The Mechanism and Factors of Enzyme Action

The Lock and Key Theory

• The Lock and Key Theory is the primary model used to explain how enzymes work.
• Each enzyme is highly specific, meaning it typically only catalyses one specific reaction.
• This is because the reactant molecule, called the substrate, must fit perfectly into a uniquely shaped "pocket" on the enzyme called the active site.
• The substrate and the active site have a complementary shape, fitting together exactly like a key entering a lock.
• Here is the step-by-step mechanism of enzyme action:
  1. The enzyme and substrate move randomly in solution and collide.
  2. The substrate binds tightly to the active site because their shapes are complementary.
  3. A temporary enzyme-substrate complex is formed.
  4. The enzyme catalyses the reaction, either breaking bonds to split the substrate or forming new bonds to join substrates together.
  5. The new products are released from the active site.
  6. The enzyme remains entirely unchanged and is free to collide with another substrate.

How Temperature Affects Enzymes

• As temperature increases, the kinetic energy of both the enzyme and substrate molecules increases.
• They move faster, leading to more frequent and successful collisions, which speeds up the rate of reaction.
• The reaction reaches its maximum rate at the optimum temperature (which is approximately $37\,^{\circ}\text{C}$ in humans).
• At low temperatures, enzymes do NOT denature; they simply have low kinetic energy, resulting in few collisions and a very slow rate of reaction.
• However, at temperatures significantly above the optimum, the intense kinetic energy makes the enzyme vibrate vigorously.
• This vibration puts severe strain on the hydrogen and ionic bonds holding the protein's 3D structure together, causing them to break.
• When these bonds break, the active site changes shape and is no longer complementary to the substrate. The enzyme is now denatured, and this permanent change means it can no longer function.

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:

$$\text{Rate} = \frac{\text{Quantity of Product Formed or Reactant Used}}{\text{Time}}$$

• 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.