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biological proteins. In cytochrome c, a common heme protein in the mitochondria, the axial ligands to the iron are occupied by histidine and methionine from the protein. Heme enzymes include calalase and peroxidase. As components of iron-sulfur centers, iron enters into multiple cluster arrangements with cysteine residues on enzymes that offer a more direct contact with the protein. These centers differ in their complexity from the simple 2Fe-2S to the more elaborate 4Fe-4S (Figure 3). Iron in these centers binds substrates as well as transfer electrons and takes part in reactions involving dehydrations and rearrangements. Enzymes with iron-sulfur centers include xanthine oxidase, succinate dehydrogenase, aconitase, and nitrogenase. A third class, represented by ribonucleotide reductase, has a FeO2 cluster with a dioxygen as a peroxide anion O2ā€” straddled between two iron centers (Figure 4). This arrangement allows the enzyme to remove a hydrogen atom from a very stable Cā€”H bond. No metal can replace iron in these complexes. Enzymes with a heme group generally are reddish-brown in color (depending on the oxidation state of the iron). The color led to early interest in these proteins and was the motivating factor behind naming heme proteins in the mitochondria 'cytochromes.' Although only a relatively few soluble enzymes have iron as a cofactor, iron is especially prominent in membrane-bound proteins that comprise electron transport pathways. Examples of the latter include the cytochromes in the mitochondria, endoplasmic reticulum, and photosystem I, II in chloroplasts. Perhaps the most unusual iron protein is ferritin, a huge multisubunit iron storage protein that has the capacity to bind more than 2500 iron atoms in its structure.

Reactivity The redox property of iron carries over to much of its chemistry as a cofactor. Iron is nearly always involved with the transfer of electrons and many times donates the electrons to a molecule of oxygen. Two important properties that fit that role are: (1) an iron atom that can readily undergo reversible valence changes from Fe2+ to Fe3+, which allows facile exchange of electrons; and (2) the ferrous-ferric ion pair has a relatively low electrochemical potential (ā€”0.1V), which allows iron to be on the high (reducing) end of an electron transport chain. In cytochrome P450 a single oxygen atom is transferred to the substrate after O2 binds to Fe(II). In the mechanism the Fe(II)ā€”O2 complex is converted into FeO, which features an Fe(V) species that attacks the substrate and incorporates the single oxygen atom into its structure. Although higher valence states such as Fe(IV) and Fe(VI) are formed by removing additional 3d electrons, only rarely are these higher valences of iron seen in biological

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