Myoglobin

Myoglobin functions as an oxygen carrier and oxygen reservoir in cardiac and skeletal muscle. It consists of a single polypeptide that is tightly folded with about 80% of it in the form of α-helices (Fig 6-2). Each stretch of α-helix is terminated by the presence of a proline residue or by a bend or loop stabilised by ionic or hydrogen bonding. The interior of the molecule is mainly composed of tightly-packed nonpolar amino acids to form a hydrophobic core. In contrast the charged residues are located on the surface, allowing them to hydrogen bond with each other and with water. The haem group is located in a cleft largely made from hydrophobic amino acids. However, there are two histidine residues included in the cleft which are involved in the binding of oxygen to the ferrous iron in the haem group. This process is called oxygenation which is different from oxidation (the loss of electrons).

FIGURE 6-2 Representations of the structure of myoglobin. Myoglobin consists of a single polypeptide which exists, in large part, in the α-helical conformation. The first panel shows the preponderance of α-helices, with each shown in a different colour. The second panel shows a simplified version of the first and emphasises the coordination of the haem group within the polypeptide chain.

[Based on Meisenberg & Simmons, 2006, Principles of Medical Biochemistry, 2nd edition, Mosby]

Haemoglobin

Haemoglobin is only found in erythrocytes, with its main function being to collect oxygen from the lungs and deliver it to the tissues. The main form of haemoglobin in adults (HbA) consists of four polypeptide chains held together by non-covalent bonding to generate a tetrameric quaternary structure (Fig 6-3). There are two types of peptides involved, α-globin and β-globin. Each globin polypeptide folds to generate a structure similar to myoglobin, with stretches of α-helix and hydrophobic residues buried in a central hydrophobic core. However, some hydrophobic residues are exposed on the surface to allow an α-globin to interact with a β-globin by hydrophobic interactions to form a dimeric structure. Each globin also has a hydrophobic haem-binding cleft similar to that in myoglobin.

FIGURE 6-3 Representations of the structure of haemoglobin. Haemoglobin is made from four subunits (two α- and two β-globins) that have a similar structure to myoglobin. The first panel shows the arrangement of the four polypeptides (α-globins in red and β-globins in blue) to form the tetrameric quaternary structure. The light green rings show the position of the haem groups carried by each subunit. The second panel emphasises the interactions between the four subunits.

[Based on Baynes & Dominiczak, 2009, Medical Biochemistry, 3rd edition, Mosby]

A functional molecule of HbA can be thought of as being made from two αβ dimers. The interactions between the globins in each dimer are primarily hydrophobic but some hydrogen and ionic bonds also occur. This results in a tight interaction between the polypeptides. In contrast, the interaction between each pair of dimers is largely through polar bonds which can be rearranged to allow the dimers to slide past each other. This ability to rearrange its organisation results in haemoglobin existing in two major forms.

The tense (T) form—in this form the two αβ dimers interact through ionic and hydrogen bonds to immobilise the polypeptides resulting in a low affinity for oxygen which is often referred to as the deoxygenated state or deoxyhaemoglobin.

The relaxed (R) form—the binding of oxygen to the T form disrupts some of the ionic and hydrogen bonds between the two dimers allowing the peptides more freedom of movement. This results in a higher affinity for oxygen resulting in the oxygenated state. The binding of oxygen to one haem group increases the affinity for oxygen of the others. This ‘communication’ between the subunits results in the phenomenon referred to as cooperative binding, where the binding of a molecule affects the subsequent binding of other molecules (Fig 6-4). Haemoglobin must bind oxygen efficiently in the lungs and release oxygen in other body tissues. Haemoglobin in the R state has a high affinity for oxygen, which is useful when in the lungs but less so in body tissues in which oxygen needs to be released. Haemoglobin in the T state has a low affinity for oxygen, allowing for efficient release in body tissues but inefficient uptake in the lungs. This problem is solved by haemoglobin making a transition from a low affinity T state to a high affinity R state as more oxygen molecules are bound.

FIGURE 6-4 The cooperative binding of oxygen to haemoglobin. Initially the haemoglobin tetramer has a low affinity for oxygen. Binding of the first oxygen increases the affinity of the remaining binding sites for the next oxygen. This continues as each successive oxygen molecule binds to the tetramer. Thus, the binding site that remains when three oxygens have bound has the highest affinity for oxygen.

[Based on Meisenberg & Simmons, 2006, Principles of Medical Biochemistry, 2nd edition, Mosby]

Fetal haemoglobin (HbF) is the main oxygen transport protein in the fetus during the last seven months of development in the uterus but is nearly completely replaced by HbA by approximately 12 weeks after birth. The structure of HbF is similar to that of HbA in that it is made from four subunits, each of which carries a haem group. However, whereas HbA consists of two α- and two β-subunits, the β-subunits are replaced by γ-subunits in HbF. These γ-subunits have a higher affinity for oxygen and thus make HbF better able to bind oxygen, giving the developing fetus better access to oxygen from the placental blood supply.