Enzymes

• role of enzymes

  biological catalysts
• intracellular enzymes

  catalase which catalyses the decomposition of hydrogen peroxide into into water and oxygen
• extracellular enzymes

  amylase which catalyses the digestion of starch into maltose

  trypsin which catalyses the hydrolysis of peptide bonds
• induced fit model

  shape of the active site is not completely complimentary to the shape of the substrate and instead conformational change once the substrate has bound to the active site to form an enzyme - substrate complex which then strains the substrate bonds lowering the activation energy so a enzyme - product complex is produced
• lock and key model

  active site has a rigid shape determined by its tertiary structure so it is only complimentary to one substrate

  the formation of the enzyme - substrate complex lowers the activation energy and the bonds in the enzyme - product complex are weak causing it to be desorbed
• factors affecting enzyme rate

  temperature

  ph

  substrate concentration

  inhibitor concentration

  enzyme concentration
• affect of substrate concentration

  increases the number of enzyme - substrate complexes formed

  rate of reaction increases

  until enzyme concentration becomes limiting

  
• affect of enzyme concentration

  increases the number of active sites

  more enzyme - substrate complexes form

  rate of reaction increases

  until substrate concentration becomes limiting

  
• affect of temperature

  kinetic energy increases

  rate increases

  above optimum -> ionic and hydrogen bonds in the tertiary structure break so the active site is no longer complimentary to the substrate causing it to denature

  
• temperature coefficient

  measures the rate of change in a reaction per 10 degrees temperature increase

  Q10 = rate of reaction at T + 10 degrees / rate of reaction at T
• affect of ph

  pepsin has a low optimum ph

  amylase has a higher optimum ph

  above and below the optimum, rate of reaction decreases as ionic and hydrogen bonds break due to the interaction of H+ and OH- ions with the tertiary structure of an enzyme
• competitive inhibitors

  bind to active sites as they have similar shapes to the substrate

  they temporarily prevent enzyme - substrate complexes from being formed until they are released from the active site

  increasing substrate concentration reduces their effect
• non-competitive inhibitors

  bind at the allosteric site

  trigger a conformational change of the active site

  increasing substrate concentration has no affect on the rate as the active site is no longer complimentary to them
• end product inhibition

  one of the products of a reaction acts as a competitive or non-competitive inhibitor for an enzyme involved in a pathway

  prevents further formation of products
• irreversible inhibitors

  permanently prevent formation of enzyme - substrate complexes

  heavy metal ions such as mercury and silver cause disulfide bonds in the tertiary structure to break

  they bond to enzymes by strong covalent bonding
• reversible inhibitors

  can be either competitive or non-competitive

  bind to enzyme temporarily by hydrogen bonding

  enzyme - substrate complexes can be formed after the inhibitor has been released
• metabolic poison

  substances that damage cells by interfering with metabolic reactions
• metabolic poison examples

  cyanide -> non-competitive and inhibits cytochrome c oxidase

  malonate -> competitive and inhibits succinate dehydrogenase

  arsenic -> competitive and inhibits pyruvate dehydrogenase
• medical drugs as inhibitors

  penicillin -> non-competitive inhibitor of transpeptidase to prevent peptidoglycan cross links in bacteria cell walls
• inactive persecutors

  non-working enzymes that are synthesised to prevent damage to cells

  example: persecutors are only removed from an inactive protease when it is needed to be activated
• cofactors

  non-protein compounds required for enzyme activity

  example: coenzymes, inorganic cofactors, prosthetic groups
• coenzymes

  organic cofactors

  do not bind permanently

  example: NAD and FAD and CoA

  derived from water soluble vitamins
• inorganic cofactors

  facilitate the temporary binding of an enzyme and a substrate

  for example: Cl- is the cofactor for amylase
• prosthetic groups

  tightly bound cofactors that are permanently apart of an enzyme's binding site

  for example: Zn 2+ for carbonic anhydrase
• desired concentration from a stock concentration

  volume of stock conc = ( required conc x final volume ) / conc of stock solution

  volume of distilled water = final volume needed - volume of stock solution