The Mechanism and Factors of Enzyme Action

How pH Affects Enzymes

• Enzymes also have an optimum pH where they are most active (for example, pH 2 for pepsin in the highly acidic stomach, but pH 7–8 for amylase in the small intestine).
• If the pH becomes too high or too low, the change in $H^+$ or $OH^-$ concentration disrupts the hydrogen and ionic bonds holding the enzyme's structure.
• This causes the protein's tertiary structure to unravel, permanently changing the shape of the active site.
• Because the active site is no longer complementary, enzyme-substrate complexes can no longer form, and the enzyme is denatured.

Calculating the Rate of Reaction

• You can calculate how fast an enzyme is working using the rate of reaction formula:

• When interpreting a curved graph, you can find the rate at a specific point by drawing a tangent to the curve and calculating its gradient ($\frac{\text{change in } y}{\text{change in } x}$).

Worked Example

In an experiment, an enzyme produces $24\,\text{cm}^3$ of oxygen gas in 2 minutes. Calculate the rate of reaction in $\text{cm}^3/\text{s}$.

Step 1: Identify the values from the question and convert units if necessary.

• Quantity of product = $24\,\text{cm}^3$
• Time = 2 minutes = $120\,\text{s}$

Step 2: Substitute the values into the rate equation.

• $\text{Rate} = \frac{24}{120}$

Step 3: Calculate the final answer with correct units.

• $\text{Rate} = 0.2\,\text{cm}^3/\text{s}$

The Role of Enzymes in Metabolism

• The energy required for all enzyme-controlled metabolic processes is provided by cellular respiration.
• Enzymes drive synthesis reactions, which build large molecules from smaller ones:
  • Carbohydrates: Glucose molecules are joined to form starch (for storage in plants), glycogen (for storage in animals), or cellulose (to strengthen plant cell walls).
  • Lipids: One molecule of glycerol combines with three molecules of fatty acids to form a lipid.
  • Proteins: Plants combine glucose with nitrate ions to make amino acids, which are then joined together to form proteins.
• Enzymes also drive breakdown (decomposition) reactions:
  • Digestion: Large, insoluble molecules (like starch, proteins, and lipids) are broken down into small, soluble molecules (glucose, amino acids, glycerol, and fatty acids) so they can be absorbed into the blood.
  • Respiration: Glucose is broken down to release energy.
  • Protein Breakdown: Excess proteins are broken down by enzymes in the liver to form urea, which is later filtered and excreted by the kidneys.