Enzymes

• Metabolism = all enzyme controlled chemical reactions in the body
• Anabolic reactions - building up molecules e.g. Protein synthesis
• Catabolic reactions - breaking down molecules e.g. digestion
• Enzyme structure & properties:

  • globular proteins
  • catalysts (biological)
• Exogonic = release energy
• Endergonic = take in energy (require)
• Enzymes act:

  • extracellular - some enzymes are secreted from cells by exocytosis and catalyse extra cellular reactions
  • intracellular, in solution - enzymes act in solution inside the cells
  • intracellular, membrane-bound - enzymes may be attached to membranes
• Lock and key model:

  • unique shape of active site means an enzyme can only catalyse one type of reaction
  • enzyme & substrate bind to form enzyme-substrate complexes
• Induced fit model:

  • enzymes shape slightly altered when binding to substrate, suggests it’s flexible and not rigid
  • enzymes shape alters slightly to accommodate substrate, strain on the substrate lowers the activation energy
• Factors affecting enzyme (T):

  • temperature - increases kinetic energy of enzyme and substrate molecules -> collide more successfully, with enough energy, more frequently -> increase rate of reaction
  • High temperature, molecules increasing in vibration breaks hydrogen bonds, changing tertiary + secondary structure -> alters shape of active site so substrate is no longer complimentary and no longer can form enzyme-substrate complexes -> enzyme denatures
  • Low temperature, enzyme inactive -> molecules have low kinetic energy
• Factors affecting enzyme activity:

  • Enzymes have an optimum pH where the rate of reaction is at its highest
  • Small pH changes cause a reversible change in enzyme activity, leading to a reduction in enzyme activity
  • Charges on the amino acid side-chains facilitate the binding of the substrate
• Low pH:

  • More acidic environment
  • Increased amount of H+ ions
  • H+ ions attracted to other negative charges in side-chains
  • Interferes with substrate binding to the active site
• High pH:

  • More alkaline environment
  • Increased amount of OH- ions
• Large shift in pH:

  • Disrupts ionic bonds and hydrogen bonds within the enzyme's tertiary and secondary structure
  • Overload of H+ and OH- ions
  • Alters the active site
  • Active site becomes non-complimentary to the substrate
  • Unable to form enzyme-substrate complexes
  • Denaturation of the enzyme occurs
• buffer ensures constant pH
• Enzyme inhibition = decrease in rate of an enzyme-controlled reaction by another molecule, inhibitor, binds with an enzyme to prevent enzyme-substrate complexes forming
• Competitive inhibitor:

  • has a similar shape to the substrate, complimentary to the active site
  • inhibits the substrate from binding and forming enzyme-substrate complexes
  • reduce the rate at which products are formed -> reduce rate of reaction
  • can be overcome by an increase in substrate concentration
• Factors affecting enzyme (S):

  • if constant, rate of reaction increases if substrate concentration increases
  • low substrate concentration, enzyme molecules have only a few substrate molecules to successfully collide with
  • high substrate concentration, active sites are filled -> increase rate of reaction (enzyme-substrate complexes formed)
  • substrate concentration is a limiting reactant
  • at critical condition, all active sites are used -> rate of reaction is at maximum
  • when active sites are full, enzyme is saturated -> line plateaus -> substrate concentration no longer limiting reactant
• Factors affecting enzymes (E):

  • enzymes can be reused once product leaves the active site
  • constant unless substrate is limited -> then plateaus
• competitive inhibitors are reversible, as if the substrate concentration increases, substrate more likely to bind to active site than inhibitor
• Non-competitive inhibitor = bind to the allosteric site, instead of the active site
• Non-competitive inhibitor:

  • inhibitor binds to allosteric site -> forms an enzyme-Inhibitor complex
  • cause conformational change to the enzymes active site -> substrate is no longer complimentary and able to bind -> unable to form enzyme-substrate complexes
• Whether non-competitive is reversible depends on the type of bonding
• Immobilised enzyme = a way of trapping enzymes within or binding enzymes to an inert matrix
• Ways to immobilise:

  • adsorption - fixed to a surface of an inert material by weak forces like hydrogen bonds
  • covalent bonding - covalently bound to an inert material
  • encapsulation - traps them in an inert matrix that forms a partially permeable membrane around them
  • cross linking - enzymes covalently bound to one another to form large clumps
• Advantages of immobilised enzyme:

  • products are not contaminated with the enzyme
  • enzymes are easily recovered for reuse
  • Increased stability
  • function over wider temperatures and pH, compared to free enzymes
  • enzymes can be easily added or removed -> greater control of reaction
  • more than one enzyme can be used at once