Overview The Sites of Heme Biosynthesis are: 1.The liver, which synthesizes a number of heme proteins (particularly cytochrome P450). 2.The bone marrow, by the erythrocyte-producing cells which are active in hemoglobin synthesis. The rate of heme synthesis in the liver is highly variable responding to changes in cellular heme pool. Heme synthesis in erythroid cells is relatively constant, and is matched to the rate of globin synthesis.

Heme synthesis The initial reaction and the last three steps in the formation of porphyrins occur in mitochondria. The intermediate steps of the biosynthetic pathway occur in the cytosol.

Detailed Reactions 1. Formation of δ-aminolevulinic acid (ALA) Glycine and succinyl CoA condense to form ALA in a reaction catalyzed by ALA synthase. This reaction requires pyridoxal phosphate as a coenzyme, and is the rate- controlling step in hepatic porphyrin biosynthesis. This reaction takes place in the mitochondrion.

Inhibition of ALA Synthase by Hemin (End Product) When porphyrin production exceeds the availability of globin, heme accumulates and is converted to hemin by the oxidation of Fe 2+ to Fe 3+. Hemin decreases the activity of hepatic ALA synthase, by decreasing the synthesis of the enzyme.

Administration of a large number of drugs leads to an increase in the ALA synthase activity These drugs are metabolized by the microsomal cytochrome P450, which is a hemoprotein oxidase system found in the liver. In response to these drugs, the synthesis of cytochrome P450 proteins increases, leading to an enhanced consumption of heme which is a component of the cytochrome P450 proteins. This causes a decrease in the concentration of heme in liver cells leading to an increase in the synthesis of ALA synthase. Effect Of Drugs On ALA Synthase Activity

2. Formation of Porphoblinogen (PBG) Two molecules of ALA condense to form porphobilinogen by ALA dehydratase. ALA dehydratase is extremely sensitive to inhibition by heavy metal ions. Lead poisoning leads to accumulation of ALA and anemia.

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3. Formation of Uroporphyrinogen  4 molecules of porphobilinogens (PBG) are condensed together by by PBG deaminase, producing the tetrapyrrole, hydroxymethylbilane.  Ring closure and isomerization of hydroxymethylbilane produces uroporphyrinogen III. This reaction is catalyzed by the enzyme uroporphyrinogen III synthase.

A (acetate): -CH 2 COOH and P (propionate): -CH 2 CH 2 COOH

4. Formation of Coproporphyrinogen Uroporphyrinogens undergo decarboxylation of their acetate groups, generating coproporphyrinogens. These reactions occur in the cytosol.

5. Formation of heme Copro-porphyrinogen III enters the mitochondrion. Two side chains are decarboxylated generating proto- porphyrinogen IX, which is oxidized to proto- porphyrin IX. The introduction of iron (as Fe +2 ) into proto-porphyrin IX occurs spontaneously, but the rate is enhanced by ferrochelatase enzyme, which is inhibited by Lead.

Porphyrias  Definition:  Porphyrias are rare inherited defects in heme synthesis, (occasionally it’s acquired).  This results in the accumulation and increased excretion of porphyrins or their precursors.

Porphyrias  Signs and symptoms result from a deficiency of the metabolic products beyond the enzymatic block or from accumulation of metabolites behind the block.  If the enzyme lesion occurs early in the enzymatic pathway prior to the formation of hydroxymethyl- bilane, this leads to accumulation of ALA and PBG.  Patients will complain of neuropsychiatric symptoms.

Porphyrias  If the enzyme block leads to the accumulation of hydroxymethylbilane, patients will complain of photosensitivity.  There are skin itches and burns (pruritis) when exposed to visible light.  These symptoms are thought to be a result of the porphyrin-mediated formation of superoxide radicals from oxygen.  These reactive oxygen species can oxidatively damage membranes.

Biosynthesis of Heme

Biochemical Causes of Major Signs & Symptoms of Porphyrias

Treatment During acute porphyria, patients require medical support, particularly for treatment of pain and vomiting. During acute porphyria, patients require medical support, particularly for treatment of pain and vomiting. The severity of symptoms of the porphyrias can be diminished by intravenous injection of hemin and glucose, which decreases the synthesis of ALA synthase. The severity of symptoms of the porphyrias can be diminished by intravenous injection of hemin and glucose, which decreases the synthesis of ALA synthase. Avoidance of sunlight and ingestion of β-carotene (a free-radical scavenger) are also helpful in porphyrias with photosensitivity. Avoidance of sunlight and ingestion of β-carotene (a free-radical scavenger) are also helpful in porphyrias with photosensitivity.

References Conversion of amino acids to specialized products; in Lippincott’s Illustrated Reviews,5 th edition. Chapter 21, page: 277. Porphyrins and bile pigments; in Harper’s Illustrated Biochemistry, 27 th edition. Chapter 31, page: page.org