In this demo we'll use two models: In the left panel a model of Deoxyhemoglobin, that is, the T-state of the system. In the right panel, a model of Carbomonoxyhemoglobin, structurally identical to Oxyhemoglobin, the R-state of the system.
1. Introduction
Hemoglobin is an allosteric system that follows the Monod-Wyman-Changeux model. Hemoglobin is a tetramer formed by four subunits. In the most abundant adult hemoglobin, Hemoglobin A1, these subunits are called a and b. Thus, quaternary structure of hemoglobin is described as a2b2.
Subunit a has 141 aminoacids, and subunit b 146. Both have a strong sequence homology with one another and with other oxygen transporter, Myoglobin, present in muscle cells. Myoglobin and hemoglobin subunits have a prosthetic group consisting in a Protoporphyrin IX coordinated to a ferrous ion in a complex of octahedral symmetry. Four of this coordinate bonds are established with the porphyrin. The other two with (a) Nitrogen of His 87 (a subunit) or His 92 b subunit; (b) an oxygen molecule, the physiological ligand, in the case of oxyhemoglobin, or remains void in the case of deoxyhemoglobin. The protein is loaded with oxygen in the lung alveoli and discharges it in the peripheral tissues.
Hemoglobin is an Allosteric System:
1. It presents positive cooperativity in oxygen binding, that gives sigmoid saturation curves (homotropic effect). This positive cooperativity does not appear either in isolated subunits or in myoglobin; then it can be concluded that depend on quaternary structure and on intersubunit interactions.
2. Cooperativity may be affected by other ligands (heterotropic effects). Thus, the proton H+, carbon dioxide CO2 and 2,3-bisphosphoglycerate enhance the sigmoid character of the dissociation curve, behaving as Allosteric Inhibitors.
3. The system is composed of four protomers (the four chains). Being slightly different, the a and b subunits behave as if they were identical.
4. The T-state corresponds to deoxyhemoglobin; the R-state to oxyhemoglobin. In the R-state, the sixth coordination position of the ferrous ion is occupied by an oxygen molecule. Other ligands, such as carbon monoxide CO, have the same effect as oxygen. Thus, carbomonoxyhemoglobin is structurally equivalent to oxyhemoglobin.
Thanks to the classical studies of Perutz we have a clear idea of the conformational transitions that occur in hemoglobin upon oxygenation. In the T-state the subunits are more tightly bound that in the R-state. Many of these bonds are salt bridges between the different subunits.
The different subunits are:
Back to the initial representation, R-state:
And T-state:
We'll study now the structural differences between the two states, looking at the intersubunit contacts.
The heme group is located in a hydrophobic pouch of the molecule, and only the propionate side chains interact with the solvent:
In both states, the ferrous ion is coordinated to the four nitrogens of the porphyrin and to the nitrogen of His 92 (helix F) in the b chain (His 87 in the a chain), the Proximal histidine. There is another histidine in the heme neighborhood, the Distal histidine.
In the R-state we can see the ligand (carbon monoxide) bound to the sixth coordination position. Also note the different diameter of the ferrous ion in R- and T-states, the ion being bigger in the T-state: