Hemoglobin is the oxygen-carrying component of RBCs[47]. This tetrameric
protein is comprised of two
a
-globin
chains and two
b
-globin
chains (Hb; a2bA2). Each globin chain is a
polypeptide folded to protect a heme moiety (protoporphorin ring with an iron(Fe)
prosthetic group for binding molecular oxygen (O2) and other ligands
(carbon dioxide;CO2[48] and nitric oxide;NO)[49]). Each globin chain
with its resident heme group is referred to as a subunit or monomer of
the Hb protein. The monomers can combine to form dimers. Under normal
conditions the a- and b-globin chains are co-ordinately expressed in adult RBCs
and they combined in a balanced fashion (2:2) to produce the Hb tetramer (a2b2)[50].
The process of subunit assembly into
Hb proceeds in the following manner:
Reaction 1. a-subunit + b-subunit
®
ab dimer.
Reaction 2. ab
dimer + ab
dimer
®
a2b2;
Hb.