Hemoglobin is found in the red blood cells of the body. The main function of hemoglobin is to transport oxygen from the lungs to the tissues and then transport CO2 back from the tissues to the lungs. One hemoglobin molecule has the ability to transport up to 4 oxygen molecules. There are two forms of hemoglobin: oxyhemoglobin, which is saturated with oxygen molecules and deoxyhemoglobin, which is unsaturated with oxygen molecules.

To see more about how the structure of hemoglobin affects its function use the chime tutorial below!



Click here to reset the view of hemoglobin .

Hemoglobin is a tetramer composed of 4 globin molecules. To display hemoglobin as 4 different peptide chains click here.

Hemoglobin is composed of two alpha-globin chains colored in blue and green
and two beta-globin chains colored in cyan and yellow

The alpha globin chain is composed of 141 amino acids and the beta globin chain is composed of 146 amino acids. Each alpha and beta globin chains bind together to form a dimer, and the two dimers bind together to form a tetramer.

Both alpha and beta globin proteins share similar secondary and tertiary structures, each with 8 helical segments. Click here to display the 8 helical segments show in white on one of the alpha globin chains.

Each globin chain also contains one heme molecule.
Click here to zoom in on the heme molecule .

The heme molecule is composed of a porphyrin ring, which contains 4 pyrrole molecules cyclically linked together, and an iron ion ligand bound in the center. To view the iron ion click here.

Now let's take a closer look at one of the alpha-globin chains to see how the heme molecule binds.

The heme molecule is located between helix E shown in green and helix F shown in red. The histidines in helix E and helix F are highly conserved. These histidines are located proximally and distally to the heme molecule and keep it in place within the globin protein by ligand binding.

The FG corner of the globin protein is also essential to hemoglobin's function and affects the conformational change between the oxygenated and deoxygenated states. The amino acids in this region are also highly conserved between species. To view the FG corner click here.

As oxygen binds to oxyhemoglobin it has a higher affinity for additional oxygen molecules, and as oxygen is released from deoxyhemoglobin it has a decreased affinity for additional oxygen molecules. This regulation is known as cooperativity and is a result of conformational changes in the structure of hemoglobin which make the heme molecule more or less accessible to oxygen. There are two structures of hemoglobin: relaxed (R) and tense (T). The conformational shift between these two structures is a result of a rotation between the alpha and beta subunits. To see this, let's look at an alpha and beta dimer

The amino acids that are involved in this conformational change in structures are located at the interface between the alpha and beta dimers and are highlighted. To zoom in click here. Several amino acids in this region are also highly conserved because they regulate oxygen affinity and cooperativity within hemoglobin.

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