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| Structure Of Heme Molecule |
Hemoglobin is made up of the
protein, globin and heme. In normal adult hemoglobin, the globin portion of
each molecule consists of four polypeptide chains: two alpha and beta chains.
These chains, in turn, are composed of 141 and 146 amino acids (Arranged in a
specific sequence), respectively. Each chain is bent and coiled. The heme group
is composed of four pyrrole rings connected by methene bridges. In the center
of this structure is an atom of iron to which oxygen is attached, when the iron
is in the ferrous state (Fe--). One heme molecule will be attached to each of
the alpha and beta chains come together to form a tetramer. The single
hemoglobin molecule, therefore, consists of two alpha chains, two beta chains,
and four heme groups (thus, four atoms of iron). Mature red blood cells are
incapable of hemoglobin synthesis. The production of heme and globin takes
place independently of each other, beginning in the polychromatic normoblast,
and ending in the reticulocyte stage. The pyrrole rings, connected by the
methene bridges, are formed in the cytoplasm and mitochondria of the red cell
precursor. The globin is manufactured in the ribosomes of the immature red
cell. The transfer of iron into the red cell precursor is carried out by
transferrin, a globulin (also called siderophilin). Each molecule of
transferrin can bind two molecule of iron. The transferrin attaches to the red
cell, and the iron passes through the membrane and into the red cell to form
the completed heme molecule each heme group then becomes attached to an alpha
or beta chain. Two of these alpha chains and two beta chains then combine with
each other to form the completed hemoglobin molecule.
