Porphobilinogen synthase
d-aminolevulinic acid dehydratase):

A. Is the control step in heme synthesis
B. Is inhibited by lead
C. Requires pyridoxal (B6) phosphate for activity
D. 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


A. The key control enzyme is ALA synthase.
B. This enzyme is very sensitive to lead.
When it is inhibited its substrate,
d-aminolevulinic acid,
C. ALA synthase requires vitamin B6, not this enzyme.
D. This is the first of the cytoplasmic enzymes
of heme synthesis.

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