Enzymes

• enzymes

  Enzymes lower the amount of activation energy that's needed, often making reactions happen at lower temperatures, speeding up the rate of reaction.
• When an enzyme fits into the enzymes active site it forms an enzyme substrate complex - this lowers the activation energy because 

  • If two substrate molecules need to be joined, being attached to the enzyme holds them close together, reducing any repulsion between the molecules so they can bond more easily.
  • If the enzymes catalyse a breakdown reaction, fitting into the active site puts a strain on bonds in the substrate , so the substrate breaks up more easily.
• Models of enzyme action:

  Lock and key model:

   where the substrate fits into the enzyme in the same way a key fits into a lock - the active site and substrate have a complementary shape. 
• Induced fit:

  As substrates bind to the active site, it changes shape slightly.

   helps explain why enzymes are so specific and only bond to one particular substrate - the substrate does not only have to be the right shape to fit the active site, it has to make the active side change shape in the right way as well.
• Enzyme properties (related to their tertiary structure)

  • Enzymes are very specific, they only usually catalyse one reaction, this is because only a complementary substrate will fit into the active site. The active site's shape is determined by the enzyme's tertiary structure (which is determined by the enzyme's primary structure). Each different enzyme has a different tertiary structure so a different shaped active site,
• If the tertiary structure of a protein is altered in any way ( changes in pH or temp) , the shape of the active site will change, this means the substrate wont fit into the active site and an enzyme substrate complex won't be formed and no longer be able to carry out its function.
• The primary structure of a protein is determined by a gene, if a mutation occurs in that gene it can change the tertiary structure of the enzyme produced.
• Factors affecting enzyme activity:

  Temperature: 

  The rate of an enzyme controlled reaction increases when the temperature is increased. More heat means more kinetic energy as molecules move faster. This makes substrate molecules more likely to collide with enzyme active sites, the energy of these collisions also increases which means each collision is likely to result in a reaction.
• • If the temperature gets too high, the reaction stops. The rise in temperature makes the enzyme molecules vibrate more, if the temperature goes above a certain level, the vibration breaks some of the bonds that keep the enzyme in shape. Active site changes shape and substrate no longer fit together = enzyme has denatured.
• change in pH:

  • Most human enzymes work best at pH 7 (neutral), exceptions like pepsin work best at pH 2 (acidic) useful as it is found in the stomach. Above and below optimum pH, the H+ and OH- ions found in acids and alkalis can disrupt the ionic and hydrogen bonds that hold the enzyme's tertiary structure in place. Enzymes are denatured and active sites change shape.
• increase substrate concentration:

  • The higher the substrate concentration, the faster the reaction, more substrate molecules mean a collision between a substrate and enzyme is more likely and so more active sites will be occupied. This is only true up until a “saturation point”, after that the enzymes have about as much as they can cope with ( all active sites are full).
• increase enzyme concentration

   the more enzyme molecules there are in a solution, the more likely a substrate molecule is to collide with one and form an enzyme-substrate complex.
• Enzyme activity can be prevented by enzyme inhibitors - molecules that bind to the enzyme that they inhibit.

  • Competitive inhibitor
  • Non - competitive inhibitor
• Competitive inhibitor:

  • have a similar shape to that of substrate molecules. They compete with substrate molecules to bind to the active site, but no reaction takes place; instead they block the active site so no substrate molecules can fit in it.
• Competitive inhibitor preventing inhibition

  • How much enzyme is inhibited is dependent on relative concentrations of the inhibitor and substrate. High concentration of the inhibitor takes up nearly all of the active site, and hardly any substrate will get to the enzyme. A higher concentration of substrate, then the substrate's chances of getting to an active site before the inhibitor increases. So increasing the concentration of substrate will increase the rate of reaction.
• Non competitive inhibitor: 

  • These bind to the enzyme away from the active site, this causes the active site to change shape so the substrate molecules can no longer bind to it. As non comp have a different shape, increasing the concentration of substrate wont prevent enzyme inhibition.