Hydroxymethylbilane

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Hydroxymethylbilane is an intermediate compound in the biosynthesis of heme, the iron-containing prosthetic group found in hemoglobin, myoglobin, and various cytochromes. It is also known as preuroporphyrinogen or HMB.

Biosynthesis

Hydroxymethylbilane is synthesized from four molecules of porphobilinogen through the action of the enzyme hydroxymethylbilane synthase (HMBS), also known as porphobilinogen deaminase. This enzyme catalyzes the head-to-tail condensation of porphobilinogen molecules to form the linear tetrapyrrole hydroxymethylbilane.

The reaction can be summarized as follows: 4 Porphobilinogen → Hydroxymethylbilane + 4 NH₃

Role in Heme Biosynthesis

Hydroxymethylbilane is a crucial intermediate in the heme biosynthetic pathway. After its formation, it is converted into uroporphyrinogen III by the enzyme uroporphyrinogen III synthase. This conversion is essential for the proper synthesis of heme and other tetrapyrrole compounds.

Clinical Significance

Deficiencies in the enzyme hydroxymethylbilane synthase can lead to a group of disorders known as acute intermittent porphyria (AIP). AIP is characterized by a buildup of porphobilinogen and other precursors, leading to a variety of symptoms including abdominal pain, neurological disturbances, and psychiatric symptoms.

See Also

• Heme
• Porphobilinogen
• Hydroxymethylbilane synthase
• Uroporphyrinogen III synthase
• Acute intermittent porphyria