Pathology > Basic Hematology > Normal Hematopoiesis > Erythrocyte Review > Hemoglobin Structure & Function

Hemoglobin: Structure & Function

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.

Hemoglobin must bind O2 at high O2 tension and release it at low O2 tension.

With deoxygenation the a1ß2 interface tightens lessening the affinity of Hgb for oxygen. This conformation is stabilized by proton binding and 2,3-DPG.

Decreasing pH strengthens the a1ß2 interface, stabilizing the low-affinity (T) conformation and releasing O2 . This is the Bohr effect.

2,3-DPG binds to hemoglobin, forming a link at the a1ß2 interface. This results in a stable low affinity (T) conformation promoting the release of O2 . 2,3-DPG also lowers the pH.

Where the O tension is low, as in the venous blood, the Bohr effect and 2,3-DPG combine to reduce the affinity of Hgb for O , releasing O to tissues.

What happens in the lung?


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