110 Section 3 ➯ Blood and Immune System
Therefore, the normal adult haemoglobin A is written as Two propionic acid (−CH2 CH2 COOH) side
HbA (α2β2). chains at position 6 and 7.
Structure of haem The iron
The iron in the haem is in ferrous (Fe2+) form.
The haem is an iron–porphyrin complex called iron–
The iron is attached to the nitrogen atom of each pyrrole
protoporphyrin IX, i.e. it consists of a porphyrin nucleus
ring.
and the iron. The structural characteristics of the haem
On the iron (Fe2+) a bond is available for loose union,
(iron–protoporphyrin IX) are given below (Fig. 3.2-9):
where:
Porphyrin nucleus In oxyhaemoglobin, O2 is attached,
In carboxyhaemoglobin, CO is attached, and so on (see
The porphyrin nucleus consists of four pyrrole rings
derivatives of Hb).
numbered I, II, III and IV, i.e. porphyrins are
tetrapyrroles. Attachment of haem to globin
The pyrrole rings are joined together by four methine
One molecule of Hb contains four units of haem, each
bridges (=CH−). The carbon atoms of methine bridges
attached to one of the four polypeptide chains constituting
are labelled α, β, γ and δ.
globin (Fig. 3.2-9D). As there are four units of haem in one
Eight side chains are attached to the pyrrole ring at posi-
molecule of Hb, so there are four iron atoms in one mole-
tions labelled 1–8. These are:
cule of Hb which can carry four molecules (eight atoms) of
– Four methyl (H3C) side chains at position 1, 3, 5 and 8.
oxygen.
– Two vinyl (−CH CH2) side chains at position 2 and 4.
FUNCTIONS OF HAEMOGLOBIN
CH CH
1. Transport of O2 from lungs to tissues
CH CH In the lungs, one molecule of O2 is attached loosely and
N N reversibly at the sixth covalent bond of each iron atom of
H H
the Hb to form oxyhaemoglobin represented as HbO2:
A B
Hb + O2 → HbO2
1 2 3 4 Deoxygenated Oxygenated
H3C CH CH2 α H 3C CH CH2 (reduced) haemoglobin haemoglobin
I H II
C
N N Oxygenation of first haem molecule in the Hb increases
δ HC
Attaches loosely
to one O2 molecule Fe++ To globin CH β the affinity of second haem for oxygen which in turn
N N increases the affinity of third haem and so on. In this
C
IV H III way, the affinity of Hb for fourth oxygen molecule is
8 H3C CH2 7 γ 6 H2C CH3 5 many times that for the first molecule.
CH2 H2C The affinity of Hb for oxygen is influenced by pH, tem-
perature and concentration of 2,3-diphosphoglycerate,
COOH C HOOC
i.e. 2,3-DPG (a product of metabolism of glucose) in the
Haem Fe2+ Fe2+ Haem RBCs.
2. Transport of CO2 from the tissues to the lungs
Hb also transports CO2 from the tissues to the lungs.
It is important to note that the CO2 from the tissues is
transported by combining with amino acids of the globin
part as shown below and not in combination with Fe2+ atom
like O2.
Haem Fe2+ D Fe2+ Haem
H H
Fig. 3.2-9 Chemistry of haemoglobin: A, structure of a pyrrole R−N + CO2 → R N
H COOH
ring; B, conventional outline of a pyrrole ring; C, arrangement
Haemoglobin Carbamino-haemoglobin
of pyrrole rings in one unit of haem (iron protoporphyrin IX) and
D, arrangement of four units of haem in one molecule of Deoxygenated Hb forms carbamino-haemoglobin more
haemoglobin. readily than oxygenated Hb. That is why venous blood
3
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Chapter 3.2 ➯ Red Blood Cells and Anaemias 111
becomes more suitable for the transport of CO2 from the pressure. It is owing to poor binding of 2,3-DPG by the γ
tissues to the lungs. polypeptide chain. Because of this, movement of oxygen
from maternal to fetal circulation is facilitated.
3. Control pH of the blood Resistance to action of alkalies is more in HbF than HbA.
This property is used in a photoelectric calorimetric
The Hb constitutes the most important acid–base buffer
method to estimate HbF in the presence of HbA.
system of blood. Hb has six times the buffering capacity
Life span of HbF is much less (1–2 week) as compared to
as compared to the plasma proteins.
that of HbA (120 days).
VARIETIES OF HAEMOGLOBIN
Haemoglobinopathies
Various varieties of Hb can be grouped as under:
Haemoglobinopathies, i.e. abnormal formation of haemo-
Physiological varieties of Hb and globin occurs due to the disorders of globin synthesis; haem
Haemoglobinopathies. synthesis being normal. Disorders of the globin synthesis
are of two main types:
Physiological varieties of haemoglobin
Formation of abnormal polypeptide chains due to substi-
Adult haemoglobin or haemoglobin A [HbA (α2β2)]. (see tution of an abnormal amino acid chain in the HbA.
page 109). Adult Hb is of two types. Example of such a disorder is haemoglobin S.
(i) Haemoglobin A [HbA (α2β2)]. It is the main form of nor- Suppression of synthesis of polypeptide chain of globin
mal adult Hb. As described on page 109, its globin part con- as seen in thalassaemia.
sists of two α and two β polypeptide chains. It is a spheroidal 1. Sickle cell haemoglobin or haemoglobin S (HbS) is the
molecule with a molecular weight of 68,000. most important haemoglobinopathy
(ii) Haemoglobin A2 [HbA2 (α2δ2)]. It is a minor compo- It occurs in 10–20% of Negroes. Sickle cell gene has
nent (about 2.5% of the total Hb) in normal adults. Its globin originated in the black population in Africa.
part consists of two α and two δ polypeptide chains. δ chains HbS is formed due to substitution of valine for glutamic
have slightly different amino acid composition (out of 146, acid at position 6 in the β chain of HbA.When HbS is
10 amino acids are different) as compared to β chains. reduced (e.g. in low O2 tension or when pH at tissue
level is low), it becomes much less soluble and precipi-
Fetal haemoglobin or haemoglobin F [HbF (α2γ2)] as the
tates into crystals within the RBCs. The crystals elon-
name indicates refers to the Hb present in the fetal RBCs
gate producing changes in shape of the cells from
and gradually disappears 2–3 months after birth.
biconcave to sickle-shaped cells (sickling) (Fig. 3.2-10).
Amount of HbF and HbA present at various stages is as The cells containing HbS are less flexible as compared to
given in Table 3.2-3. the RBCs containing HbA, hence leading to a blockade
of microcirculation.
Structure of HbF is similar to that of HbA, except that its
Sickle-shaped cells greatly increase blood viscosity
globin part consists of two α and two γ polypeptide chains
thereby decreasing the blood flow to tissues.
(in place of β chains). γ chains also have 146 amino acids but
Sickle-shaped cells are more fragile and are very liable to
its 37 amino acids are different than that of β chains.
undergo haemolysis producing the so-called sickle cell
Special features of HbF are given below: anaemia. Sickle trait is inherited as Mendelian dominant
but the full blown disease is autosomally recessive.
Affinity for oxygen in case of HbF is more than that of
Heterozygous individual with sickle cell trait rarely has
HbA, i.e., it can take more oxygen than HbA at low oxygen
Table 3.2-3 Amount of HbF and HbA at various stages Rod-like polymers
in human beings
Stage HbF (%) HbA (%)
At 20 weeks of intrauterine 94 6
O2
At birth 80 20
At 2 months after birth 50 50 Normal red cells Deoxyhaemoglobin Sickle shaped
(oxyhaemoglobin) (HbS molecule)
At 4 months after birth 10 90
Fig. 3.2-10 Mechanism of sickling of red blood cell containing
At more than 1 yr after birth <1 > 99
haemoglobin S (HbS).
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112 Section 3 ➯ Blood and Immune System
Table 3.2-4 Main differentiating features of β thalassaemia—major and minor
S. No. b thalassaemia major b thalassaemia minor
1. Thalassaemia major is also called as Mediterranean anaemia Thalassaemia minor is more common.
or Cooley’s anaemia and is less common.
2. It is inherited as a homozygous transmission (i.e. abnormal It is inherited as a heterozygous transmission (i.e. abnormal
genes are inherited from both the parents) therefore: gene is inherited from one parent), therefore:
There is complete absence of β chain synthesis. The synthesis of β chain is not completely absent (partial).
Absence of β chain synthesis results in moderate to severe Anaemia is of mild type.
anaemia.
HbF level is markedly increased. HbF level is either normal or slightly elevated.
3. The individual suffering from thalassaemia major has short The individuals suffering from thalassaemia minor comparatively
life span, i.e. (dies young 17–18 years). survive longer (up to adult) and transmit abnormal gene to
their offsprings.
severe symptoms but homozygous develop full blown 4. Carboxyhaemoglobin or carbon monoxyhaemoglobin
disease. is a compound of Hb with carbon monoxide (CO)
The individual with sickle cell trait has resistance to one
Hb + CO → COHb
type of malaria.
The affinity of Hb for CO is much more (200–250
2. Thalassaemia (Mediterranean anaemia) is a haemoglo-
times) than its affinity for oxygen. Because of this, the
binopathy characterized by following features:
CO displaces oxygen from Hb, thereby reducing the
Cause. Thalassaemia results due to defect in the synthe-
oxygen carrying capacity of the blood.
sis of polypeptide chain α and β of HbA.
5. Methaemoglobin. When reduced or oxygenated Hb is
Types. Depending upon whether α or β chains are not
treated with an oxidizing agent, e.g. potassium ferricya-
synthesized, α thalassaemia or β thalassaemia may occur,
nide, the ferrous Fe2+ is oxidized to ferric (Fe3+); the sixth
respectively. β Thalassaemia is more common and is fur-
bond is attached to OH to form the compound methae-
ther of two types: thalassaemia major and thalassaemia
moglobin. Methaemoglobin is represented as HbOH.
minor.
Disadvantages of methaemoglobin are:
Differentiating features of thalassaemia major and minor It cannot unite reversibly with gaseous oxygen; the
are depicted in Table 3.2-4. O2 of the attached OH is not given off in a vacuum.
6. Glycosylated haemoglobin is a derivative of haemoglo-
DERIVATIVES OF HAEMOGLOBIN (REACTIONS bin A present in very small amount, e.g. haemoglobin
OF HAEMOGLOBIN) A1C (HbA1C), in which glucose is attached to terminal
valine in the β chains. The level of glycosylated haemo-
Haemoglobin has the property to readily react with any
globin in the blood increases in poorly controlled
gas, other substance to form the so-called derivatives of
patients of diabetes mellitus.
haemoglobin. These include:
1. Oxyhaemoglobin. Haemoglobin reacts readily with SYNTHESIS OF HAEMOGLOBIN
oxygen to form oxyhaemoglobin which is an unstable
and reversible compound, i.e. oxygen can be released Haemoglobin is synthesized in the cytoplasm of intermedi-
from this compound. In this compound iron remains ate normoblasts.
in the ferrous state.
2. Reduced haemoglobin or deoxygenated haemoglobin Synthesis of haem
is formed when oxygen is released from the Haem is synthesized in the mitochondria. Steps of synthe-
oxyhaemoglobin.
sis are (Fig. 3.2-11):
HbO2 → Hb + O2
Succinyl-CoA (derived from the citric acid cycle in
(Oxyhaemoglobin) (Reduced haemoglobin)
mitochondria) and glycine are the starting substances in
3. Carbamino-haemoglobin is a compound of Hb with the synthesis of haem. These condense to form α-amino-
carbon dioxide β-ketoadipic acid. The condensation requires pyridoxal
HbNH2 + CO2 → HbNHCOOH phosphate for activation of glycine.
3
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