enzymes

• an enzyme is a biological catalyst that speed up a chemical reaction

  • it itself remains unchanged
  • as it isn’t used up in a reaction
• enzymes are soluble for travelling through the bloodstream
• an enzyme has a tertiary structure
• bond present in the enzymes is hydrogen ionic and disulphide
• The tertiary structure of an enzyme can be changed through

  • mutation, ( casuing a change in primary structure therefore tertiary structure)
  • denaturing
• the active site has a specific tertiary structure and therefore a specific 3D shape

  enzymes are specific molecules
• Lock and Key model

  1. tertiary structure, the active site and substrate are complementary
  2. the substrate fits into the active site and binds to form a Esc,
  3. bonds form temporarily between certain amino acids of the active site and groups on the substrate
  4. pressure is out on the bonds within the substrate breaking it apart into products
  5. after the reaction the products leave the active site and the enzyme is ready to be used again
• The lock and key model is a problem

  • proteins are not ridged
  • they can movements large and small
• Induced fit model

  1. initially the shape of the active site is not exactly complementary to that of the substrate
  2. the active site changes shape and moulds itself around the substrate molecule
  3. after the reaction the products leave the active site
  4. once the products have been released the shape of the enzymes active site remains unchanged
• the enzymes active site after the products leave return to the original shape
• the activation energy

  • this is the minimum amount of energy required for a reaction to start
• enzymes and activation energy

  • enzymes work by lowering the activation energy
  • which allows the reaction to occur at the relatively low temperatures found in living cells (37)
• enzyme activity is measured by:

  • how fast the product is made: this requires the volume/mass of product to be measured over a period of time
  • how fast the substrate is broken down/ reactants are used up: this requires the volume/mass of reactants used to be measured over a period of time
• effects of temperature on enzymes

  • optimum temperature is the one enzymes catalyse reactions best
  • increasing the temp gives both enzyme and substrate more kinetic energy
• effect of temp on enzymes

  • more kinetic energy increases the chance of substrate molecules colliding and subsequently binding with the active site of the enzyme
• effect of temp on enzymes

  • as temp increases the enzyme molecules vibrate more
• at very high temp excessive vibrations break some of the bonds (hydrogen, ionic, disulphide) holding the enzymes tertiary structure in place
• when the bonds holding the tertiary structure in place are broken by vibrations due to high temp, the shape of the active site changes and the substrate can no longer bind so no enzyme substrate complex can form and the enzyme is denatured
• the rate of reaction is still working at low temps for enzymes as they still work at cold temps
• the rate of reaction for temp on enzymes

  slows and then stops because some have started to denature as some weaker H bonds have been broken in the active site but denaturing is a process
• denatured is irreversible and stops the reaction when all are denatured
• Formation of enzyme-substrate (ES) complexes lowers activation energy of metabolic reactions.
• • Specific tertiary structure determines shape of active site, complementary to a specific substrate.
  • 
• How does substrate concentration affect rate of reaction?

  
  

  Given that enzyme concentration is fixed, rate increases proportionally to

  substrate concentration.

  Rate levels off when maximum number of ES complexes form at any given time.
• How does enzyme concentration affect rate of reaction?

  
  

  Given that substrate is in excess, rate increases proportionally to enzyme concentration

  Rate levels off when maximum number of ES complexes form at any given time.
• How could a student identify the activation energy of a metabolic

  reaction from an energy level diagram?

  
  

  Difference between free energy of substrate & peak of curve.
• the ph of a solution is its measure of h+ ions
• all enzymes have a optimum ph
• most metabolic enzymes have an optimum close to 7.

  some have extremes like pepsin ph2
• below the optimum its more acidic with more h+ ions

  and above the optimum its more alkaline with more OH- ions

  these cause the hydrogen and ionic bonds disrupted
• at extreme ph the tertiary structure of the active site changes and the substrate can no longer bind and fit into the enzymes active site
• the higher the substrate conc the higher the rate of reaction, as there is more chance of collision

  this is only true until the saturation point
• the saturation point is when all enzymes active sites are occupied at any one time
• once all the active sites are occupied and saturation point is reached,

  adding more substrate will not increase the rate of reaction as all the active sites are occupied

  now the conc of enzymes has become the limiting factor
• competitive inhibiters

  • inhibitor has a similar shape to the substrate
  • competes with substrate for the active site
  • with the inhibitor occupying the active site of the enzyme the substrate cannot bind so ESC can’t form
  • competitive inhibitors molecules move in and out of active site
• the effect of competitive inhibitors can be reduced by increasing the substrate concentration
• the inhibitor in the enzyme is called enzyme inhibitor complex
• competitive inhibitor graph

  • at the maximum rate of reaction the active sites are saturated and the ROR is constant
  • the ESCS are formed slower
• without a competitive inhibitor the maximum ROR is reached faster as there’s no competition for the active site