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Cambridge IGCSE Biology · 0610

Chapter 5: Enzymes

Definitions and importance

Catalyst
A substance that increases the rate of a chemical reaction and is not changed by the reaction.
Enzymes
Proteins that are involved in all metabolic reactions, where they function as biological catalysts. They are not changed by the reaction and can be used repeatedly.
Importance
Enzymes are crucial to all living organisms because they speed up metabolic reactions to a rate necessary to sustain life. Without them, these reactions would take too long to occur.

Enzyme action

Active site
Every enzyme contains an active site with a specific shape.
Mechanism
The shape of the active site is complementary to its substrate. The substrate enters the active site to form an enzyme-substrate complex.
Products
The substrate is broken down, and the product is released, leaving the enzyme free to bind with another substrate molecule.
Specificity
Enzymes are specific to only one type of substrate because of the complementary shape and fit of the active site with the substrate. For example, proteases break down proteins but cannot break down carbohydrates.
Lock and key mechanism showing substrate binding to complementary active site, enzyme-substrate complex formation, and product release
Diagram 1: The lock and key mechanism. A specific enzyme with a uniquely shaped active site binds a matching substrate to form an enzyme-substrate complex. Products are released and the unchanged enzyme is free to bind another substrate molecule.

Factors affecting enzyme activity: temperature

Effect of increase
As temperature increases up to the optimum temperature, the rate of reaction increases.
Kinetic energy
This is because molecules have more kinetic energy and move faster, leading to a higher frequency of effective collisions between the enzyme and substrate, forming more enzyme-substrate complexes.
Denaturation
At very high temperatures above the optimum, the enzyme becomes denatured.
Shape and fit
High temperatures cause the active site to change shape, meaning the substrate no longer fits (“shape and fit” is lost), so the rate of reaction decreases rapidly.
Graph of enzyme activity versus temperature showing gradual rise to optimum then sharp fall due to denaturation
Diagram 2: Enzyme activity vs. temperature. An asymmetrical curve shows a gradual increase in rate as temperature rises, a peak at the optimum temperature, and a very sharp drop after the optimum as the enzyme denatures.

Exam Traps

  • Do not draw a symmetrical bell curve for temperature; the pH graph is symmetrical, not the temperature graph.
  • Human enzymes typically have an optimum near 37 °C — stating "higher temperature always increases rate" loses marks.

Factors affecting enzyme activity: pH

Optimum pH
Enzymes have an optimum pH where they work best. As the pH moves away from this value (becoming too acidic or too alkaline), the rate of reaction decreases.
Denaturation
Drastic changes in pH cause the enzyme to become denatured because the shape of the active site changes, losing the complementary fit with the substrate.
Symmetrical bell-shaped graph of enzyme activity versus pH peaking at optimum pH
Diagram 3: Enzyme activity vs. pH. A symmetrical bell-shaped curve peaks at the optimum pH (e.g. pH 7 for most enzymes, or pH 2 for pepsin), with the rate decreasing as pH becomes more acidic or alkaline than the optimum.

Exam Traps

  • Pepsin works best at about pH 2 in the stomach — do not state pH 7 as optimum for every enzyme.

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