Enzymes of Heme Biosynthesis

The hematopoietic system is affected by both short- and long-term arsenic exposure. Arsenic is known to cause a wide variety of hematological abnormalities like anemia, absolute neutropenia, leucopenia, thrombocytopenia, and relative eosinophilia - more common than absolute esino-philia, basophilic stippling, increased bone marrow vascularity, and rouleau formation (Rezuke et al, 1991). These effects may be due to a direct hemolytic or cytotoxic effect on the blood cells and a suppression of erythropoiesis. The mechanism of hemolysis involves depletion of intracellular GSH, resulting in the oxidation of hemoglobin (Saha et al, 1999). Arsenic exposure is also known to influence the activity of several enzymes of heme biosynthesis. Arsenic produces a decrease in ferrochelatase, and decrease in COPRO-OX and increase in hepatic 5-aminolevulinic acid synthetase activity (Woods and Southern, 1989). Subchronic... 

Astner 1, Schulze JO, van den Heuvel J, Jahn D, Schubert WD, Heinz DW. Crystal structure of 5-aminolevuhnate synthase, the first enzyme of heme biosynthesis, and its link to XLSA in humans. EMBO J. 2005 24 3166-3177. 

Dailey HA. Enzymes of heme biosynthesis. J Biol InorgChem 1997 2 411-7. 

Wu CK, Dailey HA, Rose JP, Burden A, Sellers VM> Wang BC. The 2.0 A structure of human fer-rochelatase, the terminal enzyme of heme biosynthesis. Nat Struct Biol 2000 8 156-160. 

The cDNA of all enzymes of heme biosynthesis have been characterized and mutations responsible for any of the porphyrias have been described. They are in the overwhelming majority heterogeneous and often family specific. But in some countries, founder effects of either AIP or PV have been elucidated. Phenotype-genotype correlations have been recognized in CEP and EPP. In the last, this will be of relevance to future patient management, as a patient with a missense mutation exhibits a decreased risk to develop the life-threatening complication of terminal liver failure as a patient with a null allele mutation. 

Several enzymes of heme biosynthesis Part of urea cycle... 

Hematological Effects. The effects of lead on the hematopoietic system have been well documented. These effects, which are seen in both humans and animals, include increased urinary porphyrins, coproporphyrins, ALA, EP, FEP, ZPP, and anemia. The process of heme biosynthesis is outlined in Figure 2-10. Lead interferes with heme biosynthesis by altering the activity of three enzymes ALAS,... 

Fe/S clusters in regulatory enzymes have been proposed to act as sensors in such a manner that, upon detection of a measurand, the cluster disintegrates and activity stops. Putative examples are NO sensing by the [2Fe-2S] cluster in the terminal enzyme of heme synthesis, ferrochelatase [8], and 02 sensing by the [4Fe-4S] cluster in the regulatory enzyme of purine nucleotide biosynthesis, glutamine 5-phosphoribosyl-l-pyrophosphate amidotransferase [9], This is of course not a catalytic activity, since the cluster is destroyed in the action. 

Control of heme biosynthesis revolves around the initial enzyme, ALA-synthase. Heme, the end product of this pathway, exerts an inhibitory effect on ALA-synthase through several mechanisms. Heme, or its oxidation product hematin, activates a repressor protein that turns off ALA-synthase biosynthesis at the translation level. Erythropoietin, a protein produced by the kidneys and found in larger than normal amounts in high-altitude dwellers, counteracts the effects of the repressor protein. Erythropoietin deficiency exists in chronic kid-... 

Fertilized 16-day-old chick embryos are kept in an incubator at 37 °C with 70 % humidity. Drugs are dissolved in a small volume (0.1-0.3 ml) of 0.15 M NaCl. According to Anderson (1978) a small dose of DDC (= l,4-dihydro-3,5-dicarbethoxycollidine) is added that leads to the formation of N-methylprotoporphyrin, which is the inhibitor of ferrochelatase, the last enzyme in heme biosynthesis. Sterile injections are made when the eggs are 18 days old. After 24 h, the embryos are killed by decapitation, the livers removed, separated from the gall bladder and rinsed with saline prior to homogenization with 3 vol of 0.25 M sucrose/0.02 M Tris buffer, pH 7.4. At least six embryo livers are pooled for each determination. 

We devised a screen for isolating mutants defective in iron-dependent regulation of heme biosynthesis that did not require prior knowledge of the mechanism or of the rate-limiting steps [83]. We speculated that if the pathway as a whole were regulated by iron, a mutant defective in that control would accumulate protoporphyrin under iron limitation. Mutants defective in the heme synthesis enzymes ferrochelatase [75] or protoporphyrinogen oxidase would likely have a similar phenotype, but porphyrin accumulation would likely be independent of iron in the structural gene mutants, and those strains would also be expected to be heme auxotrophs. 

Iron insertion is the final step of heme biosynthesis, and this is controlled by the enzyme ferrochelatase. This enzyme is located on the inner face of the mitochondrial inner wall and its failure to function properly leads to a condition called erythropoietic protoporphyria, which, because protoporphyrin IX builds up in the skin, leads to light sensitivity. Protoporphyrin is also found in the red blood cells, bile, and feces. The three porphyrias that arise due to genetic malfunction of the last three enzymes in heme biosynthesis have not yet been tracked down to specific genetic abnormalities. 

Porphyrias clinical conditions resulting from genetic defects in heme biosynthesis. For the pathway of heme biosynthesis, see Porphyrins. Inborn errors have been described for 7 of the 8 enzymes in this pathway. Although no major genetic defect has been described for the first enzyme of the pathway, S-aminolevulinate synthase (EC 2.3.1.37), low activity has been reported in a case of congenital sideroblastic anemia [G. R. Buchanan et al. Blood 55 (1980) 109-115]. Heme is an essential constituent of many important enzymes and hemoproteins. Absence of heme synthesis is therefore incompatible with life, and homozygotes of inherited autosomal dominant disorders of heme synthesis are not viable, unless there is residual activity of the enzyme concerned. P. are classified as erythropoietic or hepatic, depending on whether the defect is located mainly in the erythroid cells or the liver. 

In animals the control of heme biosynthesis appears to be primarily a control on the rate of biosynthesis of the enzyme ALA-synthetase. Recent experiments [Sassa and Granick, 21] suggest that control of this enzyme occurs at both the transcription and the translation levels. Because these controls have been studied mainly in the liver, we shall discuss the evidence for the control mechanisms in this tissue, particularly the more recent work using chick embyro liver cells grown in primary culture. In later sections we shall discuss the ALA-synthetase control mechanism in the red cells for heme and hemoglobin synthesis, and the control mechanisms for chlorophyll synthesis in plants. 

Tyrosine hydroxylase (TH) is an enzyme that catalyzes the hydroxylation of tyrosine to 3,4-dihydroxypheny-lalanine in the brain and adrenal glands. TH is the rate-limiting enzyme in the biosynthesis of dopamine. This non-heme iron-dependent monoxygenase requires the presence of the cofactor tetrahydrobiopterin to maintain the metal in its ferrous state. 

A summary of the steps in the biosynthesis of the porphyrin derivatives from PBG is given in Figure 32-8. The last three enzymes in the pathway and ALA synthase are located in the mitochondrion, whereas the other enzymes are cytosolic. Both erythroid and non-erythroid ( housekeeping ) forms of the first four enzymes are found. Heme biosynthesis occurs in most mammalian cells with the exception of mature erythrocytes, which do not contain mitochondria. However,... 

ALA Synthase Is the Key Regulatory Enzyme in Hepatic Biosynthesis of Heme... 

In general, the porphyrias described are inherited in an autosomal dominant manner, with the exception of congenital erythropoietic porphyria, which is inherited in a recessive mode. The precise abnormalities in the genes directing synthesis of the enzymes involved in heme biosynthesis have been determined in some instances. Thus, the use of appropriate gene probes has made possible the prenatal diagnosis of some of the porphyrias. 

In summary, lead inhibits the activity of certain enzymes involved in heme biosynthesis, namely, 5-aminolevulinic acid dehydratase (ALAD), and ferrochelatase. As a consequence of these changes, heme biosynthesis is decreased and the activity of the rate limiting enzyme of the pathway,... 

ALAD, and ferrochelatase. Lead indirectly stimulates the mitochondrial enzyme ALAS, which catalyzes the condensation of glycine and succinyl-coenzyme A to form ALA. The activity of ALAS is the rate-limiting step in heme biosynthesis increase of ALAS activity occurs through feedback derepression. Lead... 

Acute intermittent porphyria is a dominantly inherited partial deficiency of porphobilinogen deaminase, and causes axonal polyneuropathy. Acute intermittent porphyria is caused by partial deficiency of porphobilinogen deaminase, an enzyme required for heme biosynthesis. Patients may present with acute abdominal pain, rapidly progressive sensorimotor axonal polyneuropathy or psychosis, and have elevated concentrations of the heme precursor 8-amino-levulinic acid in their urine. Symptoms may be precipitated by treatment with barbiturates or other drugs and are suppressed by treatment with hematin [59]. 

---