| Inside each red blood cell are millions of hemoglobin (Hb) molecules, which bind and store oxygen.
Hemoglobin is a large and structurally complex molecule, consisting of two alpha subunits and two beta subunits.
Each subunit is made up of a heme group and a long polypeptide, called globin, which coils around the heme group. Holding the heme in position within the folds of the globin are two histidine molecules.
In the center of the heme group is an atom of ferrous iron (Fe2+). A ring of nitrogenous compounds, which is derived from porphyrin, surrounds the iron atom. The iron atom can reversibly bond with one molecule of oxygen (O2).
Because it has four subunits, a hemoglobin molecule can reversibly bond with up to four O2 molecules.
When not bonded to O2, deoxyhemoglobin stays in a tensed state or conformation. The first O2 molecule to bond causes the oxyhemoglobin to shifts to a relaxed state. This conformation change makes it easier for other O2 molecules to bind to the other hemes, a property called cooperativity. |
