Figure 1 is a model of human deoxyhemoglobin.  It was created in RasMol version 2.6 by Roger Sayle using the pdb coordinates from the pdb file 4hhb.  The 3D coordinates were determed from x-ray crystallography by Fermi, G., Perutz, M. F., Shaanan, B., Fourme, R.: The crystal structure of human deoxyhaemoglobin at 1.74 A resolution. J Mol Biol 175 pp. 159 (1984)

An Essay on Hemoglobin Structure and Function:

Hemoglobin is the protein that carries oxygen from the lungs to the tissues and carries carbon dioxide from the tissues back to the lungs. In order to function most efficiently, hemoglobin needs to bind to oxygen tightly in the oxygen-rich atmosphere of the lungs and be able to release oxygen rapidly in the relatively oxygen-poor environment of the tissues. It does this in a most elegant and intricately coordinated way. The story of hemoglobin is the prototype example of the relationship between structure and function of a protein molecule.

Hemoglobin Structure

A hemoglobin molecule consists of four polypeptide chains: two alpha chains, each with 141 amino acids and two beta chains, each with 146 amino acids. The protein portion of each of these chains is called "globin". The a and b globin chains are very similar in structure. In this case, a and b refer to the two types of globin. Students often confuse this with the concept of a helix and b sheet secondary structures. But, in fact, both the a and b globin chains contain primarily a helix secondary structure with no b sheets.
Figure 2 is a close up view of one of the heme groups of the human a chain from dexoyhemoglobin.  In this view, the iron is coordinated by a histidine side chain from amino acid 87 (shown in green.)
Each a or b globin chain folds into 8 a helical segments (A-H) which, in turn, fold to form globular tertiary structures that look roughly like sub-microscopic kidney beans. The folded helices form a pocket that holds the working part of each chain, the heme.

A heme group is a flat ring molecule containing carbon, nitrogen and hydrogen atoms, with a single Fe2+ ion at the center. Without the iron, the ring is called a porphyrin. In a heme molecule, the iron is held within the flat plane by four nitrogen ligands from the porphyrin ring. The iron ion makes a fifth bond to a histidine side chain from one of the helices that form the heme pocket. This fifth coordination bond is to histidine 87 in the human a chain and histidine 92 in the human b chain. Both histidine residues are part of the F helix in each globin chain.

The Bohr Effect

The ability of hemoglobin to release oxygen, is affected by pH, CO2 and by the differences in the oxygen-rich environment of the lungs and the oxygen-poor environment of the tissues. The pH in the tissues is considerably lower (more acidic) than in the lungs. Protons are generated from the reaction between carbon dioxide and water to form bicarbonate:

CO2 + H20 -----------------> HCO3- + H+

This increased acidity serves a twofold purpose. First, protons lower the affinity of hemoglobin for oxygen, allowing easier release into the tissues. As all four oxygens are released, hemoglobin binds to two protons. This helps to maintain equilibrium towards the right side of the equation. This is known as the Bohr effect, and is vital in the removal of carbon dioxide as waste because CO2 is insoluble in the bloodstream. The bicarbonate ion is much more soluble, and can thereby be transported back to the lungs after being bound to hemoglobin. If hemoglobin couldn’t absorb the excess protons, the equilibrium would shift to the left, and carbon dioxide couldn’t be removed.

In the lungs, this effect works in the reverse direction. In the presence of the high oxygen concentration in the lungs, the proton affinity decreases. As protons are shed, the reaction is driven to the left, and CO2 forms as an insoluble gas to be expelled from the lungs. The proton poor hemoglobin now has a greater affinity for oxygen, and the cycle continues.

  For Further Reading: Perutz, M. F., Hemoglobin Structure and Respiratory Transport, Scientific American, volume 239, number 6, December, 1978.