Porphobilinogen (PBG) is an organic compound that occurs in living organisms as an intermediate in the biosynthesis of porphyrins, which include critical substances like hemoglobin and chlorophyll.
The structure of the molecule can be described as molecule of pyrrole with sidechains substituted for hydrogen atoms at positions 2, 3 and 4 in the ring (1 being the nitrogen atom); respectively, an aminomethyl group , an acetic acid (carboxymethyl) group , and a propionic acid (carboxyethyl) group .
In a early step of the porphyrin biosynthesis pathway, porphobilinogen is generated from aminolevulinic acid (ALA) by the enzyme ALA dehydratase.
Next in the pathway towards porphyrins, four molecules of porphobilinogen are combined into hydroxymethyl bilane by the enzyme porphobilinogen deaminase:
Acute intermittent porphyria causes an increase in urinary porphobilinogen.