Heme biosynthesis occurs in most mammalian cells except mature erythrocytes, which lack mitochondria. Approximately 85% of heme synthesis occurs in erythroid precursor cells in the bone marrow, and the majority of the remainder in hepatocytes. Heme biosynthesis is initiated by the condensation of succinyl–CoA and glycine in a pyridoxal phosphate-dependent reaction catalyzed by mitochondrial δ-aminolevulinate synthase (ALA synthase, EC 126.96.36.199).
(1) Succinyl-CoA + glycine → δ-aminolevulinate + CoA-SH + CO2
Humans express two isozymes of ALA synthase. ALAS1 is ubiquitously expressed throughout the body, whereas ALAS2 is expressed in erythrocyte precursor cells. The reaction is rate-limiting for porphyrin biosynthesis in mammalian liver. The initial product formed is α-amino-β-ketoadipate, which is rapidly decarboxylated to δ-aminolevulinate (Figure 31–6, top).
Biosynthesis of porphobilinogen. ALA synthase is present in mitochondria, ALA dehydratase in the cytosol.
Condensation of two molecules of ALA, catalyzed by cytosolic ALA dehydratase (188.8.131.52) forms porphobilinogen:
(2) 2 δ-Aminolevulinate → porphobilinogen + 2 H2O
(Figure 31–6, bottom). A zinc metalloprotein, ALA dehydratase is sensitive to inhibition by lead, as can occur in lead poisoning.
The reaction catalyzed by cytosolic hydroxymethylbilane synthase (uroporphyrinogen I synthase, EC 184.108.40.206) forms hydroxymethylbilane:
(3) 4 Porphobilinogen + H2O → hydroxymethylbilane + 4 NH3
Catalysis involves head-to-tail condensation of four molecules of porphyrobilinogen to form the linear tetrapyrrole hydroxymethylbilane (Figure 31–7, center).
Conversion of porphobilinogen to uroporphyrinogens. Linearization of porphobilinogen is catalyzed by hydroxymethylbilane synthase (also called uroporphyrinogen synthase I, or porphobilinogen deaminase).
Cyclization of hydroxymethylbilane is catalyzed by cytosolic uroporphyrinogen III synthase, EC 220.127.116.11:
(4) Hydroxymethylbilane → uroporphyrinogen III + H2O
forms uroporphyrinogen III (Figure 31–7, right). Hydroxymethylbilane can also cyclize spontaneously to form uroporphyrinogen I (Figure 31–7, left), but under normal conditions the uroporphyrinogen formed is almost exclusively the type III isomer. The type-I isomers of porphyrinogens are, however, formed in excess in certain porphyrias. Since the pyrrole rings of these uroporphyrinogens are connected by methylene (—CH2—) rather than by methyne bridges, they do not form a conjugated ring system and thus they, and indeed all porphyrinogens, are colorless. They are, however, readily auto-oxidized to their respective colored porphyrins.
All four acetate moieties of uroporphyrinogen III next undergo decarboxylation to methyl (M) substituents, forming coproporphyrinogen III in a cytosolic reaction catalyzed by uroporphyrinogen decarboxylase, EC 18.104.22.168 (Figures 31–8 and 31–9):
Decarboxylation of uroporphyrinogen III to coproporphyrinogen III. A = acetyl; M = methyl; P = propionyl.
(5) Uroporphyrinogen III → coproporphyrinogen III + 4 CO2
This decarboxylase can also convert uroporphyrinogen I, if present, to coproporphyrinogen I.
The final three reactions of heme biosynthesis take place in mitochondria. Coproporphyrinogen III enters mitochondria and is converted to protoporphyrinogen III, and then to protoporphyrin III. These reactions are catalyzed by coproporphyrinogen oxidase (EC 22.214.171.124), which decarboxylates and oxidizes the two propionic acid side chains to form protoporphyrinogen III:
(6) Coproporphyrinogen III + O2 + 2 H+ → protoporphyrinogen III + 2 CO2 + 2 H2O
This oxidase is specific for type III coproporphyrinogen, so type I protoporphyrins generally do not occur in humans.
Protoporphyrinogen III is next oxidized to protoporphyrin III in a reaction catalyzed by protoporphyrinogen oxidase, EC 126.96.36.199:
(7) Protoporphyrinogen III + 3 O2 → protoporphyrin III + 3 H2O2
The final step in heme synthesis involves the incorporation of ferrous iron into protoporphyrin III in a reaction catalyzed by ferrochelatase (heme synthase, EC 188.8.131.52):
(8) Protoporphyrin III + Fe2+ → heme + 2 H+
Figure 31–9 summarizes the stages, and their intracellular locations, in the biosynthesis of the porphyrin derivatives from porphobilinogen. For the above reactions, numbers correspond to those in Figure 31–10 and Table 31–2.
TABLE 31–2Summary of Major Findings in the Porphyriasa ||Download (.pdf) TABLE 31–2 Summary of Major Findings in the Porphyriasa
|Enzyme Involvedb ||Type, Class, and OMIM Number ||Major Signs and Symptoms ||Results of Laboratory Tests |
|1. ALA synthase 2 (ALAS2), EC 184.108.40.206 ||X-linked sideroblastic anemiac (erythropoietic) (OMIM 301300) ||Anemia ||Red cell counts and hemoglobin decreased |
|2. ALA dehydratase EC 220.127.116.11 ||ALA dehydratase deficiency (hepatic) (OMIM 125270) ||Abdominal pain, neuropsychiatric symptoms ||Urinary ALA and coproporphyrin III increased |
|3. Uroporphyrinogen I synthased EC 18.104.22.168 ||Acute intermittent porphyria (hepatic) (OMIM 176000) ||Abdominal pain, neuropsychiatric symptoms ||Urinary ALA and PBGe increased |
|4. Uroporphyrinogen III synthase EC 22.214.171.124 ||Congenital erythropoietic (erythropoietic) (OMIM 263700) ||Photosensitivity ||Urinary, fecal, and red cell uroporphyrin I increased |
|5. Uroporphyrinogen decarboxylase EC 126.96.36.199 ||Porphyria cutanea tarda (hepatic) (OMIM 176100) ||Photosensitivity ||Urinary uroporphyrin I increased |
|6. Coproporphyrinogen oxidase EC 188.8.131.52 ||Hereditary coproporphyria (hepatic) (OMIM 121300) ||Photosensitivity, abdominal pain, neuropsychiatric symptoms ||Urinary ALA, PBG, and coproporphyrin III and fecal coproporphyrin III increased |
|7. Protoporphyrinogen oxidase EC 184.108.40.206 ||Variegate porphyria (hepatic) (OMIM 176200) ||Photosensitivity, abdominal pain, neuropsychiatric symptoms ||Urinary ALA, PBG, and coproporphyrin III and fecal protoporphyrin IX increased |
|8. Ferrochelatase EC 220.127.116.11 ||Protoporphyria (erythropoietic) (OMIM 177000) ||Photosensitivity ||Fecal and red cell protoporphyrin IX increased |
Steps and cellular location of the reactions in the biosynthesis from porphobilinogen of the indicated porphyrin derivatives, notably heme.
Intermediates, enzymes and regulation of heme synthesis. The numbers of the enzymes that catalyze the indicated reactions are those used in the accompanying text and in column 1 of Table 31–2. Enzymes 1, 6, 7, and 8 are mitochondrial, but enzymes 2 to 5 are cytosolic. Regulation of hepatic heme synthesis occurs at ALA synthase (ALAS1) by a repression-derepression mechanism mediated by heme and its hypothetical aporepressor. Dashed lines indicate the negative regulation by repression. Mutations in the gene encoding enzyme 1 cause X-linked sideroblastic anemia. Mutations in the genes encoding enzymes 2 to 8 give rise to the porphyrias.
ALA Synthase Is the Key Regulatory Enzyme in Hepatic Biosynthesis of Heme
There are two isozymes of ALA synthase. ALAS1 is expressed throughout the body; ALAS2 is expressed in erythrocyte precursor cells. The reaction catalyzed by ALA synthase 1 (Figure 31–6) is rate-limiting for biosynthesis of heme in liver. It appears that heme, probably acting through an aporepressor molecule, acts as a negative regulator of the synthesis of ALAS1 (Figure 31–10). The rate of synthesis of ALAS1 thus increases greatly in the absence of heme, but diminishes in its presence. Heme also affects translation of the enzyme and its transfer from the cytosol to the mitochondrion. ALAS1 has a short half life, which is typical for an enzyme that catalyzes a rate-limiting reaction.
Many drugs whose metabolism requires the hemoprotein cytochrome P450 increase cytochrome P450 biosynthesis. The resulting depletion of the intracellular heme pool induces synthesis of ALAS1, and the rate of heme synthesis rises to meet metabolic demand. The biosynthesis of ALAS2 is not feedback regulated by heme, and therefore is not induced by these drugs.