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- BIOLOGY AND CHEMISTRY (2.3)
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B io Factsheetwww.curriculum-press.co.uk Number 175
Haemoglobin: Structure & Function
Haemoglobin is the oxygen – carrying pigment in red blood cells. 1. Oxygen diffuses across the alveoli into the haemoglobin in red cells
It is a globular protein consisting of four tightly packed polypeptide 2. Haemoglobin has a high affinity for oxygen in the high oxygen
chains. concentration that exist there
There are two identical alpha chains and two identical beta chains The point is that haemoglobin doesn’t have a high affinity no
(Fig 1). matter what the oxygen concentration is! If it did, it would never
let go of the oxygen!
Fig 1. Molecule of human haemoglobin. 3. Oxygen atoms combine with the haem groups of the
α-polypeptide chain Haem group β-polypeptide chain haemoglobin. The addition of one oxygen molecule to the first
haem group distorts the shape of the haemoglobin molecule,
making it easier for the second oxygen molecule to combine with
haem. This, in turn, makes it easier for the third oxygen molecule
to combine with a third haem group. It then becomes a little
harder for the fourth oxygen molecule to join the fourth haem
group. This trend explains the sigmoid shape of the oxygen
dissociation curve.
4. Oxyhaemoglobin is formed;
5. Haemoglobin unloads / has a low affinity for oxygen in areas eg
the respiring tissues, where oxygen is in low concentration;
6. This is a consequence of its dissociation curve i.e. small changes
in concentration gives large changes in saturation;
7. Respiration in tissues gives high CO 2 concentration, high
temperature and high H+ concentration
9. Under these conditions the dissociation curve shifts to the
β-polypeptide chain α-polypeptide chain right (Fig 2)
Fig 2. The effect of pH on the ODC (The Bohr Effect)
• The hydrophobic parts of the chains point inwards towards the
centre of the molecule and interactions between these 100
hydrophobic parts help to maintain the 3-D shape
• The hydrophilic parts point outwards and help maintain 1
saturation of Hb with oxygen (%)
haemoglobin’s solubility 80
74 2 1 pH 7.6
• Each polypeptide chain contains an iron-containing haem group
• Each haem group can bind with one oxygen molecule, so each 2 pH 7.4
60
molecule of haemoglobin can carry 4 oxygen molecules at a time. 59 3 3 pH 7.2
• The aggregation of several (4) polypeptide chains gives
haemoglobin a quaternary structure. 40
38
Typical Exam Questions
Q1. Use Fig 1 to explain why the haemoglobin molecule has a
20
quaternary structure.
Answer: It is made of several/ four polypeptides; Note it has
nothing to do with the fourth dimension of protein structure pO2/kilopascals
0 2 4 6 8 10 12 14
Q2. Why do all human haemoglobin molecules always have
At a ppO2 of 4 kilopascals, how saturated is the haemoglobin at:
identical tertiary structure?
pH 7.6 ?…………………answer = 74%
Answer: Because they contain an identical sequence of amino pH 7.4 ?…………………answer = 59%
acids and thus the bonds always occur in the same positions pH 7.2 ?…………………answer = 38%
You can see that the curve has moved to the right
By far the most common exam question concerns how haemoglobin As the tissues get more acidic, haemoglobin releases more oxygen
picks up oxygen in the lungs and offloads it at the respiring tissues. (becomes less saturated). Increasing temperature and increasing
To get good marks, precision is vital. Here are the key facts to learn. CO2 concentrations have just the same effect.
These are the conditions in the respiring tissues (high temperatures
and CO2 from respiration, high H+ concentrations from dissociation
of bicarbonate ions or production of lactic acid.
1
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