inhibitors and immobilised enzymes

A competitive inhibitor has a similar shape to the substrate and competes for the active site; its effect is reduced by adding more substrate. A non-competitive inhibitor binds at a site away from the active site (an allosteric site) and changes the active site shape, so adding substrate does not reverse it. Immobilised enzymes are fixed to a surface or trapped in alginate beads, so they can be reused and easily separated from the product, and they tend to be more stable to changes in temperature and pH. This makes them valuable in continuous industrial processes.

Examples in context

Example 1. Catalase in liver tissue. Catalase catalyses the breakdown of hydrogen peroxide, a toxic by-product of metabolism, into water and oxygen: $2\text{H}_2\text{O}_2 \rightarrow 2\text{H}_2\text{O} + \text{O}_2$. Catalase is one of the fastest known enzymes, converting millions of substrate molecules per second. In a CCEA practical, students add liver to hydrogen peroxide and measure the oxygen given off; boiling the liver first denatures the catalase, and no gas is produced, demonstrating that the active protein is essential.

Example 2. Lactase immobilised in beads. In the dairy industry, the enzyme lactase is trapped in alginate beads packed into a column. Milk is run through the column, and the lactase hydrolyses lactose into glucose and galactose, producing lactose-free milk for people who are lactose intolerant. Because the enzyme is immobilised, it is not carried away with the product, can be used continuously for long periods, and the milk is not contaminated with free enzyme. This illustrates the industrial advantages of immobilised enzymes over enzymes in free solution.

Try this

Q1. Explain why raising the temperature above the optimum decreases the rate of an enzyme-controlled reaction. [3 marks]

• Cue. The enzyme denatures, hydrogen and ionic bonds break, the active site changes shape and is no longer complementary to the substrate.

Q2. State one advantage of using immobilised enzymes in industry. [1 mark]

• Cue. They can be reused, or easily separated from the product, or are more stable.

Q3. A reaction produces 24 cubic centimetres of product in 2 minutes. Calculate the mean rate in cubic centimetres per second. [2 marks]

• Cue. $24 / 120 = 0.2 \ \text{cm}^3 \ \text{s}^{-1}$.