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Cells & Molecules > Enzymes
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  • All enzymes are globular proteins and round in shape
  • They have the suffix "-ase"
  • Intracellular enzymes are found inside the cell
  • Extracellular enzymes act outside the cell (e.g. digestive enzymes)
  • Enzymes are catalysts → speed up chemical reactions
    • Reduce activation energy required to start a reaction between molecules
    • Substrates (reactants) are converted into products
    • Reaction may not take place in absence of enzymes (each enzyme has a specific catalytic action)
    • Enzymes catalyse a reaction at max. rate at an optimum state
  • Induced fit theory
    • Enzyme's shape changes when substrate binds to active site
    • Amino acids are moulded into a precise form to perform catalytic reaction effectively
    • Enzyme wraps around substrate to distort it
    • Forms an enzyme-substrate complex → fast reaction
    • E + S → ES → P + E
  • Enzyme is not used up in the reaction (unlike substrates)

Changes in pH

  • Affect attraction between substrate and enzyme and therefore efficiency of conversion process
  • Ionic bonds can break and change shape / enzyme is denatured
  • Charges on amino acids can change, ES complex cannot form
  • Optimum pH
    • pH 7 for intracellular enzymes
    • Acidic range (pH 1-6) in the stomach for digestive enzymes (pepsin)
    • Alkaline range (pH 8-14) in oral cavities (amylase)
  • pH measures the conc. of H+ ions - higher conc. will give a lower pH

Enzyme Conc. is proportional to rate of reaction, provided other conditions are constant. Straight line
Substrate Conc. is proportional to rate of reaction until there are more substrates than enzymes present. Curve becomes constant.

Increased Temperature

  • Increases speed of molecular movement → chances of molecular collisions → more ES complexes
  • At 0-42 °C rate of reaction is proportional to temp
  • Enzymes have optimum temp. for their action (varies between different enzymes)
  • Above ≈42°C, enzyme is denatured due to heavy vibration that break -H bonds
    • Shape is changed / active site can't be used anymore

Decreased Temperature

  • Enzymes become less and less active, due to reductions in speed of molecular movement
  • Below freezing point
    • Inactivated, not denatured
    • Regain their function when returning to normal temperature
  • Thermophilic: heat-loving
  • Hyperthermophilic: organisms are not able to grow below +70°C
  • Psychrophiles: cold-loving


  • Slow down rate of reaction of enzyme when necessary (e.g. when temp is too high)
  • Molecule present in highest conc. is most likely to form an ES-complex
  • Competitive Inhibitors
    • Compete with substrate for active site
    • Shape similar to substrates / prevents access when bonded
    • Can slow down a metabolic pathway
  • [EXAMPLE] Methanol Poisoning
    • Methanol CH3OH is a competitive inhibitor
    • CH3OH can bind to dehydrogenase whose true substrate is C2H5OH
    • A person who has accidentally swallowed methanol is treated by being given large doses of C2H5OH
    • C2H5OH competes with CH3OH for the active site
  • Non-competitive Inhibitors
    • Chemical does not have to resemble the substrate
    • Binds to enzyme other than at active site
    • This changes the enzyme's active site and prevents access to it
  • Irreversible Inhibition
    • Chemical permanently binds to the enzyme or massively denatures the enzyme
    • Nerve gas permanently blocks pathways involved in nerve message transmission, resulting in death
    • Penicillin, the first of "wonder drug" antibiotics, permanently blocks pathways certain bacteria use to assemble their cell wall component (peptidoglycan)

End-product inhibition

  • Metabolic reactions are multi-stepped, each controlled by a single enzyme
  • End-products accumulate within the cell and stop the reaction when sufficient product is made
  • This is achieved by non-competitive inhibition by the end-product
  • The enzyme early in the reaction pathway is inhibited by the end-product

The metabolic pathway contains a series of individual chemical reactions that combine to perform one or more important functions. The product of one reaction in a pathway serves as the substrate for the following reaction.