B5.1: Enzymes

Overview

In this unit, you’ll dive into one of the most important chapters of Biology—enzymes. Enzymes are special proteins that act as biological catalysts, speeding up chemical reactions in your body without being used up themselves. Every time you breathe, digest food, or repair cells, enzymes are working!

Understanding enzymes will help you make sense of everything from why our bodies function more efficiently at certain temperatures to why excessive heat or acidity can disrupt the process.

What are enzymes?

Enzymes are biological catalysts made of protein molecules that bind with the substrate with a complementary shape at the active site, forming an enzyme-substrate complex

Key Terms

Terms	Definition
Active site	The specific region of the enzyme where the substrate binds
Substrate	The molecule that the enzyme catalyses
Product	The new substance formed after the enzyme reaction
Enzyme-substrate complex	The temporary complex formed when an enzyme binds to its substrate

The Lock and Key Model

A common representation of how the substrate and enzymes bind together is the ‘Lock and Key Model’. Just like its name, the substrate (the key) binds with the enzyme (the lock) at the active site (the hole) that has a complementary shape. The process can be seen below.

Source: #22 Enzymes - 'Lock and key' model, weebly

A substrate can only bind with an enzyme’s active site that has a complementary shape to it, else the substrate will not bind.

Source: Biology Enzyme, thealevelbiologist

When we say that an enzyme and its substrate have a complementary shape. This means that their shapes fit together perfectly like a lock and key. Because their shapes match exactly, the substrate can bind to the enzyme’s active site and form an enzyme-substrate complex. This is what allows the enzyme to carry out the reaction efficiently.

Factors Affecting Enzyme Activity

1) Temperature

Temperature condition	Effect on Enzyme
Too cold	Reaction is slow because particles move more slowly, and fewer collisions occur due to less random movement
Optimum Temperature	Enzyme activity is fastest
Too hot	Enzyme denatures—active site changes shape permanently and no longer fits the substrate to catalyse reactions

Higher temperatures increase kinetic energy, leading to more frequent collisions between the enzyme and the substrate. But if it's too hot, the enzyme’s structure changes irreversibly (denaturation), and it no longer works.

Source: Factors affecting enzymes' performance, evolvingscience

Here is an example graph you may encounter in your exam. According to the graph, as the pH increases, enzyme activity also increases, reaching its highest point at the optimum pH at 40ºC. However, beyond this point, the enzyme activity decreases. This is because the active site shape has changed permanently, and the enzyme can no longer catalyse reactions by binding with the substrate. This means that the enzyme has been denatured.

2) pH

pH condition	Effect on Enzyme
Too low (acidic)	Denatures enzyme
Optimum pH	Enzyme activity is fastest (e.g, pepsin in the stomach works best at pH 2)
Too high (alkaline)	Denatures enzyme

pH can interfere with the ionic and hydrogen bonds that hold the enzyme’s structure. When disrupted, the active site shape changes, and the enzyme can no longer bind to the substrate.

Here is an example graph you may encounter in your exam. This graph shows the relationship between the rate of reaction and pH of the enzymes pepsin and amylase. For pepsin, its optimum pH is 2, as shown when the rate of reaction is highest. While for amylase, its optimum pH is 7, as shown when the rate of reaction is highest. However, both before and after these optimum points, the rate of reaction of the enzyme drops as it has been denatured. This means that the enzyme can no longer bind to the substrate and catalyse reactions.

Common Enzymes in the Body

Enzyme	Substrate	Product	Found in
Amalyse	Starch	Maltose	Saliva, pancreas
Protease	Proteins	Amino acids	Stomach, pancreas
Lipase	Lipids	Fatty acids & Glycerol	Small intestine, pancreas

Tips & Tricks

LOCK AND KEY → Use this phrase to remember specificity.
DENATURATION is permanent → It ruins enzyme activity for good.
Enzyme names usually end in -ase (e.g., lipase, amylase, protease).
Practice drawing and labeling enzyme diagrams (substrate, active site, product).
Know the optimum conditions for enzymes and be able to explain changes using scientific language (e.g., collision theory, shape and fit).

Written by Xavier Le

Edited by Quinn Luong