Porphobilinogen synthase (d-aminolevulinic acid dehydratase):

    1. Is the control step in heme synthesis
    2. Is inhibited by lead
    3. Requires pyridoxal (B6) phosphate for activity
    4. Is present in the cytosol
  1. A, B and C
  2. A and C
  3. B and D
  4. D only
  5. All of the above


    1. The key control enzyme is ALA synthase.
    2. This enzyme is very sensitive to lead. When it is inhibited its substrate, d-aminolevulinic acid, accumulates.
    3. ALA synthase requires vitamin B6, not this enzyme.
    4. This is the first of the cytoplasmic enzymes of heme synthesis.

Go to the next question