Heme synthesis & disorders.ppt

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HEME SYNTHESIS DISORDERS

• M.Prasad Naidu
• MSc Medical Biochemistry,
• Ph.D.Research Scholar

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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.

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• 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.

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• 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.

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• 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.

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• 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 (-CHCH2) group

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• 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.

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• 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.

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• 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.
• The enzyme contains zinc and is inhibited by
  Lead.

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• 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.

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• 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-synth
  ase) and Uroporphyrinogen-lll-cosynthase.
• During this deamination reation 4 molecules of
  ammonia are removed.

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• 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.

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• 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.

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• 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.

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• 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).

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• 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.

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• 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.

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• 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.

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• 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.

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• 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.

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• 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.

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• 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.

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• 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.

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• Porphyrias may be broadly grouped into 3 types
• Hepatic porphyrias
• b. Erythropoietic porphyrias
• c. porphyrias with both erythropoietic and
  hepatic abnormalities.

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• 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
  d-aminolevulinate.
• The urine gets darkened on exposure to air due to
  the conversion of porphobilinogen to porphobilin
  and porphyrin.

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• 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
  .

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• 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.

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• Treatment
• Acute intermittent porphyria is treated by
  administration of hematin which inhibits the
  enzyme ALA synthase and the accumulation of
  porphobilinogen.

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• 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.

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• 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).

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• 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.

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• 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.

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• 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.

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• Treatment
• Infusion of hematin is used to control this
  disorder.
• Hematin inhibits ALA synthase and thus reduces
  the accumulation of various intermediates.

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• 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.

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• 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
  flourescence.

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• 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.

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• 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.

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• Diagnosis of porphyrias
• To demonstrate porphyrins, UV flourescence is the
  best technique.
• The presence of porphyrin precursor in urine is
  detected by Ehrlichs reagent.
• When urine is observed under ultraviolet light
  porphyrins if present, will emit strong red
  flourescence.

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