Enzyme Action

• Enzymes:

  • biological catalysts that speed up chemical reactions
  • can be intracellular (within cells) or extracellular (outside cells)
  • globular proteins (soluble in water)
• • The active site of an enzyme has a specific shape to fit a specific substrate
  • The tertiary structure of the active site is complementary to the structure of the substrate
  • Therefore enzymes are highly specific
• Enzymes lower the activation energy of a reaction by providing an alternate reaction pathway.
• Lock-and-key model:

  • substrate fits perfectly into the active site, forming an enzyme-substrate complex
  • the reaction takes place and the products are released
  • the shape of active site remains unchanged when the substrate binds to it
• Induced fit model:

  • as the substrate approaches the enzyme, the active site changes shape slightly so that it is complementary to the substrate
  • the substrate has to be the right shape + change the shape of active site in the right way
  • this explains why enzymes are so specific
• How enzyme-substrate complex lowers activation energy:

  • when joining substrate molecules, being attached to the enzyme holds them closer together, reducing any repulsion between the molecules, so they can bond more easily.
  • when breaking down substrate molecules, fitting into the active site puts a strain on bonds in the substrate, so they break up more easily.
• Temperature: higher = faster rate

  • molecules have more kinetic energy so moves faster -> collides more frequently
  • molecules collide with more energy -> more likely to overcome activation energy
  • temp too high: bonds holding the enzyme's tertiary structure in place breaks, causing the active site to change shape -> enzyme is denatured
• pH: closer to optimum = faster rate

  • above/below optimum pH: H+ and OH- ions found in acids and alkalis can break ionic bonds and hydrogen bonds that hold the enzyme's tertiary structure in place, causing the active site to change shape -> enzyme is denatured
• Enzyme concentration: higher = faster rate

  • More enzymes -> more active sites available, substrates are more likely to collide with one and form an enzyme-substrate complex
  • Amount of substrate is limited: adding enzymes have no further effect
• Substrate concentration: higher = faster rate

  • more substrates -> more enzyme-substrate complexes can be formed
  • up till the point of saturation: all active sites are full, so adding substrates have no further effect
• Enzyme inhibition:

  • competitive inhibition: inhibitors have a similar shape to the substrate, so compete with substrates for active sites.
  • non-competitive inhibition: inhibitors bind to the enzyme at an alternative site, changing the shape of the active site, which prevents substrates from binding to it.
• Enzyme inhibition - increasing substrate concentration:

  • increases rate in competitive inhibition, as substrates are more likely to collide with enzymes than inhibitors.
  • has no effect in non-competitive inhibition, as the shape of active site remains changed so enzyme-substrate complexes are still unable to form.