• Are biological catalysts that help to speed up biological chemical reactions. Without them life would not be possible.
  • Enzymes do not start a chemical reaction rather they just speed up the rate of a reaction and they do this by lowering the activation energy of a reaction.
  • Enzymes do not get used up in a chemical reaction and so can be reused.


  • Enzymes are proteins and so are sensitive to temperature and pH changes. Depending on the enzyme in question, if the temperature or pH is not at its optimum level, there is the possibility that the enzyme could become denatured. Denaturation occurs when the protein/enzyme loses its structure due to changes in temperature or pH in this case – think of when you fry an egg, clear white ‘liquid’ changes to solid white. This is an example of denaturation.
  • The key with an enzyme is its active site. Its active site has a shape that is complementary with the substrate that it will catalyse. There are two models that describe the relationship between the active site and the substrate and they are the lock and key model and induced fit hypothesis.


  • The shape of the active site is determined primarily by the tertiary structure of the protein, which is why the bonding and interactions between the R groups of the amino acids must be correct. If the interactions are wrong, then the shape of the active site will be wrong, and so the enzyme won’t be able to bind its substrate.
  • But NOTE, errors in the primary structure and secondary structure can also effect the function of the enzyme.

REMEMBER structure = function.


(Image from: https://commons.wikimedia.org/w/index.php?curid=1267900)