Redox partner

We will use here the main results obtained for two complex and distinct situations the structural and spectroscopic information gathered for D. gigas [NiFe] hydrogenase and AOR, in order to discuss relevant aspects related to magnetic interaction between the redox centers, intramolecular electron transfer, and, finally, interaction with other redox partners in direct relation with intermolecular electron transfer and processing of substrates to products. 

In some cases, small biological redox partner proteins such as heme-containing cytochromes, ferredoxins comprising an iron-sulfur cluster, or azurin with a mononuclear Cu site have been used as natural mediators to facilitate fast electron exchange with enzymes. A specific surface site on the redox protein often complements a region on the enzyme surface, and enables selective docking with a short electron tunneling... 

Outer-sphere electron transfer reactions involving the [Co(NH3)6]3+/2+ couple have been thoroughly studied. A corrected [Co(NH3)6]3+/2+ self-exchange electron transfer rate (8 x 10-6M-1s-1 for the triflate salt) has also been reported,588 which is considerably faster than an earlier report. A variety of [Co(NH3)g]3+/2+ electron transfer cross reactions with simple coordination compounds,589 organic radicals,590,591 metalloproteins,592 and positronium particles (electron/ positron pairs)593 as redox partners have been reported. 

The catalytic activity of CYP enzymes requires functional coupling with its redox partners, cytochrome P450 NADPH oxidoreductase (OR) and cytochrome bs. Measurable levels of these two proteins are natively expressed in most cell lines. Therefore, introduction of only the CYP cDNA is generally needed for detectable catalytic activity. However, the levels of expression of the redox partner proteins may not support maximal CYP catalytic activity, and therefore enhancement of OR levels may be desirable. This approach has been used successfully with an adenovirus expression system in LLC-PKi cells [12],... 

Back electron transfer takes place from the electrogenerated reduc-tant to the oxidant near the electrode surface. At a sufficient potential difference this annihilation leads to the formation of excited ( ) products which may emit light (eel) or react "photochemical ly" without light (1,16). Redox pairs of limited stability can be investigated by ac electrolysis. The frequency of the ac current must be adjusted to the lifetime of the more labile redox partner. Many organic compounds have been shown to undergo eel (17-19). Much less is known about transition metal complexes despite the fact that they participate in fljjany redox reactions. 

A redox reaction is a special case of the equilibrium reaction of A + B in Equation 13.1 B is now a reducible group in a biomolecule with an EPR spectrum either in its oxidized or in its reduced state (or both), and A is now an electron or a pair of electrons, that is, reducing equivalents provided by a natural redox partner (a reductive substrate, a coenzyme such as NADH, a protein partner such as cytochrome c), or by a chemical reductant (dithionite), or even by a solid electrode ... 

The aim in solution studies on metalloprotein is to be able to say more about intermolecular electron transfer processes, first of all by studying outer-sphere reactions with simple inorganic complexes as redox partners. With the information (and experience) gained it is then possible to turn to protein-protein reactions, where each reactant has its own complexities... 

Rate Constants and Reactivity. Electron-transfer reactions of plastocyanin (and other metalloproteins) are so efficient that only a narrow range of redox partners (having small driving force) can be employed. Rates are invariably in the stopped-flow range, Table I. Unless otherwise stated parsley plastocyanin... 

Other redox partners Co(bipy)33+ (oxidant) and Ru(NH3)s py2+ (reductant) are likewise partially blocked by Pt(NH3)6 +. Interestingly the reaction of cytochrome c(II) with PCu(II) is also blocked by Pt(NH3)5 +, thus identifying this as a site for electron transfer with cytochrome c. This observation is con-sis tant with a preliminary report of NMR results (19). The blocking is in fact more extensive than that observed with the above complexes, which is reasonable in view of the larger size of cytochrome c. Reaction with the negatively charged dipicol-inate oxidant, Co(dipic)2, was similarly investigated, where separate association of the oxidant with Pt(NH3)6 + can be... 

The effect is of similar magnitude to that observed for blocking by the 3+ redox inactive Co (1 3)53+ ( 30% decrease). Use of Cr(III) modified protein has no effect on the reaction with Fe(CN)53- as oxidant. These observations (21) support the belief that positive and negative redox partners utilize different functional sites on the protein for electron transfer. 

It is assumed in these experiments that the modification closest to the electron-transfer site will have most effect on rate constants. Rate constants are enhanced for 3+ and retarded for 3- redox partners. With Co(phen)33+ and Fe(CN)53 as oxidants it has been demonstrated that both react at the exposed heme edge of cytochrome c (23). The exposed heme edge is also relevant with PCu(II) as oxidant, Table III. With all three oxidants... 

A.G. Sykes I agree with Dr. Sutin that a 16% effect is small. Recent work has however shown that in our experiments the Cr binds at two or more sites on the plastocyanin (analyses confirm that there is attachment of one Cr per molecule of protein), and it is necessary therefore for us to elaborate on the original Farver and Pecht results. Our evidence is based on detailed kinetic studies with redox partners such as Co(phen>3 + when kinetic plots are found to be biphasic. It is concluded that the Cr attached at... 

The concentration of the Ti(C>2 ) species is solvent dependent. Thus, the solvent (or H20) may play the role of a redox partner. 

At this stage it was uncertain what the negative volumes of activation really meant since overall reaction volumes were not available. There was, however, data, now in the literature (140), that suggested that the oxidation of [Ru(NH3)6]2+ to [Ru(NH3)6]3+ is accompanied by a volume decrease of ca. 30 cm3 mol-1, which would mean that the activation volumes quoted above could mainly arise from volume changes associated with the oxidation of the ruthenium redox partner. 

Scott, E.E., White, M.A., He, Y.A., Johnson, E.F., Stout, C.D. and Halpert, J. R. (2004) Structure of mammalian cytochrome P450 2B4 complexed with 4-(4-chlorophenyl) imidazole at 1.9-A resolution insight into the range of P450 conformations and the coordination of redox partner binding. The Journal... 

The heterogeneous standard (or conditional) rate constant k° measures the intrinsic ability of a species (say, Ox) to exchange electrons with the electrode in order to convert to its redox partner (say, Red). A species with a large k° will convert to its redox partner on a short time scale a species with a small k° will convert to its redox partner on a long time scale. 

From a structural viewpoint, the Fe4S4 cluster can be considered to originate from the interpenetration of Fe4 and S4 tetrahedra. Scheme 2 depicts the molecular structures of the two redox partners [Fe4S4-(S-Ph)4]27[Fe4S4(S-Ph)4]3-.la... 

Despite the fact that it has not been possible to isolate any redox partners of this compound, it is evident that at least the monocation... 

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