1.4.2 - many proteins are enzymes

•    Introduction to enzymes 

  • Each enzyme lowers the activation energy of the reaction it catalyses  —> speed up rate of reaction
  • Enzymes are biological catalysts; they catalyse a wide range of intracellular (within cells) and extracellular (outside cells) reactions that determine structures and functions from cellular to whole-organism level. 
• Lock and key model

  • Old, outdated
  • Active site is a fixed shape / doesn’t change shape; it is complementary to one substrate
  • After a successful collision, an enzyme-substrate complex forms leading to a reaction.
• Induced fit model 

  • More recent , accepted.  1. Before reaction, enzyme active site not completely complementary to substrate / doesn’t fit substrate. 2. Active site shape changes as substrate binds and enzyme-substrate complex forms. 3. This stresses / distorts bonds in the substrate leading to a reaction.
• The specificity of enzymes 

  • Enzymes have a specific shaped tertiary structure and active site.(Sequence of amino acids (primary structure) determines tertiary structure. )

  • Active site is complementary to a specific substrate.

  • Only this substrate can bind to the active site, inducing fit and forming an enzyme-substrate complex
• Factors affecting rate of enzyme-controlled reactions

  ENZYME CONCENTRATION. 

  • As the enzyme concentration increases, the rate of reaction increases.

  • Enzyme conc. = limiting factor (substrate in excess)

  THE ENZYME CONCENTRATION INCREASES THE RATE OF REACTION AS

  • More enzymes more available active sites

  SO..

  • More successful E-S collisions and E-S complexes 
  • At a certain point, rate of reaction plateaus
  • Substrate conc. = limiting factor (all substrates in use)
• SUBSTRATE CONCENTRATION.

  • As the substrate concentration increases, the rate of reaction increases.

  • Substrate concentration = limiting factor (too few (not enough) enzyme molecules to occupy all active sites)

  THE INCREASE IN SUBSTRATE CONCENTRATION INCREASES THE RATE OF REACTION AS MORE SUBSTRATES WILL OCCUPY THE ACTIVE SITES RESULTING IN

  • More successful E-S collisions and E-S complexes.

  • At a certain point, rate of reaction plateaus

  • Enzyme conc. = limiting factor (all active sites saturated; excess substrate)
• TEMPERATURE.

  • As the temperature increases until optimum, the rate of reaction increases.

  THIS IS DUE TO AN •Increase in kinetic energy

  RESULTING IN.... •More successful E-S collisions and E-S complexes

  • As the temperature increases above optimum, the rate of reaction falls.
  • Because the enzymes denature; tertiary structure and active site change shape 

     (hydrogen / ionic bonds break)

  • resulting in fewer E-S collisions and E-S complexes 

   (since the substrate can no longer bind to the active site)

  •   When all the enzymes are denatured, the Rate of reaction is 0. 
•        pH.

  • If the pH is above or below the optimum pH, the rate of reaction decreases.
  • This is because enzymes denature; tertiary structure and active site change shape (hydrogen and ionic bonds break)
  • therefore complementary substrates can no longer bind to active site
  • resulting in fewer E-S collisions and E-S complexes
  • pH = - log(10) [H+ ]
• Concentration of competitive inhibitors.

  • Competitive inhibitors decrease the rate of reaction. 
  • Because they have a similar shape to substrate
  • And they compete for / bind to / block active site so 

    substrates can’t bind

  • Resulting in fewer E-S complexes
  • Increasing substrate conc. reduces effect of inhibitor 

  (level of inhibition dependent on relative concentrations of substrate and inhibitor)
• Concentration of non-competitive inhibitors.

  • Non-competitive inhibitors decrease the rate of reaction.
  • Because they bind to site away from the active site (allosteric site)
  • Causing the enzyme tertiary structure/active site change shape 

     so substrate can’t bind to active site

  • This results in there being fewer E-S complexes
  • Increasing substrate concentration has no effect on rate of reaction as it causes permanent change to the active site.
• structure of non-competitive and competitive inhibitors

  A) allosteric site

  B) non-competitive inhibitor

  C) competitive inhibitor

  D) substrate

  E) active site