Enzyme Structure (College Board AP® Biology)

Forming an Enzyme-Substrate Complex

• Enzymes are globular proteins required in metabolic pathways

  • Virtually every metabolic reaction within living organisms is catalyzed by an enzyme – enzymes are therefore essential for life

• Enzymes have an active site where specific substrates bind forming an enzyme/substrate complex

• The active site of an enzyme has a specific shape to fit a specific substrate

• Extremes of heat or pH can change the shape of the active site, preventing the substrate binding – this is called denaturation

• Substrates collide with the enzyme's active site and this must happen at the correct orientation and speed in order for a successful reaction to occur

• An enzyme-substrate complex forms when an enzyme and its substrate join together

• The enzyme-substrate complex is only formed temporarily (a few nanoseconds) before the enzyme catalyzes the reaction and the product(s) are released

Formation of an Enzyme / Substrate Complex Diagram

The active site of an enzyme has a specific shape to fit a specific substrate (when the substrate binds an enzyme-substrate complex is formed)

• The specificity of an enzyme is a result of the complementary nature between the shape of the active site on the enzyme and its substrate(s)

• The lock and key hypothesis likens this relationship to the fitting of a key into a specific lock

• The shape of the active site is determined by the complex tertiary structure of the protein that makes up the enzyme:

  • Proteins are formed from chains of amino acids held together by peptide bonds

  • The order of amino acids determines the shape of an enzyme molecule

  • If the order is altered, the resulting 3D shape changes

Enzyme Specificity Diagram

An example of enzyme specificity – the enzyme catalase can bind to its substrate hydrogen peroxide as they are complementary in shape, whereas DNA polymerase is not