Complexes

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  Coordination complexes in biochemistry
Approximately one-third of the chemical elements are present in living organisms. Many of these are metallic ions whose function within the cell depends on the formation of d-orbital coordination complexes with small molecules such as porphyrins (see below). These complexes are themselves bound within proteins (metalloproteins) which provide a local environment that is essential for their function, which is either to transport or store diatomic molecule (oxygen or nitric oxide), to transfer electrons in oxidation-reduction processes, or to catalyze a chemical reaction. The most common of these utilize complexes of Fe and Mg, but other micronutrient metals including Cu, Mn, Mo, Ni, Se, and Zn are also important.  

Hemoglobin
Hemoglobin is one of a group of heme proteins that includes myoglobin, cytochrome-c, and catalase. Hemoglobin performs the essential task of transporting dioxygen molecules from the lungs to the tissues in which it is used to oxidize glucose, this oxidation serving as the source of energy required for cellular metabolic processes. Hemoglobin consists of fourglobin protein subunits (depicted by different colors in this diagram) joined together by weak intermolecular forces. Each of these subunits contains, buried within it, a molecule ofheme, which serves as the active site of oxygen transport. [image source]

The presence of hemoglobin increases the oxygen carryng capacity of 1 liter of blood from 5 to 250 ml. Hemoglobin is also involved in blood pH regulation and CO2 transport. Heme itself consists of an iron atom coordinated to a tetradentate porphyrin. When in the ferrous (Fe2+ state) the iron binds to oxygen and is converted into Fe3+. Because a bare heme molecule would become oxidized by the oxygen without binding to it, the adduct must be stabilized by the surrounding globin protein. In this environment, the iron becomes octahedrally-coordinated through binding to a component of the protein in...
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