Enzymes

Cards (29)

  • Enzymes are proteins that act as biological catalysts for intra & extracellular reactions to determine structure & function
  • Enzymes affect the metabolism of cells & whole organisms
  • The specific tertiary structure of enzymes determines the shape of the active site, which is complementary to a specific substrate
  • Formation of enzyme-substrate (ES) complexes lowers the activation energy of metabolic reactions
  • Example of an enzyme that catalyses intracellular reactions: Catalase catalyses the decomposition of hydrogen peroxide into water + oxygen
  • Examples of enzymes that catalyse extracellular reactions:
    • Amylase: carbohydrase catalyses digestion of starch to maltose in saliva/small intestine lumen
    • Trypsin: pancreatic endopeptidase catalyses hydrolysis of peptide bonds in small intestine lumen
  • Induced fit model of enzyme action:
    • Shape of active site is not directly complementary to substrate & is flexible
    • Conformational change enables ES complexes to form when substrate adsorbs
    • This puts strain on substrate bonds, lowering activation energy
    • Bonds in enzyme-product complex are weak, so product desorbs
  • Lock and key model of enzyme action:
    • Active site has a rigid shape determined by tertiary structure, complementary to 1 substrate
    • Formation of ES complex lowers activation energy
    • Bonds in enzyme-product complex are weak, so product desorbs
  • Factors affecting the rate of enzyme-controlled reactions:
    • Enzyme concentration
    • Substrate concentration
    • Concentration of inhibitors
    • pH
    • Temperature
  • How substrate concentration affects rate of reaction:
    • Rate increases proportionally to substrate concentration until the maximum number of ES complexes form
  • How enzyme concentration affects rate of reaction:
    • Rate increases proportionally to enzyme concentration until the maximum number of ES complexes form
  • How temperature affects the rate of enzyme-controlled reactions:
    • Rate increases as kinetic energy increases & peaks at optimum temperature
    • Above optimum, denaturation occurs as ionic & H-bonds in structure break
  • Temperature coefficient (Q10):
    • Measures the change in rate of reaction per 10°C temperature increase
    • Q10 = R2 / R1 (where R represents rate)
  • How pH affects rate of reaction:
    • Enzymes have a narrow optimum pH range
    • Outside the range, denaturation occurs as H+ / OH- ions interact with H-bonds & ionic bonds in structure
  • Competitive inhibitors:
    • Bind to the active site, temporarily preventing ES complexes from forming
    • Increasing substrate concentration decreases their effect
  • Non-competitive inhibitors:
    • Bind at allosteric binding site, triggering a conformational change of the active site
    • Increasing substrate concentration has no impact on their effect
  • End-product inhibition:
    • One of the products of a reaction acts as a competitive or non-competitive inhibitor for an enzyme involved in the pathway, preventing further formation of products
  • Irreversible inhibitors:
    • Permanently prevent formation of ES complexes
    • Heavy metal ions e.g. mercury, silver cause disulphide bonds in tertiary structure to break
  • Reversible inhibitors:
    • May be competitive or non-competitive
    • Bind to enzyme temporarily, allowing ES complexes to form after the inhibitor is released
  • Metabolic poison:
    • Substance that damages cells by interfering with metabolic reactions, usually an inhibitor
  • Examples of metabolic poisons:
    • Cyanide: non-competitive, irreversible, inhibits cytochrome c oxidase
    • Malonate: competitive, inhibits succinate dehydrogenase
    • Arsenic: competitive, inhibits pyruvate dehydrogenase
  • How some medicinal drugs act as inhibitors:
    • Penicillin: non-competitive inhibitor of transpeptidase
    • Ritonavir: inhibits HIV protease
  • Inactive precursors in metabolic pathways:
    • Some enzymes in metabolic pathways are synthesised as inactive precursors to prevent damage to cells
    • One part of the precursor acts as an inhibitor, and ES complexes form when it is removed
  • Cofactors:
    • Non-protein compounds required for enzyme activity
    • Include coenzymes, inorganic cofactors, and prosthetic groups
  • Coenzymes:
    • Organic cofactors that do not bind permanently
    • Often transport molecules or electrons between enzymes
  • Inorganic cofactors:
    • Facilitate temporary binding between substrate and enzyme
    • Often metal ions e.g. Cl- is the cofactor for amylase
  • Prosthetic groups:
    • Tightly-bound cofactors that act as a permanent part of the enzyme’s binding site
    • Example: Zn2+ for carbonic anhydrase
  • Producing a desired concentration of solution from a stock solution:
    • Volume of stock solution = required concentration x final volume needed concentration of stock solution
    • Volume of distilled water = final volume needed - volume of stock solution
  • what is the temperature coefficient?
    The temperature coefficient for a biological reaction is the ratio between the rates of that reaction at two different temperatures