Reduction mechanism, cytochrome

Oxygen reduction Models have been created for cytochrome oxidase, which have provided insights into the oxygen activation and reduction mechanism. 

Reduction of the heme iron of cytochromes P450 to the ferrous state 3 is necessary for the binding and subsequent activation of atmospheric dioxygen. Initially, two electrons are derived from NAD(P)H by flavin adenine dinucleotide (FAD)-containing proteins and then are used sequentially via one-electron transfers. AU cytochromes P450 can be divided into two main classes with respect to the reduction mechanism and the structure of their immediate redox partner. The first class includes most soluble... 

The results are consistent [148] with a mechanism involving one-electron transfer from reduced flavoprotein to quinone. The resulting semiquinones from naphthoquinone and menadione subsequently transfer an electron to oxygen benzosemi-quinone does not. A similar one-electron reduction mechanism apparently operates with cytochrome-hj reductase. 

Table IV also lists the structure analyses of three bacterial cytochromes. Logically these should be delayed to the second half of the chapter, but the results have had such an important influence on thinking about oxidation-reduction mechanisms that their discussion here is mandatory. A key structure study from an evolutionary standpoint was that of R. rvhrum by Salemme, Kraut, and colleagues at the University of California, San Diego (30,31). That study established in one stroke that the eukaryotic cytochrome fold also extended to a bacterial cytochrome and included the cytochromes of photosynthesis as well as respiration. The fact that this structural homology had been predicted on the basis of amino acid sequence comparisons (32) did not lessen the excitement of seeing direct confirmation from the molecular model. Only one of three possible conclusions can be drawn ...

Recent chemical evidence has made it apparent that both of the two detailed oxidation-reduction mechanisms which have been proposed for cytochrome must be abandoned the Winfield mechanism espoused for eukaryotic c, and the heme crevice hydrogen bond network proposed for bacterial c-i. Each mechanism has been undermined by the discovery of unworkable amino acids at key positions in species for which the proposed scheme should hold. A fresh approach is needed. 

Mayer, B., Heinzel, B.. Klatt, P., John M., Schmidt, K., and Bohme, E. (1992). Nitric oxide synthase-catalyzed activation of oxygen and reduction of cytochromes Reaction mechanisms and possible physiological implications. J. Cardiovasc. Pharmacol. 20, S54-S56. 

Shiro, Y., M. Fujii, T. lizuka, S.I. Adachi, K. Tsukamoto, K. Nakahara, and H. Shoun (1995). Spectroscopic and kinetic studies on reaction of cytochrome P450nor with nitric oxide. Implication for its nitric oxide reduction mechanism. J. Biol. Chem. 270, 1617-1623. 

Multiple Flash Reduction of Cytochrome be. A Q cycle model involving interheme transfer implies that the two hemes of cyt be should be reducible by successive light flashes (7) if their reoxidation is inhibited by NQNO, as was proposed for its mechanism of action. The effect of two successive saturating laser flashes, and of five xenon flashes (Fig. 3), on the amplitude of cyt be reduction was tested in the presence and absence of NQNO, and at 9 C to slow the reoxidation, conditions that should allow the maximum cytochrome reduction. It was not possible to obtain an increase in the amplitude of be reduction beyond 0.6-0.7 heme/flash on... 

The organization of xanthine oxidase appears to be quite complex. There is evidence that various substrates are not bound at the same site, and that the primary reaction of different substrates may occur with various ones of the cofactors. The oxidation of purines and aldehydes is inhibited by pteridyl aldehyde and by cyanide, but these reagents do not affect the oxidation of DPNH. It is possible that these inhibitors influence substrate binding sites and primary electron transport, respectively, and that the oxidation of DPNH involves a different binding site and avoids the cyanide-sensitive electron transport mechanism, which may well involve iron. Xanthine oxidase, and probably all flavoproteins, require —SH groups, but a definite function for these groups cannot be ascribed at this time. Similarly, various factors influence the reactions with oxidants differentially. Cyanide inhibits cytochrome reduction, but not the reactions with 0 or dyes. Reduction of either cytochrome c or nitrate depends upon the presence of molybdenum. These observations... 

In addition to catalyzing the oxidation of many compounds, LiP is also able to catalyze reductive reactions in the presence of electron donors such as EDTAor oxalate (Fig. 7) (6, 77). Veratryl alcohol is a free radical mediator in these reactions. The electron donors appear to be oxidized by a LiP generated veratryl alcohol cation radical. The resulting anion radical can catalyze the reduction of good electron acceptors such as cytochrome c, nitroblue tetrazolium, and oxygen. Evolution of CO2 from EDTA or oxalate effectively drives the reductive reactions. Similar reactions have also been observed with the manganese dependent peroxidases in the presence of quinones (20). Early work performed in our laboratory showed that these reductive mechanisms are not involved in TNT reduction. However, these reactions may be involved in other steps in TNT metabolism. 

FIGURE 21.20 A model for the mechanism of O9 reduction by cytochrome oxidase. 

The mechanism of reduction of dioxygen by fully reduced cytochrome oxidase. Correlation of room and low temperature studies. G. M. Clore, Rev. Inorg. Chem., 1980,2, 343-360 (52). 

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