Haemoglobin

Haemoglobin (also hemoglobin, or abbreviated Hb) is a protein which is used in red blood cells to store and transport oxygen.

Haemoglobin is made up of four polypeptide subunits, two alpha (α) subunits and two beta (β) subunits. Each of the four subunits contains a heme molecule, where the oxygen itself is bound through a reversible reaction, meaning that a haemoglobin molecule can transport four oxygen molecules at a time.

The reversible nature of the binding of oxygen allows for both the uptake of oxygen in the lungs and its release in body tissues.

The heme molecules each contain a single central iron atom and are responsible for giving the red colour to haemoglobin, and thus to the blood as a whole. 

The four subunits of Hemoglobin are similar to myoglobin (Berg et al., 2011: 207). Myoglobin is a single polypeptde, existing in either the deoxymyoglobin form (not bound to oxygen) or the oxymyoglobin form (bound to oxygen) (Berg et al., 2011: 204). Myoglobin contains heme (Berg et al., 2011: 207). Heme contains a central iron atom surrounded by protopyrophyrin, which is the organic component (Berg et al., 2011: 207). When O2 binds to the iron atom (the iron must be in the Fe2+ state for O2 to bind), the iron atom actually moves from outside of the plane of the porphyrin to within the plane of the porphyrin (Berg et al., 2011: 208).

The binding of 02 in hemoglobin is cooperative, meaning that the binding of 02 in each of the one subunit is not independent of the binding at other subunits (Berg et al., 2011: 207). Eventhough myoglobin has a higher affinity for 02 than hemoglobin, hemoglobin is more effective and efficient at delievering oxygen to tissues (Berg et al., 2011: 208). In lungs, 98% of hemoglobin is saturated, whereas in the tissues, only 32% of hemoglobin is saturated (Berg et al., 2011: 208). This means that in the tissues, 66% of hemoglobin subunits released their oxygen. In contrast, in lungs, 98% of myoglobin would be saturated, and 91% of myoglobin would be saturated in tissues (Berg et al., 2011: 208). Compared to myoglobin, hemoglobin has a much more complete

Hemoglobin has a T and R state. In the T (tense) state, or deoxygenated state, the binding sites of hemoglobin are constrained (Berg et al., 2011: 210). In the R (relax) state, or oxygenated state, the binding sites are less constrained, making it easier for the hemoglobin subunits to bind to oxygen (Berg et al., 2011: 210). There are two models that attempt to explain the cooperativity of hemoglobin. The first model is the concerted, or MWC model. This model proposes that whenever an 02 molecule binds to a subunit of hemoglobin, it shifts the equilibrium between the T and R states (Berg et al., 2011: 211). According to this model, when none of the hemoglobin subunits are bound to oxygen, the T state of the protein is favored (Berg et al., 2011: 211). As more and more sites are bound to oxygen, the reaction shifts to favor the R state (Berg et al., 2011: 211). A transition from the T state to the R state will increase the binding affinity of the other sites for 02. The sequential model, on the other hand, suggests that you don't have to have a conversion from the T state to the R state to increase the affinity of other binding sites (Berg et al., 2011: 211). A mixture of both models explains what is observed about hemoglobin cooperativity better than either one of these models can achieve on its own. It is observed that when 3 out of 4 subunits of hemoglobin are bound to 02, the protein is almost always in the R state (Berg et al., 2011: 211). Another observation is that when 1 out of 4 hemoglobin subunits are bound to 02, the protein is almost always in the T state (Berg et al., 2011: 211). 

References

Berg J., Tymoczko J and Stryer L. (2011) Biochemistry, 7th edition, England: W.H. Freeman and Company