Hemoglobin subunit binding O2
For hemoglobin, its function as an oxygen-carrier in the blood is fundamentally linked to the equilibrium between the two main states of its quaternary structure, the unliganded "deoxy" or "T state" versus the liganded "oxy" or "R state". The unliganded (deoxy) form is called the "T" (for "tense") state because it contains extra stabilizing interactions between the subunits. In the high-affinity R-state conformation the interactions which oppose oxygen binding and stabilize the tetramer are somewhat weaker or "relaxed". In some organisms this difference is so pronounced that their Hb molecules dissociate into dimers in the oxygenated form. Structural changes that occur during this transition can illuminate how such changes result in important functional properties, such as cooperativity of oxygen binding and allosteric control by pH and anions. Hemoglobin is definitely not a pure two-state system, but the T to R transition provides the major, first-level explanation of its function.
