Enzymes—catalysts of life

At any one time in the body there are thousands of chemical reactions happening. In some cells there may be hundreds of different reactions occurring simultaneously. How then does each cell control which reactions occur—when, where, and how fast? How does the cell stop them from interfering with each other? More basically, how does the cell make them happen at all? Glucose in a beaker will not release its energy, nor turn into starch on its own. What is required to make these reactions happen?

The answer to all of the above questions is: with enzymes.

Enzymes are complex protein molecules which act as biological catalysts. A catalyst is a substance which speeds up a reaction, but which is not used up in the reaction itself.

Enzymes can work fast and efficiently. The enzyme you will be using in the experiment, catalase, can be involved in up to 5 600 000 reactions per minute.

For each separate type of chemical reaction in the body there is a separate enzyme. This is because each enzyme has a particular shape which matches up exactly with the reacting substance or substances (the substrate). The substrate locks in to a particular place in the enzyme called the active site in a process often described as ‘lock and key’ (see the animations and associated figures in 'More images'). In this way, one kind of enzyme has only one job, so the cell can control which reactions occur by controlling which enzymes are available at the time.

Some enzymes operate inside a cell, for example, those that speed up the breakdown of glucose to release energy. These enzymes are called intracellular enzymes. Others, such as digestive enzymes, work outside cells, and are called extracellular enzymes. In the experiment 'Investigating a common enzyme' you will be working with an intracellular enzyme.

By having chemical reactions catalysed by enzymes, and by having a separate enzyme for each reaction type, the cell can control the complex array of reactions in an orderly fashion.