Types of hemoglobin

Hemoglobin consists of four subunits (globin
chains), two pairs of identical polypeptides,
each polypeptide being attached to a heme
group with an oxygen-binding site. The different
kinds of hemoglobins, which are characteristic
of different stages of development
(embryonic, fetal, and postnatal), differ in the
composition of their chains: the hemoglobin of
adults (HbA) contains two ! and two " chains
(!2"2). A small proportion of adult hemoglobin
has two ! and two # chains (HbA2: !2#2).
Hemoglobin formed during the fetal period
(HbF) contains two ! and two $ chains (!2$2). In
the embryonic stage, % chains are joined to & or $
chains (Hb Gower 1: %2&2, and Hb Portland:
%2$2); two ! and two & chains form Hb Gower 2
(!2&2).