Enzyme Function

Enzyme Function

  • Globular proteins that act as biological catalysts
  • Speed up chemical reactions that would normally be slow at cell temperature
  • Precise 3-D shape includes a depression surface; active site
  • Amino acids involved on active site whilst other remain 3-D shape


Lock and Key theory:

  • The molecule (substrate) must have a specific, complimentary shape, to fit into the active site.
  • Substrate forms temporary bonds with amino acids of the active site to create an enzyme-substrate complex
  • Enzyme holds the substrate so that they react more quickly
  • When reaction has finished, the products are released, leaving the enzyme unchanged


Induced Fit theory:

  • Active site is flexible
  • When the substrate (s) enter the active site the enzyme changes shape slightly
  • When reaction finishes and the enzyme returns to its normal shape
  • Only a specific substrate will induce the change in shape of an enzyme’s active site


Activation Energy:

  • To convert substances into products bonds must change (within and between molecules)
  • Breaking chemical bonds requires energy to start the reaction – activation energy
  • Heating a substrate or using an enzyme would give this energy
  • Heat agitates atoms that become unstable and start the reaction
  • Enzymes reduce the time this process takes


  • The specific shape of the enzyme and the substrate is such that electrically charged groups on their surfaces interact
  • Attraction of oppositely charged groups may distort the shape of the substrate and assist in breaking/making bonds
  • Sometimes the amino acids contain acidic side chains which may provide conditions favourable for the reaction
  • Substrate molecules are sometimes altered, not added to or broken down, these are isomerase enzymes


Intracellular Inside the cells
Extracellular Outside the cells
Catabolic Breaking down reactions
Anabolic Building up reactions



Finding rates of enzyme-controlled reactions:

  • Rate of reaction measured by: the quantity of substrate used or the quantity of product formed in a given time
  • The reaction will firstly occur quickly however as the substrate is used up there are fewer substrate molecules to bind with the enzyme and the reaction slows down
  • The rapid slope is the initial rate of reaction


Enzyme and Substrate concentration; rate of reaction:

  • Initial rate of reaction is directly proportional to the enzyme concentration because the more enzyme that is present the greater the number of active sites
  • It will continue in this linear fashion if there is an excess of substrate
  • Reaction can be limited by the enzyme if there is increased substrate concentrations but not enough active sites