Formed directly by the condensation of porphobilinogens:

A. Uroporphyrinogen I
B. Uroporphyrin III
  1. A only
  2. B only
  3. Both A and B
  4. Neither A or B

Right!

Uroporphyrinogen I is the product of uroporphyrinogen I synthase. Uroporphyrinogen I does not give rise to heme. In the presence of uroporphyrinogen III cosynthase, an intermediate in the reaction rearranges, forming uroporphyrinogen III as the major product. Uroporphyrinogen III is an intermediate in heme synthesis. Uroporphyrin III is not.

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