<P> The iron ion may be either in the Fe or in the Fe state, but ferrihemoglobin (methemoglobin) (Fe) cannot bind oxygen . In binding, oxygen temporarily and reversibly oxidizes (Fe) to (Fe) while oxygen temporarily turns into the superoxide ion, thus iron must exist in the + 2 oxidation state to bind oxygen . If superoxide ion associated to Fe is protonated, the hemoglobin iron will remain oxidized and incapable of binding oxygen . In such cases, the enzyme methemoglobin reductase will be able to eventually reactivate methemoglobin by reducing the iron center . </P> <P> In adult humans, the most common hemoglobin type is a tetramer (which contains four subunit proteins) called hemoglobin A, consisting of two α and two β subunits non-covalently bound, each made of 141 and 146 amino acid residues, respectively . This is denoted as α β . The subunits are structurally similar and about the same size . Each subunit has a molecular weight of about 16,000 daltons, for a total molecular weight of the tetramer of about 64,000 daltons (64,458 g / mol). Thus, 1 g / dL = 0.1551 mmol / L. Hemoglobin A is the most intensively studied of the hemoglobin molecules . </P> <P> In human infants, the hemoglobin molecule is made up of 2 α chains and 2 γ chains . The gamma chains are gradually replaced by β chains as the infant grows . </P> <P> The four polypeptide chains are bound to each other by salt bridges, hydrogen bonds, and the hydrophobic effect . </P>

Oxygen binds reversibly to the iron of hemoglobin