- In a living cell, thousands of chemical reactions are taking place. These processes are called metabolism. Most of the reactions of metabolism cannot occur without globular proteins called enzymes.
- Enzymes are biological catalysts. They speed up chemical reactions without being used up. They do this by lowering the activation energy needed for a reaction to start. This allows reactions like photosynthesis and respiration to occur at a suitable rate.
- Enzymes control different reactions. Anabolic reactions build up molecules, eg proteins. Catabolic reactions break down molecules, eg digestion.
- Enzymes are different from other catalysts because they work thousands of times faster, they are specific (one enzyme can only work on one type of substance) and they are sensitive to conditions (temperature, pH, chemicals).
How enzymes work
There are two models on how enzymes work. The out-dated model is the lock and key model which said that substrate fits into the active site like a key fits into a lock. When the substrate fits, the chemical reaction occurs and products are released.
This model has been replaced by induced-fit model.
The accepted model of how enzymes work is the induced-fit model. In this model, each type of enzyme has a specific area called the active site which is shaped so that the substrate can fit into it. When the substrate joins the active site, it causes the enzyme to change shape slightly, causing the chemical reaction the enzyme is catalysing to occur. The products are released and the enzyme returns to its normal shape ready for another reaction.
