Hemoglobin (Hgb) is a large protein (66.7 kD) coupledto
four porphyrins or heme moities.The globin portion of Hgb
consists of four polypeptide chains ( a with 141aa and ß
with 146aa )arranged in pairs forming a tetramer. Each globin
chain is covalently attached to a heme moiety.
The bonds between a and ß chains are weaker than
between similar globin chains, forming a natural cleavage
plane, the a1ß2 interface, important
for oxygen binding and release.
When this cleavage is open [R (relaxed) state] oxygen can
bind (high oxygen affinity). When the two a1ß2
interfaces are closely bound [T (taut) state] the Hgb molecule
has a low affinity for oxygen.
The binding of oxygen rotates the globin chains, moving
the ß chains together and sliding the a1ß2
interfaces apart (the R state) thus increasing the oxygen
affinity of Hgb.