Students should be able to describe the nature of enzyme molecules and…

The idea

Enzymes are proteins folded into a precise three-dimensional shape. This shape creates a small region called the active site, which is complementary to one specific substrate. The substrate binds, forming an enzyme–substrate complex, and the reaction is catalysed. Because activity depends on shape, temperature and pH matter. Raising temperature increases kinetic energy and collision frequency, so rate rises until the optimum; beyond it, bonds holding the tertiary structure break, the active site denatures and substrate no longer fits, so rate falls sharply. pH changes alter charges and bonding within the protein, again distorting the active site. Each enzyme has an optimum pH; pepsin works in acidic stomach conditions while intestinal enzymes prefer alkaline conditions. Describing means stating what enzymes are and explaining how these two variables change activity.

What earns the marks

• Enzymes are proteins that function as biological catalysts, speeding up reactions without being used up.
• The active site is a specific region whose shape is complementary to a particular substrate.
• Substrate binding forms an enzyme–substrate complex, lowering the activation energy of the reaction.
• Increasing temperature raises kinetic energy and collision frequency, increasing rate up to an optimum.
• Above the optimum, bonds maintaining tertiary structure break, the active site changes shape and the enzyme denatures.
• pH affects bonding and charges in the protein, altering active-site shape and therefore activity.
• Each enzyme has an optimum pH, illustrated by pepsin in acidic stomach conditions and intestinal enzymes in alkaline conditions.

Common mistakes and how to put them right

• Saying enzymes are killed by high temperature.

  Put it right: Enzymes are molecules, so the correct idea is that they are denatured, meaning the active site loses its shape.

• Claiming the substrate is denatured.

  Put it right: Only the enzyme's active site changes shape, so the correction is that denaturation refers to the enzyme protein.

• Stating that all enzymes have the same optimum pH.

  Put it right: Optima differ, for example pepsin is adapted to acidic conditions.

Exam tips

• Sketch the rate–temperature curve and label optimum, rising rate and denaturation to structure a written answer.
• Use the phrase 'complementary shape' when linking active site to substrate rather than vague wording such as 'fits'.
• When explaining pH, refer to bonds or charges in the protein changing shape of the active site, not to temperature effects.