1. Heme is essential for hemoglobin and myoglobin function in oxygen transport, as well as enzymes involved in electron transport and chemical catalysis. 2. Heme is synthesized through 8 steps beginning with glycine and succinyl CoA, with the rate-limiting first step occurring in mitochondria and intermediate steps in cytosol. 3. Deficiencies in heme synthesis enzymes can cause porphyrias, characterized by excess porphyrin precursors in urine and sometimes feces, with acute intermittent porphyria presenting as acute abdominal pain.

1. Heme Biosynthesis
by
Dr. Santhosh Kumar N
Associate Professor
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2. • Hb found in RBCs (erythrocytes) & carriers of O2 from the lungs to the tissues &
carries CO2 tissue to the lungs.
• Normal value – 14 to16 gm/dl (male)
13 to 15 gm/dl (female)
 Hemoglobin are conjugated proteins (Heme + globulin)

3. Functions of Heme
• Derivative of porphyrin (Ferroprotoporphyrin).
• Heme is the prosthetic group of several proteins & enzymes
– Transport of gases (Hemoglobin & Myoglobin)
– Electron transfer (cytochrome-c,a,a3,b, involve ETC)
– Chemical catalysis (tryptophan pyrrolase)
– Detoxification (cytochrome-P450)
– Antioxidant defence (Catalase, Peroxidase)

4. Heme production:
• Erythroid cell (bone marrow- (85%) & liver (15%)
• Produced by combination of porphyrin with iron
• Porphyrin are cyclic compounds formed by fusion 4
pyrrole rings linked by methenyl (=CH-) bridges, with
iron atom.
M: Methyl, V: Vinyl, P: Propionyl

5. Heme Biosynthesis

6. • In heme synthesis:
• The first & last 3 reactions occurs in mitochondria &intermediate
reactions takes place in cytosol
• Starting material of the heme synthesis- Glycine & Succinyl CoA.

7. Step -1: ALA Synthesis
(In mitochondria)
• Condensation of Succinyl-CoA & glycine
in the presence of PLP to form delta amino
levulinic acid (δ-ALA) by ALA synthase.
• Anemia maybe manifested by Vit-B6
deficiency
• Rate-limiting enzyme of the pathway.
PLP: Pyridoxal phosphate

8. Step 2: Formation of PBG
(In the cytoplasm)
• 2 moles of ALA are condensed to form
porphobilinogen (PBG) by ALA
dehydratase (zinc contain)
• Inhibited by lead.

9. Step 3: Formation of UPG
(In the cytoplasm)
• Condensation of 4 PBG occurs in a head-to-tail manner to
form linear tetra pyrrole (OH-methylbilane (HMB). By
PBG-deaminase (Uroporphyrin -I synthase or HMB
synthase).
• HMB will cyclise to form uroporphyrinogen–I further
converted to uroporphyrinogen-III by
uroporphyrinogen–III co-synthase.
Methylene
bridges

10. Step 4: Synthesis of CPG
(In the cytoplasm)
• UPG-III is converted to coproporphyrinogen
(CPG-III) by uroporphyrinogen
decarboxylase.
• The acetate groups (CH2–COOH) are
decarboxylated to methyl (CH3) groups (4CO2
are eliminated).

11. Step 5: Synthesis of PPG
(In mitochondria)
• CPG is oxidized to proto-porphyrinogen- III
by coproporphyrinogen oxidase.
• 2 propionic acid side chains are oxidatively
decarboxylated to vinyl groups.
• Oxidase enzyme acts only on type - III series
2
2

12. Step 6: Generation of Protoporphyrin
(In mitochondria)
• The PPG-III is oxidized by proto-
porphyrinogen oxidase to form proto
porphyrin-III.
• Protoporphyrin-IX is thus formed.
Methylene
bridges (–CH2)
Methenyl
bridges (–CH=)
oxidized
Colored

13. Step 7: Generation of Heme
(In mitochondria )
• Formation of heme is the attachment of ferrous
iron to the protoporphyrin by heme synthase or
ferrochelatase.
(Ferrous)
(red in color)

14. • Iron atom has six coordination bonds
• Four bonds formed b/w iron & nitrogen
• 5th bond b/w nitrogen atom of histidine residue of globin polypeptide chain(proximal
histidine F-8)
• 6th bond with oxygen & side chain of another histidine residue of globin chain (Distal
histidine, F7)

15. • When the ferrous iron (Fe++) in heme gets
oxidized to ferric (Fe+++) form – hematin (dark
brown)
• Which loses the property of carrying the oxygen.
2+

16. 2CO2
4H+

17. Regulation of Heme synthesis
1. ALA synthase is regulated by repression mechanism.
• Heme inhibits the synthesis of ALA synthase by acting as a co-repressor.
2. ALA synthase is also allosterically inhibited by hematin.
• Excess of heme, the Fe++ is oxidized to Fe+++ (ferric), thus forming hematin.
3. The compartmentalization 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 the steps 2,3 and 4 are in
cytoplasm.

18. 4. Drugs like barbiturates induce heme synthesis.
– Barbiturates require the heme containing cytochrome p450 for their metabolism.
– Out of the total heme synthesized, two thirds are used for cytochrome p450
production.
5. Lead inhibits ferrochelatase and ALA dehydratase .
6. INH (Isonicotinic acid hydrazide)
 Decreases the availability of pyridoxal phosphate may also affect heme synthesis.

19. Disorders of Heme Biosynthesis
(Porphyrias)

20. • Porphyrias are rare inherited (autosomal) or acquired disorder due to deficiencies of
enzymes in heme synthesis.
• This leads to accumulation & increased excretion of porphyrins or porphyrin precursors
(ALA & PBG) in the urine & feces.
• Porphyrin precursors are also excreted in urine under normal conditions,
– PBG - 2mg in urine/ day.
– ALA - 1.7mg in urine/ day.

21. Classification
• The porphyrias are classified as
– Erythropoietic porphyria &
– Hepatic porphyrias,
Depending on whether the enzyme deficiency occurs in the erythropoietic cells
of the bone marrow or in the liver.

22. Acute intermittent porphyria
– Inherited as an autosomal dominant trait
» Pass from one parent onto their child
» Pass from both parents onto their child -autosomal recessive trait
– ALA & PBG are elevated in blood & urine
» PBG deaminase (UPG-I synthase) defect
– On standing – urine turns colorless to more color
» Due to photo-oxidation of PBG to porphobilin

23. • No photosensitivity.
– Porphyrin are not excreted or elevated in blood
• Most common patient with acute abdominal pain & vomiting
• More frequency in female than in males
– Female sex hormones have a stimulatory effect on ALA synthase
• Neuropsychiatric manifestation
– Fluctuating BP & may present sensory & motor disturbances, confusion.

26. Diagnosis of Porphyrias
• UV fluorescence is the best technique to demonstrate porphyrins.
• The presence of porphyrin precursor in urine is detected by Ehrlich's test.
• When urine is observed under UV light; porphyrins if present, will emit
strong red fluorescence.

27. Soret Band
• All porphyrins will have an absorption band near 400nm; this distinguishing
band is called the Soret band.
• UP, CP and PP show Soret bands at 406, 400 and 408 respectively.
• Heme does not possess this property.

28. Urinary Excretion of Prophyrins
Urinary Excretion Disorder
ALA (δ- amino levulinic acid )
PBG (Porphobilinogen )
Acute intermittent porphyria (AIP)
CP (coproporphyrinogen) Erythropoeitic porphyria
UP (Uroporphyrinogen) Acquired porphyrias
PP (Protoporphyrin ) Acquired porphyrias

29. What is hematin
Q: Functions of Heme
Heme is the prosthetic group of Hemoglobin & Myoglobin,
cytochrome-P450 cytochrome-c,a,a3,b, (Electron transfer
& involve ETC), tryptophan pyrrolase, Catalase &
Peroxidase)
Q: Major substrates for heme syn -Glycine & Succinyl CoA.
Regulatory enzyme in heme synthesis
ALA dehydratase & Ferrochelatase inhibited by Lead
ALA synthatase
Fe2+ of heme is oxidized to Fe+++ (ferric)
On standing Acute intermittent porphyria patients
urine turns colorless to more color why ?
Due to photo-oxidation of PBG
to porphobilin

30. • Q: 10 year old man presented with l week history of episodic severe abdominal pain
associated with vomiting 3-4 times per day and dark reddish in urine. He had 2 episodes
of similar abdominal pain along with generalized epileptic for which he was on treatment.
On physical examination, neurological & abdominal examination was unremarkable.
There was no photosensitivity.
Lab examination shown Hb of 11 gms% and urine was strongly positive for
phorphobilinogen (PBG).
– Suggest the probable diagnosis
– What is the biochemical basis for the above mentioned lab findings
– Explain the pathway implicated in this condition

31. Over thinking is the biggest waste of
human energy.
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