Uroporphyrinogen III synthase (congenital erythropoietic porphyria)
Catalyzes cyclization of the linear tetrapyrrole, hydroxymethylbilane, to the macrocyclic uroporphyrinogen III, the branch point for the various sub-pathways leading to the wide diversity of porphyrins. Porphyrins act as cofactors for a multitude of enzymes that perform a variety of processes within the cell such as methionine synthesis (vitamin B12) or oxygen transport (heme).
Porphyrin metabolism; protoporphyrin-IX biosynthesis; coproporphyrinogen-III from 5-aminolevulinate: step 3/4.
Defects in UROS are the cause of congenital erythropoietic porphyria (CEP) [MIM:263700]; also known as Gunther disease. Porphyrias are inherited defects in the biosynthesis of heme, resulting in the accumulation and increased excretion of porphyrins or porphyrin precursors. They are classified as erythropoietic or hepatic, depending on whether the enzyme deficiency occurs in red blood cells or in the liver. The manifestations of CEP are heterogeneous, ranging from nonimmune hydrops fetalis due to severe hemolytic anemia in utero to milder, later onset forms, which have only skin lesions due to cutaneous photosensitivity in adult life. The deficiency in UROS activity results in the non-enzymatic conversion of hydroxymethylbilane (HMB) into the uroporphyrinogen-I isomer. Note=Severe congenital erythropoietic porphyria is associated with non-immune hydrops fetalis, a generalized edema of the fetus with fluid accumulation in the body cavities due to non-immune causes. Non-immune hydrops fetalis is not a diagnosis in itself but a symptom, a feature of many genetic disorders, and the end-stage of a wide variety of disorders.
Belongs to the uroporphyrinogen-III synthase family.