Question 7 of 13

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Porphobilinogen synthase (delta-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.

A. 1, 2 and 3

B. 1 and 3

C. 2 and 4

D. 4 only

E. All of the above

The correct answer is (C).

Comments:

1. The key control enzyme is ALA synthase.

2. This enzyme is very sensitive to lead. When it is inhibited its substrate, delta-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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Last modified 1/5/95