Hemoglobin (Hgb) is coupled to four iron-binding,methene-linked tetrapyrrole rings(heme). The α (16p13.3; 5'-ζ-pseudoζ-pseudoα2-pseudoα1-α2-α1-θ1-3') and β (11p15.4) globin loci determine the basic hemoglobin structure.The globin portion of Hgb consists of twoαchains and two β chains arranged in pairsforming a tetramer. Each of the four globin chainscovalently associates with a heme group. The bonds between α and β chains are weaker than between similar globin chains, therebyforming a cleavage plane that is important for oxygen binding and release. High affinityfor oxygen occurs upon relaxation of theα1-β2 cleavage plane. When the twoα1-β2 interfaces are closely bound, hemoglobin has a lowaffinityfor oxygen. Hb A,which containstwo α chains plustwo β chains,comprises 97% of totalcirculatingnhemoglobin. The remaining 3% of totalcirculating hemoglobin iscomprised of Hb A-2,which consists of twoαchains plustwo δ chains, and fetal hemoglobin (Hb F),which consists of two α chainstogetherwith two γ chains.
