What are enzymes?

Other enzymes do their job inside the cell. They are called intracellular enzymes. Their job is to speed up the chemical reactions occurring in our cells. But they do more than just speed up the reactions; they also control them.

At this moment, thousands of chemical reactions are taking place in your body. Each reaction is controlled by a particular enzyme. Our enzymes make sure that the right reactions occur in the right place and at the right time.

An enzyme-controlled reaction

Here is an example of a reaction which is controlled by an enzyme:

Maltase (enzyme)

Maltose (substrate) Glucose (product)

The substance which the enzyme acts on is called the substrate - in this case maltose. The new substance or substances formed as a result of the reaction are the products. In this case there is just one product: glucose. The enzyme catalysing this particular reaction is maltase.

This reaction will go in either direction. In other words the reaction is reversible: maltose can be turned into glucose, or glucose into maltose. The enzyme will work either way. If there is a lot of maltose present compared with glucose, the reaction will go from left to right; if there is a lot of glucose present compared with maltose, it will go from right to left. Most metabolic reactions are reversible.

The properties of enzymes

Enzymes have five important properties:

. They are always proteins

-This is one reason why we need proteins in our food-

2. They are specific in their action

-What this means is that each enzyme controls one particular reaction, or type of reaction. Thus maltase will only act on maltose, and surcease on sucrose.

3. They can be used over again

-This is because they are not altered by the reaction in which they take part. However, an enzyme molecule eventually runs down and has to be replaced.

4. They are destroyed by heating

-This is because enzymes, in common with all proteins, are destroyed by heating. This is called denaturation. Most enzymes stop working if the temperature rises above about 45°C

5. They are sensitive to pH

-the term pH refers to degree of acidity or alkalinity of a solution. Most intracellular enzymes work best in neutral conditions, i.e. conditions that are neither acidic nor alkaline.

How do enzymes work?

Molecules are constantly moving about and bumping into each other. Now when a substrate molecule bumps into a molecule of the right enzyme, it fits into a depression on the surface of the enzyme molecule. This depression is called the active site. The reaction then takes place and the molecules of the product leave the active site, freeing it for another substrate molecule.

The active site of a particular enzyme has a specific shape into which only one kind of substrate will fit. The substrate fits into the active site rather like a key fits into a lock. This is why enzymes are specific in their action.

When an enzyme is denatured no longer fits. A change in the pH has a similar effect.