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.
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.
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