Enzymes

• What is an enzyme?

  An enzyme is a protein that acts as a catalyst in biological reactions, speeding up the rate of the reaction without being consumed in the process.
• Enzymes speed up the rate of chemical reactions in organisms by acting as catalysts (they do not get used up in the reaction).
• Enzymes catalyse specific metabolic reactions at a cellular level (for example respiration) and extracellularly, such as during digestion.
• Enzymes are proteins with a 3D structure, they have an active site which has a specific shape, that can only bind to a certain substrate.
• When an enzyme binds to a substrate, an enzyme-substrate complex is formed because the shape of the active site is complimentary to that substrate, and after the reaction has occurred, the products leave the active site and the enzyme goes on to make more enzyme-substrate complexes.
• How do enzymes speed up the rate of metabolic reactions?

  They lower the activation energy of the reaction
• Enzymes speed up the rate of metabolic reactions by lowering the activation energy of a reaction.
• Enzymes


  Speed up the rate of metabolic reactions by lowering the activation energy of a reaction
• Enzymes allow reactions to happen at a lower temperature


  Than they would without the enzyme being present
• How enzymes work


  • They either hold substrates close together, reducing repulsion and allowing them to bond more easily
  • They put more strain on the bonds of a substrate allowing them to break apart more easily
• What are the two main enzyme models?

  The lock and key model and the induced-fit model
• The lock and key model is the original model which explained how enzymes work
• The lock and key model states that the shape of an active site is exactly complimentary to the specific substrate molecule and the substrate fits in exactly when they collide, forming an enzyme-substrate complex
• The induced fit model is based on new molecular evidence that suggests that the shape of the active site of an enzyme is not exactly complimentary to the substrate molecule.
• In the induced-fit model, when the substrate collides with the active site, it can change shape slightly to fit around the substrate and form an enzyme-substrate complex.
• What is the difference between the lock and key model and the induced-fit model?

  Lock and key model states that there is complimentary binding between enzyme and substrate, whereas the Induced-fit model states that the enzyme changes shape to accommodate substrate.
• What factors affect enzyme action?

  Enzyme concentration, substrate concentration, temperature and pH
• An increasing enzyme concentration increases the number of active sites available for the substrate to collide with, therefore more enzyme-substrate complexes can form.
• How does increasing enzyme concentration affect the rate of reaction?

  It will increase until the amount of substrate becomes the limiting factor, as there are more enzymes than substrate, so increasing the enzyme concentration will have no more effect on the rate (levels off).
• What does this image show?

  How increasing enzyme concentration will increase the rate of reaction until the amount of substrate becomes the limiting factor (levels off).

  
• How would you calculate how fast the rate is changing on a graph?

  Gradient
• Increasing the substrate concentration will increase the rate of reaction as there are more substrate molecules, therefore Meaning more collisions occur and more enzyme-substrate complexes will form.
• Why does the rate of reaction begin to slow?

  The enzyme concentration becomes a limiting factor, when all of the enzyme active sites are occupied (reached saturation point), increasing the substrate concentration will have no further effect on the rate

  
• What effect does increasing temperature have on enzymes rate of reaction?

  Increases, this is because there is more kinetic energy, causing the molecules to move faster, and therefore increases the number of collisions and enzyme-substrate complexes formed.
• What is optimum temperature?

  The temperature at which an enzyme reaches its maximum rate of reaction
• Each enzyme has an optimum temperature, and once this has been reached, increasing the temperature further causes the rate of reaction to decrease
• Why does the graph decrease after 40 degrees?

  At high temperatures, the enzyme molecules vibrate too much, and the bonds maintaining the tertiary structure are broken, this causes the active site to change, and no more enzyme-substrate complexes can be formed. Therefore causing the rate of reaction to slow

  
• At high temperatures, the enzyme molecules vibrate too much, and the bonds maintaining the tertiary structure are broken, this causes the active site to change and thus meaning that no more enzyme-substrate complexes can be formed as the substrate no longer fits. This enzyme is permanently denatured, causing the reaction to stop.
• What is optimum pH?

  The pH at which enzymes reach their maximum reaction rate
• All enzymes have an optimum pH level, with most enzymes working best at pH 7. However some enzymes, such as pepsin in the stomach, work better at lower pH‘s as they function in acidic environments.
• Enzymes are proteins so if the pH changes from neutral (pH=7) then it will affect the ionisation of amino acids within the protein. If there is not enough H+ or OH- ions present, then the enzyme’s shape may alter, preventing it from functioning correctly. The enzyme becomes denatured and loses activity.
• If the pH increases above 8, the enzyme becomes overly alkaline, and the hydrogen bonding between the R groups breaks down, changing the shape of the enzyme. Again, the enzyme cannot bind to the substrate, and therefore the reaction stops.
• Increasing the concentration of the substrate increases the number of collisions per second between the substrate and the enzyme, leading to more successful collisions and hence a higher rate of reaction.
• If the pH increases above 8, the hydrogen atoms on the carboxyl group become negatively charged, repelling other negative charges around them. As a result, the enzyme unfolds and its shape alters, making it unable to bind to the substrate. It is now denatured and cannot catalyse any reactions.
• Denaturing an enzyme means that the enzyme has lost its ability to catalyse reactions because its three dimensional shape has been altered by heat, chemicals or other factors. It is irreversible.
• When the substrate concentration reaches saturation point, all available binding sites on the enzyme are occupied by substrates, and any further increase in substrate concentration has no effect on the rate of reaction.
• Above and below the optimum pH for each enzyme, the H+ ions and the OH- ions disrupt the ionic and hydrogen bonds holding the enzymes tertiary structure in place
• Why does the reaction rate decrease either side of the graph?

  Above and below the optimum pH, the H+ ions and the OH- ions disrupt the ionic and hydrogen bonds holding the enzymes tertiary structure in place, the active site can then change shape, meaning that substrates no longer fit and enzyme-substrate complexes cannot be formed

  
• At extremes of the pH spectrum, the active site changes shape and no more enzyme-substrate complexes can be formed as the substrate no longer fits. This enzyme is now permanently denatured, causing the reaction to stop.
• How do H+ ions affect the tertiary structure of enzymes?

  It interacts with the charges on the enzymes amino acids, causing them to become positively charged, meaning that they will repel each other and change/break the tertiary structure.