Electron transfer iron proteins

Nitrogen Fixation in Nature The nitrogenase enzyme is a two-component protein that consists of an electron-transfer Fe protein and a catalytic protein [85]. Three different nitrogenase enzymes are known, which differ primarily in the nature of the putative active site within the catalytic protein. The most common form is the MoFe protein, in which the active site for nitrogen reduction, the so-called FeMo cofactor (FeMoco), is composed of seven irons, one molybdenum, and nine sulfides... 

For the identification of low-spin iron(Il), a low value of the quadrupole splitting is generally not enough. As an example, the Mossbauer spectrum of the reduced low-spin ferrous form of cytochrome C552 is shown in Figure 10. The reduced heme iron has 5 = 0.46 nun s and AEq = -Fl.30mms . These values are characteristic for the low-spin ferrous forms of several electron transfer heme proteins like cytochromes c and... 

In nature there are only two major types of electron-carrier sites in addition to the blue copper proteins and the Cua site, viz. cytochromes and iron-sulphur clusters [161,162]. The cytochromes consist of an iron ion bound to a porphyrin ring. Two axial ligands coii5)lete the octahedral coordination sphere. During electron transfer, iron alternates between Fe(II) and Fe(III). Several types of cytochromes exist in biological systems, depending on the substituents on the por-... 

To reassess the electron-transfer sequence on the acceptor side of photosystem I, Shinkarev et al used the above calculated average lifetimes for the back-reactions involving FeS-X and FeS-A and applied the known electron-transfer rate-vx.-distance relationship" and by taking into consideration of the asymmetrical position of iron-sulfur clusters FeS-A and FeS-B relative to FeS-X as determined by X-ray crystallography. The work of Moser on electron transfer in proteins deduced that the rate of electron transfer between the electron carriers decreases exponentially with distance. According to the relationship established by Moser etal, a change in distance of 1.7 A would result in a one-order change in the electron-transfer rate in proteins. 

Robust voltammetry and in situ STM to molecular resolution have been achieved when the Au(lll)-electrode surfaces are modified by linker molecules, Fig. 8-10, prior to protein adsorption. Comprehensive voltammetric data are available for horse heart cyt and P. aeruginosa The latter protein, which we address in the next Section, has in a sense emerged as a paradigm for nanoscale bioelectrochemistry. We address first briefly two other proteins, viz. the electron transfer iron-sulfur protein Pyrococcus furiosus ferredoxin and the redox metalloenz5mie Achromobacter xylosoxidans copper nitrite reductase. 

Complex I, also referred to as the NADH dehydrogenase complex, catalyzes the transfer of electrons from NADH to UQ. The major sources of NADH include several reactions of the citric acid cycle (see pp. 284-287), and fatty acid oxidation (Chapter 12). Composed of at least 25 different polypeptides, complex I is the largest protein component in the inner membrane. In addition to one molecule of FMN, the complex contains seven iron-sulfur centers (Figure 10.2). Iron-sulfur centers, which may consist of two or four iron atoms complexed with an equal number of sulfide ions, mediate 1-electron transfer reactions. Proteins that contain iron-sulfur centers are often referred to as nonheme iron proteins. Although the structure and function of complex I are still poorly understood, it is believed that NADH reduces FMN to FMNH2. Electrons are then transferred from FMNH2 to an iron-sulfur center, 1 electron at a time. After transfer from one iron-sulfur center to another, the electrons are eventually donated to UQ (Figure 10.3). 

A commonexperimental strategy for studying electron transfers between proteins uses a metal-substituted heme protein as one of the reactants. In particular, the substitution of zinc for iron in one of the porphyrin redox centers allows facile initiation of electron transfer through photoexcitation of the zinc porphyrin (ZnP). The excited zinc porphyrin, ZnP in Equation (6.32),... 

Early reports on interactions between redox enzymes and ruthenium or osmium compounds prior to the biosensor burst are hidden in a bulk of chemical and biochemical literature. This does not apply to the ruthenium biochemistry of cytochromes where complexes [Ru(NH3)5L] " , [Ru(bpy)2L2], and structurally related ruthenium compounds, which have been widely used in studies of intramolecular (long-range) electron transfer in proteins (124,156-158) and biomimetic models for the photosynthetic reaction centers (159). Applications of these compounds in biosensors are rather limited. The complex [Ru(NHg)6] has the correct redox potential but its reactivity toward oxidoreductases is low reflecting a low self-exchange rate constant (see Tables I and VII). The redox potentials of complexes [Ru(bpy)3] " and [Ru(phen)3] are way too much anodic (1.25 V vs. NHE) ruling out applications in MET. The complex [Ru(bpy)3] is such a powerful oxidant that it oxidizes HRP into Compounds II and I (160). The electron-transfer from the resting state of HRP at pH <10 when the hemin iron(III) is five-coordinate generates a 7i-cation radical intermediate with the rate constant 2.5 x 10 s" (pH 10.3)... 

R 121 J.M. Nocek, K. Huang and B.M. Hoffman, An Approach to NMR Treatment of Structural Perturbation in Paramagnetic Proteins too Big for Solution Structure Determination , p. 227 R 122 I. Bertini, F. Capozzi and C. Luchinat, Electronic Isomerism in Oxidized High-Potential Iron-Sulfur Proteins Revisited , p. 272 R 123 C.O. Fernandez and A.J. Vila, Paramagnetic NMR of Electron Transfer Copper Proteins , p. 287 Vol. 859, 2003 Oriental Foods and Herbs... 

In 1993, we reported [19] reversible electron transfer between electrodes and the iron heme protein myoglobin imbedded in cast multi-lameUar liquid crystal films of didodecyldimethylammonium bromide (DDAB). Heretofore, reversible electron transfer from electrodes to myoglobin in solution had been accomplished only for highly purified myoglobin solutions on specially cleaned indium tin oxide electrodes [20,21]. If enhanced electron transfer for proteins in surfactant or lipid films were to prove general, it might help solve longstanding problems in protein electrochemistry. 

Iron Sulfur Compounds. Many molecular compounds (18—20) are known in which iron is tetrahedraHy coordinated by a combination of thiolate and sulfide donors. Of the 10 or more stmcturaHy characterized classes of Fe—S compounds, the four shown in Figure 1 are known to occur in proteins. The mononuclear iron site REPLACE occurs in the one-iron bacterial electron-transfer protein mbredoxin. The [2Fe—2S] (10) and [4Fe—4S] (12) cubane stmctures are found in the 2-, 4-, and 8-iron ferredoxins, which are also electron-transfer proteins. The [3Fe—4S] voided cubane stmcture (11) has been found in some ferredoxins and in the inactive form of aconitase, the enzyme which catalyzes the stereospecific hydration—rehydration of citrate to isocitrate in the Krebs cycle. In addition, enzymes are known that contain either other types of iron sulfur clusters or iron sulfur clusters that include other metals. Examples include nitrogenase, which reduces N2 to NH at a MoFe Sg homocitrate cluster carbon monoxide dehydrogenase, which assembles acetyl-coenzyme A (acetyl-CoA) at a FeNiS site and hydrogenases, which catalyze the reversible reduction of protons to hydrogen gas. 

The side chains of the 20 different amino acids listed in Panel 1.1 (pp. 6-7) have very different chemical properties and are utilized for a wide variety of biological functions. However, their chemical versatility is not unlimited, and for some functions metal atoms are more suitable and more efficient. Electron-transfer reactions are an important example. Fortunately the side chains of histidine, cysteine, aspartic acid, and glutamic acid are excellent metal ligands, and a fairly large number of proteins have recruited metal atoms as intrinsic parts of their structures among the frequently used metals are iron, zinc, magnesium, and calcium. Several metallo proteins are discussed in detail in later chapters and it suffices here to mention briefly a few examples of iron and zinc proteins. 

The most conspicuous use of iron in biological systems is in our blood, where the erythrocytes are filled with the oxygen-binding protein hemoglobin. The red color of blood is due to the iron atom bound to the heme group in hemoglobin. Similar heme-bound iron atoms are present in a number of proteins involved in electron-transfer reactions, notably cytochromes. A chemically more sophisticated use of iron is found in an enzyme, ribo nucleotide reductase, that catalyzes the conversion of ribonucleotides to deoxyribonucleotides, an important step in the synthesis of the building blocks of DNA. 

All these intermediates except for cytochrome c are membrane-associated (either in the mitochondrial inner membrane of eukaryotes or in the plasma membrane of prokaryotes). All three types of proteins involved in this chain— flavoproteins, cytochromes, and iron-sulfur proteins—possess electron-transferring prosthetic groups. 

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