Unit 2: Section 3

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Action of Enzymes

Enzymes are biological catalysts

  • They catalyse metabolic reactions in your body e.g. digestion and respiration.
  • Enzyme action can be intracellular - within cells, or extracellular - outside cells.
  • Enzymes are globular proteins.
  • Enzymes have an active site, which has a specific shape. The active site is the part of the enzyme where the substrate molecules bind to.
  • The specific shape of the active site is determined by the enzyme's tertiary structure.
  • For the enzyme to work, the substrate has to fit into the active site (its shape has to be complementary). If the substrate shape doesn't match the active site, the reaction won't be catalysed. This means that enzymes work with very few substrates - usually only one.
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Action of Enzymes

Activation energy

  • In a chemical reaction, a certain amount of energy needs to be supplied to the chemicals before the reaction will start. This is called the activation energy - it's often provided as heat.
  • Enzymes reduce the amount of activation energy that's needed, often making reactions happen at a lower temperature than they could without an enzyme. This speeds up the rate of reaction.
  • When a substance binds to an enzyme's active site, an enzyme-substrate complex is formed. It's the formation of the enzyme-substrate complex that lowers the activation energy. This is because:
  • If two substrate molecules need to be joined, attaching to the enzyme holds them close together, reducing any repulsion between the molecules so they can bond more easily.
  • If the enzyme is catalysing a breakdown reaction, fitting into the active site puts a strain on bonds in the substrate. This strain means the substrate molecule breaks up more easily.
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Factors Affecting Enzyme Activity


  • The rise in temperature makes the enzyme's molecules vibrate more.
  • If the temperature goes above a certain level, this vibration breaks some of the bonds that hold the enzyme in shape.
  • The active site changes shape and the enzyme and substrate no longer fit together.
  • At this point, the enzyme is denatured - it no longer functions as a catalyst.


  • All enzymes have an optimum pH value. Most human enzymes work best at pH7, but there are exceptions.
  • Pepsin, for example, works best at acidic pH 2, which is useful because it's found in the stomach.
  • Above and below the optimum pH, the H+ and OH- ions found in acids and alkalis can mess up the ionic bonds and hydrogen bonds that hold the enzyme's tertiary structure in place.
  • This makes the active site change shape, so the enzyme is denatured.
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Factors Affecting Enzyme Activity

Enzyme concentration

  • The more enzyme molecules there are in a solution, the more likely a substrate is to collide with one and form an enzyme-substrate complex. So increasing the concentration of the enzyme increases the rate of reaction.
  • But, if the amount of substrate is limited, there comes a point when there's more than enough enzyme molecules to deal with all the available substrate, so adding more enzyme has no further effect.

Substrate concentration

  • The higher the substrate concentration, the faster the reaction - more substrate molecules means a collision between substrate and enzyme is more likely and so more active sites will be used.
  • This is only true up until a 'saturation' point though. After that, there are so many substrate molecules that the enzymes have about as much as they can cope with (all the active sites are full), and adding more makes no difference.
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Factors Affecting Enzyme Activity

Cofactors and coenzymes

  • Some cofactors are inorganic molecules. They work by helping the enzyme and substrate to bind together.
  • They don't directly participate in the reaction so aren't used up or changed in any way.
  • Some cofactors are organic molecules - these are called coenzymes.
  • They participate in the reaction and are changed by it.
  • They often act as carriers, moving chemical groups between different enzymes. They're continually recycled during this process.

Competitive inhibition

  • Competitive inhibitor molecules have a similar shape to that of the substrate molecules.
  • They compete with the substrate molecules to bind to the active site, but no reaction takes place.
  • Instead they block the active site, so no substrate molecules can fit in it.
  • How much the enzyme is inhibited depends on the relative concentration of the inhibitor and substrate.
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Factors Affecting Enzyme Activity

Non-competitive inhibition

  • Non-competitive inhibitor molecules bind to the enzyme away from its active site.
  • This causes the active site to change shape so the substrate molecules can no longer bind to it.
  • They don't 'compete' with the substrate molecules to bind to the active site because they are a different shape.
  • Increasing the concentration of substrate won't make any difference - enzyme activity will still be inhibited.
  • If they're strong covalent bonds, the inhibitor can't be removed easily and the inhibition is irreversible.
  • If they're weaker hydrogen bonds or weak ionic bonds, the inhibitor can be removed and the inhibition is reversible.
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Factors Affecting Enzyme Activity

Some metabolic poisons are enzyme inhibitors

  • Cyanide is an irrecersible inhibiroe of cytochrome c oxidase, an enzyme that catalyses respiration reactions. Cells that can't respire die.
  • Malonate inhibits succinate dehydrogenase (which also cayalyses respiration reactions).
  • Arsenic inhibits the action of pyruvate dehydrogenase, yet another enzyme that catalyses respiration reactions.

Some drugs work by inhibiting enzymes

Some medicinal drugs are enzyme inhibitors, for example:

  • Some antiviral drugs e.g. reverse transcriptase inhibitors inhibit the enzyme reverse transcriptase, which catalyses the replication of viral DNA. This prevents the virus from replicating.
  • Some antibiotics e.g. penicillin inhibits the enzyme transpeptidase, which catalyses the formation of proteins in bacterial cell walls. This weakens the cell wall and prevents the bacterium from regulating its osmotic pressure. As a result the cell bursts and the bacterium is killed.
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