Structure. synthesis and porphyrias of heme. In service of medicne.

1. V.S.RAVIKIRAN, MSc.
STRUCTURE OF HEMOGLOBIN

2. V.S.RAVIKIRAN, MSc.,
Department of Biochemistry,
ASRAM Medical college,
Eluru-534005.AP, India.
vsravikiran2013@gmail.com

3. STRUCTURE OF HEMOGLOBIN

4. STRUCTURE OF HEMOGLOBIN
• Normal level of Hemoglobin (Hb) in blood in
males is 14–16 g/dL and in females, 13–15
g/dL. Hb is globular in shape.
• The adult Hb (HbA) has 2 alpha chains and 2
beta chains.
• Molecular weight of HbA is 67,000 Daltons
(66,684 to be exact).

5. STRUCTURE OF HEMOGLOBIN

6. STRUCTURE OF HEMOGLOBIN
• Hb F (fetal Hb) is made up of 2 alpha and 2
gamma chains.
• Hb A2 has 2 alpha and 2 delta chains.
• Normal adult blood contains 97% HbA, about
2% HbA2 and about 1% HbF.

7. STRUCTURE OF HEME
• Alpha chain gene is on chromosome 16 while
the beta, gamma and delta chains are on
chromosome 11.
• Each alpha chain has 141 amino acids. The
beta, gamma and delta chains have 146 amino
acids.

8. STRUCTURE OF HEME
• There are 36 histidine residues in Hb
molecule; these are important in buffering
action.
• The 58th residue in alpha chain is called distal
histidine, because it is far away from the iron
atom. The 87th residue in alpha chain is called
proximal histidine, because it lies nearer to
the iron atom

9. Attachment of Heme with Globin
Chain
There are 4 heme residues per Hb molecule, one
for each subunit in Hb.
The iron atom of heme occupies the central
position of the porphyrin ring.
The reduced state is called ferrous (Fe++) and the
oxidized state is ferric (Fe+++).
The ferrous iron has 6 valencies and ferric has 5
valencies.
In hemoglobin, iron remains in the ferrous state.

10. Linkage of heme with globin. Pink circle represents the globin chain.
Blue rectangle represents the protoporphyrin ring.

11. STRUCTURE OF HEME
• Hemoglobin is a conjugated protein having
heme as the prosthetic group and the protein,
globin.
• It is a tetrameric protein with 4 subunits, each
subunit having a prosthetic heme group and
the globin polypeptide.

12. STRUCTURE OF HEME
• The polypeptide chains are usually two alpha
and two beta chains.
• Hemoglobin has a molecular weight of about
67,000 Daltons.
• Each gram of Hb contains 3.4 mg of iron.

13. Heme is present in:
• a. Hemoglobin
• b. Myoglobin
• c. Cytochromes
• d. Peroxidase
• e. Catalase
• f. Tryptophan pyrrolase
• g. Nitric oxide synthase.

14. Heme
• Heme is produced by the combination of iron
with a porphyrin ring.
• Chlorophyll, the photosynthetic green
pigment in plants is magnesium-porphyrin
complex.

15. Structure of Heme
• Heme is a derivative of the porphyrin.
• Porphyrins are cyclic compounds formed by
fusion of 4 pyrrole rings linked by methenyl
(=CH-) bridges
Pyrrole ring

16. Structure of Heme
• Since an atom of iron
is present, heme is a
Ferroprotoporphyrin.
• The pyrrole rings are
named as I, II, III, IV
and the bridges as
alpha, beta, gamma
and delta.
• The possible areas of
substitution are
denoted as 1 to 8.
Porphyrin ring

17. Structure of Heme
• When the
substituent groups
have a symmetrical
arrangement (1, 3, 5,
7 and 2, 4, 6, 8) they
are called the I
series.
• The III series have an
asymmetrical
distribution of
substituent groups
(1, 3, 5, 8, and 2, 4,
6, 7).

18. Structure of Heme
• Type III is the most predominant in biological
systems. (It is also called series 9,)
a. Propionyl (–CH2–CH2–
COOH) group.
b. Acetyl (–CH2–COOH)
group.
c. Methyl (–CH3) group
d. Vinyl (–CH=CH2)
group.
Complete structure of
heme is shown in Figure.
Structure of heme

19. Structure of Heme

20. BIOSYNTHESIS OF HEME
• Heme can be synthesized by almost all the
tissues in the body.
• Heme is synthesized in the normoblasts, but
not in the matured erythrocytes.
• The pathway is partly cytoplasmic and partly
mitochondrial.

22. Step 1: ALA Synthesis
Condensation of succinyl CoA and glycinein the presence of pyridoxal
phosphate to form delta amino levulinic acid (ALA).

23. Step 2: Formation of PBG
ALA dehydratase
- lead+ Zn
In the cytoplasm, two molecules of ALA are condensed to form
porphobilinogen (PBG).
(monopyrrole)

24. 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 (-CH2-).
PBG-deaminase
uroporphyrinogen III synthase

25. Step 3: Formation of UPG
Condensation occurs in a headto-tail manner, so that a
linear tetrapyrrole is produced; this is named as
hydroxy methyl bilane (HMB).

26. Step 4: Synthesis of CPG
uroporphyrinogen
decarboxylase
The UPG-III is next converted to coproporphyrinogen (CPG-III) by
decarboxylation. The acetate groups (CH2–COOH) are
decarboxylated to methyl (CH3) groups

27. Step 5: Synthesis of PPG
• Further metabolism takes place in the mitochondria
Two propionic acid side chains are oxidatively
decarboxylated to vinyl groups
coproporphyrinogen
oxidase

28. Step 6: Generation of PP
The protoporphyrinogen-III is oxidized to protoporphyrin-III (PPIII).
The methylene bridges (–CH2) are oxidized to methenyl bridges
(–CH=) and colored porphyrins are formed. Protoporphyrin-9 is
thus formed.
protoporphyrinogen
oxidase

29. Step 7: Generation of Heme
• The last step is the attachment of ferrous iron to the
protoporphyrin in mitochondria. Iron atom is coordinately
linked with 5 nitrogen atoms (4 nitrogen of pyrrole rings of
protoporphyrin and 1st nitrogen atom of a histidine residue
of globin). The remaining valency of iron atom is satisfied
with water or oxygen atom.
Heme synthase or
ferrochelatase
(- lead)
iron atom is co-
nately linked with
nitrogen atoms

30. Summary of heme biosynthesis. The numbers denote the
enzymes. Part of synthesis is in mitochondria, and the rest in
cytoplasm.

31. Regulation of Heme Synthesis
ALA synthase is regulated by repression
mechanism. Heme inhibits the synthesis of
ALA synthase by acting as a co-repressor.
Repression by heme on the enzymes responsible for
heme synthesis

32. Regulation of Heme Synthesis
ALA synthase is also allosterically inhibited by
hematin.
The compartmentalization of the enzymes of
heme synthesis. The rate-limiting enzyme is in
the mitochondria.
Drugs like barbiturates induce heme synthesis
and require the heme containing cytochrome
P450 for their metabolism.

33. Regulation of Heme Synthesis
The steps catalyzed by ferrochelatase and
ALAdehydratase are inhibited by lead.
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
administration of glucose to relieve the acute
attack of porphyrias.

34. Disorders of Heme Synthesis
• Porphyrias are a group of inborn errors of
metabolism associated with the biosynthesis
of heme. (Greek ‘porphyria’ means purple).
• These are characterized by increased
production and excretion of porphyrins and or
their precursors (ALA + PBG).
• Most of the porphyrias are inherited as
autosomal dominant traits.

35. Porphyrias may be broadly grouped
into 3 types:
• a. Hepatic porphyrias
• b. Erythropoietic porphyrias
• c. Porphyrias with both erythropoietic and
hepatic abnormalities.
This classification is based on the major site, where the
enzyme deficiency is manifested. The clinical manifestations
vary.
Porphyrias in general, are not associated with anemia.

36. Enzyme
deficiencies in
porphyrias

37. Features of important types of
porphyrias

38. Porphyria cutanea tarda

39. Porphyria cutanea
tarda. (A-B) Sun-
exposed hands of a PCT
patient showing areas
of atrophy and scarring.
(C) Urine from a
symptomatic PCT
patient and a healthy
control in daylight (left)
and under ultraviolet
light (right). The PCT
urine has an orange-red
color in daylight that
fluoresces red under
ultraviolet light.

40. Urine may appear purple during an
attack or after standing in the light

41. Congenital Erythropoietic Porphyria
When UV light is reflected on to teeth a red
fluorescence is seen; this is called
erythrodontia.

42. Erythropoietic
protoporphyria.
(A) An EPP patient after sun
exposure. Note the reddish
and swollen appearance of
her face and (B) scarring and
thickening of the skin on the
dorsum of her hand because
of multiple sun/light
exposures.

43. Repeated ulceration and scarring may cause mutilation
of nose, ear and cartilage. This may mimic leprosy.

44. Acquired Porphyrias
• A characteristic difference from congenital porphyrias is that
there is associated anemia in the acquired variety.

45. Acquired Porphyrias
Causes of acquired porphyrias (cont.)

46. Urinary excretion of porphyrias

47. THE END
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