Electron transfer with cytochromes

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... 

Steady state kinetics and protein-protein binding measurements have also been reported for the interaction of these mutant cytochromes with bovine heart cytochrome c oxidase [120]. The binding of cytochrome c variants to the oxidase occurred with increasing values of Kj in the order He (3 x 10 Mol L ) < Leu = Gly < wild-type < Tyr < Ser (3 x 10 molL ). Steady-state kinetic analysis indicated that the rate of electron transfer with cytochrome c oxidase increased in the order Ser < He < Gly < Leu < Tyr < wild-type, an order notably different from that observed for a related analysis of the oxidation of these mutants by cytochrome c peroxidase [85]. This difference in order of mutant turnover by the oxidase and peroxidase may arise from differences in the mode of interaction of the cytochrome with these two enzymes. 

The 2,6-dipicolinic acid complex [Co(dipic)2] has recently been used as a lipophilic oxidant to study electron transfer with cytochromes c and cssl and with the blue copper centre in stellacyanin, plastocyanin and azurin penetration of the complex towards the metal centre of the protein is thought to aid electron transfer.257,271... 

In a recent report, it was demonstrated that adsorption of 4,4 -bipyridine on platinum led to quasi-reversible rates of electron transfer with cytochrome c as evidenced by cyclic voltammetry. However, the concentration of 4,4 -bipyridine required to produce this electrochemical response was five times that which is required at gold electrodes. This difference was ascribed to the difference in the tendency of 4,4 -bipyridine to adsorb at gold and platinum electrodes. These results indicate that the use of 4,4 -bipyridine may be applicable to other solid electrodes as well for the study of cytochrome c electron transfer reactions. 

The pioneering works of Hill and Eddows have opened the way to realize fast and efficient electron transfer of enzymes at the electrode surface. They modified a gold electrode with 4,4 -bipyrydyl, an electron promoter, not a mediator since it does not take part in electron transfer in the potential region of interest, to accomplish rapid electron transfer of cytochrome [1], Their work has triggered intensive investigation of electron transfer of enzymes using modified electrodes [2]. 

In the pursuit of mechanisms for electron transfer, various cytochromes and enzymes have been isolated and purified. A list of proteins implicated in the electron transfer processes with reduction of electron acceptor is given in Table 16.5. Although reductases have considerable specificity for reduction, it is apparent that the low-potential multiheme cytochromes interface with numerous different electron acceptors. 

ATP is synthesized. Addition of cyanide (CN ), which blocks electron transfer between cytochrome oxidase and 02, inhibits both respiration and ATP synthesis, (b) Mitochondria provided with succinate respire and synthesize ATP only when ADP and P, are added. Subsequent addition of venturicidin or oligomycin, inhibitors of ATP synthase, blocks both ATP synthesis and respiration. Dinitrophenol (DNP) is an uncoupler, allowing respiration to continue without ATP synthesis. 

Many of the reactions of the plastocyanins and azurins with other redox proteins follow Marcus behaviour.946 These reactions all show a single mechanism of electron transfer, with no kinetic selectivity and no specific interactions between the proteins. The notable exception to this behaviour is cytochrome / (c552), where a specific interaction occurs,934 appropriate for its natural redox partner. Equation (48) represents a probable sequence of electron carriers, although it is difficult to extrapolate conclusions to the membrane-bound proteins. 

The reaction of the fully reduced enzyme with 02 does not follow the pathway described above. Instead, loss of compound A is followed by the production of compound B through the intermediate formation of another compound (II), which may be equivalent to the peroxo compound C, but having reduced cytochrome a and CuA. The transient nature of this species II compared to compound C would result from the possibility of electron transfer from cytochrome a and CuA, to give compound B. The fact that cytochrome a and CuA are at least partially oxidized in compound B is shown by the appearance of the ESR signal (in 30-50% intensity) characteristic of the fully oxidized enzyme. Intermediates beyond compound B in the oxidation sequence have also been described. In one of these, the CuB2+ is ESR-detectable. 

However, the changes in environment which occurred with the change from a reductive to an oxidative atmosphere rendered iron sulfide-based redox systems inconvenient, as they were very sensitive to (irreversible) oxidation. We saw in earlier chapters the facile formation of porphyrin and phthalocyanines from relatively simple precursors, and these systems were adopted for the final steps of electron transfer in oxidative conditions. The occurrence of iron centres in planar tetradentate macrocycles is ubiquitous, and metalloproteins containing such features are involved in almost every aspect of electron transfer and dioxygen metabolism. A typical example is seen in the electron transfer protein cytochrome c (Fig. 10-10). 

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