4 - enzymes

• Enzymes are proteins that function as biological catalysts. They catalyse or speed up the rate of chemical reactions. They remain chemically unchanged at the end of the reaction
• A catalyst is a substance that can speed up a chemical reaction, without itself being chemically changed at the end of the reaction.

  • biological catalysts are produced by living cells
• activation energy refers to the energy that is needed to start a chemical reaction
• chemical reaction can be said as the process of rolling a huge stone up a hill so that it rolls down and breaks into tiny pieces.

  • activation energy is needed to roll the stone (reactant) up the hill. once over the hill, the rest of the reaction occurs. the stone (reactant) rolls down and breaks into tiny pieces (products)
• an enzyme speeds up the rate of a specific metabolic reaction by decreasing the activation energy required.
• enzymes catalyse digestion.

  • large, insoluble food substances need to be hydrolysed into small, soluble molecules, to pass through the plasma membrane of small intestinal cells (absorption)
• digestive enzymes include:

  • amylase - starch to maltose
  • maltase - maltose to glucose
  • protease - proteins to amino acids
  • lipase - fats to fatty acids and glycerol
• enzymes are used in all metabolic reactions that occur in cells. They can be categorised into:

  • anabolic
  • catabolic
• an anabolic reaction is the synthesis of complex molecules from simpler molecules

  • e.g. synthesis of proteins from amino acids
• a catabolic reaction is the breaking of complex molecules into simpler molecules

  • e.g. catalyse breaks down hydrogen peroxide into oxygen and water
• • maltase: maltose
  • carbohydrases: carbohydrates
  • proteases: proteins
  • lipases: lipids
• characteristics of enzymes

  • enzymes speed up chemical reactions
  • enzymes are required in minute amounts
  • enzymes are highly specific in action - each chemical reaction in a cell is catalysed by only one unique enzyme
• substrate: the substance on which enzymes act
• enzyme reactions depend on the presence of active sites

  • active sites are grooves on the surface of an enzyme molecule into which the substrate molecules with the matching shape can fit
• Lock-and-key

  • the enzyme is the lock, the substrate is the key
  • when a substrate binds to the active site, a temporary molecule, enzyme-substrate complex is formed
• • reactions take place at the active site to convert the substrate molecules into product molecules
  • the product molecules separate from the enzyme
  • the enzyme molecule remains unchanged and is free to combine again with more substrate molecules
• Lock-and-key

  1. an enzyme has a specific 3d shape which contains an active site
  2. only the substrate with a 3d shape complementary to that of the active site can fit into the enzyme to form an enzyme substrate complex
  3. chemical reaction occurs and the substrate is converted into products
  4. the products detach from the active site
• denaturation is the change in the 3d structure of an enzyme or any other soluble protein, caused by heat or chemicals such as acids or alkalis

  • when an enzyme is denatured, there is loss/ change in the active site
  • the substrate can no longer fit into the enzymes active site
  • no reaction can take place
• A limiting factor refers to a factor that directly affects the rate of a chemical reaction
• enzymes are affected by temperature and pH
• optimum temperature is when the enzyme is most active

  • for most human enzymes, optimum temperature is around 40-45 degrees celcius

  beyond optimum temperature, enzyme activity decreases rapidly

  • high temperatures break the bond within the enzyme and changes its 3d shape
  • active site of the enzyme loses its original shape
  • enzymes are now denatured
• enzymes are inactive at very low temperatures

  • at low temperatures, the kinetic energy of molecules is low
  • enzyme and substrate molecules move slowly
  • chances of substrate molecules colliding with enzymes are low

  as temperature increases, the enzyme activity increases

  • increase in temperature = increase in kinetic energy
  • increase in rate of effective collision
  • increased rate of formation of enzyme substrate complex
• enzymes are affected by pH

  • different enzymes have different optimum pHs, at which they are most active
  • extreme changes in pH denature the enzyme and causes it to lose function
• effect of substrate concentration

  • at a low concentration of the substrate, the enzyme occurs in excess, therefore all substrate molecules bind to active sites immediately
  • as substrate concentration increases, the frequency of effective collisions between the active site of enzymes and substrates increases.
  • at high concentration of substrate, the rate of enzyme reaction plateaus
  • all available active sites are occupied and engaged in the reaction. the enzymes are saturated
  • enzyme concentration is the limiting factor
• effect of enzyme concentration

  • at low enzyme concentrations, the rate of reaction increases as enzyme concentration increases
  • more active sites are available to bind with the substrate molecules, therefore the frequency of effective collisions increases
  • increase in formation of enzyme substrate complex = increase in products formed per unit time
  • as enzyme concentration increases, the rate of reaction plateaus
  • number of substrate molecules is fixed and there is an excess of enzyme molecules
  • substrate concentration is the limiting factor