Iron sulfur centers, complex

More complex iron-sulfur centers in the MoFe-protein of Mo-nitrogenase are discussed next. 

Studies (see, e.g., (101)) indicate that photosynthesis originated after the development of respiratory electron transfer pathways (99, 143). The photosynthetic reaction center, in this scenario, would have been created in order to enhance the efficiency of the already existing electron transport chains, that is, by adding a light-driven cycle around the cytochrome be complex. The Rieske protein as the key subunit in cytochrome be complexes would in this picture have contributed the first iron-sulfur center involved in photosynthetic mechanisms (since on the basis of the present data, it seems likely to us that the first photosynthetic RC resembled RCII, i.e., was devoid of iron—sulfur clusters). 

This chapter will focus on simple and complex iron-sulfur-con-taining proteins isolated from sulfate reducing bacteria (SRB), in order to review the following topics types and distribution of proteins metal clusters involved and their association with other centers and... 

Fe-4S] + + clusters are certainly the most ubiquitous iron-sulfur centers in biological systems. They play the role of low potential redox centers in ferredoxins, membrane-bound complexes of the respiratory... 

Despite its weakness, the anisotropy of the g tensor of iron-sulfur centers can be used to determine the orientation of these centers or that of the accommodating polypeptide in relation to a more complex system such as a membrane-bound complex. For this purpose, the EPR study has to be carried out on either partially or fully oriented systems (oriented membranes or monocrystals, respectively). Lastly, the sensitivity of the EPR spectra of iron-sulfur centers to structural changes can be utilized to monitor the conformational changes induced in the protein by different factors, such as the pH and the ionic strength of the solvent or the binding of substrates and inhibitors. We return to the latter point in Section IV. 

Similar difficulties have been encountered in the case of complex enzymes such as fumarate reductase and nitrate reductase from E. coli, in which substituting certain Cys ligands led to the loss of several if not all the iron-sulfur centers (171, 172). However, in the case of nitrate reductase, which possesses one [3Fe-4S] and three [4Fe-4S] centers, it was possible to remove selectively one [4Fe-4S]... 

XOD is one of the most complex flavoproteins and is composed of two identical and catalytically independent subunits each subunit contains one molybdenium center, two iron sulfur centers, and flavine adenine dinucleotide. The enzyme activity is due to a complicated interaction of FAD, molybdenium, iron, and labile sulfur moieties at or near the active site [260], It can be used to detect xanthine and hypoxanthine by immobilizing xanthine oxidase on a glassy carbon paste electrode [261], The elements are based on the chronoamperometric monitoring of the current that occurs due to the oxidation of the hydrogen peroxide which liberates during the enzymatic reaction. The biosensor showed linear dependence in the concentration range between 5.0 X 10 7 and 4.0 X 10-5M for xanthine and 2.0 X 10 5 and 8.0 X 10 5M for hypoxanthine, respectively. The detection limit values were estimated as 1.0 X 10 7 M for xanthine and 5.3 X 10-6M for hypoxanthine, respectively. Li used DNA to embed xanthine oxidase and obtained the electrochemical response of FAD and molybdenum center of xanthine oxidase [262], Moreover, the enzyme keeps its native catalytic activity to hypoxanthine in the DNA film. So the biosensor for hypoxanthine can be based on... 

Xanthine oxidoreductase (XOR) is a molybdenum-containing complex homodimeric 300-kDa cytosolic enzyme. Each subunit contains a molybdopterin cofactor, two nonidentical iron-sulfur centers, and FAD (89). The enzyme has an important physiologic role in the oxidative metabolism of purines, e.g., it catalyzes the sequence of reactions that convert hypoxanthine to xanthine then to uric acid (Fig. 4.36). 

Iron-sulfur centers form NO complexes that require the disruption of the complex and almost certainly at least some unfolding of the polypeptide. Reversibility of NO binding in these proteins is not enough to regain function it would be necessary to reform the clusters and partially refold the proteins. Although this is not impossible, it is clearly a more difficult process to imagine. 

Pyridine nucleotide-dependent flavoenzyme catalyzed reactions are known for the external monooxygenase and the disulfide oxidoreductases However, no evidence for the direct participation of the flavin semiquinone as an intermediate in catalysis has been found in these systems. In contrast, flavin semiquinones are necessary intermediates in those pyridine nucleotide-dependent enzymes in which electron transfer from the flavin involves an obligate 1-electron acceptor such as a heme or an iron-sulfur center. Examples of such enzymes include NADPH-cytochrome P4S0 reductase, NADH-cytochrome bs reductase, ferredoxin — NADP reductase, adrenodoxin reductase as well as more complex enzymes such as the mitochondrial NADH dehydrogenase and xanthine dehydrogenase. 

Oxidation-Reduction Reactions The NADH dehydrogenase complex of the mitochondrial respiratory chain promotes the following series of oxidation-reduction reactions, in which Fe3+ and Fez+ represent the iron in iron-sulfur centers, Q is ubiquinone, QH2 is ubiquinol, and E is the enzyme ... 

Purine nucleotides are degraded by a pathway in which they lose their phosphate through the action of 5 -nucleotidase (Fig. 22-45). Adenylate yields adenosine, which is deaminated to inosine by adenosine deaminase, and inosine is hydrolyzed to hypoxanthine (its purine base) and D-ribose. Hypoxanthine is oxidized successively to xanthine and then uric acid by xanthine oxidase, a flavoenzyme with an atom of molybdenum and four iron-sulfur centers in its prosthetic group. Molecular oxygen is the electron acceptor in this complex reaction. 

With the exception of coenzyme Q, all members of this chain are pro teins. These may function as enzymes as is the case with the dehy drogenases, they may contain iron as part of an iron-sulfur center, they may be coordinated with a porphyrin ring as in the cytochromes, or they may contain copper, as does the cytochrome a + a3 complex. 

NADH dehydrogenase and succinate dehydrogenase also contain Fe atoms that are bound by the S atoms of cysteine residues of the protein, in association with additional, inorganic sulfide atoms. Structures of these complexes are shown in figure 10.19. Succinate dehydrogenase has three iron-sulfur centers, one with a [2Fe-2S] cluster, one with [4Fe-4S], and one with a cluster containing 3 Fe atoms and 3 (or possibly 4) sulfides. Iron-sulfur centers undergo one-electron oxidation-reduction reactions. 

Complex I The NADH Dehydrogenase Complex. The NADH dehydrogenase complex is the largest complex in the mitochondrial inner membrane. It has some 26 different polypeptides, including approximately seven iron-sulfur centers and a flavoprotein with bound FMN. NADH probably reacts with the FMN, reducing it to FMNH2, and the reduced flavin then transfers electrons to an iron-sulfur center (fig. 14.9). Electrons then move from one iron-sulfur center to another, and eventually to UQ. 

Electron transfer through complex I. The reduction of FMN to FMNH2 by NADH requires the uptake of one proton from the matrix. FMNH2 subsequently transfers electrons to a series of iron-sulfur centers and releases protons to the solution in the intermembrane space. When the non-sulfur centers reduce UQ to UQH2, two more protons are taken up from the matrix. 

Electron transfer to 02 occurs stepwise, through a series of flavoproteins, cytochromes (heme-proteins), iron-sulfur proteins, and a quinone. Most of the electron carriers are collected in four large complexes, which communicate via two mobile carriers— ubiquinone (UQ) and cytochrome c. Complex I transfers electrons from NADH to UQ, and complex II transfers electrons from succinate to UQ. Both of these complexes contain flavins and numerous iron-sulfur centers. Complex III, which contains three cyto-... 

The reaction center of photosystem I is larger and more complex. It contains two large polypeptides and at least seven other smaller subunits. The reactive chlorophyll a dimer P700 resides on the two main polypeptides, along with about 60 additional molecules of chlorophyll a, two quinones, and an iron-sulfur center. 

The Z scheme. [(Mn)4 = a complex of four Mn atoms bound to the reaction center of photosystem II Yz = tyrosine side chain Phe a = pheophytin a QA and Qb = two molecules of plastoquinone Cyt b/f= cytochrome hf,f complex PC = plastocyanin Chi a = chlorophyll a Q = phylloquinone (vitamin K,) Fe-Sx, Fe-SA, and Fe-SB = iron-sulfur centers in the reaction center of photosystem I FD = ferredoxin FP = flavoprotein (ferredoxin-NADP oxidoreductase).] The sequence of electron transfer through Fe-SA and Fe-SB is not yet clear. 

Oxidation of NADH begins with complex I, also termed NADH dehydrogenase or NADH ubiquinone oxidoreductase. It contains 25 polypeptide chains, flavine mononucleotide (FMN), and several iron-sulfur centers. The function of this complex is to reduce a substance called ubiquinone (UQ or CoQ), whose structure is shown in Figure 17.5. UQ is not protein bound and can move about freely. In the process of reducing UQ, the NADH is oxidized to NAD+. It is now accepted that in complex I, NADH first reduces FMN, and the resulting FMNH2 then transfers its electrons through at least three iron-sulfur centers to UQ. As the electrons pass from NADH to UQ, two to four protons are extruded from the mitochondrial matrix across the inner membrane. 

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