Haemoglobin

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

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).

Fig. 1 - Diagram of a haemoglobin molecule showing two alpha and beta chains each having a haem group

Details to the structure of haemoglobin can be summarized as:

• A 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

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 1).

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.

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

$\mathrm{Hb}+4{\mathrm{O}}_2\rightleftharpoons {\mathrm{HbO}}_8$

Each haem group binds to one oxygen molecule, ${\mathrm{O}}_2$ (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 acclimatization - 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 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.

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.

Fig. 3 - A hypothetical graph showing a linear relationship between partial pressure of oxygen and haemoglobin saturation

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.

Fig. 4 - Labelled diagram of the haemoglobin dissociation curve

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:

$\mathrm{Hb}+4{\mathrm{O}}_2\rightleftharpoons {\mathrm{HbO}}_8$

• Unloading - the backward reaction with HbO8 dissociating into $\mathrm{Hb}+4{\mathrm{O}}_2$
• Loading - the forward reaction where $\mathrm{Hb}+4{\mathrm{O}}_2$ to form ${\mathrm{HbO}}_8$
• Transport - ${\mathrm{HbO}}_8$ 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.

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).

Fig. 5 - The partial pressure of oxygen and haemoglobin saturation of the dissociation curve where unloading occurs is highlighted in green

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 cooperativity.

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.

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).

Fig. 6 - Graph with two different haemoglobin dissociation curves next to each other. Note the dissociation curve at higher carbon dioxide levels has a rightwards shift

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).

Fig. 7 - Graph with two curves lying next to each other. Note the green curve laying leftward of the blue curve

A leftward shift means that the haemoglobin has a higher affinity for oxygen. A lower ${\mathrm{PO}}_2$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 PO_2, allowing the muscle cells to continue respiring anaerobically.

Rightward shift

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

Fig. 8 - 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:

$$\begin{gathered} 1){\mathrm{H}}_2\mathrm{O}+{\mathrm{CO}}_2\rightleftharpoons {\mathrm{H}}_2{\mathrm{CO}}_3 \\ 2){\mathrm{H}}_2{\mathrm{CO}}_3\rightleftharpoons {\mathrm{H}}^{+}+{{\mathrm{HCO}}_3}^{-} \end{gathered}$$

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.