Uroporphyrinogen III

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Hydroxymethylbilan

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Uroporphyrinogen III

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Uroporphyrinogen-III-synthesis-from-pre-uroporphyrinogen

Uroporphyrinogen III is a crucial biochemical compound in the heme biosynthesis pathway, which is fundamental for the production of hemoglobin, myoglobin, and various other heme-containing enzymes. It is a tetrapyrrolic compound, part of the porphyrin family, which are characterized by their unique structure consisting of four modified pyrrole subunits interconnected at their α carbon atoms via methine bridges (=CH-).

Biosynthesis

Uroporphyrinogen III is synthesized from porphobilinogen through a series of enzymatic reactions. The first step involves the condensation of four molecules of porphobilinogen to form a linear tetrapyrrole, hydroxymethylbilane, catalyzed by the enzyme porphobilinogen deaminase. This linear tetrapyrrole is then cyclized by the enzyme uroporphyrinogen III synthase to form uroporphyrinogen III. This cyclization is crucial as it introduces asymmetry into the molecule, making it distinct from uroporphyrinogen I, which is symmetric and not used in heme biosynthesis.

Function

The primary function of uroporphyrinogen III is to serve as a precursor in the synthesis of heme. Following its formation, uroporphyrinogen III undergoes decarboxylation and modification steps to eventually form protoporphyrin IX, the immediate precursor to heme. Heme is an essential component of various proteins, including oxygen transport proteins like hemoglobin and myoglobin, and enzymes such as cytochrome c oxidase involved in cellular respiration.

Clinical Significance

Alterations in the biosynthesis of uroporphyrinogen III can lead to several types of porphyria, which are disorders resulting from the buildup of porphyrins or their precursors due to deficiencies in the enzymes of the heme biosynthesis pathway. For example, a deficiency in uroporphyrinogen III synthase activity can lead to congenital erythropoietic porphyria (CEP), a rare and severe form of porphyria characterized by photosensitivity, hemolytic anemia, and reddish-brown discoloration of the urine.

See Also

• Porphyrin
• Heme synthesis
• Porphyria
• Hemoglobin
• Myoglobin