Nonheme-iron proteins

A fundamental and still unanswered question in iron metabolism is how iron is released from hTi and when and where is it reduced to the ferrous state before being transported across the endosomal membrane to the cytoplasm by divalent metal transporter DMTl Experiments carried out at endosomal conditions indicate that iron released from Tf to most physiological iron binders is easily reducible. However, the time required for the release of iron from transferrin as Fe to physiological chelators, even at endosomal pH, is much longer ( 6 min) than that compared to the cell-cycling time of transferrin, which may be as little as 1-2 min. Transferrin completes some 100-200 cycles of iron uptake, transport and delivery to cells during its lifetime in the circulation, thus demanding a sophisticated and efficient iron-release process. 

A salient feature of the receptor-mediated transferrin-to-cell endocytic cycle in human iron metabolism is the persistence of the transferrin-transferrin receptor 

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Rieske, J. S. In Nonheme Iron Proteins Role in Energy Conservation San Pietro, E., Ed. The Antioch Press Yellow Springs, OH, 1965, 461-468. 

The general influence of covalency can be qualitatively explained in a very basic MO scheme. For example, we may consider the p-oxo Fe(III) dimers that are encountered in inorganic complexes and nonheme iron proteins, such as ribonucleotide reductase. In spite of a half-filled crystal-field model), the ferric high-spin ions show quadrupole splittings as large as 2.45 mm s < 0, 5 = 0.53 mm s 4.2-77 K) [61, 62]. This is explained... 

Rbo is a homodimeric protein, each subunit of which contains two distinct mononuclear nonheme iron centers in separate domains (Fig. 10.4) (Coehlo et al. 1997). Center I contains a distorted rubredoxin-type [Fe(SCys)4] coordination sphere. [Fe(SCys)4] sites in proteins are known to catalyze exclusively electron transfer, which is, therefore, the putative function for center I. Center II contains a unique [Fe(NHis)4(SCys)] site that is rapidly oxidized by 0, and is, therefore, the likely site of superoxide reduction (Lombard et al. 2000). A blue nonheme iron protein, neelaredoxin (Nlr) from Desulfovibrio gigas (Silva et al. 1999), contains an iron center closely resembling that of Rbo center II (Table 10.1). The blue color is due to the oxidized (i.e., Fe(III)) form [Fe(NHis)4(SCys)] site, which, in both Nlr and Rbo, has a prominent absorption feature at -650 nm. Reduction of center II to its Fe(II) form fully bleaches its visible absorption. These absorption features have been used to probe the reactivity of Rbo with superoxidie. 

Probing Metalloproteins Electronic absorption spectroscopy of copper proteins, 226, 1 electronic absorption spectroscopy of nonheme iron proteins, 226, 33 cobalt as probe and label of proteins, 226, 52 biochemical and spectroscopic probes of mercury(ii) coordination environments in proteins, 226, 71 low-temperature optical spectroscopy metalloprotein structure and dynamics, 226, 97 nanosecond transient absorption spectroscopy, 226, 119 nanosecond time-resolved absorption and polarization dichroism spectroscopies, 226, 147 real-time spectroscopic techniques for probing conformational dynamics of heme proteins, 226, 177 variable-temperature magnetic circular dichroism, 226, 199 linear dichroism, 226, 232 infrared spectroscopy, 226, 259 Fourier transform infrared spectroscopy, 226, 289 infrared circular dichroism, 226, 306 Raman and resonance Raman spectroscopy, 226, 319 protein structure from ultraviolet resonance Raman spectroscopy, 226, 374 single-crystal micro-Raman spectroscopy, 226, 397 nanosecond time-resolved resonance Raman spectroscopy, 226, 409 techniques for obtaining resonance Raman spectra of metalloproteins, 226, 431 Raman optical activity, 226, 470 surface-enhanced resonance Raman scattering, 226, 482 luminescence... 

VI. NITRIC OXIDE COMPLEXES OF OTHER NONHEME IRON PROTEINS... 

Other enzymes that are not obviously related to the dioxygenases have at least superficially similar metal sites. The fatty acid desaturase of the endoplasmic reticuluum is a nonheme iron protein and requires both oxygen and reducing equivalents for activity (Strittmatter and Enoch, 1978). It is not known whether this enzyme forms a nitroxyl complex, but rat liver microsomes containing the enzyme form an S = nitroxyl adduct when treated with nitrite and dithionite. 

Iron-containing superoxide dismutases are present in many species of bacteria (Hassan and Fridovitch, 1978). These nonheme iron proteins have a characteristic set of EPR lines split about g = 4.2 in the ferric state, arising from the middle Kramers doublet of a rhombic high-spin site. Ferrous iron superoxide dismutase forms an S = I complex with NO that resembles the lipoxygenase-NO adduct by EPR criteria (I. Fridovich, T. Kirby, and J. C. Salerno, (1978) unpublished observations). 

Geng, Y. J., Petersson, A. S., Wennmalm, A., and Hansson, G. K. (1994). Cytokine-induced expression of nitric oxide synthase results in nitrosylation of heme and nonheme iron proteins in vascular smooth muscle cells. Exp. Cell Res. 214, 418-428. 

Pellat, C., Henry, Y., and Drapier, j. C. (1990). IFN-gamma-activated macrophages Detection by electron paramagnetic resonance of complexes between L-arginine-derived nitric oxide and nonheme iron proteins. Biochem. Biophys. Res. Commun. 166, 119-125. 

Where A2 and B are the flavoprotein (Fp) and the small nonheme iron protein, respectively. 

Noncompetitive inhibition 476,477 Nonheme iron proteins. See Iron-sulfur and diiron proteins Nonlinear equations 460 Nonmetallic ions, ionic radii, table 310 Nonproductive complexes 475 Norepinephrine (noradrenaline) 553,553s in receptor 555s Nuclear envelope 11... 

The most clearly documented role lor selenium is as a necessary component of glutathione peroxidase. Selenium is also involved in the functions of additional enzymes, e.g.. type I iodoihvronine deiodinase. leukocyte acid phosphatase, and glucuronidases. A role for selenium in electron transfer has been suggested as has involvement in nonheme iron proteins. Selenium and vitamin b appear to be necessary lor proper functioning of lysosomal membranes. A role for selenium in metabolism of thyroid hormone has been continued. 

The biochemical importance of flavin coenzymes ap-pears to be their versatility in mediating a variety of redox processes, including electron transfer and the activation of molecular oxygen for oxygenation reactions. An especially important manifestation of their redox versatility is their ability to serve as the switch point from the two-electron processes, which predominate in cytosolic carbon metabo-lism, to the one-electron transfer processes, which predomi-nate in membrane-associated terminal electron-transfer pathways. In mammalian cells, for example, the end products of the aerobic metabolism of glucose are C02 and NADH (see chapter 13). The terminal electron-transfer pathway is a membrane-bound system of cytochromes, nonheme iron proteins, and copper-heme proteins—all one-electron acceptors that transfer electrons ultimately to 02 to produce H20 and NAD+ with the concomitant production of ATP from ADP and P . The interaction of NADH with this pathway is mediated by NADH dehydrogenase, a flavoprotein that couples the two-electron oxidation of NADH with the one-electron reductive processes of the membrane. 

Iron-containing proteins are classified as either heme proteins or nonheme iron proteins. The former contain iron that is coordinated to a porphyrin... 

P700 (a special chlorophyll a molecule) serves as the reaction center of photosystem I, and a bound form of ferredoxin (ferredoxin-reducing substance) may be the electron acceptor. Electrons flow subsequently to NADP through ferredoxin (a nonheme iron protein) and a flavoprotein. 

Bartsch, R. G. Nonheme iron proteins and Chromatium iron protein. In Bacterial Photosynthesis, H. Gest, A. San Pietro, and L. P. Vernon, eds., Antioch Press, Yellow Springs, Ohio, pp. 315—326 (1963). 

Ulmer, D. D. and B. L. Vallee Optically active chromophores. III. Heme and nonheme iron proteins. Biochemistry 2, 1335—1340 (1963). 

The electron-transport chain contains a number of iron-sulfur proteins (also known as nonheme iron proteins). The iron atoms are bound to the proteins via cysteine —S— groups and sulfide ions one such 4-Fe cluster is shown in Fig. 14-1. These proteins mediate electron transport by direct electron transfer changes in oxidation state of the iron in iron-sulfur proteins can be monitored by electron spin resonance spectroscopy (ESR). 

For the purpose of determining the iron-coordination structure of nonheme iron proteins, reconstitution experiments from apoprotein and other constituents are an elegant approach which can inductively indicate the original iron structure. 

Both of these systems activate O2 via a two-equivalent reduction (without the protein catalysts this would give HOOH). In contrast, pyrocatechol dioxygenase (PDO) (a nonheme iron protein) activates 2 without a reductive cofactor (equation 106). 

B. Redesign of Nonheme Iron Proteins. In heme protein redesign described above, the heme prosthetic group largely dictates the active site structure. Redesign focuses mainly on the proximal and distal sides of the heme, causing minimal effects on the overall protein scaffolds. This is not necessarily the case for nonheme metalloproteins in which metal sites are not as dominant and small changes may have more dramatic effects on the protein folds and stability. 

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