This document provides information on heme synthesis and disorders of heme synthesis (porphyrias). It describes the 7 step process of heme biosynthesis, which takes place partly in the cytoplasm and mitochondria. The key steps involve the formation of porphobilinogen (PBG), uroporphyrinogen, coproporphyrinogen, protoporphyrinogen, and finally heme through the insertion of iron. Regulation and some specific porphyrias are also outlined, including acute intermittent porphyria, congenital erythropoietic porphyria, porphyria cutanea tarda, hereditary coproporphyria, and variegate porphyria.

1. HEME SYNTHESIS &
DISORDERS
M.Prasad Naidu
MSc Medical Biochemistry,
Ph.D.Research Scholar

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.  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.

5.  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.

7.  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.

8. • 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

10. 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.

11.  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.

13. 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.

14. 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.

15.  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.

16. 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.

17.  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.

18. 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.

19. 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).

20.  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.

22. 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.

23.  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.

25.  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.

26.  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.

27. 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.

28.  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.

30. 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.

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

33. 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.

34.  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.

35.  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.

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

37. 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.

38. 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).

39. 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.

40.  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.

41. 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.

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

43. 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.

44. 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

45. 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.

46.  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.

47. 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.

49. Thank you