Enzymes

• What are enzymes?

  • Enzymes combine with substrate molecules at the active site to produce a product.
  • Enzymes are tertiary proteins
  • Globular shape.
  • Biological catalysts that speed up the rate of metabolic reactions.
• What hypothesis explains how enzymes interact with substrates?

  Lock and key hypothesis
• What shape do enzymes have?

  Spherical globular shape
• How do enzymes achieve specificity?

  Each enzyme reacts with particular substrates
• What forms when a substrate binds to an enzyme active site?

  Enzyme-substrate complex

  
• What does the original lock and key hypothesis suggest?

  Exact fit between substrate and active site
• What type of reaction do anabolic and catabolic enzymes perform?

  • Anabolic: Build larger products from smaller substrates
  • Catabolic: Break large substrate molecules into smaller products
• What is the function of lysozyme?

  • To destroy pathogenic bacteria by breaking down their cell walls
  • Found within tears and other secretitons
• How does lysozyme break down bacterial cell walls?

  By breaking glycosidic bonds between the amino acid bonds chains found within that polysacaride cell wall
• What does the induced fit hypothesis suggest about enzymes?

  Active site changes shape to fit substrate
• What type of bonds maintain the 3D shape of enzymes?

  Disulphide bonds

  
• What are the properties of enzymes when reactions take place?

  • Enzymes are specific; each enzyme will catalyse only one particular reaction.
  • Enzymes are very efficient and have a high turnover number; this means that they can convert many molecules of substrate into product per unit time.
• What is activation energy for enzymes?

  • Chemical reactions need energy to start them off
  • The activation energy is the energy needed to break existing chemical bonds inside molecules.
  • In the body enzymes lower the activation energy of a reaction.
  • This reduces the input of energy needed to allow reactions to take place; which means they can take place at lower temperatures.
• How does temperature have an effect on enzymes?

  • Increase in temperature = molecules greater kinetic energy.
  • Enzyme and substrate molecules move more quickly, increasing collisions; forming more successful enzyme-substrate complexes.
  • Increasing the temperature of an enzyme controlled reaction results in an increase in rate of reaction (product is formed at an increased rate).
  • Rate of reaction doubles for each 10°C rise in temperature.
  • Continuing until the optimum temperature is reached.
  • For most enzymes the optimum temperature is 40°C.
• How does pH affect enzyme activity?

  • Small changes in pH (within this narrow range) can affect the rate of reaction without affecting enzyme structure.
  • Small changes outside the optimum range can cause reversible changes in enzyme structure; this results in inactivation.
  • Extremes of pH can denature an enzyme.

  
• What happens when substrate concentration increases?

  • Enzyme molecule can be re-used, so only a low enzyme concentration is needed to catalyse a large number of reactions.
  • The number of substrate molecules that one enzyme molecule can turn into products in a given time is called the turn-over number.

  
• What is the turnover number of catalase?

  • 40 million molecules per second
  • Catalase breaks down the highly toxic waste, hydrogen peroxide. As the enzyme concentration increases, there are more active sites available and therefore the rate of reaction increases.
• What is a competitive inhibitor?

  • Structurally similar to substrate, blocks active site
  • It can fit in the active site instead of the substrate molecule.
  • A competitive inhibitor prevents enzymesubstrate complexes forming
• How can the effect of a competitive inhibitor be reduced?

  • By increasing substrate concentration, the enzyme is more likely to collide with a substrate molecule and form a successful enzyme-substrate complex.
• What is a non-competitive inhibitor?

  • Do not bind to the active site; they bind to any other part of the enzyme (allosteric site).
  • Altering the overall shape of the enzyme molecule, including the active site.
  • The substrate molecule can no longer fit into the active site.
• What happens to enzyme activity with non-competitive inhibitors?

  • Increasing the substrate concentration will not increase the rate of reaction in this case as the substrate can no longer fit into the enzyme’s active site.
  • Successful enzyme-substrate complexes cannot form.
• What are the advantages of using immobilised enzymes?

  • No product contamination
  • Can be recovered and reused
  • Requires small enzyme quantity
  • Greater stability at higher temperatures
  • Catalyse reactions over wider pH range
  • Multiple enzymes can be used
  • Greater process control
  • Suitable for continuous processes
• What is the role of lactase in the dairy industry?

  Breaks down lactose into glucose and galactose

  
• How do biosensors work?

  • Biosensors detect biologically important molecules very rapidly, even at low concentrations.
  • Used to measure blood glucose concentration in individuals suffering from diabetes.
  • Use immobilised enzymes on a gel membrane.
  • Detects a chemical change, as substrate is converted to product, and a transducer converts this chemical change into an electrical signal which can be amplified and viewed on a display.
• What is the product formed when urea is detected by a biosensor?

  • The biosensor above detects urea molecules.
  • Small urea molecules diffuse across the partially permeable membrane and form enzyme-substrate complexes with immobilised urease.
  • The product formed is ammonium ions; the transducer converts this into an electrical signal.
  • The signal is amplified and reading is shown on the display.
• What is the function of the transducer in a biosensor?

  Converts chemical change into electrical signal
• How do enzymes react at lower temperatures?

  • At 25°C kinetic energy is low.
  • The enzyme and the substrate molecules collide less often.
  • Fewer successful enzymesubstrate complexes form.
  • The product is produced slowly.
  • Enzyme activity is low.
• How do enzymes react around optimum temperatures?

  • At 37°C kinetic energy is higher.
  • The enzyme and the substrate molecules collide more often.
  • More successful enzyme-substrate complexes form.
  • The product is produced more quickly (the curve is steeper between 0 and 20 minutes).
  • Enzyme activity levels off between 20 and 60 minutes as substrate concentration becomes a limiting factor (substrate molecules have been converted into product).
• What is the effect on enzymes at high temperatures?

  • At 60°C product is initially formed very quickly due to very high kinetic energy levels.
  • The enzymes quickly become denatured as vibrations break hydrogen bonds within the active site of the enzyme, causing the shape of the active site of the enzyme to change.
  • Less product is formed as successful enzyme-substrate complex cannot form.
  • Unconverted substrate molecules remain.
• How do enzymes work with pH?

  • To form an enzyme-substrate complex the charges on the amino acid side-chains of the active site must attract charges on the substrate molecule.
  • The charges of the enzyme’s active site are affected by free hydrogen (H+ ) and hydroxyl (OH- ) ions.
  • If, for example, there are too many H+ ions (too acidic) the active site and substrate may end up with the same charge.
  • The enzyme active site and substrate would repel one another.
• What happens when enzyme concentration remains constant?

  • The rate of reaction will increase as the substrate concentration increases.
  • The reaction will level off once all the active sites are occupied; the number of available active sites becomes a limiting factor at higher substrate concentrations.
• What are immobilised enzymes?

  • Immobilised enzymes are fixed, bound or trapped on an inert matrix. An example is alginate beads.
  • Enzymes can also be immobilised on a membrane.