Effect of pH on enzyme action

• what is pH?

  • a measure of the acidity or alkalinity (GCSE)
  • the measure of hydrogen ion (H+ or proton) concentration
  • low pH - high H+ concentration
  • high pH - low H+ concentration
• what are acids and alkaline in context of protons?

  • alkalines are proton acceptors
  • acids are proton donors (they can dissociate in solution forming H+)
• how does pH affect bonds?

  • the presence (or absence) of H+ can affect the bonds which stabilise the secondary, tertiary and quaternary structure of protein
  • H+ will be attracted to the slightly negative atoms (O or N) in proteins, or the negatively charged amino acids in an ionic bond
• which bonds are affected by pH?

  • hydrogen
  • ionic
  • this has implications for protein structure and function
• what bonds are not affected by pH?

  • covalent bonds (peptide, disulphide)
• if an enzymes active site relies on amino acids with charged R-groups, suggest how changes in pH could result in the loss of enzyme function?

  • changing the pH will alter the charges around the active site (e.g. H+ will cluster around negatively charged R-groups), which might interfere with the binding of the substrate to the active site.
  • therefore can't form enzyme-substrate complexes
• is denaturation by changing pH reversible?

  • small changes in pH either side of the optimum pH do not denature enzymes as the bonds disrupted can reform if the pH returns to normal
  • an extreme change away from optimum pH will cause permanent denaturation as hydrogen and ionic bonds will be broken and may reform in the wrong place, changing overall tertiary structure
• what are the effects of acidity on enzyme structure?

  • there is a low pH, so a high H+ concentration
  • the enzyme will form more bonds externally rather than internally
  • can't change a covalent bond
  • negatively charged R-groups and O/N atoms are attracted to the H+
  • causes disruption of hydrogen bonds and ionic bonds within secondary and tertiary structure
• what are the effects of alkaline on enzyme structure?

  • high pH, low H+ concentration
  • forms too many internal H (or covalent) bonds
  • some bonds that did exist between H+ and negatively charged R-groups and O/N atoms are broken
  • this may cause additional hydrogen bonds and ionic bonds to form within the protein therefore disrupting secondary and tertiary structure
• why would pepsin and trypsin denature if they swapped locations?

  • they would denature because they have opposite pH environments - pepsin works in acidic (2) pH and would form too many internal hydrogen and covalent bonds, changing the tertiary structure, and trypsin works in alkaline (6-9) pH and would form too many external hydrogen bonds, changing the tertiary structure
  • they would be unable to carry out their function and form enzyme-substrate complexes
• what is the calculation for pH?

  pH = -log10 [H+]
• what is the calculation for H+?

  [H+] = 10 (to the power of -pH)
• what kind of a scale is pH?

  logarithmic scale
• what happens to the hydrogen ion concentration if the pH increases by 1?

  decreases by *10
• why happens to the hydrogen ion concentration if the pH decreases by 2?

  increases by *100
• what happens to the hydrogen ion concentration if the pH decreases by 3?

  increases by *1000
• what happens to the hydrogen ion concentration if the pH increases by 4?

  decreases by *10000