High-potential iron proteins redox properties

High-potential iron proteins, 45 313-314, 344 cluster stability, 45 324-332 function, 45 315-316 residues, 45 322-344 structure and, 45 317-322 redox properties, 45 333-344 solvent accessibility, 45 330, 332-333 source and function, 45 314-316 structure, 45 316-322 hydrogen bonding and, 45 321-322 intermolecular aggregation, 45 322 primary, 45 317-318 secondary and tertiary, 45 318-321... 

High-potential iron proteins (HiPIPs) comprise a subset of the 4Fe-4S cluster family of metalloproteins that are characterized by a positive reduction potential, F , in the range of +50 to +450 mV. This class is differentiated from the 4Fe-4S centers in low-potential ferredoxins, which show a negative E, tjrpically varying between -100 and -650 mV. The origin of these distinct redox properties has been rationalized in terms of the three-state hypothesis of Carter (1), summarized in Scheme 1, and can be attributed to the stability of the common [Fe4S4(SR)4] state. 

An unusual [2Fe-2S] ferredoxin with unique spectroscopic properties exists in association with cytochromes b and c, and is involved in respiratory electron transport in mitochondria, chloroplasts and certain bacteria. When isolated, the complex contains two b hemes, one c, heme and the 2Fe-2S protein. The 2Fe-2S protein from the bct complex (Sections 62.1.5.2.3 and 62.1.5.2.5) was purified from bovine mitochondria by Rieske et al.,162 and is referred to as the Rieske iron-sulfur protein. The properties of this protein have been reviewed763 and its topography in mitochondrial ubiquinol-cytochrome c reductase has been described.764 They have high redox potentials in the range+150-330 mV. 

While not the most toxic, plutonium is the most likely transuranium element to be encountered. Plutonium commonly exists in aqueous solution in each of the oxidation states from III to VI. However, under biological conditions, redox potentials, complexa-tion, and hydrolysis strongly favor Pu(IV) as the dominant species (27, 28). It is remarkable that there are many similarities between Pu(IV) and Fe(III) (Table I). These include the similar charge per ionic-radius ratios for Fe(III) and Pu(IV) (4.6 and 4.2 e/k respectively), the formation of highly insoluble hydroxides, and similar transport properties in mammals. The majority of soluble Pu(IV) present in body fluids is rapidly bound by the iron transport protein transferrin at the site which normally binds Fe(III). In liver cells, deposited plutonium is initially bound to the iron storage protein ferritin and... 

Iron comprises approximately 4.7% of the Earth s crust. The enormous quantities of this metal in the earth core are prerequisite for the magnetic field that shields the planet from cosmic radiation and enables life. The ubiquitous availability of iron and its ability to adjust its oxidation state, redox potential and electron spin state makes it suited to participate in a large number of chemical reactions. Thus, iron has become essential for animals, plants, fungi and most bacteria, ivhere it functions in a ivide variety of iron-dependent enzymes and metal proteins. To avoid deficiency symptoms, mechanisms have evolved in these organisms to maintain iron homeostasis in situations of scarce supply, but also to avoid oxidative stress as mediated by Fenton chemistry ivhen supply is excessive. In industry, iron is used in over 2500 varieties of steel, each with different physical properties. In fact, annual steel production is almost as high as that of all other metals combined hence the environmental effects of iron must also be considered. 

---