of 49 /49

Click here to load reader

Heme synthesis & disorders

Embed Size (px)



Text of Heme synthesis & disorders

Page 1: Heme synthesis & disorders


M.Prasad NaiduMSc Medical Biochemistry,Ph.D.Research Scholar

Page 2: Heme synthesis & disorders

HEME SYNTHESISHEME SYNTHESISHeme 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.

Page 3: Heme synthesis & disorders

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.

Page 4: Heme synthesis & disorders
Page 5: Heme synthesis & disorders

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.

Page 6: Heme synthesis & disorders
Page 7: Heme synthesis & disorders

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.

Page 8: Heme synthesis & disorders

• Hans Fischer synthesised heme in laboratory in 1920(Nobel prize, 1930).

• The usual substitutions are :a.propionyl (-CH2-CH2-COOH) groupb. acetyl (-CH2-COOH) groupc. methyl (-CH3) group d. vinyl (-CH=CH2) group

Page 9: Heme synthesis & disorders
Page 10: Heme synthesis & disorders

Biosynthesis of HemeHeme 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.

Page 11: Heme synthesis & disorders

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.

Page 12: Heme synthesis & disorders
Page 13: Heme synthesis & disorders

Step 2: Formation of PBGNext 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.

Page 14: Heme synthesis & disorders

Step 3: Formation of UPGCondensation 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.

Page 15: Heme synthesis & disorders

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.

Page 16: Heme synthesis & disorders

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.

Page 17: Heme synthesis & disorders

Step 5: synthesis of PPGFurther 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.

Page 18: Heme synthesis & disorders

Step 6: Generation of PPThe 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.

Page 19: Heme synthesis & disorders

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).

Page 20: Heme synthesis & disorders

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.

Page 21: Heme synthesis & disorders
Page 22: Heme synthesis & disorders

Regulation of Heme synthesis ALA synthase is regulated by repression


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.

Page 23: Heme synthesis & disorders

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.

Page 24: Heme synthesis & disorders
Page 25: Heme synthesis & disorders

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.

Page 26: Heme synthesis & disorders

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.

Page 27: Heme synthesis & disorders

Shunt BilirubinWhen 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.

Page 28: Heme synthesis & disorders

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.

Page 29: Heme synthesis & disorders
Page 30: Heme synthesis & disorders

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.

Page 31: Heme synthesis & disorders

• Porphyrias may be broadly grouped into 3 types:

a.Hepatic porphyrias b. Erythropoietic porphyrias

c. porphyrias with both erythropoietic and hepatic abnormalities.

Page 32: Heme synthesis & disorders
Page 33: Heme synthesis & disorders

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.

Page 34: Heme synthesis & disorders

It is usually expressed after puberty in humans.

Clinical featuresThe 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.

Page 35: Heme synthesis & disorders

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.

Page 36: Heme synthesis & disorders

Treatment:Acute intermittent porphyria is treated by

administration of hematin which inhibits the enzyme ALA synthase and the accumulation of porphobilinogen.

Page 37: Heme synthesis & disorders

Congenital erythropoietic porphyriaThis 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.

Page 38: Heme synthesis & disorders

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).

Page 39: Heme synthesis & disorders

Porphyria cutanea tardaThis 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.

Page 40: Heme synthesis & disorders

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.

Page 41: Heme synthesis & disorders

Hereditary coproporphyriaThis 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.

Page 42: Heme synthesis & disorders

Treatment :Infusion of hematin is used to control this

disorder.Hematin inhibits ALA synthase and thus

reduces the accumulation of various intermediates.

Page 43: Heme synthesis & disorders

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.

Page 44: Heme synthesis & disorders

Protoporphyria This disorder is also known as

erythropoietic protoporphyria.The disease is due to a deficiency in


Protoporphyrin IX accumulates in erythrocytes, bone marrow, and plasma.

Patients are photosensitive.

Reticulocytes and skin biopsy exhibit red flourescence.

Page 45: Heme synthesis & disorders

Acquired porphyriasThe 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.

Page 46: Heme synthesis & disorders

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.

Page 47: Heme synthesis & disorders

Diagnosis of porphyriasTo 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.

Page 48: Heme synthesis & disorders
Page 49: Heme synthesis & disorders

Thank youThank you