General Chemistry

The Heme Group

In hemoglobin, each subunit contains a heme group, which is displayed using the ball-and-stick representation in Figure 2. Each heme group contains an iron atom that is able to bind to one oxygen (O2) molecule. Therefore, each hemoglobin protein can bind four oxygen molecules.

One of the most important classes of chelating agents in nature are the porphyrins. A porphyrin molecule can coordinate to a metal using the four nitrogen atoms as electron-pair donors, and hence is a polydentate ligand (see Figure 1). Heme is a porphyrin that is coordinated with Fe(II) and is shown in Figure 4.

Ball and Stick 2D Chemdraw
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Figure 4

On the left is a three-dimensional molecular model of heme coordinated to the histidine residue (a monodentate ligand, see Figure 1) of the hemoglobin protein. On the right is a two-dimensional drawing of heme coordinated to the histidine residue, which is part of the hemoglobin protein. In this figure, the protein is deoxygenated; i.e., there is no oxygen molecule bound to the heme group.

Note: The coordinate-covalent bonds between the central iron atom and the nitrogens from the porphyrin are gold; the coordinate-covalent bond between the central iron atom and the histidine residue is green. In the three-dimensional model, the carbon atoms are are gray, the iron atom is dark red, the nitrogen atoms are dark blue, and the oxygen atoms are light red. The rest of the hemoglobin protein is purple.

In the body, the iron in the heme is coordinated to the four nitrogen atoms of the porphyrin and also to a nitrogen atom from a histidine residue (one of the amino-acid residues in hemoglobin) of the hemoglobin protein (see Figure 4). The sixth position (coordination site) around the iron of the heme is occupied by O2 when the hemoglobin protein is oxygenated.

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This page created by Matt Traverso, Washington University in St Louis.
© 2004, Washington University.
Materials and Information present may be reproduced for educational purposes only.

Revised: 2004-08-08