Welcome to the World of Enzymes!

In this chapter, we are going to explore enzymes, which are some of the most important molecules in your body. Think of enzymes as the "tiny workers" or "biological tools" that make sure your body functions quickly and efficiently. Without them, the chemical reactions that keep you alive would happen so slowly that your body simply couldn't survive!

This chapter is part of your study on Cells, molecules and transport. While we've looked at the basic building blocks of life in previous chapters, here we focus on how those molecules are actually put together or broken down.

1. What are Enzymes?

Enzymes are biological catalysts. A catalyst is something that speeds up a chemical reaction without being used up itself. In human biology, enzymes are made of proteins.

Key things to remember about enzymes:

  • They speed up reactions.
  • They are not changed by the reaction (they can be used over and over again).
  • They are highly specific—this means one enzyme usually only works for one specific reaction.

How they work: The "Lock and Key" Analogy

Imagine a lock and a key. Only one specific key fits into a specific lock. Enzymes work the same way:

1. The Substrate: This is the molecule the enzyme is going to act upon (the "key").
2. The Active Site: This is a specially shaped "pocket" on the enzyme where the substrate fits (the "lock").
3. The Enzyme-Substrate Complex: This is formed when the substrate binds to the active site.
4. The Product: The enzyme either breaks the substrate apart or joins molecules together to create a new product.

Quick Review: Enzymes reduce the energy needed for a reaction to start, making life processes like digestion and DNA replication possible.

2. Important Enzymes You Need to Know

The Edexcel syllabus mentions several specific enzymes. You will see these again in the "Nutrition and Energy" and "Internal Transport" chapters, but here is a quick list of the "workers" you should recognise:

  • Amylase and Maltase: Break down starch into glucose.
  • Pepsin and Trypsin: (Proteases) Break down proteins into amino acids.
  • Lipases: Break down lipids (fats) into fatty acids and glycerol.
  • DNA Polymerase: Involved in DNA replication (building new DNA strands).

3. Factors Affecting Enzyme Activity

Enzymes are quite sensitive! If the environment around them changes too much, they stop working. Here are the four main factors that affect how fast an enzyme works:

A. Temperature

As temperature increases, enzyme activity increases because molecules move faster and collide more often. However, if it gets too hot, the enzyme's protein structure breaks down. This is called denaturation. The active site changes shape, and the substrate can no longer fit.

The "Optimum" temperature: This is the temperature where the enzyme works fastest (usually around \(37^\circ C\) in humans).

B. pH (Acidity/Alkalinity)

Most enzymes have an optimum pH. For example, pepsin in the stomach likes acidic conditions (low pH), while trypsin in the small intestine likes slightly alkaline conditions. If the pH moves too far away from the optimum, the enzyme will denature.

C. Substrate Concentration

If you add more substrate, the rate of reaction increases because there are more molecules for the enzymes to work on. However, eventually, all the active sites will be busy (saturated), and the rate will level off.

D. Inhibitors

Inhibitors are molecules that stop enzymes from working. There are two types:

1. Competitive Inhibitors: These have a similar shape to the substrate and "compete" for the active site, blocking it.
2. Non-competitive Inhibitors: These bind to another part of the enzyme, causing the active site to change shape so the substrate no longer fits.

4. Immobilised Enzymes

In industry and medicine, we often "trap" enzymes so we can use them more easily. These are called immobilised enzymes.

How are they made?

A common method is to trap the enzymes in alginate beads. You might do a practical where you mix an enzyme with sodium alginate and drop it into calcium chloride to create tiny jelly-like beads containing the enzyme.

Advantages of Immobilised Enzymes:

  • Easy Recovery: You can easily filter the beads out of the product once the reaction is finished.
  • Stability: Trapping them makes them more resistant to changes in temperature and pH.
  • No Contamination: The enzyme doesn't end up mixed in with the final product (e.g., your milk or juice).
  • Continuous Use: You can use the same enzymes over and over again, saving money.

Real-World Applications:

1. Lactose-free milk: Lactase enzymes are immobilised in beads. Milk is poured over them, and the lactase breaks down the lactose sugar into glucose and galactose.
2. Sucrose conversion: Converting sucrose into glucose and fructose (which are sweeter and used in food production).
3. Glucose testing strips: Used by people with diabetes to measure blood sugar levels quickly and accurately.

5. Required Practicals (Core Skills)

For your exams, you need to understand how to investigate these concepts. You may be asked to describe these experiments:

Investigating Temperature and pH

Independent Variable: The temperature (using a water bath) or the pH (using buffer solutions).
Dependent Variable: The rate of reaction (e.g., how quickly starch disappears or how much oxygen gas is produced).
Control Variables: Concentration of enzyme, concentration of substrate, and volume of solutions.

Investigating Immobilised Enzymes

You may be asked to describe how to prepare alginate beads. Remember to mention using a syringe to drop the mixture into calcium chloride and then rinsing the beads before use.

Common Mistake to Avoid: Never say an enzyme is "killed" by heat. Enzymes are molecules, not living things! Always use the word denatured.

Chapter Summary

1. Enzymes are protein catalysts that speed up reactions using an active site.
2. They are specific: the substrate must fit the active site (Lock and Key).
3. Heat and extreme pH can denature enzymes (change their shape).
4. Inhibitors stop enzymes from working by blocking or changing the active site.
5. Immobilised enzymes are trapped (e.g., in alginate beads) for industrial use like making lactose-free milk.