Sign up to access the complete lesson and track your progress!
Unlock This CourseEnzymes are amazing biological molecules that speed up chemical reactions in living things. Think of them as the ultimate helpers in your body - they make reactions happen thousands of times faster than they would naturally. Without enzymes, the chemical reactions in your cells would be so slow that life as we know it couldn't exist!
Every enzyme is made of proteins, which are long chains of smaller units called amino acids. These chains fold up into specific 3D shapes and this shape is absolutely crucial for how the enzyme works.
Key Definitions:
Enzymes are proteins with a very specific 3D shape. This shape is formed when long chains of amino acids fold up in a particular way. The folding creates pockets, grooves and bumps on the enzyme's surface. One of these special areas is called the active site - this is where all the action happens!
The active site is like a special workspace on the enzyme. It's perfectly shaped to fit with specific substrate molecules. Think of it like a jigsaw puzzle piece - only the right substrate will fit properly into the active site.
The active site has a very specific shape and chemical environment. It contains amino acids that can interact with the substrate in different ways - some might be positively charged, others negatively charged and some might be neutral. This creates the perfect conditions for the chemical reaction to occur.
The 3D shape of the active site determines which substrates can bind. Only molecules with the complementary shape can fit properly.
The active site provides the right chemical conditions (pH, charge distribution) for the reaction to happen quickly.
The substrate binds temporarily, undergoes the reaction, then the products are released, leaving the enzyme unchanged.
Scientists have developed two main models to explain how enzymes work with their substrates. Both help us understand the relationship between enzyme structure and function.
This is the simpler model to understand. Imagine the enzyme as a lock and the substrate as a key. Just like a key must have exactly the right shape to fit into a lock, the substrate must have exactly the right shape to fit into the enzyme's active site.
In this model, the active site has a fixed, rigid shape that perfectly complements the substrate. The substrate fits into the active site like a key fits into a lock. Once the reaction occurs, the products no longer fit the active site shape, so they are released.
This is a more modern and accurate model. It suggests that both the enzyme and substrate are slightly flexible. When the substrate approaches the active site, both molecules change shape slightly to achieve a perfect fit.
Unlike the rigid lock and key model, the induced fit model shows that enzymes are flexible. The active site changes shape when the substrate binds, creating an even better fit. This is like a glove that moulds to fit your hand perfectly when you put it on.
Let's look at some enzymes you encounter every day to see how structure and active sites work in real life.
Amylase is found in your saliva and helps break down starchy foods like bread and potatoes. Its active site is perfectly shaped to bind to starch molecules and cut them into smaller sugar molecules.
Found in saliva and pancreatic juice. Starts working as soon as you put starchy food in your mouth!
Breaks down large starch molecules into smaller maltose molecules that can be absorbed.
Has a groove that fits perfectly around starch chains, positioning them for cutting.
Catalase is found in nearly all living cells and breaks down hydrogen peroxide, which is toxic, into harmless water and oxygen. You can see this enzyme in action when you put hydrogen peroxide on a cut - the bubbling is oxygen being released!
Liver cells contain huge amounts of catalase because the liver processes many toxins that produce hydrogen peroxide. The enzyme's active site contains iron atoms that help break the O-O bond in hydrogen peroxide. This is why liver tissue bubbles vigorously when hydrogen peroxide is added - millions of catalase enzymes are working simultaneously!
Because enzyme function depends so heavily on their 3D shape, anything that changes this shape can affect how well they work or stop them working altogether.
When enzymes are exposed to extreme conditions like high temperature or wrong pH, they can lose their specific 3D shape. This is called denaturation and it usually means the enzyme stops working.
High temperatures make the protein chains vibrate more, potentially breaking the bonds that hold the enzyme in its correct shape. This is why cooking food changes its texture - many enzymes get denatured!
The relationship between enzyme structure and function is one of the most important concepts in biology. If the active site changes shape even slightly, the substrate might not fit properly anymore and the enzyme won't work efficiently.
Some people are born with genetic mutations that affect enzyme structure. For example, people with phenylketonuria (PKU) lack a properly functioning enzyme called phenylalanine hydroxylase. The active site of this enzyme is altered, so it can't process the amino acid phenylalanine properly. This shows how crucial correct enzyme structure is for health.
One of the most remarkable things about enzymes is how specific they are. Each enzyme typically works with only one substrate or a small group of very similar substrates. This specificity comes from the precise shape and chemical properties of the active site.
The active site of each enzyme has evolved to be perfectly complementary to its substrate. This means not only does the shape fit perfectly, but the chemical properties (like charge and polarity) also match up perfectly.
Think of enzyme specificity like molecular recognition. The enzyme can 'recognise' its correct substrate among thousands of other molecules in the cell, just like you can recognise a friend's face in a crowd.