ATP and Exercise
Biochemical Tests

Other Sections

Cell Biology
Human Biology
Plant Biology


How They Work

Enzymes are both proteins and biological catalysts. Their function in the body is to vastly increase the rate of reaction between chemicals: thus making life possible.

Enzymes work by reducing the activiation energy of a reaction, this means that with an enzyme present less energy is needed for the reaction to happen and hence more reaction can occur.

For an enzyme reaction to happen the substrate (chemicals that are being reacted) goes to the active site (where the part of the enzyme that does the work). Crucially, it is the enzyme's shape that is involved.

The induced fit hypothesis explains how the enzymes work. The active site of an enzyme is the bit where reactions take place. And the substrate the chemical which the enzyme is reacting. In this hypothesis, the two do not fit perfectly, and so the shape of the active site changes slightly to fit. The reaction takes place and it returns to the original shape.

induced fit hypothesis diagram

Factors Affecting Enzyme Activity

Temperature affects the rate of activity of an enzyme. To begin with; when temperature is increased the rate of reaction goes up because the enzyme and substrates are moving about more and have more energy. However after an optimum point the activity decreases, this is because the enzyme denatures when hydrogen bonds break and the enzyme looses its distinctive shape. A similar effect is seen with pH.

Increasing substrate concentration will increase the reaction rate. But eventually this rate will level out because the enzymes can only do so much work and the maximum number are being used up.

Enzyme activity can be reduced by the presence of inhibitors; there are two types.

  • A competitive inhibitor has a similar structure to the substrate so fits into the active site so the enzyme is not catalysing the actual substrate all the time, the rate of reaction is reduced.
  • A non-competitive inhibitor binds to part of the enzyme. This alters its shape and therefore stops it from working. Poisons are often this type of inhibitor, for example: cyanide.
  • Another way of affecting the rate is molecules binding to the allosteric site on an enzyme. This is another part (not the active site) that tells the enzyme to work or not. The presence of these molecules will inhibit or activate the enzyme. However only a few very special enzymes do this.

    Digestive Enzymes

    Because they are neat and easy to use; we will look at the different digestive enzymes in the table below.

    SubstanceEnzymesWhat Happens
    Carbohydrates Amylase
    The salivary amylase begins digestion in the mouth, but not much happens since food only stays there for a short while, but it keeps the mouth clean

    Amylase from the pancreas breaks down starch into maltose.

    When the maltose gets into the ileum, the last stage of digestion takes place and the disaccharides are broken into glucose so they can be absorbed by active transport.
    Endopeptiases These are produced in the pancreas and also as pepsin in gastric juice. This enzyme slices up polypeptides into smaller ones of about 6-12 monomers. It does this at different points depending on the enzyme.

    This also leaves alot of ends to help the next enzyme.
    Exopeptidases These proteins live in the ileum membrane, they finish off the job and break the proteins into individual amino acids; which can now be absorbed into the blood stream.
    Triglycerides Lipase Digestion of triglycerides involves more than just enzymes, so is covered in more detail on digestion.

    Updated: 15 May 2011.