Wednesday, August 01, 2007

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

4 comments :

  1. Why is it that mammalian red blood cells lack a nucleus but reptilian and avian cells do not?

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  2. Hemoglobin! My favorite protein! It binds O2! It lets O2 dissociate when appropriate! Dissociation is appropriately modified by pH, temperature, and CO2 levels! Also adjusted in an autoregulatory fashion by BPG! Binds and transports CO2! Binds and transports NO! Buffers pH changes by binding H+! Makes blood that attractive crimson color! Slices! Dices!

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  4. Why is it that mammalian red blood cells lack a nucleus but reptilian and avian cells do not?

    Well, Gregory Lab notes that there is a ... strong positive relationship between the sizes of nuclei (black ellipses), and thus DNA contents, and of red blood cells (clear ellipses) in vertebrates.

    See especially the beautiful 5th figure set. It shows that the really small blood cells are without nuclei.

    Naively, small blood cells should be more efficient (larger area/volume ratio, less viscosity losses, and smaller blood vessels can grow closer to oxygen consumers). But interestingly, ... in mammals, among whom red blood cells are enucleated (i.e., do not contain genomes), there is still a positive correlation between genome size and cell size.

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