3.1.4.2 Many Proteins are enzymes

• Enzymes act as biological catalysts as each enzyme lowers activation energy of reaction it catalyses, to speed up the rate of reaction
• Enzymes catalyse a wide range of intracellular and extraacellular reactions that determine strcuture and functions from cellular to whole organism level
• The induced fit model of enzyme action is:

  1. The substrate binds to the active site of an enzyme but not completely complementary
  2. Causes the active site to change shape so it is complementary to the substrate
  3. So enzyme-substrate complexes form
  4. Causing bonds in the substrate to distort, lowering the activation energy
• Models of enzyme action have changed over tim as initally it was the lock and key model which is now outdated, stating that the active site a fixed shape, complementary to one substrate
• The specificity of enzymes is that they have a specific tertiary structure which determines the shape of the active site, which is dependent on the sequence of amino acids or the primary structure. The active site is now complementary to a specific substrate and only this substrate can bind to the active site, inducing fit and forming an enzyme-substrate complex.
• As enzyme conc. increases, rate of reaction increases

  • Enzyme conc. = limiting factor (excess substrate)
  • More enzymes so more available active sites
  • So more enzyme substrates complexes form

  At a certain point, the rate of reaction stops increasing and plateus

  • Substrate conc. = limiting factor, where all substrates are in use
• As substrate conc. increases, the rate of reaction increases

  • Substrate conc. = limiting factor, this is because there are too few enzyme molecules to occupy all active sites
  • More enzyme substrate complexes form

  At a certain point, the rate of reaction stops increasing and plateaus

  • Enzyme conc. = limiting factor
  • As all active sites are saturated and occupied at a given time
• As temp. increases up to optimum, the rate of reaction increases

  • More kinetic energy
  • So more enzyme substrate complexes form

  As temp. increases above optimum, the rate of reaction decreases

  • Enzymes denature so the tertiary structure and active site change shape
  • As hydrogen and ionic bonds break
  • So active site no longer complementary bind
  • So fewer enzyme substrate complexes form
• As pH increases/ decreases above/ below an optimum, rate of reaction decreases

  • Enzymes denature so the tertiary structure and active site change shape
  • As hydrogen and ionic bonds break
  • So active site no longer complementary
  • So fewer enzyme substrate complexes form
• As the concentration of competitive inhibitor increases, the rate of reaction decreases

  • Similar shape to the substrate
  • Competes for or binds to and blocks the active site
  • So substrates can't bind and fewer enzyme substrate complexes form

  Increasing substrate conc. reduces the effect of inhibitors
• As concentration of non competitive inhibitor increases, rate of reaction decreases

  • Binds to site other than the active site known as the allosteric site
  • This changes the enzymes tertiary structure and the active site shape
  • So active site no longer complementray to substrate
  • So substrates can't bind so fewer enzyme substrate complexes form

  Increasing substrate conc. has no effect on rate of reaction as change to active site is permanent