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Haemoglobin

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Biology

Haemoglobin transports oxygen from the lungs to the rest of the body and carbon dioxide from respiring cells to the lungs.

Haemoglobin is a red, proteinous pigment found in red blood cells.

Oxygen cannot dissolve well in the blood plasma; consequently, it needs to be carried by haemoglobin throughout the body. As a pigment, haemoglobin also gives red blood cells their colour.

Haemoglobin structure

Let’s break the word ‘haemoglobin’ into two components - ‘haemo’ and ‘globin’. ‘Haemo’ represents the haem group, whereas ‘globin’ represents the protein. Thus, you may understand that haemoglobin consists of the haem groups and the protein chains (Figure 1).

Figure 1. Diagram of a haemoglobin molecule shows two alpha and beta chains, each with a haem group.

Details to the structure of haemoglobin can be summarised as:

  • A very large, globular, conjugated protein - where the suffix ‘globin’ comes from
  • The quaternary structure comprises four polypeptide chains - two alpha (alpha-globin) and two beta chains (beta-globin).
  • Four haem groups loosely each bound to one terminal polypeptide chain - carry four oxygen molecules in total (eight oxygen atoms), forming oxyhaemoglobin

Study tip: A quaternary protein structure consists of multiple tertiary structures combined. Do refer to the ‘Tertiary and quaternary protein structure’ document to refresh your memory on this!

Haem group structure

The haem group is the main contributor to the function of haemoglobin, as the haem group helps to carry oxygen molecules. Each haemoglobin molecule has four haem groups - one haem group at the terminal of each polypeptide chain (Figure 2).

Figure 2. Diagram of a haem group.

The haem group consists of two components:

  • A porphyrin ring (in blue) - provides structural support for the iron as it holds the iron in place in the protein chain.
  • An iron ion in the centre (in beige) - oxygen-carrying component of haem as oxygen binds to it during transport.

Below is a visual aid on the chemistry behind how oxygen binds to the iron ion of haem. Iron oxidises in the process.

Haemoglobin [+] diagram showing the binding of oxygen to haem [+] StudySmarter

Figure 3. Diagram showing the binding of oxygen to haem. CC BY-NC-SA 3.0 US Source: chem.libretexts

Haemoglobin binds reversibly to oxygen during transport. Once oxygen is bound to haemoglobin, it can also unbind afterwards, as stated by the equation:

Each haem group binds to one oxygen molecule, (i.e., two oxygen atoms). Given that there are four haemoglobin groups, one haemoglobin carries four oxygen molecules (i.e., eight oxygen atoms) in total.

Haemoglobin concentration definition

Haemoglobin concentration is the amount of haemoglobin present in one’s blood. The normal range for haemoglobin concentration in adult males is 8.7-11.2 mmol/L and in adult females is 7.4-9.9mmol/L.

Haemoglobin concentration increases beyond the normal range in a process called acclimatisation - the body adapts to low oxygen levels in regions of high altitudes by producing more haemoglobin. Therefore, the increase in haemoglobin increases the number of ‘oxygen carriers’, allowing more oxygen in the atmosphere to be carried in the blood.

Haemoglobin concentration beyond the normal range in humans signals certain health conditions. Anaemia is the condition when the haemoglobin concentration is too low. In contrast, haemoglobin concentration that is too high is a result of polycythaemia - a condition where the body produces too many red blood cells.

Haemoglobin dissociation curve

Now that you’ve learned about the structure of haemoglobin and refreshed your memory about its function, you may question how these concepts relate. Can the role of haemoglobin in the transport of oxygen be quantified?

To answer the above questions, you will learn about the haemoglobin dissociation curve.

The haemoglobin dissociation curve is a graph of haemoglobin (Hb) saturation on the y-axis against partial pressure of oxygen (PO2) on the x-axis.

Haemoglobin saturation - the amount of oxygen bound to the haemoglobin compared to the total number of binding sites available.

Partial pressure - the concentration of oxygen in the body.

The shape of the curve

It would be expected that when Hb saturation in the blood increases, the partial pressure will increase, creating a linear regression such as in Figure 4. This is NOT the case.

Haemoglobin [+] Hypothetical linear curve [+] StudySmarter

Figure 4. A hypothetical graph showing a linear relationship between partial pressure of oxygen and haemoglobin saturation. Source: StudySmarter Originals.

Instead, the graph takes a ‘weird’ shape known as a sigmoid (S-shaped) curve (Figure 5). This shape is crucial in the rationale behind oxygen transport.

Haemoglobin [+] labelled diagram of the haemoglobin dissociation curve [+] StudySmarter

Figure 5. Labelled diagram of the haemoglobin dissociation curve. Source: StudySmarter Originals.

Unloading means the release of oxygen into cells, whereas ‘loading’ is when oxygen from the lungs binds to haemoglobin for its transport—blood enclosed in blood vessels transport oxygen in the haemoglobin.

The first thing to notice is that the graph in Figure 5 is split into three areas by two dotted lines:

  • Area leftwards to the dotted line at 50% haemoglobin (Hb) saturation - the unloading of oxygen
  • Area rightwards to the dotted line at almost 100% Hb saturation - the loading of oxygen
  • Area in between the two dotted lines - oxygen transport in blood vessels

Furthermore, if you could recall the reversible nature of oxygen binding to haemoglobin, as stated by the equation:

  • Unloading - the backward reaction with HbO8 dissociating into
  • Loading - the forward reaction where to form
  • Transport - is maintained

You can see how the dissociation curve and the state of haemoglobin interrelate - the left and rightmost regions of the curve denote the unloading and loading of oxygen, respectively. In contrast, the central region of the curve represents the transport of oxygen.

Study tip: Should you need more help remembering the reversible nature of oxygen binding to haemoglobin, the word ‘dissociation’ from the haemoglobin dissociation curve shows that oxygen can dissociate from haemoglobin.

We will learn in detail the unloading and loading of oxygen indicated by the dissociation curve.

Unloading and loading of oxygen

Unloading is the process where oxygen gets released into respiring cells. One can identify the region of the haemoglobin dissociation curve where unloading occurs (Figure 6).

Haemoglobin [+] curve with the region where unloading occurs highlighted [+] StudySmarter

Figure 6. The partial pressure of oxygen and haemoglobin saturation of the dissociation curve where unloading occurs is highlighted in green. Source: StudySmarter Originals.

Unloading

Unloading happens at a low partial pressure of oxygen until 50% saturation. This process occurs in metabolically active and aerobically respiring cells. At low partial pressure, the affinity of haemoglobin for oxygen is low, making oxygen difficult to bind to haemoglobin. You may notice that the graph moves from less steep to increasing steepness until the 50% mark. This is due to a phenomenon called positive cooperability.

Remember how four oxygen molecules can bind to one haemoglobin molecule? It is difficult for the first oxygen to bind to haemoglobin. Yet, as soon as the first oxygen molecule binds, the quaternary structure of the haemoglobin is altered. This will make it easier for the other oxygen molecules to bind. Hence why the curve slowly transitions to loading as PO2 increases. This is positive cooperability!

Loading

On the other hand, loading happens at a high PO2 (95% saturation) until the maximum haemoglobin saturation. This process occurs in the lungs, where partial pressures of oxygen are the highest. At a high PO2, the affinity of haemoglobin for oxygen increases, making the oxygen binding to haemoglobin in the pulmonary capillaries easy. The plateau of the sigmoid curve is beneficial in loading, as oxygen can be loaded even though the PO2 may drop slightly, such as entering a stuffy room.

Study tip: It is easy to get confused between ‘unloading’ and ‘loading’ or which regions of the sigmoid curve represent these.

Bohr shift

What would happen to the curve if there was an increased CO2 concentration? High CO2 levels would shift the sigmoid curve shift rightwards. This phenomenon is the Bohr shift (Figure 7).

Haemoglobin [+] graphs describing the bohr shift [+] StudySmarter

Figure 7. Graph with two different haemoglobin dissociation curves next to each other. Note the dissociation curve at higher carbon dioxide levels has a rightwards shift. Source: StudySmarter Originals.

The Bohr shift would mean a higher partial pressure to achieve 50% saturation. In other words, the affinity of haemoglobin for oxygen drops, causing haemoglobin to be loaded with oxygen less readily. Instead, the body unloads oxygen for respiring cells more efficiently, allowing these cells to continue aerobic respiration and produce ATP.

Variants of haemoglobin

The activity of haemoglobin variants can be identified by comparing the dissociation curve of the variant with the curve of adult human haemoglobin.

Leftward shift

Certain variants of haemoglobin experience a leftward shift when compared to adult human haemoglobin (Figure 8).

Haemoglobin graphs describing the leftward shift StudySmarter

Figure 8. Graph with two curves lying next to each other. Note the green curve laying leftward of the blue curve. Source: StudySmarter Originals.

A leftward shift means that the haemoglobin has a higher affinity for oxygen. A lower is needed to achieve a 50% saturation. In other words, oxygen loads haemoglobin more readily as oxygen latches onto the haemoglobin more easily.

Organisms with low oxygen supply from the environment have haemoglobin that experiences a leftward shift to ensure their cells still receive sufficient oxygen. Such haemoglobin includes foetal haemoglobin alongside the haemoglobin of alpacas and lugworms (i.e. Arenicola). Note here that the foetus has its subtype of haemoglobin as the PO2 of the placenta that applies oxygen to the foetus is low.

Another proteinous pigment that acts as an oxygen carrier in humans is myoglobin. Myoglobin is found in the muscle filaments. The curve of myoglobin also has a leftward shift compared to haemoglobin - it has a higher affinity for oxygen than haemoglobin. This allows myoglobin to store oxygen molecules and only release oxygen at a very low PO2, allowing the muscle cells to continue respiring anaerobically.

Rightward shift

Other haemoglobin variants experience a rightward shift compared with adult human haemoglobin (Figure 9).

Haemoglobin Graphs describing the rightward shift StudySmarter

Figure 9. Graph with two curves lying next to each other. Notice how the yellow curve lies leftward of the blue curve?

A rightward shift means a haemoglobin variant with a lower oxygen affinity - a higher PO2 is needed to achieve 50% saturation. Simply speaking, oxygen loads haemoglobin less readily but gets unloaded to respiring tissues more readily.

Organisms with such haemoglobin variants include organisms with high oxygen demand or organisms with a high metabolic rate. Small and active animals such as birds and mice will experience a rightward shift in the haemoglobin dissociation curve.

Does haemoglobin take part in the transport of carbon dioxide in the blood?

Now that you know haemoglobin transports oxygen, you might wonder if it transports carbon dioxide as well.

Haemoglobin does transport carbon dioxide, but the amount of carbon dioxide transported by haemoglobin is low. This is at around 15% of the total PCO2. Carbon dioxide binds to another site in the haemoglobin molecule, the amino-terminal, instead of haem. This forms a complex termed carbaminohaemoglobin.

Instead, most carbon dioxide is transported as the soluble bicarbonate HCO3- ion. Carbon dioxide will react with water, forming carbonic acid. Carbonic acid then dissociates into bicarbonate and an H+ via an enzyme called carbonic anhydrase.

The chemical equations are as follows:

As the reactions are reversible, carbon dioxide can regenerate from the bicarbonate ion to be exhaled through the lungs.

Problems with the oxygen transport

The haem group is relatively reactive, meaning that it can also bind other substances, not only oxygen.

The most significant problem would be the binding of carbon monoxide to haemoglobin. This is because carbon monoxide has a higher affinity for haemoglobin than oxygen. Carbon monoxide will latch onto the haemoglobin more readily than oxygen. The binding of the carbon monoxide molecule to the haem group is irreversible. This means that the carbon monoxide cannot dissociate from the haem group once it binds, and the haem group cannot carry oxygen. Thus, carbon monoxide poisoning results in insufficient oxygen being carried in red blood cells even if the PO2 is high.

Haemoglobin - Key takeaways

  • Haemoglobin is a quaternary protein made up of four polypeptide chains. Each chain possesses a haem group that is responsible for carrying oxygen. The haem group consists of an iron ion surrounded by a porphyrin ring.
  • Adult males and females have different ranges for their haemoglobin levels. Acclimatisation, where the body adapts to high altitudes with low partial pressure of oxygen, increases haemoglobin levels.
  • The haemoglobin dissociation curve represents the activity of haemoglobin. Haemoglobin gets unloaded at low partial pressures and loaded at high partial pressures of oxygen. The curve shifts rightwards at high partial pressures of carbon dioxide, a phenomenon known as the Bohr shift.
  • The activity of haemoglobin variants can be determined by the direction of shift compared to haemoglobin. Haemoglobin variants adapted to the low partial pressure of oxygen and myoglobin have a leftward shift, whereas haemoglobin variants adapted to animals with high metabolic rates have a rightward shift.

Haemoglobin

The average haemoglobin level in adults is 8.7-11.2 mmol/L for males and 12-16 g/dL for females.

When the haemoglobin levels are below the average range, this is a medical condition called anaemia.

The main treatment for low haemoglobin would be iron supplements.

There are many explanations for high haemoglobin. These include acclimatisation at higher altitudes or polycythaemia where the body produces too many red blood cells.

The part of the blood that transports the most oxygen is the haemoglobin.

Only a very small percentage of oxygen is transported by the plasma as oxygen is poorly soluble.

The pulmonary vein transports oxygenated blood from the lungs to the heart.

Final Haemoglobin Quiz

Question

Suggest why haemoglobin in blood is necessary to carry oxygen.


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Answer

Oxygen cannot dissolve well in blood plasma.

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Question

Haemoglobin is a tertiary structure made of four polypeptide chains - two alpha (alpha-globin) and two beta chains (beta-globin). (True/ False)

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Answer

False - haemoglobin is a quaternary structure


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Question

Explain why haemoglobin can carry eight oxygen atoms.


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Answer

Haemoglobin consists of four oxygen binding sites as it is able to carry four oxygen molecules. As each oxygen molecule consists of two oxygen atoms, eight oxygen atoms make up the four oxygen molecules.


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Question

Haemoglobin concentration increases beyond the normal range in a process called acclimatisation, whereby the body adapts to high oxygen levels in regions of high altitudes by producing more haemoglobin. (True/ False)


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Answer

False - in acclimatisation, the body adapts to LOW oxygen levels in regions of high altitudes by producing more haemoglobin


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Question

What do haemoglobin saturation and partial pressure mean in terms of the haemoglobin dissociation curve?


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Answer

  • Haemoglobin saturation -  how much oxygen, in percentage, is bound to haemoglobin compared to the total number of binding sites available.
  • Partial pressure - the concentration of oxygen in the body

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Question

Select the letter that suggests the loading of oxygen in the lungs.


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Answer

c

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Question

Describe the term ‘positive cooperability’.


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Answer

Positive cooperability relates to the binding of oxygen to haemoglobin. Four oxygen molecules can bind to one haemoglobin molecule. It is very difficult for the first oxygen molecule to bind as haemoglobin is not good for binding. However, once the first oxygen molecule binds, haemoglobin changes into a shape that is easier for the other oxygen molecules to bind.


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Question

Explain the haemoglobin dissociation curve change when the PCO2 (partial pressure of carbon dioxide) is high.


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Answer

When the partial pressure of carbon dioxide is high, cells are actively respiring. The curve then shifts to the right, meaning that the affinity of haemoglobin for oxygen drops. Haemoglobin unloads oxygen more readily to respiring tissues to continue with aerobic respiration.

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Question

Explain why the haemoglobin dissociation curve of foetal haemoglobin has a leftwards shift.


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Answer

The foetus obtains oxygen form the mother’s blood via the placenta, which has a low partial pressure of oxygen. Hence, the haemoglobin dissociation curve has a leftward shift as foetal haemoglobin has a higher affinity for oxygen, meaning that even at low partial pressures of oxygen foetal haemoglobin loads with oxygen readily. 


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Question

Explain the leftward shift of the myoglobin dissociation curve, relating your answer to the function of myoglobin.


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Answer

Myoglobin is another oxygen-carrying protein found in muscle fibers. It has a higher affinity for oxygen, which explains its leftwards shift. As such, myoglobin only unloads oxygen to respiring tissues when the partial pressure of oxygen is very low. This is to prolong aerobic respiration.


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Question

Explain why the haemoglobin dissociation curve of birds has a rightwards shift.


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Answer

Birds are small animals with high metabolic rates, meaning that the cells are actively respiring. As the production of carbon dioxide increases as a result, this causes the curve to shift to the right. Their haemoglobin then has a lower affinity for oxygen, meaning that oxygen is unloaded to respiring tissues.


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Question

Describe how carbon dioxide gets transported in the haemoglobin.


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Answer

Carbon dioxide binds to the amino-terminal of the polypetides making up haemoglobin, forming a complex known as carbaminohaemoglobin.


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Question

Describe how carbon dioxide gets transported in the plasma.

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Answer

Carbon dioxide gets transported in the plasma as bicarbonate ions. Carbon dioxide reacts with water in the plasma to form carbonic acid. Carbonic acid then dissociates to form proton and bicarbonate via the action of carbonic anhydrase. As the reactions are reversible, bicarbonate ions get reform into carbon dioxide and expelled from the lungs.


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Question

Similar to oxygen, carbon monoxide binds to the haem groups of haemoglobin (True/ False)


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Answer

True

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Question

The binding of carbon monoxide to oxygen is irreversible (True/ False)


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Answer

True

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