Internal electron transfer

The subunits of CODH/ACS have been isolated (see earlier discussion). The isolated a subunit contains one Ni and four Fe and has spectroscopic properties (186) similar to those of Cluster A, the active site of acetyl-CoA synthesis (212). Unfortunately, it has no ACS activity. Therefore, ACS activity may reside in the a subunit or it may require both the a and the fi subunits. If Clusters B and/or C of the B subunit are involved in acetyl-CoA synthesis, one possible role could be in electron transfer. Although acetyl-CoA synthesis and the CO/ exchange reactions do not involve net electron transfer, both of these reactions are stimulated by ferredoxin, indicating that internal electron transfer within CODH/ACS may be required during the reaction (121). Further studies with the isolated subunits and the reconstitu-... 

Bureik, M., Schiffler, B., Hiraoka, Y. et al. (2002) Functional expression of human mitochondrial CYP11B2 in fission yeast and identification of a new internal electron transfer protein, etpl. Biochemistry, 41 (7), 2311—2321. 

The two-electron oxidation of sulfite generates a Molv Fem state, which converts to a Mov-Fen state. Cytochrome c then oxidizes this state to Mov-Fem, which then undergoes another internal electron transfer (k3) to form MoVI-Fen. A second oxidation by cytochrome c forms MoVI-Fem, which completes the catalytic cycle. 

In addition, the direct electrostatic interaction between adsorbates has been treated . At intermediates distances of the order of a surface lattice constant, Norskov, Holloway, and Lang report that this interaction can give rise to substantial (> 0.1 eV) interaction energies, when both adsorbates in question induce electron transfer to or from the surface or have a large internal electron transfer. 

The very rapid reaetion (3.15) with a large — AG can thus be measured. We therefore have an effective method for generating very rapidly in situ a powerful reducing or oxidizing agent. One of the most impressive applications of these properties is to the study of internal electron transfer in proteins. 

In the first step, considerable amounts of the final product are produced as well as smaller amounts of a transient in which the oxidation states are incorrect . Internal electron transfer redresses this imbalance. The species Ru(bpy)3+ produced must be removed rapidly (by scavenging with edta) so that it cannot oxidize the Ru(II) protein and interfere with the final step.See Sec. 5.9. Some other examples of the application of the photolytic method to a... 

At 25°C and p = 0.1 M, the values of are 10 -10 M and those of k are 10 -10 s depending on the identity of L. The internal electron transfer rate in an outer-sphere complex can thus be analyzedwithout considering work terms or, what is equivalent, the equilibrium controlling the formation of precursor complex. This favorable situation is even improved when the metal centers are directly bridged. The relative orientation of the two metal centers in a well-established geometry can be better treated than in the outer-sphere complex (Sec. 5.8). 

There has been nothing like the enthusiasm for the application to these systems of the theoretical equations, which we have noted in the previous sections and will encounter in the next. Nevertheless, a number of features are present which are qualitatively consistent with the discussions in Sec. 5.8.1 and which are in part illustrated in Table 5.11. There is a correlation of rate constant with the driving force of the internal electron transfer. -pjjg p. itro-phenyl derivative is a poorer reducing agent when protonated and k is much less than for the unprotonated derivative. Consequently disproportionation (2A 3) becomes important. Although there are not marked effects of structural variation on the values of A , the associated activation parameters may differ enormously and this is ascribed to the operation of different mechanisms."" The resonance-assisted through-chain operates with the p-... 

More subtle factors that might affect k will be the sites structures, their relative orientation and the nature of the intervening medium. That these are important is obvious if one examines the data for the two copper proteins plastocyanin and azurin. Despite very similar separation of the redox sites and the driving force (Table 5.12), the electron transfer rate constant within plastocyanin is very much the lesser (it may be zero). See Prob. 16. In striking contrast, small oxidants are able to attach to surface patches on plastocyanin which are more favorably disposed with respect to electron transfer to and from the Cu, which is about 14 A distant. It can be assessed that internal electron transfer rate constants are =30s for Co(phen)3+, >5 x 10 s for Ru(NH3)jimid and 3.0 x 10 s for Ru(bpy)3 , Refs. 119 and 129. In the last case the excited state Ru(bpy)3 is believed to bind about 10-12 A from the Cu center. Electron transfer occurs both from this remote site as well as by attack of Ru(bpy)j+ adjacent to the Cu site. At high protein concentration, electron transfer occurs solely through the remote pathway. 

A large positive AV is associated with the internal electron transfer within the outer-sphere complex between Co(NH3)5X + and Fe(CN). I. Krack and R. van Eldik, Inorg. Chem. 25, 1743 (1986) Y. Sasaki, K. Endo, A. Nagasawa and K. Saito, Inorg. Chem. 25, 4845 (1986). This value is not easy to interpret. 

M. Fabian and co-workers have studied the protein s role in internal electron transfer to the catalytic center of cytochrome c oxidase using stopped-flow kinetics. Mitochondrial cytochrome c oxidase, CcO, an enzyme that catalyzes the oxidation of ferrocytochrome c by dioxygen, is discussed more fully in Section 7.8. In the overall process, O2 is reduced to water, requiring the addition of four electrons and four protons to the enzyme s catalytic center. Electrons enter CcO from the cytosolic side, while protons enter from the matrix side of the inner mitochondrial membrane. This redox reaction. 

Tyrosyl radical Fe(TV)Tyr is formed by an internal electron transfer in the peroxidase Compound I ... 

These data are rationalized in terms of the Shannon-Swan rule and the low first ionization potential of the Cu(II) ion compared to the Zn(II) ion. This difference allows the Zn(U) complex to fragment by the route, OE - EE + OE , (i.e., a radical loss from the molecular ion). The fragmentation scheme of the Cu(II) complex follows the route, EE - EE + EE (i.e., a neutral, stable molecule loss from the molecular ion), which suggests an internal electron transfer and cleavage of the Cu-S bond in the molecular ion (612). 

That NO is a better rc-acceptor ligand than N2R parallels the greater electronegativity of O over NH and the greater Lewis acidity of NO+ over N2R+. However, the organodiazenido ligand renders the metal more susceptible to internal electron transfer (Mn—N2R+ - M"+2—N2R ) and to attack by electrophiles at Na (equations 110 and 111). 

Although such reactions have been known for a long time, it appears a somewhat neglected area of study. Most attention has been on cysteine and its oxidation to the disulfide which is catalyzed by metal ions, in particular CuI[ (see Section 20.2.2.2.2) and FeI,[.81 The likely intermediates in these reactions are metal-cysteine complexes which undergo internal electron transfer. As noted earlier (Section 20.2.2.2.2), penicillamine differs from cysteine in its reactivity and gives rise to mixed valence species. More recently Mn11 has also been found to catalyze the oxidations of Cys and Pen. 

The formation of some Ru(III) complexes upon photodecarbonylation followed by an internal electron transfer can be expressed as follows... 

Shown in Figures 5-7 are the redox pathways for xanthine oxidase, sulfite oxidase, and nitrate reductase (assimilatory and respiratory), respectively. These schemes address the electron and proton (hydron) flows. The action of the molyb-doenzymes is conceptually similar to that of electrochemical cells in which half reactions occur at different electrodes. In the enzymes, the half reactions occur at different prosthetic groups and intraprotein (internal) electron transfer allows the reactions to be coupled (i.e., the circuit to be completed). In essence, this is the modus operandi of these enzymes, which must be determined before intimate mechanistic considerations are seriously addressed. 

The rate of electron transfer that occurs to/from the metal center is high. Structure based modeling of the active site of human MnSOD [40], which includes calculating the energies of both the oxidized and reduced states with either water or hydroxide bound to the metal, suggests the rate of this internal electron transfer is enhanced by electron-relaxation effects. In addition, a 0.17 V redox potential is calculated, a value that is low compared with the experimental values of 0.31 V fori . coli and 0.26 V for B. stearothermophilus, respectively. A potential of —0.30 V seems to be optimal as it lies midway between the redox potentials of the two half reactions of the dismutation process [41],... 

Dithiete can be used as an oxidant for the thiometalates and oxythiometalates, and thus induced internal electron-transfer reactions provide an efficient method for synthesizing selected metal dithiolene and oxo-containing... 

Recent results in this laboratory have demonostrated the conversion of ortho- and para-, but not m< ta-hydroxyphenyl radicals into the phenoxyl radical. Furthermore, an internal electron transfer from the nitro-group to the bromide of p-nitrobenzyl bromide radical anion, to produce p-nitrobenzyl radical and bromide ion, has been observed. 

Wilson EK, Bellelli A, Liberti S, et al. Internal electron transfer and structural dynamics of cdl nitrite reductase revealed by laser CO photodissociation. Biochemistry 1999 38 7556-64. 

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