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How They Work

Enzymes are proteins that act as biological catalysts which means their function in the body is to vastly increase the rate of reaction between chemicals, that would not happen otherwise: thus making life possible.

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

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

The induced fit hypothesis explains how the enzymes work. The active site of an enzyme is the area where reactions take place. And the substrate is the chemical that the enzyme is reacting together. According to this explanation for how enzymes work, the two do not fit perfectly, and so the shape of the active site changes slightly to fit. The reaction takes place and the enzyme returns to its original shape.

induced fit hypothesis diagram

Factors Affecting Enzyme Activity

Temperature has an important influence on the rate of activity of an enzyme. To begin with, if you increase temperature: the rate of reaction goes up because the enzyme and substrates are moving about more and have more energy. However if you go beyond an optimum point the activity decreases, this is because the enzyme denatures as hydrogen bonds break and the enzyme looses its distinctive shape. A similar effect is seen when pH is changed.

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 it can fit in to the active site and it stops the enzyme 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 correctly. Many poisons are 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 of how important they are; 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 the process of active transport.
    Endopeptidases 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 a lot 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: 30 December 2015.