Enzymes
Introduction


Do you use 'biological' washing powder to get your clothes clean? If you do, you can thank molecules called enzymes for helping to break down any stains. In this unit we will find out about enzymes, how they work, and how their effectiveness is affected by changes in temperature and pH (acidity or alkalinity).

What are enzymes?
Enzymes are large, soluble protein molecules. We call them 'biological catalysts' because all living things rely on them to catalyse the reactions that keep them alive. Each chemical reaction that takes place in your body is catalysed by a specific
enzyme
An enzyme is a biological catalyst which speeds up reactions in living things. An enzyme is a protein molecule with a specific shape to accommodate reactant molecules.
enzyme
. For example, an enzyme in your saliva, called amylase, starts off the breakdown of starch in food.

We also have an enzyme called catalase in many of our cells, especially the liver. It breaks down hydrogen peroxide, a poison that can build up in cells.

hydrogen peroxide    water  +  oxygen
2 H2O2(aq)    2 H2O(l)  +  O2(g)

Look at the reaction in Fig.1 below:

Figure 1.   The enzyme catalase in liver breaks down hydrogen peroxide.
Display as fullscreen
Which of the following tests would identify the gas given off in the above reaction?
  • Click here to mark the question


Enzymes are remarkably efficient catalysts. Each enzyme
molecule
A molecule is a group of two or more atoms bonded together.
molecule
can catalyse the reaction of thousands, or even millions, of
reactant
A reactant is a substance we start with before a chemical reaction.
reactant
molecules every second.

Explaining how enzymes work
Enzymes are in fact proteins (see the unit Food Chemistry). Each enzyme molecule is made up of protein chains folded and wrapped into intricate shapes. Each enzyme has a shape that matches the molecules it helps to react together (called the substrates). These reactant molecules fit into 'active sites' on the enzyme, which effectively brings them together so that they can react. The fit between the active sites and the
substrate
A substrate is a reactant molecule in an enzyme-catalysed reaction.
substrate
molecules is highly specific, rather like a lock and key. The reaction then takes place and then the products leave the active sites, so that the enzyme can catalyse more reactions.

Look at the model in Fig.2 below:

Figure 2.   The 'lock and key' mechanism of enzyme action.
Display as fullscreen


Each enzyme works best at an optimum temperature. The optimum temperature for most enzymes in the human body is around 40 °C.

Figure 3.   The effect of temperature on enzyme-catalysed reactions.
Display as fullscreen
What happens to the rate of an enzyme-catalysed reaction as you increase the temperature?
  • Click here to mark the question


The characteristic shape of the graph in Fig.3 above seems strange at first sight. After all, most reactions speed up as the temperature rises. However, when an enzyme gets too hot, it loses its special shape and stops working. We say that the enzyme is denatured.

The extra energy at temperatures of 45 °C and above makes the enzyme vibrate more rapidly, and the shaking breaks some of the weak bonds holding it in shape. Now the reactant molecules (substrates) no longer fit snugly into the active sites and the reaction is not catalysed. It's as if you try to use your front-door key, but someone has changed the lock!

Enzymes also work best at an optimum
pH value
The pH value of a solution is a number on a scale indicating how strongly acidic or alkaline the solution is. The lower the pH value, the more acidic it is. pH values less than 7 are acidic, pH values over 7 are alkaline, and a pH value of 7 indicates a neutral solution.
pH value
. This varies from enzyme to enzyme, depending on the conditions in which the enzyme operates. For example, those in the acidic environment of the stomach work well at low pH values. In contrast, look at the way pH affects the amylase enzyme in your saliva:

Figure 4.   The effect of pH on the enzyme amylase.
Display as fullscreen
What is the optimum pH of amylase?
  • Click here to mark the question
Summary


Enzymes are biological catalysts made up of protein molecules.

Each reaction in the body is catalysed by a specific enzyme.

Enzymes work best at an optimum temperature (usually around 40 °C). They become ineffective at higher temperatures as their special shape changes. They become denatured, and substrate molecules no longer fit snugly into the active sites on the enzyme.

Enzymes also work best at an optimum pH value, which varies from enzyme to enzyme.

Exercises
1. Match the enzyme to its location.
  • Catalase
    Amylase
  • Click here to mark the question
2. The breakdown of hydrogen peroxide is catalysed by an enzyme in potato, as well as by manganese(IV) oxide. Which statement is true?
  • Click here to mark the question
Figure 5.  
Display as fullscreen
3. Which of the curves in Fig.5 shows how the effectiveness of an enzyme varies with temperature?
  • Click here to mark the question
4. What do we call the temperature at which an enzyme is most effective?
  • temperature
  • Click here to mark the question
Figure 6.  
Display as fullscreen
5. Look at the graph in Fig.6. Match the enzyme to the conditions in which it works best.
  • A
    B
    C
  • Click here to mark the question
6. Explain why excessive heating will denature an enzyme.
  • Click here to mark the question
Well done!
Try again!