Enzymes as biological catalysts, the induced-fit model, the effect of temperature, pH, substrate and enzyme concentration, and the action of inhibitors.

What this dot point is asking

WJEC wants you to explain how enzymes work as biological catalysts by lowering activation energy, describe the induced-fit model, and explain how temperature, pH, substrate and enzyme concentration and inhibitors affect the rate of an enzyme-controlled reaction. You also need to be able to read rate graphs and calculate an initial rate from a tangent.

How enzymes work

Every chemical reaction needs an input of energy, the activation energy, to get started. Enzymes provide an alternative reaction pathway with a lower activation energy, so a far greater proportion of substrate molecules have enough energy to react at body temperature. They do this without raising the temperature, which would damage cells.

Each enzyme has a region called the active site with a specific three-dimensional shape determined by its tertiary structure. In the lock-and-key model the substrate fits an exactly complementary active site like a key in a lock. The more accurate induced-fit model says the active site changes shape slightly as the substrate binds, moulding around it to form an enzyme-substrate complex and putting strain on the substrate's bonds, which lowers the activation energy and speeds the reaction. Because the shape of the active site depends on the protein's tertiary structure, anything that disrupts that structure (heat, extreme pH) destroys catalytic activity.

Factors affecting enzyme activity

Temperature

A useful rule is that the rate roughly doubles for each $10\,^\circ\text{C}$ rise up to the optimum (the temperature coefficient $Q_{10} \approx 2$). Beyond the optimum (around $40\,^\circ\text{C}$ for many human enzymes) the increasing vibration of atoms breaks the weak bonds holding the tertiary structure, the active site loses its complementary shape, and the enzyme is denatured permanently.

pH

Most human enzymes have an optimum near pH 7, but pepsin in the stomach works best near pH 2. Moving away from the optimum changes the charge on amino acid side chains, breaking ionic bonds and hydrogen bonds in the active site.

Substrate and enzyme concentration

At low substrate concentration the rate is limited by how often substrate meets an active site, so adding substrate increases the rate. Once every active site is constantly occupied the enzyme is saturated and the rate plateaus. Raising enzyme concentration raises the plateau, provided substrate is not limiting.

Inhibitors

• Competitive inhibitors have a shape similar to the substrate and bind to the active site, blocking it. Their effect is reduced by adding more substrate because substrate then outcompetes the inhibitor.
• Non-competitive inhibitors bind to a site away from the active site (the allosteric site), changing the enzyme's overall shape so the active site no longer fits the substrate. Adding more substrate does not reverse this.

Examples in context

Example 1. Catalase and hydrogen peroxide. Catalase is found in liver and potato tissue and breaks down toxic hydrogen peroxide into water and oxygen: $2\text{H}_2\text{O}_2 \rightarrow 2\text{H}_2\text{O} + \text{O}_2$. Because it releases a measurable gas, it is the classic enzyme for rate experiments. Heating the catalase to $80\,^\circ\text{C}$ before adding peroxide gives no oxygen, demonstrating denaturation, while boiling the peroxide alone still works, showing the temperature effect is on the enzyme.

Example 2. Statins as competitive inhibitors. Statin drugs lower blood cholesterol by competitively inhibiting HMG-CoA reductase, the enzyme that controls cholesterol synthesis in the liver. The statin molecule resembles the enzyme's natural substrate and binds the active site, reducing cholesterol production. This is a real medical use of competitive inhibition and a favourite WJEC application context.

Try this

Q1. State what is meant by the active site of an enzyme. [1 mark]

• Cue. The region of the enzyme with a shape complementary to the substrate, where the substrate binds.

Q2. Explain why adding more substrate reverses the effect of a competitive inhibitor but not a non-competitive inhibitor. [2 marks]

• Cue. A competitive inhibitor competes for the active site, so more substrate outcompetes it; a non-competitive inhibitor changes the active site shape, so extra substrate cannot bind.

Q3. A reaction reaches an initial rate of $1.8$ cm cubed per second. Suggest two changes to the conditions that would increase this initial rate, and explain each. [4 marks]

• Cue. Raise temperature towards the optimum (more kinetic energy, more successful collisions); increase substrate concentration (more enzyme-substrate complexes per second until saturation).