4) Enzymes

• enzymes

  • biological catalyst which speeds up metabolic reactions in living organisms
  • do not get used up in the reaction

  
• How is the activation energy lowered:

  • if two substrate molecules need to be joined: the enzyme holds them together so that they can make a bond more easily
  • if the enzyme is catalysing a breakdown: fitting into the active site puts a strain on the bonds of the substrate so that they can be broken more easily
• Lock and key Model

  1. substrate binds to enzyme forming an enzyme substrate complex
  2. The binding of the substrate and enzyme places stress of the glucose-fructose bond and breaks bond
  3. Products are released and the enzyme is free to bind to another structure
• Intracellular enzyme

  • made and retained inside the cell

  catalase - breaks down the hydrogen peroxide into water and oxygen
• Extracellular enzyme - act outside the cell that produce and secrete them

  • amylase works outside of cells in saliva to catalyse the hydrolysis of starch into maltose
  • trypsin - produced by the pancreas into the small intestine to break down proteins into smaller polypeptides
• Structure of enzyme:

  globular proteins

  • 3D shape
  • soluble in water

  contain active site

  • specific shape
  • where substrate binds to
• Induced fit model:

  • active site is not complementary, the active site changes shape, so that the substrate can fit
• induced fit vs lock and key

  • active site does not change shape around the substrate
  • already fits the substrate (complementary before binding)
• Enzymes and temperature:

  1. as temperature increases, so does kinetic energy and he number of successful collisions increasing the rate of reaction
  2. optimum temperature, where enzymes activity is at its highest
  3. temperature is too high, vibration break bonds (hydrogen) that hold the enzyme together. Enzymes denature (irreversible change to shape of active site)
• Temperature coefficient Q10:

  The Q10 value for a reaction shows how much the rate of reaction changes when the temperature is raised by 10 degrees

  at temperatures before the optimum Q10 value of 2 means that a rate doubles every 10 degrees increase
• Q10 = rate at higher temperature/rate at lower temperature
• enzymes and pH:

  1. this is optimum pH
  2. as you move away from optimum pH, enzymes start to denature and enzyme activity decreases

  different enzymes have different pH's

  
• Enzyme concentration:

  as long as substrate remains in excess, increased concentration does not affect rate of reaction

  • as enzyme conc increases, rate of reaction increases so more enzyme-substrate complex are formed
  • as enzyme conce increases further, graph plateaus as all substrate being used
  • substrate conc is limiting factor
• Substrate concentration:

  1. as substrate concentration increases the number of collisions increases , this means the rate of reaction increases
  2. active site become occupied and graph plateaus (V-max)
  3. enzyme conc is the limiting factor

  
• co-enzyme - organic groups

  co-factos - inorganic groups
• co-factors:

  • some enzymes will only work if there is another non-protein structure bound to them called a co-factor.
  • help the enzyme and substrate bind together but do not directly participate in the reaction
  • they are not used up or changed in any way

  
• Co-enzymes:

  • participate in the reaction and are changed by it
  • often act as carriers moving chemical groups between enzymes
  • they are continually recycled during the process

  
• Reversible inhibition

  • weaker hydrogen bonds or weak ionic bonds
  • inhibitor can be removed
• Non-reversible inhibition

  • strong covalent bonds
  • the inhibitor cannot be removed easily
• Competitive inhibitors:

  • have a similar shape to the substrate
  • bind to the active site but no reaction takes places, instead active site is blocked

  
• Non-competitive inhibitors:

  • bind to a site other than the active site (allosteric site)
  • causes the active site to change shape so that the substrate molecules can no longer fit

  
• comparing inhibitors

  no inhibitor:

  1. not all active site is taken up. Increase substrate concentration = increase rate of reaction
  2. every active sit is saturated. All substrate in active site. = Vmax
  3. Excess substrate. no increase in rate of reaction.
• comparing inhibitors

  competitive inhibitor:

  1. A competitive inhibitor binds to the enzyme’s active site, and slows down the process. 
  2. The substrate will soon find an active site. The effect of a competitive inhibitor is to slow down the process.
• Comparing inhibitors

  non-competitive inhibitors:

  1. The shape of the active site changes because the non-competitive inhibitor has bound to an allosteric site; preventing an enzyme substrate complex from forming = less products formed
  2. The reaction rate cannot reach the same point because the non-competitive inhibitors have changed the shape of some of the active sites.