Iron coupling, electronic

Protons are in general indispensable for the dismutation of superoxide (Eq. (4)). Also in the case of its dismutation catalyzed by a metal center, two protons are needed for the dissociation of the product (H2O2) from the metal center (Scheme 9). Therefore, a complex which can accept two protons upon reduction and release them upon oxidation is an excellent candidate for SOD activity. The studies on proton-coupled electron transfer in Fe- and Mn-SODs 48), demonstrated that the active site of MnSOD consists of more than one proton acceptor (Scheme 10). Since the assignment of species involved in proton transfer is extremely difficult in the case of enzymatic systems, relevant investigations on adequate model complexes could be of vast importance. H2dapsox coordinates to Fe(II) in its neutral form, whereas in the case of Fe(III) it coordinates in the dapsox form. Thus, oxidation and reduction of its iron complex is a proton-coupled electron transfer process 46), which as an energetically favorable... 

The term mixed valence state has been used in two connotations (1) to mean two coupled irons with different formal charge (e.g., Fe Fe ) and (2) to mean two (or more) irons coupled and electronically delocalized to have fractional formal charge (e.g., 2Fe from delocalized Fe -t- Fe " "). We use mixed valence only in the former meaning. 

In addition to NAD and flavoproteins, three other types of electron-carrying molecules function in the respiratory chain a hydrophobic quinone (ubiquinone) and two different types of iron-containing proteins (cytochromes and iron-sulfur proteins). Ubiquinone (also called coenzyme Q, or simply Q) is a lipid-soluble ben-zoquinone with a long isoprenoid side chain (Fig. 19-2). The closely related compounds plastoquinone (of plant chloroplasts) and menaquinone (of bacteria) play roles analogous to that of ubiquinone, carrying electrons in membrane-associated electron-transfer chains. Ubiquinone can accept one electron to become the semi-quinone radical ( QH) or two electrons to form ubiquinol (QH2) (Fig. 19-2) and, like flavoprotein carriers, it can act at the junction between a two-electron donor and a one-electron acceptor. Because ubiquinone is both small and hydrophobic, it is freely diffusible within the lipid bilayer of the inner mitochondrial membrane and can shuttle reducing equivalents between other, less mobile electron carriers in the membrane. And because it carries both electrons and protons, it plays a central role in coupling electron flow to proton movement. 

FIGURE 19-9 IMADH ubiquinone oxidoreductase (Complex I). Complex I catalyzes the transfer of a hydride ion from NADH to FMN, from which two electrons pass through a series of Fe-S centers to the iron-sulfur protein N-2 in the matrix arm of the complex. Electron transfer from N-2 to ubiquinone on the membrane arm forms QH2, which diffuses into the lipid bilayer. This electron transfer also drives the expulsion from the matrix of four protons per pair of electrons. The detailed mechanism that couples electron and proton transfer in Complex I is not yet known, but probably involves a Q cycle similar to that in Complex III in which QH2 participates twice per electron pair (see Fig. 19-12). Proton flux produces an electrochemical potential across the inner mitochondrial membrane (N side negative, P side positive), which conserves some of the energy released by the electron-transfer reactions. This electrochemical potential drives ATP synthesis. 

Effect of Pressure on Proton-Coupled Electron Transfer Reactions of Seven-Coordinate Iron Complexes in Aqueous Solution It has been shown that seven-coordinate Fe(III) diaqua complexes consisting of a pentaaza macrocyclic ligand possess superoxide dismutase (SOD) activity, and therefore could serve an imitative SOD function.360 Choosing appropriate chemical composition of a chelate system yielded suitable pKa values for the two coordinated water molecules so that the Fe(III) complexes of 2,6-diacetylpyridine-bis(semicarbazone) (dapsox) and 2,6-diacetylpyridine-bis(semioxamazide) (dapsc) (see Scheme 7.12) would be present principally in the highly active aqua-hydroxo form in solution at physiological pH.361... 

RNRs catalyze the reduction of ribonucleotides to deoxyribonucleotides, which represents the first committed step in DNA biosynthesis and repair.These enzymes are therefore required for all known life forms. Three classes of RNRs have been identified, all of which turn out to be metalloenzymes. The so-called class I RNRs contain a diiron site (see Cobalt Bn Enzymes Coenzymes and Iron-Sulfur Proteins for the other two types of RNRs). As diagrammed in Figure 5, these enzymes generate first a tyrosyl radical proximal to the diiron site in the protein subunit labeled R2, and then a thiyl radical in an adjacent subunit (Rl) that ultimately abstracts a hydrogen atom from the ribonucleotide substrate. This controlled tyrosine/thiol radical transfer must occur over an estimated distance of 35 A, and a highly choreographed proton-coupled electron transfer (PCET) mechanism across intervening aromatic residues has been proposed. Perhaps, even more remarkably,... 

Electrons in the iron-sulfur clusters of NADH-Q oxidoreduetase are shuttled to coenzyme Q. The flow of two electrons from NADH to coenzyme Q through NADH-Q oxidoreduetase leads to the pumping offour hydrogen ions out of the matrix of the mitochondrion. The details of this process remain the subject of active investigation. However, the coupled electron- proton transfer reactions of Q are crucial. NADH binds to a site on the vertical arm and transfers its electrons to FMN. These electrons flow within the vertical unit to three 4Fe-4S centers and then to a bound Q. The reduction of Q to... 

White A. F. and Yee A. (1985) Aqueous oxidation-reduction kinetics associated with coupled electron-cation transfer from iron-containing silicates at 25°C. Geochim. Cosmochim. Acta 49, 1263—1275. 

Figure 18.10 Coupled electron-proton transfer reactions through NADH-Q oxidoreductase. Electrons flow in Complex I from NADH through FMN and a series of iron-sulfur cluster to ubiquinone (Q). The electron flow results in the pumping of four protons and the uptake of two protons from the mitochondria matrix. [Based on U, Brandt et al, FEB5 Letters 54S(2003) 9-17, Figure 2.]...

In this study, heterogeneous electron-transfer kinetics were measured for the following Se(VI)/Se(IV), As(V)/As(III), Fe(CN)5 /Fe(CN)5 ", and Fe(III)/Fe(II). All experiments were done at pH 6.0 with the exception of the iron couple, which was done at pH 3.0. Using electron-transfer kinetic constants, aqueous diffusion coefficients, aqueous concentrations, starting potentials, and a constant double-layer capacitance model, values for the change of EMF as a function of time for a platinum electrode were calculated numerically. The result of this simulation was then compared to the observed potentiometric response for a solution of the same concentration. 

The values and sources of diffusion data used in this study are given in Table I. The values of heterogeneous electron transfer rate constants are given in Table II. Rate constants were not measured above pH 3.0 for the iron couple due to the low solubility of ferric hydroxide. 

Whipple ER (1968) Quantitative Mossbauer Spectra and Chemistry of Iron Earth and Atmospheric Science. PhD dissertation, Massachusetts Irrstitute of Technology, Cambridge, 187 p White AF, Yee A (1985) Aqueous oxidation-reduction kinetics associated with coupled electron-cation transfer from iron-containing silicates at 25°C. Geochim Cosmochim Acta 49 1263-1275 Winchell AN (1925) Studies in the mica group. Am J Sci 5lh series 9 309-327... 

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