2.4 - enzymes

• enzymes = 3D globular proteins that are biological catalysts, speeding up chemical reactions by lowering the activation energy
• active site = the part of the enzyme that binds to the substrate and catalyses the reaction
• substrate = the molecule that is changed into the product in a reaction
• amino acids and R groups form a specific shape for a specific substrate
• induced fit model = enzyme changes shape (conformational change) to fit the substrate and then reverts back
• competitive inhibitors

  • have a shape similar to the active site
  • binds to the active site, preventing the substrate from entering
  • increasing the substrate concentration reduces the effect of the inhibitor
• non competitive inhibitors

  • bind to the allosteric site ( not the active site)
  • causes a conformational change to the active site so the active site is no longer complementary to the substrate
  • increasing concentration has no effect on inhibition - max rate of reaction cannot be achieved
• co factors = bind to the allosteric site
• co enzymes = similar structure to the substrate, so they can bind to the active site
• intracellular = inside the cell e.g. lysosomes
• extracellular = outside the cell e.g. trypsin
• enzyme + substrate >>> ES complex >>> enzyme + products
• Apoenzyme (protein portion) + Cofactor = Holoenzyme
• effect of temperature:

  • increasing temp = more kinetic energy, increasing the frequency of collisions, resulting in an increase in the number of E-S complexes and therefore an increase in the rate of the reaction and more product formation
  • temperature past optimum = reaction rate decreases rapidly as the enzyme denatures and stops working - hydrogen, disulfide and ionic bonds are broken
  • decreasing temp = rate of reaction decreases due to low kinetic energy meaning fewer ES complexes are formed -below freezing point, enzymes are inactivate
• temperature coefficient (Q10)= measure of the rate of change of a biological or chemical system as a consequence of increasing the temperature by 10 °C.
• Q10 = rate of reaction at (x + 10)°C / rate of reaction at x°C
• rate of reaction = product/time
• effect of pH

  • Most enzymes function efficiently over a narrow pH range
  • having a pH above or below the optimum pH of an enzyme, denatures the enzyme due to disruption to the hydrogen and ionic bonds
  • Most enzymes have an optimum pH of about 7.
  • Some enzymes function in acidic or alkaline pH – e.g. digestive enzymes
• effect of substrate concentration

  • increasing substrate concentration increases collisions between S and E molecules occur more often
  • More ES complexes form – more product formed – reaction rate increases
  • Increasing substrate further – a point is reached where the rate reaches a maximum value
  • All the enzymes are forming ES complexes - all the active sites are occupied at all times
  • Any further increase in substrate will not have an effect on reaction rate because the enzyme concentration is the limiting factor
• effect of enzyme concentration

  • As enzymes increase, more active sites become available
  • More ES complexes are formed - increase in reaction rate
  • If enzymes increased further – all the active sites will be occupied by S molecules
  • The rate is will eventually decrease as the substrate is the limiting factor
• vitamins = organic co factors + co enzymes
• minerals = inorganic co factors
• allosteric site = a site on the enzyme that is not the active site