Protein which reversibly bind iron

A low molecular weight protein, different from metallothionine which reversibly binds iron with high affinity has been isolated from rabbit reticulocyte cytosol (54, 55, 56). Although very little is yet known about its physiological properties, the molecular weight is around 6000, and iron appears to be reversibly bound under physiological conditions. This protein may be able to mobilize iron from the plasma membrane and donate it for heme and ferritin biosynthesis (56), but no definitive physiological role for siderochelin has been established. 

The best characterized and thus prototypical of the binuclear iron-oxo proteins is hemerythrin (Hr), an oxygen carrier from some classes of marine invertebrates, such as sipunculids. The protein consists of several identical 13 kDa subunits which contain two iron atoms and reversibly bind one molecule of O2. Myohemerythrins contain a single 13 kDa subunit (25,26,27). 

Myoglobin is a protein of molecular weight of about 17,000 with the protein chain containing 153 amino acid residues folded about the single heme group. This restricts access to the iron atom (by a second heme) and reduces the likelihood of formation of a hematin-like Fe(III) dimer. The micro environment is similar to that in Cytochrome c, but there is no sixth ligand (methionine) to complete the coordination sphere of the iron atom. Thus there is a site to which a dioxygen molecule may reversibly bind. 

Catalases and peroxidases. Many iron and copper proteins do not bind 02 reversibly but "activate" it for further reaction. We will look at such metallo-protein oxidases in Chapter 18. Here we will consider heme enzymes that react not with 02 but with peroxides. The peroxidases,1943 which occur in plants, animals, and fungi, catalyze the following reactions (Eq. 16-6,16-7) ... 

It must be noted that the reversible binding of molecular oxygen by these haemoproteins can only take place when the iron is in the II oxidation state. Thus the ferric state, with which this article is mainly concerned, is not of direct biological importance in this type of haemo-protein. However, ferrohaemoglobin and ferromyoglobin are readily oxidised in vitro to the ferric or met-form, and most studies of the structure, physical properties and reactivity of these proteins have been performed on the ferric form. The ferric form is in fact a very useful probe, since its electronic properties are extremely sensitive to the environment of the metal. 

In the native deoxy form the active site consists of an iron(II) protoporphyrin IX encapsulated in a water resistant pocket and bound to the protein through the imidazole group of the histidine residue F8. The ferrous ion is 5-coordinate which allows the reversible binding of molecular oxygen in the sixth coordination site as shown in Fig. 17-E-7. 

Cyanide is the CN ion. At physiological pH, it exists as HCN in the body. The mechanism of cyanide toxicity is believed to be the inactivation of iron IB (ferric) enzymes in the body. The inhibition of cytochrome oxidase, which disrupts mitochondrial oxidative phosphorylation, is thought to be the most important mechanism for cyanide toxicity (Warburg, 1911 Keilin, 1929 DiPahna, 1971 Agency for Toxic Substances and Disease Registry, 1997). Cyanide also binds to the hemoglobin in erythrocytes (Farooqui and Ahmed, 1982). The binding of cyanide to Fe(III) enzymes and proteins is reversible (DiPahna, 1971 Way, 1984). 

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