In heme biosynthesis, joining of four porphobilinogen molecules creates a uroporphyrinogen molecule. There are two possible forms the uroporphyrinogen can have. Uroporphyrinogen III is an asymmetric compound. It arises from action of uroporphyrinogen I synthase AND urophophyrinogen III cosynthase. If only the first enzyme is active, the symmetric compound, uroporphyrinogen I is produced instead. In the hereditary condition called congenital erythropoietic porphyria, the urophophyrinogen III cosynthase is defective and the symmetric type I porphyrins accumulate, causing the urine to turn red, the skin to become photosensitive, and the teeth to become fluorescent. Since insufficient heme is produced, anemia results.

1. Porphyrin and Chlorophyll Metabolism

2. Photosynthetic Pigments