enzymes

• Enzymes are biological catalysts that speed up the rate of reactions by lowering the EA of the reaction and are chemically unaltered at the end of the reaction & thus can be reused, and are effective in small amounts
• Enzymes are soluble in water as they are globular proteins with most of their hydrophilic amino acids on their external surface and most of their hydrophobic amino acids buried in their interior.
• eznymes exhibit specifity

  1. absolute specifity: cat specific rxn
  2. grp specifity: attacks 1 type of chem bond
• Contact residues: found in the active site – help to position the substrate in the correct orientation via H, I, HI

  2. Catalytic residues: found in the active site – have specific R groups which act on bonds in the substrate and help to catalyse the conversion of substrate to product

  3. Structural residues: interact with each other to maintain the overall 3D conformation of the protein
• Proximity effects: temporary binding of substrates in close proximity in the enzyme active site incrrases chance of a reaction

  2. Orientation effects: substrates are held by enzymes in their active sites in an orientation that will enable the bonds in substrates to be exposed to chemical attack

  3. Strain effects: slight distortion of substrates when they bind to the enzyme active site, strains bonds in substrates that need to be broken for products to form, increasing chance of beakage
• Temperature coefficient, Q10 = Rate of reaction at (x+10) /Rate of reaction at x
• Beyond the optimum temperature,

  ➔ kinetic energy of enzyme and substrate molecules continue to increases

  ➔ intramolecular vibrations increases

  ➔ H, I, HI between R groups that maintain the

  3D conformation of the enzyme are disrupted, specific conformation of active site is lost and the enzyme denatures

  ➔ substrate no longer complementary to the shape and charge of active site and cannot bind to it

  ➔ rate of enzyme-substrate complex formation decreases and rate of reaction decreases.
• *At optimum pH,

  ➔ conformation of enzyme active site is most ideal for substrate binding and rate of reaction is highest
• *As pH deviates from the optimum,

  ➔ excess H+ or OH- ions affects the ionisation of the R-groups of the amino acids residues as

  excess H+ results in –COO- groups becoming -COOH and excess -OH- results in -NH3

  + becoming –NH2

  ➔ thus ionic bonds and hydrogen bonds that maintain the conformation of the enzyme active site is

  disrupted and the enzyme denatures

  ➔ thus the interaction between substrate and catalytic residues in the active site of enzyme is disrupted

  ➔ thus the rate of enzyme-substrate complex formation decreases

  ➔ and rate of reaction/product formation decreases
• Michaelis constant (Km): substrate concentration at which reaction proceeds at half its max. rate

  ➔ low Km – high affinity between enzyme & substrate

  ➔ high Km – low affinity between enzyme & substrate
• Inhibitor/Activator binds to allosteric site of the enzyme. This results in conformational change in enzyme.

  • Binding of inhibitor stabilises enzyme in an inactive state

  • ➔ shifts curve to the right

  • Binding of activator stabilises enzyme in an active state

  ➔ shifts curve to the left
• non-comp Inhibitor binds to site other than active site

  ➔This results in a conformational change in the enzyme active site

  ➔Thus substrate can no longer bind to active site

  ➔rate of reaction decreases

  ➔Hence the inhibitor effectively decreases the available

  [enzyme] as it forms an inactive enzyme-inhibitor complex

  ➔Hence the effects of the inhibition cannot be overcome by increasing [substrate]
• Substrate binding stabilizes the enzyme in the active conformation and opposes the effect of the

  inhibitor. This allows Vmax to be

  reached at high substrate concentration.

  • Binding of substrate in allosteric enzymes exhibit cooperativity. Binding of a substrate to the first

  subunit, changes the conformation of the other subunits such that it is easier to accept subsequent substrates.

  Hence the rate against substrate concentration plot is sigmoidal.
• competitive:

  Vmax remains the same

  ➔ Km increases

  non-comp

  Vmax decreases

  ➔Km remains the same
• rate of rxn increases proportionally w substrate

  conc
• competitive inhi

  can be overcome by increasing [s] as increases chance of sub binding to AS instead of inhi binding