Hemoglobin

 
The following text is slightly modified from Horton et al. (2007) Principles of Biochemistry.

In vertebrates, O₂ is bound to molecules of hemoglobin for transport in red blood cells, or erythrocytes. Viewed under a microscope, a mature mammalian erythrocyte is a biconcave disk that lacks a nucleus or other internal membrane-enclosed compartments (right). A typical human erythrocyte is filled with
approximately 3 × 10⁸ hemoglobin molecules.

Hemoglobin is more complex than myoglobin because it is a multisubunit protein. In adult mammals, hemoglobin contains two different globin subunits called α-globin and β-globin. Hemoglobin is an α₂β₂ tetramer, which indicates that it contains two α chains and two β chains. Each of these globin subunits is similar in structure and sequence to myoglobin, reflecting their evolution from a common ancestral globin gene in primitive chordates.


Each of the four globin chains contains a heme prosthetic group identical to that found in myoglobin. The α and β chains face each other across a central cavity (above). The tertiary structure of each of the four chains is almost identical to that of myoglobin (left). The α chain has seven helices, and the β chain has eight. (Two short α helices found in β-globin and myoglobin are fused into one larger one in α-globin.) Hemoglobin, however, is not simply a tetramer of myoglobin molecules. Each α chain interacts extensively with a β chain, so hemoglobin is actually a dimer of αβ subunits. The presence of multiple subunits is responsible for oxygen-binding properties that are not possible with single-chain myoglobin.

The structure of hemoglobin was solved by Max Perutz [Nobel Laureates].



©Laurence A. Moran and Pearson Prentice Hall 2007