Enzymes, Substrates & Products (AQA A-Level Biology): Revision Notes
Enzymes, Substrates & Products
What are enzymes?
Enzymes are biological catalysts that speed up biochemical reactions in living organisms. All enzymes are proteins, which means they can be detected using the Biuret test - when mixed with an enzyme solution and Benedict reagent, the solution turns violet, confirming the presence of proteins.
The Biuret test is a reliable method for detecting proteins in biological samples. The characteristic violet colour change occurs because the Biuret reagent reacts with peptide bonds present in all proteins, including enzymes.
Living cells contain hundreds of different enzymes, each with a specific role. These enzymes vary considerably in size, from relatively small molecules to enormous protein complexes, but they all share the common feature of being proteins with complex three-dimensional structures.
The active site and enzyme function
Each enzyme possesses a unique tertiary structure that determines its function. Within this structure lies a crucial region called the active site - a specifically shaped pocket or groove on the enzyme's surface where biochemical reactions occur.
The active site is the part of an enzyme molecule where the substrate (the reactant molecule) binds during a biochemical reaction. The active site and substrate have complementary shapes, meaning they fit together precisely, but they are not identical shapes.
When an enzyme catalyses a reaction, the following sequence occurs:
- The substrate molecule collides with the enzyme's active site
- The substrate binds to form an enzyme-substrate complex
- This complex is unstable and quickly breaks down
- The products are released, and the enzyme remains unchanged
- The enzyme is now free to bind with another substrate molecule
This process significantly reduces the activation energy required for the reaction to proceed, allowing it to occur more rapidly under the mild conditions found in living cells.
Remember that enzymes are not consumed in the reaction - they remain unchanged and can be reused repeatedly. This is what makes them such efficient catalysts in biological systems.
Enzyme specificity
Enzymes demonstrate remarkable specificity - each enzyme catalyses only one type of chemical reaction with one specific type of substrate. This specificity arises from the precise shape and chemical properties of the active site.
Worked Example: Enzyme Specificity
Amylase is an enzyme that breaks down starch into maltose.
- Substrate: Starch (a polysaccharide)
- Product: Maltose (a disaccharide)
- Specificity: Amylase will only hydrolyse starch and cannot break down other substances, even chemically similar ones like cellulose or glycogen
This demonstrates how the precise shape of amylase's active site is complementary only to starch molecules.
The biochemical reactions occurring within a single cell are incredibly complex, with hundreds of different reactions happening simultaneously. Each reaction is controlled by its own specific enzyme, preventing unwanted chemical interactions and maintaining cellular organisation.
Key Points to Remember:
- Enzymes are proteins that can be identified using the Biuret test, which produces a violet colour
- The active site is the specific region where substrates bind, and it has a complementary shape to the substrate
- Enzyme specificity means each enzyme catalyses only one type of reaction with one specific substrate
- The induced-fit model explains how both enzyme and substrate change shape slightly when they bind together
- Enzyme-substrate complexes form temporarily before breaking down to release products and regenerate the enzyme