Hemoglobin alpha, zeta, mu, theta, and related Hb subunits
Hb is the oxygen transport protein of erythrocytes. It is an allosterically modulated heterotetramer. Hemoglobin A (HbA) is the most common Hb in adult humans, and is formed from two alpha-chains and two beta-chains (alpha2beta2). An equilibrium exists between deoxygenated/unliganded/T(tense state) Hb having low oxygen affinity, and oxygenated /liganded/R(relaxed state) Hb having a high oxygen affinity. Various endogenous heterotropic effectors bind Hb to modulate its oxygen affinity and cooperative behavior, e.g. hydrogen ions, chloride ions, carbon dioxide and 2,3-bisphosphoglycerate. Hb is also an allosterically regulated nitrite reductase; the plasma nitrite anion may be activated by hemoglobin in areas of hypoxia to bring about vasodilation. Other Hb types are: HbA2 (alpha2delta2) which in normal individuals, is naturally expressed at a low level; Hb Portland-1 (zeta2gamma2), Hb Gower-1 (zeta2epsilon2), and Hb Gower-2 (alpha2epsilon2), which are Hbs present during the embryonic period; and fetal hemoglobin (HbF, alpha2gamma2), the primary hemoglobin throughout most of gestation. These Hbs types have differences in O2 affinity and in their interactions with allosteric effectors.