Hemoglobin derivative and disorders

1. Haemoglobin
Presentation:
ShahzebHussain
Clinical
Biochemistry

2. Haemoglobin
Introduction
 Hemoglobin is an oxygen-binding protein found in erythrocytes that transports oxygen from the
lungs to tissues.
Hemoglobin is formed by the combination of heme with globin (protein). Globin is made up of four
polypeptide chains (an oligomeric protein). Two of these polypeptides are known as alpha (α) and
the other two are known as beta (β).
Each alpha chain has 141 amino acids and each beta chain has 146 amino acids, which are arranged
in a definite sequence. Its molecular weight is 65,000.

3. Hemoglobin
Due to the characteristic folding of its tertiary structure each polypeptide forms a cup like structure
with a pocket like area where the prosthetic group, heme is buried. Heme has iron, which is linked to
the imidazole nitrogen of the histidine in positions 58 and 87 of the alpha chains. In the beta chain
the heme iron is linked with histidine in positions 92 and 63. Altogether there are four heme groups
in one hemoglobin molecule.

4. Hemoglobin
Composition
Heme:
It is an iron-porphyrin compound. It is the prosthetic group embedded in the packet like structure
formed by folding of the hemoglobin tertiary structure.
Porphyrin
Porphyrin is a complex compound with a tetrapyrrole ring structure.
Early stage of erythrocyte cells contain porphyrin, during the course of their development, porphyrin
is converted to heme by addition of iron and then to hemoglobin by addition of protein, globin.

5. Haemoglobin
Function
Haemoglobin has two main functions in human body.
1. It gives the red colour to the blood.
2. Its main function is to transfer oxygen from lungs to all parts of body and take carbon dioxide
back from body to lungs to expel out.

6. Metabolism
The major source of heme is hemoglobin found in RBC’s. When the RBC end its life after 120 days
the hemoglobin molecule is degraded. The amino acids from the globin and iron are recycled while
the porphyrin is degraded. Bilirubin is the end product of heme metabolism.

7. Derivatives
There are some derivatives of normal Hb that arise due
to metabolic changes in the RBC.
The various hemoglobin derivatives are:
1. Oxyhemoglobin (HbO2):
The main function of hemoglobin is to transport
oxygen from the lung to the tissues. In lungs the
partial pressure of oxygen is 100 mm of Hg, at this
pressure hemoglobin is 95-96% saturated with
oxygen. On binding with O2 in the lungs
hemoglobin is converted to oxy-hemoglobin
(Hb02). O2 is bound to heme iron.
Hb + O2 → HbO2

8. Derivatives
2. Reduced Hemoglobin (HHb):
Oxy-hemoglobin moves to the tissue where the partial pressure of O2 is 26 mm of Hg due to which
oxygen is released into the tissues and in turn H+ binds to Hb and forms reduced hemoglobin.
HbO2 + H+ → HHb + O2
3. Carbaminohemoglobin:
Hemoglobin also binds to CO2 in the tissues. CO2 is bound to the α-amino group at the N-terminal
end of each of the four polypeptide chains of hemoglobin to form carbaminohemoglobin. As one
CO2 binds O2 is released.

9. Derivatives
4. Methemoglobin:
In RBC the iron of hemoglobin is normally in ferrous (Fe2+) form, but
it is readily oxidized to the ferric (Fe3+) form by hydrogen peroxide
formed by RBC cell metabolism, to yield methemoglobin. Ferric iron is
incapable of binding O2 therefore the functions of hemoglobin are
disturbed. Normally 1.7 to 2.4 % of total hemoglobin will be in the
form of met-hemoglobin. Increase in the percent of methemoglobin is
prevented by the peroxidase action of a naturally occurring peptide
known as glutathione present in the RBC. Methemoglobin is dark
brown in colour.

10. Derivatives
5. Carboxyhemoglobin:
Oxy-hemoglobin can bind to carbon monoxide (CO). Even normal, non- oxygenated hemoglobin
can bind with CO to form carboxyhemoglobin. [Hb + CO → HbCO]. CO has got an affinity of 200
times more than that of O2 towards Hb. Hemoglobin can bind more readily to CO than to O2. Even
if there is a little amount of CO in air, it can displace oxyHb to form carboxyHb. Due to this there will
be tissue hypoxia because the oxygen binding capacity is reduced and there is also reduced O2
releasing capacity i.e. it cannot release O2 though it may be bounded to O2.

11. Abnormality in haemoglobin
1. HbS or Sickle Cell Hemoglobin:
Sickle cell hemoglobin (HbS) arises due to the defect in β chain in which glutamic acid present at
the 6th position is replaced by valine. Valine is also present naturally at position one. These two valine
residues form hydrophobic interaction producing a sticky patch on HbS. Due to this replacement
there is a sticky patch on HbS which appears on the oxy HbS. There is a complementary site to this
sticky patch on deoxy HbS and also on deoxy HBA.

12. Abnormality in haemoglobin
When hemoglobin molecules combine together in chains they form precipitates of HbS. The precip-
itate formed in the RBC sinks down and the biconcave shape of RBC is converted to sickle shape.
The life span of RBC is reduced to less than half (about 30 days). HbS is very unstable, due to which
there is excessive hemolysis. This results in anemia called sickle cell anemia. The physiological
changes observed in sickle cell anemia are – physical exertion, weakness, short of breath, leukemia
and heart murmurs.

13. Abnormality in haemoglobin
HbM or Methemoglobin:
The defect lies both in α and β chains. This is due to replacement of histidine residue in 58th position
in α chain and 63rd position in β chain. Due to this replacement, the iron (Fe) present in the ferrous
state is oxidized to ferric state. This ferric iron cannot bind oxygen. Therefore the oxygen carrying
capacity is disrupted leading to anemia and hypoxia (low O2 to tissues).

14. Abnormality in haemoglobin
Thalassemia’s:
The defect in thalassemia’s is the decreased rate of synthesis of one of the polypeptide chains of the
globin molecule. One of the chains is synthesized in less amounts than the other due to the defect
in DNA.
There are two types of thalassemia:
1. β-thalassemia:
β-thalassemia occurs due to the decreased synthesis rate of P-chain of globin. Due to the deficiency
of P-chain the a-chains either combine among themselves forming a-4-globin or it can combine
with γ or δ chains, thereby forming more of HbA2 and HbF. This results in the impairment of the
transport of O2 by Hb resulting in hypoxia.

15. Abnormallty in haemoglobin
There are very low levels of Hb i.e. 2-3 g/100ml (hypochromic cells). The life span of such RBC is
greatly reduced. The symptoms include anemia, growth retardation, wasting and fever.
2. α-thalassemia:
α-thalassemia occurs due to the decreased rate of synthesis of α-chain of globin. But this is rarely
seen due to the presence of two pairs of genes for a chain in the Hb gene. Due to lack of a chain,
the β chain may combine either with δ, γ or among itself forming β4, or β2δ2 or β2γ2.

16. Abnormality in haemoglobin
Pyruvate Kinase Deficiency:
It is an inherited autosomal recessive genetic disorder which affects the survival of red blood cells.
Pyruvate kinase deficiency is the second most common cause of enzyme- deficient hemolytic
anemia, following G6PD deficiency. A variety of mutations can lead to lowered production, activity, or
stability of pyruvate kinase, an enzyme essential to glycolysis. A total lack of this enzyme’s activity will
be lethal.
Red blood cells use glycolysis as their sole energy source. In pyruvate kinase deficiency, the last step
(phosphoenolpyruvate converted to pyruvate) of glycolysis is unable to occur.

17. Abnormality in haemoglobin
A discrepancy between red blood cell energy requirements and ATP generating capacity produces
irreversible membrane injury resulting in cellular distortion, rigidity and lysis. This leads to premature
erythrocyte destruction by the spleen and liver.

18. Thank you
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