Enzymes

• What are enzymes?

  globular proteins that act as biological catalysts. They can be reused so are effective in small quantities and speed up reactions by lowering the activation energy
• What is activation energy?

  energy needed to start a chemical reaction
• What three things are needed for a reaction to take place?

  1) substrates must collide with the active site with sufficient kinetic energy to alter the arrangement of atoms

  2) energy of products must be lower than substrates

  3) activation energy
• How do enzymes speed up chemical reactions?

  they lower the activation energy so reactions can take place at lower temperatures. Thus, metabolic processes occur rapidly at human body temperature
• Describe the structure of an enzyme

  - specific 3D shape as a result of primary structure

  - have an active site which is the functional region of an enzyme made up of a small number of amino acids
• What is meant by the term 'substrate'?

  a molecule on which an enzyme reacts by fitting into the active site to form an enzyme-substrate complex
• Draw and label an enthalpy change diagram

  
• Describe the induced fit model of enzyme action.

  1) the active site forms as the enzyme and substrate interact

  2) the enzyme's general shape changes in the presence of a substrate

  3) enzyme is flexible and can mould itself around substrate

  4) after the products are formed, the enzyme resumes its original shape
• How does an enzyme lower the activation energy of a reaction?


  the enzyme puts strain on the substrate molecules, distorting the bonds in the substrate and lowering the activation energy required to break the bonds
• Describe the lock and key model of enzyme action

  the shape of the substrate is exactly complementary to the active site of the enzyme
• What is a limitation of the lock and key model of enzyme action?

  it suggests enzymes are rigid, which is not the case as other molecules can bind to sites other than the active site
• Name two ways to measure an enzyme-catalysed reaction

  -measure the formation of products

  -measure the disappearance of substrate
• How can you calculate the rate of change of a reaction from a graph?

  draw a tangent at a point and calculate its gradient
• Explain the effect of increasing temperature on the rate of reaction before optimum temperature

  kinetic energy of molecules increases= more frequent and effective collisions between enzyme and substrate= more enzyme-substrate complexes form= rate of reaction increases

  
• Why does the rate of reaction slow at high temperatures?

  hydrogen and other bonds in enzyme begin to break= active site changes shape= substrate fits less easily= fewer enzyme-substrate complexes formed= slower rate of reaction
• Define denaturation

  The irreversible, structural change in an enzyme that makes it unable to catalyse due to the substrate no longer fitting its active site
• Why is the human body temperature 37 degrees, whereas the optimum temperature of most enzymes is 40 degrees?

  -high temperatures increase metabolic rate but require more energy to maintain temperatures

  - some proteins may denature at higher temperatures

  -at high temperatures, any further increase (e.g. due to illness) might denature enzymesSee an expert-written answer!We have an expert-written solution to this problem!
• How can a change in pH affect the rate of reaction?

  -a change in pH alters the charges on amino acids that make up the active site. As a result, the substrate can no longer attach to the active site

  -a change in pH may cause bonds maintaining the enzyme's tertiary structure to break. The active site changes shape

  As a result, any change in pH will reduce the rate of reaction
• Why do enzymes work efficiently at low concentrations?

  they are not used up in chemical reactions
• Describe and explain the effect of increasing enzyme concentration on the rate of reaction

  A) if there is an excess of substrate, an increase in the amount of enzyme will lead to a proportionate increase in rate of reaction because there are more substrate than active sites can cope with

  B) if all substrate molecules can bind to the active sites at the same time, then rate of reaction is at its maximum as all active sites are filled

  C) if there are not enough substrates to fill all the active sites, then an increase in enzymes will have no effect on the rate of reaction as there are already enough active sites to accommodate all substrate molecules

  
• Why does the rate of reaction eventually level off?

  all available substrate is already being acted upon as rapidly as it can be by enzymes
• Describe and explain the effect of increasing substrate concentration on rate of reaction

  A) if amount of enzyme is fixed, an increase in substrate concentration is proportional to rate of reaction as active sites are available to be filled

  B) if all substrate molecules are being acted upon, the rate of reaction is at its maximum as the enzymes are working as rapidly as possible with all active sites filled

  C) if there is an excess of substrate, the rate of reaction will level off as all active sites are filled

  
• What are enzyme inhibitors?

  Substances that directly or indirectly interfere with the functioning of the active site of an enzyme and so reduce its activity
• What are competitive inhibitors?

  have a molecular shape similar to substrate and can occupy the active site of an enzyme, competing with the substrate for space in the active site
• What happens to the effect of the competitive inhibitor when substrate concentration increases?

  the effect of the competitive inhibitor is reduced. Because the inhibitor is not permanently bound, another molecule can take its place when it leaves. Eventually, all substrate molecules will occupy an active site. When inhibitor is bonded to active site, it block substrate from combining with the enzyme
• What are non-competitive inhibitors?

  attach to binding site on enzyme which is not the active site. This alters the shape of the enzyme and its active site, so substrate can no longer occupy it and enzyme can't function
• Does an increase in substrate concentration alter the effect of a non-competitive inhibitor?

  the substrate and inhibitor are not competing for the same site, so an increase in substrate concentration doesn't decrease the effect of the inhibitor
• What is a metabolic pathway?


  sequence of enzymatically catalysed chemical reactions in a cell
• Describe end-product inhibition

  end product increases above normal = greater inhibition of enzyme A= less end product produced= concentration of end product reduces to normal

  end product decreases below normal= less inhibition of enzyme A= more end product produced= concentration of end product increases to normal