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Enzymes

Enzymes are proteins that catalyse chemical reactions. To 'catalyse' a reaction means to speed it up.

Almost every reaction that takes place in living things has an enzyme to catalyse it. Without enzymes, many of these reactions would happen too slowly and the organism would not be able function properly.

Diagram of an enzyme. It is a yellow blobby shape. At the top it has a large cavity, with curvy edges (this is the active site).

A simplified diagram of an enzyme

Enzymes are often referred to as 'biological catalysts'. 'Biological' because they are proteins (which are biological molecules), and 'catalysts' because they speed up chemical reactions (without being used up in the process).

Enzymes are specific

Each enzyme has one specific reaction that it catalyses.

In this reaction, one or more substances - called the substrates - are converted to one or more other substances - called the products.

Diagram showing an enzyme-catalysed reaction. The diagram is in the form of a chemical equation. On the left hand side there are diagrams of two molecules, with a plus sign between them. These are labelled, "Substrates". On the right hand side, there are diagrams of two different molecules with a plus sign between them. These are labelled, "Products". There is an arrow pointing from the left hand side to the right hand side. It has a diagram of an enzyme over it. This is labelled, "Enzyme - Catalyses the reaction".

Every enzyme has one specific reaction that it catalyses.

The substrates bind to a site on the enzyme called the active site. The active site has a complementary shape to the substrates (a complementary shape is a shape that fits together with another shape).

Once the substrates have bound to the active site, the reaction takes place, converting the substrates to products. The products then leave the active site.

Diagram showing the process of an enzyme controlled reaction. The first part of the diagram shows the substrates entering the enzyme's active site. The second part shows the substrates in the active site. There is an arrow between these two parts labelled, "1. Substrates bind to the enzyme's active site.". The third part shows the products in the enzyme's active site. There is an arrow from the second part to the third labelled, "2. Enzyme catalyses the reaction in which the substrates are converted to products.". The fourth part shows the products leaving the active site. There is an arrow from the third part to the fourth labelled, "3. Products leave the enzyme's active site.".

Substrates bind to the enzyme's active. The substrates are then converted to products, which then leave the enzyme's active site.

Some reactions have only one substrate or only one product.

The lock and key model is a model of how enzymes work

Scientists have come up with a model, called the lock and key model, to explain how enzymes work.

The lock and key model says that the active site of an enzyme has an exactly complementary shape to that enzyme's substrates. This allows the substrates to fit perfectly into the active site, like a key in a lock.

Diagram of the lock and key model. The title of the diagram says, "The Lock and Key Model". Under that, there is a diagram of a key in a lock. Under that, there is a diagram of substrates in the active site of an enzyme.

The lock and key model says that the substrates fit perfectly in the enzyme's active site, like a key in a lock.

The lock and key model has limitations

The way that scientists test a model is by using the model to make predictions and then comparing those predictions to what is actually observed.

If the model's predictions closely match the observations, then this suggests it is a good model (it is an accurate description of reality).

If this is the case, then scientists continue to test more and more of the model's predictions. If the model's predictions continue to closely match the observations, then the scientists can be increasingly confident that the model is an accurate description of reality.

Flowchart showing the process of modelling. The title says, "Using Models in Science". The flowchart says that you start by developing a model. Then you use the model to make predictions. Then you compare the predictions to observations. Then the flowchart asks if the predictions match the observations. If YES, then it says to make more predictions, and it points back to "Use the model to make predictions". If NO, it says to develop a new model, and it points back to "Develop a model".

The process of developing and testing a scientific model.

Many predictions from the lock and key model closely match what is observed. However, there are some situations where the lock and key model's predictions do not match what is observed. This suggests that there are better ways of describing how enzymes work.

Over time, scientists have developed other models of enzyme action. The predictions made from these models more closely match the observations. You do not need to know the details of these other models for this course.

Names of enzymes often end in '-ase'

Enzymes often have names that end in -ase.

For example, there is a digestive enzyme called amylase which catalyses the break down of starch.

Flashcards

Flashcards help you memorise information quickly. Copy each question onto its own flashcard and then write the answer on the other side. Testing yourself on these regularly will enable you to learn much more quickly than just reading and making notes.

1/3

What are enzymes?

2/3

How do enzymes work?

3/3

What is the 'lock and key' model?

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