enzymes

• enzyme attaches to the substrate and the molecules produced are products
• molecules must have a certain amount of energy before they can react (activation energy) and enzymes make the reaction happen faster by lowering the activation energy of the reaction
• • active site attaches to the substrate molecule - enzyme - substrate complex
  • the tertiary structure of the active site is complementary to the substrate, so fits perfectly. each enzyme is specific for the molecule
• once the substrate has bonded to the active site the amino acids on the surface of the active site can form temporary bonds with the substrate

  • these bonds between the active site and substrate help to lower the activation energy of the reaction so therefore the enzyme helps to increase the rate of reaction

  the enzymes then catalyses the reaction to form enzyme - product complex and the products are released from the active site
• any molecules that don't have at least the activation energy, cannot react

  enzymes provide a pathway for reactions with a lower activation energy barrier
• enzymes found in living organisms - biological catalysts
• lock and key hypothesis

  • when scientists first looked at enzymes they thought the tertiary structure of the active site was fixed and does not change shape.
  • the substrate slots perfectly into the active site
• induced fit hypothesis

  • scientists then realised the tertiary structure of the active site changes shape as the substrate approaches
  • as the substrate forms bonds with the amino acids in the active site, the tertiary structure so the active site moulds itself tightly around the substrate
  • this change in the tertiary structure ensures the active site fits perfectly to the substrate
  • molecules that are not the substrate cannot form the correct bonds to the correct amino acids so the tertiary structure doesn't change so the active site does not adjust to fit the molecule
• initial rate - large amount of product being produced in a short amount of time

  reaction slowing down - less product being formed, rate of reaction has decreased

  reaction stopped - no more product being made, reaction has stopped

  A) initial rate

  B) reaction slowing down

  C) reaction stopped

  
• • at the start there is a large amount of substrate molecules so a high frequency of successful collisions between the substrate and active site - rapid initial rate of reaction
  • as the reaction takes place, some substrate is converted to product, so amount of substrate molecules fall so the chances of the substrate and active site colliding decreases - reaction slows down
  • at a certain point all of the substrate molecules have been converted to product so no more substrate molecules left to collide with the active site - reaction stops
• a tangent is used to measure the rate of reaction

  • work out length of X and Y
  • then divide Y/X

  tangent line should be long so it can be read accurately
• rate of an enzyme controlled reaction depends on the frequency of successful collisions between the substrate molecules and active site

  • 'frequency' - number of collisions per second
• to measure temperature..

  1. select an enzyme
  2. measure the amount of product produced with time
  3. repeat using a range of temperatures
  4. draw tangents to measure the rate of reaction at each temperature
  5. the tangent needs to be drawn at the same time point on each graph
• if the temperature is too hot the enzyme can denature
• after an enzyme has denatured, it cannot renature if we cool it back down. the tertiary structure has changed so much and cannot be reversed
• 1. both the enzyme and substrate have high kinetic energy so move rapidly and the chances of collisions increases
  2. at the optimum temperature there are the maximum frequency of collisions between the substrate and the active site (optimum human temperature is 40 degrees)
  3. at very high temperatures the enzymes are vibrating rapidly which causes the hydrogen bonds to break so the tertiary structure of the enzyme begins to change. so the active site changes shape and is no longer complementary to the substrate

  A) rate of reaction increases

  B) rate of reaction decreases

  
• the pH depends on the concentration of hydrogen ions

  • low pH = high concentration of H+
  • high pH = low concentration of H+
• H+ ions can form temporary bonds with the 'R' groups of amino acids within the active site which then form temporary bonds with the substrate. so changing the pH can prevent the 'R' groups from bonding with the substrate so will reduce how effectively the substrate bonds to the active site - reduced rate of reaction
• each enzymes works fastest at a specific optimum pH

  • e.g. amylase (mouth) works best at pH 7
  • if the pH changes the rate of reaction decreases
• ionic bonds form between 2 charged 'R' groups

  • a change in the pH can break the bonds which are essential for the enzymes tertiary structure
  • changes the active site making it less likely that the substrate will attach successfully
  • if the pH changes too much the the active site may change so much and is no longer complementary. enzyme has denatured
• working out the pH of a solution - pH = log (H+)

  • log = button on calculator
  • H+ = concentration
• substrate concentration

  • low substrate concentration - low frequency of successful collisions between the substrate and active site. rate of reaction is low
  • high substrate concentration - high frequency of successful collisions between the substrate and active site. rate of reaction is high.
  • if there is too much substrate there comes a point where the rate of reaction stops increasing any further as the enzyme is working at its fastest rate (V max)
• enzyme concentration

  • low enzyme concentration - all the active sites will be colliding with the substrate molecules, so a large number number of substrates are unable to collide with the active sites as they are limited. the rate of reaction is low
  • high enzyme concentration - more enzyme so the frequency of successful collisions between the substrate and active site will increase. rate of reaction will increase
• if there is no free active site for the enzymes to collide with, the rate of reaction will not increase any further. the enzyme is saturated
• the substrate is not the only molecule that can bind to the active site

  • competitive inhibitor

  it is similar but not identical to the active site but because its not the substrate no reaction happens

  the molecule then leaves the active site
• by having the competitive inhibitor occupy the active site for a short period of time it prevents the actual substrate from binding to the active site

  • this lowers the rate of reaction
  • inhibits the reaction
• competitive inhibitors compete with the substrate for the active site
• you can reduce the effect of the competitive inhibitor by increasing the substrate concentration, this is because if there are more substrate molecules they are more likely to collide with the active site.
• competitive inhibitors

  • prevents the substrate from binding and forming an enzyme substrate complex
  • as more substrate concentration increases the less likely the competitive inhibitor is to bind to the active site

  
• some competitive inhibitors are irreversible, once it has entered the active site it won't leave

  however, some competitive inhibitors are temporary
• non competitive inhibitors do not bind to the active site of the enzyme

  • they bind to a different site (allosteric site) and this causes the tertiary structure of an enzyme to change
  • the shape of the active site changes so it is not complementary to the substrate
  • the substrate therefore cannot bind to form the enzyme substrate complex so the rate of reaction decreases
• non competitive inhibitor

  • not a similar structure to the active site
  • cannot be overcome by increasing the substrate concentration as non competitive inhibitors causes the shape of the active site to change
  • therefore the substrate cannot bind successfully with the active site