Heme synthesis & disorders


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Heme synthesis & disorders

  1. 1. HEME SYNTHESIS & DISORDERS M.Prasad Naidu MSc Medical Biochemistry, Ph.D.Research Scholar
  2. 2. HEME SYNTHESIS  Heme is the most important porphyrin containing compound.  Heme is a derivative of the porphyrin.  Porphyrins are cyclic compounds formed by fusion of 4 pyrrole rings linked by methenyl (=CH-) bridges.
  3. 3.  Metal ions can bind with nitrogen atoms of pyrrole rings to form complexes.  Since an atom of iron is present, heme is a ferroprotoporphyrin.  The pyrrole rings are named as l, ll, lll, lV and the bridges as alpha, beta, gamma and delta.
  4. 4.  Naturally occurring porphyrins contain substituent groups replacing the 8 hydrogen atoms of the porphyrin nucleus.  When the substituent groups have a symmetrical arrangement (1, 3, 5, 7 and 2, 4, 6, 8) they are called the I series –type l porphyrins.  The lll series have an asymmetrical distribution of substituent groups (1, 3, 5, 8 and 2, 4, 6, 7)- type ll porphyrins.
  5. 5.  Type lll is the most predominant in biological systems.  It is also called series 9, because fischer, the pioneer in porphyrin chemistry has placed it as the 9th in a series of 15 possible isomers.  Hans Fischer, the father of porphyrin chemistry, proposed a short hand model for presentation of porphyrin structures.
  6. 6. • Hans Fischer synthesised heme in laboratory in 1920(Nobel prize, 1930). • The usual substitutions are : a.propionyl (-CH2-CH2-COOH) group b. acetyl (-CH2-COOH) group c. methyl (-CH3) group d. vinyl (-CH=CH2) group
  7. 7. Biosynthesis of Heme  Heme can be synthesised by almost all the tissues in the body.  Heme is primarily synthesised in the liver and the erythrocyte-producing cells of bone marrow (erythroid cells).  Heme is synthesised in the normoblasts, but not in the matured ones.
  8. 8.  The pathway is partly cytoplasmic and partly mitochondrial. Step 1: ALA synthesis  The synthesis starts with the condensation of succinyl CoA and glycine in the presence of pyridoxal phosphate to form delta amino levulinic acid (ALA).  The enzyme ALA synthase is located in the mitochondria and is the rate-limiting enzyme of the pathway.
  9. 9. Step 2: Formation of PBG  Next few reactions occur in the cytoplasm.  Two molecules of ALA are condensed to form porphobilinogen (PBG).  The condensation involves removal of 2 molecules of water and the enzyme is ALA dehydratase.  Porphobilinogen is a monopyrrole.
  10. 10. Step 3: Formation of UPG  Condensation of 4 molecules of the PBG, results in the formation of the first porphyrin of the pathway, namely uroporphyrinogen(UPG).  The pyrrole rings are joined together by methylene bridges, which are derived from alpha carbon of glycine.
  11. 11.  When the fusion occurs, the lll series of isomers are predominantly formed; and only the lll series are further used.  This needs 2 enzymes which catalyse the reactions; PBG-deaminase (Uroporphyrinogen- l-synthase) and Uroporphyrinogen-lll- cosynthase.  During this deamination reation 4 molecules of ammonia are removed.
  12. 12. Step 4: synthesis of CPG  The UPG-lll is next convertedto coproporphyrinogen (CPG-lll) by decarboxylation.  Four molecules of CO2 are eliminated by uroporphyrinogen decarboxylase.  The acetate groups (CH2-COOH) are decarboxylated to methyl (CH3) groups.
  13. 13.  Step 5: synthesis of PPG  Further metabolism takes place in the mitochondria.  CPG is oxidised to protoporphyrinogen (PPG- lll) by coproporphyrinogen oxidase.  Two propionic acid side chains are oxidatively decorboxylated to vinyl groups.
  14. 14. Step 6: Generation of PP  The Protoporphyrinogen-lll is oxidised by the enzyme protoporphyrin-lll (PP-lll) in the mitochondria.  The oxidation requires molecular oxygen.  The methylene bridges (-CH2) are oxidised to methenyl bridges (-CH=) and coloured porphyrins are formed.  Protoporphyrin-9 is thus formed.
  15. 15. Step 7: Generation of Heme  The last step in the formation of heme is the attachment of ferrous iron to the protoporphyrin.  The enzyme is heme synthase or ferrochelatase which is also located in mitochondria.  Iron atom is co-ordinately linked with 5 nitrogen atoms (4 nitrogen of pyrrole rings of protoporphyrin and 1st nitrogen atom of a histidine residue of globin).
  16. 16.  The remaining valency of iron atom is satisfied with water or oxygen atom.  When the ferrous iron (Fe++) in heme gets oxidised to ferric (Fe+++) form, hematin is formed, which loses the property of carrying the oxygen.  Heme is red in colour, but hematin is dark brown.
  17. 17. Regulation of Heme synthesis  ALA synthase is regulated by repression mechanism.  Heme inhibits the synthesis of ALA synthase by acting as a co-repressor.  ALA synthase is also allosterically inhibited by hematin.  When there is excess of free heme, the Fe++ is oxidised to Fe+++(ferric), thus forming hematin.
  18. 18.  The compartmentalisation of the enzymes in the synthesis of heme makes it easier for the regulation.  The rate-limiting enzyme is in the mitochondria.  The steps 1,5,6, and 7 are taking place inside mitochondria, while steps 2,3 and 4 are in cytoplasm.
  19. 19.  Drugs like barbiturates induce heme synthesis.  Barbiturates require the heme containing cytochrome p450 for their metabolism.  Out of the total heme synthesised, two thirds are used for cytochrome p450 production.  The steps catalysed by ferrochelatase and ALA dehydratase are inhibited by lead.
  20. 20.  INH (Isonicotinic acid hydrazide) that decreases the availability of pyridoxal phosphate may also affect heme synthesis.  High cellular concentration of glucose prevents induction of ALA synthase.  This is the basis of glucose to relieve the acute attack of porphyrias.
  21. 21. Shunt Bilirubin  When 15N or 14C labelled glycine is injected, this is incorporated into heme and into RBCs.  After 100-120 days, when RBCs are lysed, the radiolabelled Hb level is decreased, along with consequent rise in radioactive bilirubin.  However, about 15% of radioactive bilirubin is excreted within about 10 days.  This is called Shunt bilirubin.
  22. 22.  This is the formation of bilirubin from heme in bone marrow, without being incorporated into Hb.  This is the result of ineffective erythropoiesis.  In porphyrias, especially in the erythropoietic varieties, the shunt biliribin will be increased.
  23. 23. Disorders of Heme synthesis  Porphyrias are group of inborn errors of metabolism associated with the biosynthesis of heme.(Greek ‘porphyria’ means purple).  These are characterised by increased production and production and excretion of porphyrins and/or their precursors (ALA + PBG).  Many of the porphyrias are inherited as autosomal dominant traits.
  24. 24. • Porphyrias may be broadly grouped into 3 types: a. Hepatic porphyrias b. Erythropoietic porphyrias c. porphyrias with both erythropoietic and hepatic abnormalities.
  25. 25. Acute intermittent porphyria This disorder occurs due to the deficiency of the enzyme uroporphyrinogen l synthase. Acute intermittent porphyria is characterised by increased excretion of porphobilinogen and δ- aminolevulinate. The urine gets darkened on exposure to air due to the conversion of porphobilinogen to porphobilin and porphyrin.
  26. 26.  It is usually expressed after puberty in humans. Clinical features  The symptoms include abdominal pain, vomiting and cardiovascular abnormalities.  The neuropsychiatric disturbances observed in these patients are believed to be due to reduced activity of tryptophan pyrrolase (caused by depleted heme levels), resulting in the accumulation of tryptophan and 5- hydroxytryptamine.
  27. 27.  These patients are not photosensitive since the enzyme defect occurs prior to the formation of uroporphyrinogen.  The symptoms are more severe after administration of drugs (e.g. barbiturates) that induce the synthesis of cytochrome P450.  This is due to the increased activity of ALA synthase causing accumulation of PBG and ALA.
  28. 28. Treatment:  Acute intermittent porphyria is treated by administration of hematin which inhibits the enzyme ALA synthase and the accumulation of porphobilinogen.
  29. 29. Congenital erythropoietic porphyria  This disorder is due to a defect in the enzyme uroporphyrinogen lll cosynthase.  It is a rare congenital disorder caused by autosomal recessive mode of inheritance, mostly confined to erythropoietic tissues.
  30. 30. Clinical features :  The patients are photosensitive (itching and burning of skin when exposed to visible light) due to the abnormal porphyrins that accumulate.  Increased hemolysis is also observed in the individuals affected by this disorder.  The individuals excrete uroporphyrinogen l and coproporphyrinogen l which oxidize respectively to uroporphyrin l and coproporphyrin l (red pigments).
  31. 31. Porphyria cutanea tarda  This is a chronic disease caused by a deficiency in uroporphyrinogen decarboxylase.  It is the most common porphyria.  It is also known as cutaneous hepatic porphyria.  It is usually associated with liver damage caused by alcohol overconsumption or iron overload.
  32. 32.  Uroporphyrin accumulates in the urine. Clinical features:  Cutaneous photosensitivity is the most important clinical manifestation of these patients.  Liver exhibits flourescence due to high concentration of accumulated porphyrins.
  33. 33. Hereditary coproporphyria  This disorder is due to a defect in the enzyme coproporphyrinogen oxidase.  Coproporphyrinogen lll and other intermediates (ALA and PBG) of heme synthesis prior to the blockade are excreted in urine and feces.  Patients are photosensitive.  They exhibit the clinical manifestations observed in the patients of acute intermittent porphyria.
  34. 34. Treatment :  Infusion of hematin is used to control this disorder.  Hematin inhibits ALA synthase and thus reduces the accumulation of various intermediates.
  35. 35. Variegate porphyria  It is an acute disease caused by a deficiency of protoporphyrinogen oxidase.  Protoporphyrinogen IX and other inermediates prior to the block accumulate in the urine.  The urine of these patients is coloured.  Patients are photosensitive.
  36. 36. Protoporphyria  This disorder is also known as erythropoietic protoporphyria.  The disease is due to a deficiency in ferrochelatase.  Protoporphyrin IX accumulates in erythrocytes, bone marrow, and plasma.  Patients are photosensitive.  Reticulocytes and skin biopsy exhibit red
  37. 37. Acquired porphyrias  The porphyrias may be acquired due to the toxicity of several compounds.  Exposure of the body to heavy metals (e.g. lead ), toxic compounds (e.g. hexachlorobenzene) and drugs (e.g. griseofulvin) inhibits many enzymes in heme synthesis.
  38. 38.  These include ALA dehydratase, uroporphyrin l synthase and ferrochelatase.  Ferrochelatase and ALA dehydratase are particularly sensitive to inhibition by lead.  Protoporphyrin and ALA accumulate in urine.
  39. 39. Diagnosis of porphyrias  To demonstrate porphyrins, UV flourescence is the best technique.  The presence of porphyrin precursor in urine is detected by Ehrlich’s reagent.  When urine is observed under ultraviolet light; porphyrins if present, will emit strong red flourescence.
  40. 40. Thank you