Explain the mechanism of enzyme action including the active site and specificity, how enzymes are denatured, the effects of temperature, substrate concentration and pH, and the importance of enzymes in synthesis and breakdown reactions.

What this dot point is asking

Edexcel statements 1.7 to 1.12 want you to explain how an enzyme works using the active site and specificity, explain denaturation as a change in the shape of the active site, describe the effects of temperature, substrate concentration and pH, carry out rate calculations, and explain why enzymes matter in both building up and breaking down molecules. Statement 1.10 is the core practical on the effect of pH on enzyme activity.

How enzymes work

Each enzyme has a region called the active site, a precise shape into which only one substrate fits, like a key into a lock. When the substrate binds, an enzyme-substrate complex forms, the reaction happens, and the products are released, leaving the enzyme free to act again. Because the active site shape is specific, an enzyme that breaks down starch will not break down protein. This is enzyme specificity.

Enzymes both build up large molecules from small ones (for example joining glucose into starch, or amino acids into proteins) and break down large molecules into small ones (starch into sugars, proteins into amino acids, lipids into fatty acids and glycerol). The same lock and key idea applies to both kinds of reaction.

Denaturation

The effect of temperature

As temperature rises, enzyme and substrate molecules move faster and collide more often, so the rate increases, up to the optimum temperature (around $37\,^{\circ}\text{C}$ for human enzymes). Above the optimum, the rising energy breaks the bonds that hold the enzyme's shape, the active site changes, and the enzyme denatures, so the rate falls sharply. The graph rises to a peak and then drops.

The effect of pH

Each enzyme works fastest at an optimum pH. Stomach protease (pepsin) works best in acid (about pH $2$), while amylase in the mouth and small intestine prefers a neutral or slightly alkaline pH. Moving away from the optimum, in either direction, changes the active site and eventually denatures the enzyme, so a pH graph is a peak centred on the optimum.

The effect of substrate concentration

At low substrate concentration, adding more substrate gives more collisions, so the rate increases. Once every active site is occupied as fast as it can work, the enzymes are the limiting factor, so adding more substrate makes no further difference and the rate levels off (plateaus).

Rate calculations

Try this

Q1. Explain why an enzyme that breaks down starch cannot break down protein. [2 marks]

• Cue. The active site of the starch enzyme has a specific shape; only starch fits it, so protein (a different shape) cannot bind. Enzymes are specific.

Q2. A student measures enzyme activity at pH $4$, $7$ and $10$ and finds it is fastest at pH $7$. Explain this result. [2 marks]

• Cue. pH $7$ is the optimum for this enzyme; at pH $4$ and pH $10$ the active site changes shape, so fewer enzyme-substrate complexes form and the rate is lower.