enzymes

• what are enzymes

  • proteins that act as biological catalysts for intracellular and extracellular reactions to determine structure + function - they affect metabolism of cells and the whole organism
  • specific tertiary structure determines shape of active site
  • formation of ESC lowers activation energy of metabolic reactions
• catalase - intracellular

  • catalyses decomposition of hydrogen peroxide (which causes oxidative stres) into water and oxygen
  • h2o2 —> h2o + o2
• amylase + trypsin - extracellular

  • amylase catalyses digestion of starch —> maltose in saliva and small intestine lumen
  • trypsin catalyses hydrolysis of peptide bonds in small intestine lumen
• factors affecting rate of enzyme controlled reactions

  • enzyme concentration
  • substrate concentration
  • concentration of inhibitors
  • pH
  • temperature
• temperature coefficient

  Q10 = (rate of reaction at higher temperature) / (rate of reaction at lower temperature)

  • describes how much the rate of a reaction changes when the temperature increases by 10°C
• competitive inhibitors

  • bind to active site, since they have similar shape to substrate
  • prevent ESC from being formed
  • increasing substrate concentration decreases their effect
• non-competitive inhibitors

  • bind to allosteric binding site
  • triggers conformational change of the active site
  • increasing substrate concentration has no impact on their effect
• end product inhibitors

  • one of the products or an enzyme involved in the pathway acts as an inhibitor
  • prevents further formation of products
• anabolic reactions

  form biological molecules that make up living tissues, e.g production of collagen
• catabolic reactions

  break down biological molecules that provide energy, e.g in respiration
• activation energy

  lowest amount of energy needed for a reaction to occur, enzymes catalyse reactions by lowering the activation energy
• how does enzyme specificity arise

  active site has a unique shape that only matches certain substrates, so enzymes only catalyse one type of reaction
• apoenzyme

  inactive enzyme
• holoenzyme

  active enzyme + cofactor
• proenzyme

  if a precursor is activated by a change in tertiary structure
• irreversible inhibiton

  • permanently prevents formation of ESC
  • heavy metal ions eg. mercury cause disulphide bonds in tertiary structure to break
  • bind to enzymes by strong covalent bonds
• metabolic poison 


  • substance that damages cells by interfering with metabolic reactions
  • usually an inhibitor
• reversible inhibition

  • can be competitive/non-competitive
  • temporarily binds to enzymes e.g H-bonds
  • ESC can form after inhibitor is released
• examples of metabolic poisons

  cyanide - non-competitive, irreversible, inhibits cytochrome c oxidase

  malonate - competitive, reversible, inhibits succinate dehydrogenase

  arsenic - competitive, inhibits pyruvate dehydrogenase
• how do some medicinal drugs act as inhibitors

  • penicillin - non-competitive inhibitor of transpeptidase to prevent formation of murein cross links in bacterial cell wall
  • ritonavir - inhibits HIV protease to prevent of assembly of of new virions
• inactive precursors in metabolic pathways

  • to prevent damage to cells, some enzymes in mp are synthesised as inactive precursors eg protease
  • one part of the precursor acts as an inhibitor, ESC form when its removed
• cofactor

  non-protein compounds required

  for enzyme activity:

  • coenzymes
  • inorganic cofactors
  • prosthetic groups
• coenzymes

  • organic cofactors - do not bind permanently, often transport molecules or electrons between enzymes
  • frequently derived from water soluble vitamins eg the hydrogen acceptor NAD is derived from niacin
• inorganic cofactors

  1. facilitate temporary binding between substrate and enzyme
  2. often metal ions, eg. Cl- is the cofactor for amylase
• prosthetic groups

  tightly bound cofactors act as a permanent part of enzymes’ binding site, e.g Zn2+ for carbonic anhydrase
• Vmax

  maximum rate of reaction