Porphobilinogen

Porphobilinogen (PBG) is a pyrrole-containing compound that plays a crucial role in the biosynthesis of heme, a component of hemoglobin, myoglobin, cytochrome c, and other heme-containing compounds. It is an intermediate in the synthesis of porphyrin, which is converted into protoporphyrin IX and then into heme.

Biosynthesis

Porphobilinogen is synthesized from aminolevulinic acid (ALA) through the action of the enzyme ALA dehydratase. Two molecules of ALA are condensed to form porphobilinogen. This reaction is the first common step in the biosynthesis of both chlorophyll and heme.

Role in Heme Synthesis

In the heme synthesis pathway, four molecules of porphobilinogen are sequentially condensed to form the tetrapyrrole hydroxymethylbilane (also known as preuroporphyrinogen) through the action of the enzyme porphobilinogen deaminase. Hydroxymethylbilane is then converted into uroporphyrinogen III, which is further processed to form protoporphyrin IX and ultimately heme.

Clinical Significance

Abnormalities in the metabolism of porphobilinogen can lead to a group of disorders known as porphyrias. These disorders are characterized by the accumulation of porphyrins or porphyrin precursors, such as porphobilinogen, in the body. Symptoms can range from skin sensitivity to sunlight to neurological complications.

One of the most common tests for acute porphyria is the Watson-Schwartz test, which detects the presence of porphobilinogen in urine. An elevated level of urinary porphobilinogen is indicative of an acute porphyria.

See Also

• Heme synthesis
• Porphyria
• Watson-Schwartz test