2.5 Enzymes

• Enzyme: 

  • A globular protein that increases the rate of a biochemical reaction by lowering the activation energy threshold (i.e. a biological catalyst)
• Substrate:

  • Reactant in a biochemical reaction
• Polar regions of amino acids attract substrate and active site of the enzyme.
• Active site:

  • Region on the surface of an enzyme to which substrates bind and which catalyses the reaction.
• • Once a substrate has been locked into the active site, the reaction is catalysed
  • The products are released and the enzyme is used again
• Lock and Key Hypothesis:

  • Substrate and active site match each other in two ways:

  1. Structurally: The 3D structure of the active site is specific to the substrate. Substrates that don't fit won't react
  2. Chemically: Substrates that are not chemically attracted to the active site won't be able to react
• Induced fit model:

  • If the lock and key model were true, one enzyme would catalyse only one reaction
  • In actuality, some enzymes can catalyse multiple reactions
  • As the substrate approaches the enzyme, it induces a conformational change in the active site - it changes shape to fit the substrate
  • This stresses the substrate, reducing the activation energy of the reaction
• • The coming together of a substrate molecule and an active site is known as a collision
  • Collisions are the result of the random movements of both substrate and enzyme
• • The substrate may be at any angle to the active site when the collision occurs
  • Successful collisions are ones in which the substrate and active site happen to be correctly aligned to allow binding to take place
• • Most enzyme reactions occur when the substrates are dissolved in water
  • All molecules dissolved in water are in random motion, with each molecule moving separately
• • If not immobilized the enzyme can move too, however enzymes tend be larger than the substrate(s) and therefore move more slowly
• • The three-dimensional conformation of proteins is stabilised by bonds or interactions between R groups of amino acids within the molecule.
  • Most of these bonds and interactions are relatively weak and they can be disrupted or broken. This results in a change to the conformation of the protein, which is called denaturation.
  • A denatured protein does not normally return to its former structure – the denaturation is permanent. Soluble proteins often become insoluble and form a precipitate.
• • Heat can cause denaturation: vibrations within the molecule breaks intermolecular bonds or interactions.
  • Extremes of pH can cause denaturation: charges on R groups are changed, breaking ionic bonds within the protein or causing new ionic bonds to form.
• • After denaturation, the substrate can no longer bind to the active site of the enzyme
• • Changing the pH will alter the charge of the enzyme, which in turn will protein solubility and may change the shape of the molecule
  • Changing the shape or charge of the active site will diminish its ability to bind to the substrate, halting enzyme function
  • Enzymes have an optimum pH and moving outside of this range will always result in a diminished rate of reaction
  • Different enzymes may have a different optimum pH ranges
• • Increasing substrate concentration increases the rate of reaction
  • At the optimum concentration of substrate molecules, all active sites are full and working at maximum efficiency
  • Any increase in concentration beyond the optimum will have no added effect as there are no extra active sites to be used.
• • Low temperatures result in insufficient thermal energy for the activation of a given enzyme-catalysed reaction to be achieved
  • Increasing the temperature will increase the speed and motion of both enzyme and substrate, resulting in higher enzyme activity
  • This is because a higher kinetic energy will result in more frequent collisions between enzyme and substrate
• • At an optimal temperature (may differ for different enzymes), the rate of enzyme activity will be at its peak
  • Higher temperatures will cause enzyme stability to decrease, as the thermal energy disrupts the hydrogen bonds holding the enzyme together
  • This causes the enzyme (particularly the active site) to lose its shape, resulting in a loss of enzyme activity (denaturation)
• Enzymes are widely used in the food industry, e.g:

  • fruit juice, pectin to increase the juice yield from fruit
  • Fructose is used as a sweetener, it is converted from glucose by isomerase
  • Rennin is used to help in cheese production
• Detergents contain proteases and lipases to help breakdown protein and fat stains.
• Enzymes are used to breakdown the starch in grains into biofuels that can be combusted.
• In the textiles industry enzymes help in the processing of fibres, for example, polishing cloth to make it appear more shiny.
• Paper production uses enzymes to help in the pulping of wood.
• In the brewing industry enzymes help a number of processes including the clarification of the beer.
• In Medicine & Biotechnology enzymes are widely used in everything from diagnostic tests to contact lens cleaners to cutting DNA in genetic engineering.
• Advantages of enzyme immobilization:

  • Concentration of substrate can be increased as the enzyme is not dissolved – this increases the rate of reaction
  • Recycled enzymes can be used many times, immobilized enzymes are easy to separate from the reaction mixture, resulting in a cost saving.
  • Separation of the products is straight forward (this also means that the the reaction can stopped at the correct time).
  • Stability of the enzyme to changes in temperature and pH is increased reducing the rate of degradation, again resulting in a cost saving.
• Lactose intolerance:

  • Can cause allergies in some people
  • Often because they are unable to produce the enzyme lactase in sufficient quantities
  • Most people produce less lactase as they get older
  • In some regions such as europe, a mutation has allowed lactose production to continue into adulthood
  • The mutation is not present in people who are lactose intolerant
• Production of Lactose-free milk:

  • Lactase obtained from commonly from yeast (bacteria is an alternative)
  • Lactase is bound to the surface of alginate beads
  • Milk is passed (repeatedly) over the beads
  • The lactose is broken down into glucose and galactose
  • The immobilized enzyme remains to be used again and does not affect the quality of the lactose free milk
• Other uses of lactose free milk:

  • As a means to increase the sweetness of milk (glucose and galactose are sweeter in flavour), thus negating the need for artificial sweeteners
  • As a way of reducing the crystallisation of ice-creams (glucose and galactose are more soluble than lactose)
  • As a means of shortening the production time for yogurts or cheese (bacteria ferment glucose and galactose more readily than lactose)