Hydroxymethylbilane

Hydroxymethylbilane (HMB) is a biochemical compound that plays a crucial role in the heme biosynthesis pathway. It is an intermediate compound formed during the synthesis of porphobilinogen to heme, a vital component of hemoglobin, myoglobin, and various cytochromes.

Structure and Properties

File:Hydroxymethylbilane.png

Chemical structure of hydroxymethylbilane

Hydroxymethylbilane is a linear tetrapyrrole molecule, consisting of four pyrrole rings linked by methine bridges (-CH=). Each pyrrole ring carries a hydroxymethyl group (-CH2OH) at its alpha position. The molecule is highly polar due to the presence of multiple hydroxyl (-OH) groups, which also makes it soluble in water.

Biosynthesis

Hydroxymethylbilane is synthesized from porphobilinogen by the enzyme porphobilinogen deaminase (PBGD), also known as hydroxymethylbilane synthase. This reaction involves the stepwise condensation of four molecules of porphobilinogen, with the loss of four molecules of ammonia.

Role in Heme Biosynthesis

In the heme biosynthesis pathway, hydroxymethylbilane is converted into uroporphyrinogen III by the enzyme uroporphyrinogen III synthase. This is a crucial step as uroporphyrinogen III is the first cyclic compound in the pathway, which is further converted into coproporphyrinogen III, and eventually into heme.

Clinical Significance

Deficiency of the enzyme PBGD leads to the accumulation of porphobilinogen and hydroxymethylbilane, causing a group of disorders known as porphyria. The most common type associated with PBGD deficiency is acute intermittent porphyria (AIP), characterized by neurovisceral attacks and increased excretion of porphobilinogen and hydroxymethylbilane in urine.

See Also

• Heme
• Porphyrin
• Porphobilinogen deaminase
• Acute intermittent porphyria

References

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