Uroporphyrinogen

Uroporphyrinogens are tetrapyrroles with four propionic acid groups and four acetic acid groups.

There are four forms, which vary based upon the arrangements of the groups ("P") and ("A"):
 * In the "I" variety, clockwise, the order repeats four times: AP-AP-AP-AP.
 * In the "III" variety, the fourth is reversed: AP-AP-AP-PA.
 * The "II" and "IV" varieties can be created synthetically, but do not appear in nature.

In the synthesis of porphyrin, uroporphyrinogen III is created by the enzyme Uroporphyrinogen III synthase, and the enzyme Uroporphyrinogen III decarboxylase converts it into coproporphyrinogen III.