Chapter 4: Enzymes

• Catalyst:

  • A substance that speeds up a chemical reaction
  • remains chemically unchanged at the end of the reaction
• What are Enzymes:

  1. an enzyme is a protein
  2. a biological catalyst
  3. speeds up chemical reactions by lowering the activation energy
  4. remains chemically unchanged at the end of the reaction
  5. Required in minute amounts
  6. can be reused
• Properties of Enzymes:

  1. speeds up chemical reaction
  2. required in minute amounts : enzymes remain unchanged at the end of a chemical reaction. Hence, only a small amount of enzymes required for a enzyme reaction
  3. Highly specific : enzymes have a specific active site with a specific shape.
  4. affected by temperature and pH
• Activation energy :

  The energy needed to start a chemical reaction
• Enzymes lower activation energy required for a specific chemical reaction
• Lock


  Enzyme
• Key


  Substrate
• Active site


  Site on the enzyme where the substrate binds
• The active site has a specific shape
• The substrate has a complementary shape to the active site
• Substrate binds to the active site


  Forms an enzyme-substrate complex
• Enzyme reaction


  1. Enzyme-substrate complex

  2. New products are formed

  3. New products leave active sites

  4. Enzymes remain chemically unchanged and are free to take part in another reaction
• Digestion :

  • Some food molecules are large and insoluble in water
  • These molecules cannot diffuse across the cell membrane
  • They have to be digested into smaller, simpler molecules in order to do so
• Amylase: digests starch to maltose

  Maltase : digests maltose to glucose

  Protease : digests proteins to amino acids

  Lipase : Digests fats to fatty acids and glycerol
• Reactions enzyme catalyse:

  Break down of complex substances. For example, breakdown of starch during digestion

  Build up of complex substances. For example, building proteins from amino acids
• Enzymes

  • Speed up chemical reactions
  • Required in minute amounts
  • Remain unchanged at the end of a chemical reaction
  • Highly specific
  • Affected by temperature
  • Affected by pH
• Enzymes are Highly specific

  Enzymes have a specific active site with a specific shape

  Lock and Key hypothesis explains the high specificity of enzymes
• Enzymes remain unchanged at the end of a chemical reaction
• Enzyme specificity:

  • one enzyme only reacts with a specific substrate
  • an enzyme is specific due to its three-dimensional shape
• Factors affecting enzyme function:

  1. Enzyme concentration
  2. substrate concentration
  3. Temperature
  4. pH
• Enzyme concentration:

  As enzyme increases = reaction rate increase

  more enzymes = more frequently collide with substrate

  Reaction rate levels off:

  substrate becomes limiting factor

  not all enzyme molecules can find substrate
• Substrate concentration:

  As substrate increase = reaction rate increase

  more substrate = more frequently collide with enzyme

  Reaction rate levels off:

  • all enzymes have active site engaged
  • enzyme is saturated (limiting factor)
  • maximum rate of reaction
• low substrate -> reaction rate directly proportional to substrate concentration

  increasing substrate concentration -> rate levels off because more active sites occupied

  with all active sites occupied, the rate reaches a maximum (curve is constant)
• Temperature :

  affected when there are changes in temperature

  temperature.

  Low - inactive

  Optimum - temperature when the activity is the highest. Enzyme is the most active

  High - denatured

  Denatured - shape of the active site is changed because bonds in the enzymes are broken
• Denaturation - High temperatures above optimum:

  • when an enzyme is denatured, there is loss or change in the active site
  • the substrate can no longer fit into enzyme active site.
  • Hence, no reaction can take place
• Denaturation:

  • change in 3D structure of enzyme caused by heat or chemicals
  • bonds in the enzyme are broken
  • when bonds are broken it will not work
• Enzymes affected by temp:

  • inactive at very low temperature
  • kinetic energy is low at low temperature. Enzymes and substrates move very slowly
  • Hence, number of collisions of substrates with enzymes are very low
  • as temperature rises, enzyme activity increases. Enzyme and substrate move faster
  • there is more successful collisions of enzymes and substrate
  • increases rate of formation of enzyme - substrate complex
• As temperature increases by 10 degree Celsius, the rate of reaction doubles
• high temperatures (after optimum temp):

  • at very high temperatures, enzyme activity decreases
  • high temperature breaks the bond within the enzyme and changes its 3D-shape
  • the active site of the enzyme loses its original shape
  • enzyme is now denatured
• Define the term 'enzyme'

  biological catalyst that lowers activation energy of a chemical reaction. Speeds up chemical reactions, chemically unchanged at the end of reaction