It consists of the protein globin (polypeptide) united with the pigment haem (heme). there are 4 haem to the one molecule of haemoglobin, contains 4 iron atoms and can carry 4 molecules (8 atoms) of oxygen. Haemoglobin reacts with oxygen very rapidly requiring less than 0.01 second. Similarly, deoxygenation of haemoglobin is also very rapid.
Molecular weight of haemoglobin is 68,000.
β’Note – Approx. 0.3 gm of haemoglobin is destroyed and
0.3 gm synthesized every hour.
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STRUCTURE
l. Haem is an iron containing porphyrin, called iron protoporphyrin IX.
The porphyrin nucleus is tetrapyrrole
i.e. it consists of 4 ‘pyrrole rings’ joined together by
4 methine (= CH-) bridges.
The pyrrole rings are
numbered I, II, III and IV; the carbon atoms of the
methine bridges are labelled a, ~, y and o; the position
on pyrrole rings to which side chains are attached are
numbered 1 to 8. The side chains at 1, 3, 5 and 8 position
are methyl (-CH:0; 2 and 4 are vinyl (-CH= CH2
); 6 and
7 are propionic acidπ (-CH2.CH2.COOH).
2. πThe iron in haem is in the ferrous (Fe2+) form.
The
iron is attached to the ‘N’ of each pyrrole ring. Each
Fe2+ combines loosely and reversibly with one molecule
of oxygen. Combination of haem with oxygen is
called oxygenation and not oxidation, because, after
combination with oxygen, iron in the haem stays in Fe2+
state. Therefore, the oxygen does not become ionic oxygen
but is carried as molecular
oxygen.
3.Globin is a protein built
from 4 polypeptide chains,
two ‘ a ‘ and two ‘W chains.
Therefore, the normal
adult haemoglobin (HbA)
is written as HbA (CJ.i~2).
Of two a-chains each
contains 141 amino-acids
and of two ~-chains each
contains 146 amino-acids.
Each polypeptide chain is associated with one haem
group. Thus, there are 4 haem to the one molecule
of haemoglobin, contains 4 iron atoms and can carry
4 molecules (8 atoms) of oxygen.
4. Oxygenation of 1st haem molecule in haemoglobin,
increases the affinity of 2nd haem for oxygen and
oxygenation of 2nd haem increases the affinity of the ” ~–
3rd and so on. Therefore, the affinity of haemoglobin β’
for the 4th oxygen molecule is many times that of the
1st. This shifting affinity of haemoglobin for oxygen
results in:
(i) Sigmoid shape of oxygen-haemoglobin dissociation
curve.
(ii) Haemoglobin reacts with oxygen very rapidly requiring less than 0.01 second. Similarly, deoxygenation of haemoglobin is also very rapid.
5. Molecular weight of haemoglobin is 68,000.
SOME IMPORTANT DEFINITIONS
1. OXYHAEMOGLOBIN – Haemoglobin reacts with oxygen to form oxyhaemoglobin
and is represented as Hb02.
The affinity of haemoglobin for oxygen is influenced
by pH, temperature and concentration of 2, 3, diphosphoglycerate (2,3 DPG) in the RBCs, a product of metabolism
of glucose.
As concentration of 2,3 DPG rises, the affinity
of haemoglobin for oxygen falls and the oxygen-haemoglobin
dissociation curve is shifted to the right; as a result more
oxygen is released by blood to the tissues.
Important Notes
1. At high altitude (e.g. 5000 metres above sea level)
2,3 DPG concentration in RBCs increases by 50%
and this makes more oxygen available to the
tissues.
2. Stored blood loses its 2,3 DPG and oxygen affinity
of haemoglobin increases resulting in less release
of oxygen.
2. CARBAMINO-HAEMOGLOBIN – Carbon dioxide reacts with haemoglobin to form cnrbnminohaemoglobi11.
CO2 + HbNHz HbNH COOH
3. REDUCED (DEOXYGENATED) HAEMOGLOBIN – Haemoglobin from which oxygen has been removed
is called reduced or deoxygenated haemoglobin and is
represented as Hb.
4. CARBOXY HAEMOGLOBIN
or CARBON MONOXY HAEMOGLOBI
Carbon monoxide (CO) reacts with haemoglobin to
form carboxy haemoglobin or carbon monoxy haemoglobin.
The affinity of haemoglobin for CO is 210 times than its
affinity for oxygen which consequently displaces oxygen
on haemoglobin, reducing the oxygen carrying capacity
of blood.
5. METHAEMOGLOBIN – When either reduced or oxygenated haemoglobin is
exposed to various drugs or oxidising agents, the ferrous
(Fe2+) is oxidised to ferric (Fe3+) form and the compound
is called methaemoglobin. It is represented as HbOH.
Disadvantages β§
(i) It cannot unite reversibly with gaseous oxygen.
(ii) It is dark coloured and when it is present in large
quantities (more than 1.5% gm/ dL) in circulation it
resembles cyanosis i.e. blue colouration of skin.
Some oxidation of haemoglobin to methaemoglobin
occurs normally, but an enzyme in RBCs, the
NADH (dihydronicotinamide adenine dinucleotide)-
methaemoglobin reductase system, converts methaemoglobin back to haemoglobin. Congenital absence of this system causes hereditanJ metliaemog lobillemia, a fatal
condition.
Haemoglobin level and NORMAL VALUES β§
1. At birth: 23 gm/ dL, because RBC count is more.
2. At the end of 3 months: 10.S gm/ dL, as an infant is totaUy on milk feed which is devoid of iron.
3. After 3 months, haemoglobin increases gradually and at the end iof 1 year it becomes 12.S gm/dL.
4. Adults Males : 14-18 gm/ dL (Average: 15.S gm/dL) Females: 12-15.5 gm/dL (Average: 14 gm/ dL) Clinically 14.8 gm/ dL haemoglobin irrespective of sex is regarded as 100% haemoglobin. When blood is equilibrated with 100% oxygen (p02 = 760 mmHg), the normal haemoglobin becomes 100% saturated. 1 gm/ dL haemoglobin when fully saturated combines with 1.34 mLoxygen, therefore, haemoglobin concentration is an index of oxygen-Carrying Capacity of blood.
Normal values – ma/es: 21 mL/ dL,femnles: 18 mL/ dL.
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FUNCTIONS OF HAEMOGLOBIN
1. Facilitate transport of oxygen from lungs to the
tissues.
2. Facilitate transport of CO2 from the tissues to the lungs .
3. It acts as an excellent acid-base buffer, being a protein . It is responsible for 70% buffering power of
whole blood.
4. It has additional nitric oxide (NO) binding site on
the ~-chain which is increased by 0 2. Therefore,
haemoglobin binds with NO in the lungs and releases
it in the tissues where it promotes vasodilation.
DISADVANTAGES OF ‘FREE’ HAEMOGLOBIN
(Why haemoglobin is contained within the RBCs?)
1. If haemoglobin was dissolved in the plasma (called free
haemoglob;in) it would lead to:
(i) increase in the viscosity of plasma, hence of whole
blood, causing BP to rise, and
(ii) increase in the osmotic pressure of plasma to
100 mmHg.
(i) and (ii) interfere with the mechanism of fluid
exchange between capillaries and tissue spaces.
2. Loss of free haemoglobin by the kidneys in urine
(hemoglolmmria)) also results in kidney damage.
3. Free haemoglobin is taken up and rapidly destroyed by
the tissue-macrophage system.
SYNTHESIS OF HAEMOGLOBIN
Synthesis of haemoglobin requires the provision of
nutrients e.g. proteins, vitamins, minerals (specially iron).
It only takes place in the developing RBCs .
Factors controlling haemoglobin formation
1. Role of proteins – A low protein intake decreases
haemoglobin regeneration even in the presence of
excess of iron; the limiting factor here is lack of globin
formation.
2. Role of Minerals
(i) Iron
(a) it helps in formation of haem;
(b) iron content of haemoglobin is 0.33%, therefore
100 mL of blood containing 15 gm of haemoglobin contains 15 x 0.33/ 100 = approx. 50 mg of iron.
(c) as life span of RBC is 120 days, therefore, 0.8%
(1/ 120 x 100) of total blood haemoglobin
contained in 50 mL (6 litres x 0.8%) of blood,
is destroyed daily, releasing approx. 25 mg of
iron. This iron is reused for fresh synthesis of
haemoglobin.
(ii) Copper – helps in promoting the absorption,
mobilization and utilization of iron. Very little
copper is required; adequate amount occurs in
diet and most iron preparations contain traces of
copper.
(iii) Cobalt – is necessary for manufacture of
vitamin B12 by bacterial action in the lumen of
GIT. It also increases the production of a hormone
erythropoietin which, in turn, stimulates
the development of RBCs.
(iv) Calcium – increases iron absorption from GIT.
3. Role of vitamins – Vitamin C, vitamin B12 and folic
acid help in synthesis of nucleic acid which in tum, is
required for the development of RBCs.
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