Enzymes (College Board AP® Biology): Study Guide

Enzyme structure and properties

• 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 high specificity to the reactions they catalyze and the substrates they bind to

  • They have an active site with a specific shape that is complementary to a specific substrate

• When a substrate binds, it forms an enzyme-substrate complex

Enzyme-substrate complex

• For an enzyme-mediated chemical reaction to occur, the shape and charge of the substrate must be compatible with the active site of the enzyme

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

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

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

• Enzymes are recyclable; they are not used up as part of the reactions they catalyze, they can be used again for future reactions

Enzymes as Biological Catalysts

• Enzymes are biological catalysts

  • ‘Biological’ because they function in living systems

  • ‘Catalysts’ because they speed up the rate of chemical reactions without being used up or changed

• All chemical reactions are associated with energy changes

• For a reaction to proceed there must be enough activation energy

• Activation energy is the amount of energy needed by the substrate to become just unstable enough for a reaction to occur and for products to be formed

  • Enzymes speed up chemical reactions because they influence the stability of bonds in the reactants

  • The destabilization of bonds in the substrate makes it more reactive

• Enzymes work by lowering the activation energy of a reaction and in doing so they provide an alternative energy pathway