CB1.2 — Enzymes (Edexcel 1SC0)

Enzymes are biological catalysts — proteins that speed up chemical reactions without being used up. They are central to digestion, respiration, photosynthesis and almost every cellular process.

Lock-and-key model

Each enzyme has an active site whose shape is complementary to a specific substrate. The substrate fits into the active site to form an enzyme-substrate complex; the reaction proceeds; the product is released; the enzyme is unchanged.

The specificity of an enzyme comes from its active site shape, which is determined by its amino acid sequence and 3D folding.

Effect of temperature

• Increasing temperature increases molecular kinetic energy → more frequent enzyme-substrate collisions → faster reaction up to the optimum temperature.
• Above the optimum, increased temperature disrupts the hydrogen bonds holding the enzyme's 3D structure → the active site shape changes → enzyme is denatured → rate drops sharply.
• Human enzymes typically have an optimum around 37 °C.

Effect of pH

• Each enzyme has an optimum pH (e.g. pepsin in the stomach: pH 2; salivary amylase: pH 7).
• pH too high or too low disrupts hydrogen bonds and ionic interactions in the enzyme's structure → active site changes shape → enzyme denatured.

Calculating rate of reaction

$$\text{rate} = \frac{1}{\text{time taken}}$$

If a reaction takes 20 s: rate = 1/20 = 0.05 s⁻¹. If measuring volume of gas produced, rate = volume ÷ time (cm³/s).

Required practical

Edexcel tests the amylase + starch + iodine practical: iodine turns blue-black when starch is present; colour change to orange when starch is broken down. Variables: temperature of water bath; pH using buffer solutions; concentration of enzyme/substrate.