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

Right

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.

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