Uroporphyrinogen I
Uroporphyrinogen I is an isomer of uroporphyrinogen III, a metabolic intermediate in the biosynthesis of heme. A type of porphyria is caused by production of uroporphyrinogen I instead of III.In living organisms, uroporphirinogen I occurs as a side branch of the main porphyrin synthesis pathway. In the normal pathway, the linear tetrapyrrole precursor preuroporphyrinogen is converted by the enzyme uroporphyrinogen-III cosynthase into the cyclic uroporphyrinogen III; which is then converted to coproporphyrinogen III on the way to porphyrins like heme. Uroporphyrinogen I is instead produced spontaneously from preouroporphyrinogen when the enzyme is not present.
The difference between the I and III forms is the arrangement of the four carboxyethyl groups and the four carboxymethyl groups. The non-enzymatic conversion to uroporphyrinogen I results in the sequence AP-AP-AP-AP, whereas the enzymatic conversion into uroporphyrinogen III lead to reversal of one AP-group and hence an AP-AP-AP-PA arrangement.
If synthesized, uroporphyrinogen I is then converted into coproporphyrinogen I by the same enzyme that acts on the III form; but that product, which is cytotoxic, then accumulates causing the pathology congenital erythropoietic porphyria.
