The hemoglobin molecule(right) is essentially four myoglobin molecules put together. Each hemoglobin molecule has four separate protein chains: two a chains and two b chains. Each of these chains is folded in the same way as the myoglobin molecule, and the four chains are nested against one another as four subunits of a compact molecule. When the hemoglobin molecule picks up four 0₂ molecules at the lungs, the subunits shift slightly so the two b units are a little closer together. When the 0₂ Molecules are turned over to myoglobin at the tissues for storage, the four hemoglobin subunits shift back to their original arrangement. In effect, the hemoglobin molecule is a machine that closes and opens when it binds and releases oxygen. Interactions between subunits also make binding of oxygen to hemoglobin an all-or-nothing proposition. Once an 0₂ molecule has bound to one of the four heme groups, a subunit shift makes it much easier to add the other three 0₂ molecules. Conversely, once one 0₂ molecule has been released at the tissues, the other three fall away more easily.' This makes the hemoglobin molecule easy to load with 0₂ at the lungs and easy to strip of its cargo at the tissue, properties desirable in an oxygen carrier.

Above: The four chains of the hemoglobin molecule each are folded like that of myoglobin, and then are packed together into a compact unit. The four heme pockets are exposed on the outside of the molecule where they are available for binding four O₂ molecules.