Cytochromes reduction

Shimao M, Onishi S, Kato N, Sakazawa C (1989) Pyrroloquinolme qumone-dependent cytochrome reduction in polyvinyl alcohol-degrading Pseudomonas sp. strain VM15C. Appl Environ Microbiol 55 275-278... 

The addition of NADH to membranes from S. acidocaldarius (DSM 639) results in the reduction of cytochromes. Antimycin A does not affect cytochrome reduction, while the absence of complete reduction in the presence of cyanide suggests the presence of a branched electron transport chain [65]. 

Hess measured the apparent equilibrium constant for reduction of cytochrome by ferroeyanide as a function of pH 185), and found that the actual reduction step did not involve a proton and was entirely independent of pH. The free energy of reduction, and hence the reduction potential, change with pH only because the ratio of amounts of state III and IV changes with pH. The observed decrease of cytochrome reduction potential of 60 mV per pH unit above pH 8 is exactly what would be calculated from the simple Nernst equation. 

Nishio IN and Whitmarsh J (1993) Dissipation of the proton electrochemical potential in intact chloroplasts. II. The pH gradient monitored by cytochrome/reduction kinetics. Plant Physiol 101 89-96... 

Using the methylviologen cation radical (MV +) formed by pulse radiolysis, monophasic kinetics of cytochrome reduction are observed with a rate constant of 4.5 X 108 M 1/s (1.1 X 108 M 1/s on a per heme basis) at pH 8.0 with the Hildenborough cytochrome (36). This very fast second-order process approaches the diffusion controlled limit. Moreover, the reverse reaction can be estimated to be 7.8 X 104 M-1/s, which suggests that the reaction takes place primarily with the highest potential heme (the A E 0 between heme I and MV + is 190 mV, consistent with an equilibrium constant of approximately 103). Interestingly, the kinetics with MV + are ionic strength dependent, which is consistent with a plus-plus interaction,... 

Pulich (4) had observed the autooxidation of low potential cytochrome c in extracts of Nostoc when supplied with NADPH under aerobic conditions and he suggested that cytochrome reduction was catalysed by ferredoxin-NADP oxidoreductase in his preparation. We could easily repeat Pulich s observation using purified proteins but found the rate of cytochrome reduction was very low. The addition of crude ferredoxin to the reaction mixture greatly stimulated the rate of electron transfer. When extracts of the soluble proteins of cyanobacteria are chromatographed on DEAE cellulose ion exchange columns, two forms of ferredoxin are occasionally observed. When cells are grown with sufficient iron, one finds a ferredoxin which elutes from the column al 0.4 to 0.5 M NaCl and which is usually identified with electron transfer from Photosystem I to FNR. This is called ferredoxin I. 

Occasionally a second ferredoxin which elutes at lower ionic strength (0.2 - 0.3 M NaCl) is observed and this is called ferredoxin II. We found that ferredoxin II was ten times as active as ferredoxin I. Hutson et al. (5) had found that these ferredoxins differed by a factor of two in supporting the reduction of NADP by Photosystem I and differed less in the decarboxylation of pyruvate by the phosphoroclastic system of Clostridium pasteruanum. We applied the ferredoxin II fraction from the ion exchange column to a hydrophobic interaction column and resolved it into three distinct isozymes. One of these was very active in the cytochrome reduction activity, the second less active and the third isoz3nne was inactive. This specificity of interaction of one ferredoxin with low potential... 

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... 

Temperature Dependence of Reduction of Cvtochrome In contrast to oxidation of c cytochromes, reduction of cyt. bH was strongly dependent on temperature, as shown in figure 2A. To obtain the temperature dependency of... 

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... 

Compounds having the 16,17 ketal, eg, budesonide, amcinonide, fluocinonide, halcinonide, triamcinolone acetonide, and flurandrenohde, also undergo metabohsm by routes that parahel that of cortisol metabohsm. Unsymmetrical acetals such as budesonide are also metabolized by routes not available to the more metabohcahy stable symmetrical 16a,17a-isopropyhdiene-dioxysubstituted compounds (desonide, flunisohde, and triamcinolone acetonide). Isozymes within the cytochrome P450 3A subfamily are thought to catalyze the metabohsm of budesonide, resulting in formation of 16a-hydroxyprednisolone and 6P-hydroxybudesonide (19,20) (Fig. 3) in addition to the more common metabohc steps (oxidation via reduction of A, etc). 

The abihty of iron to exist in two stable oxidation states, ie, the ferrous, Fe ", and ferric, Fe ", states in aqueous solutions, is important to the role of iron as a biocatalyst (79) (see Iron compounds). Although the cytochromes of the electron-transport chain contain porphyrins like hemoglobin and myoglobin, the iron ions therein are involved in oxidation—reduction reactions (78). Catalase is a tetramer containing four atoms of iron peroxidase is a monomer having one atom of iron. The iron in these enzymes also undergoes oxidation and reduction (80). 

Clotrimazole and other azole derivatives have a different mode of action than the polyenes, eg, amphotericin B. The latter biad to the ergosterol present ia the membranes of yeasts and fungi, but azole derivatives inhibit the cytochrome P-450 dependent biosynthesis of ergosterol (8—11). This inhibition not only results in a reduction of ergosterol, but also in an accumulation of C-14 methyl sterols. They disturb membrane permeabiUty, inhibit cell rephcation, and are basically responsible, in combination with the reduction of ergosterol levels, for the antifungal action. 

Miconazole. Miconazole nitrate [22832-87-7] (Fig. 2), the 1-phenethyl-imidazole derivative first described in 1969, interferes at low doses with the cytochrome P-450 dependent ergosterol biosynthesis in yeasts and fungi. The result is accumulation of C-14 methylated sterols on the one hand and reduction of the ergosterol levels in the membranes on the other hand (12). Analogous to clotrimazole, this leads to a disturbance in the membranes it results in inhibition of ceU repHcation, mycelium development (in C. albicans) and finally, ceU death. High concentrations of miconazole, which may be achieved with topical use, disturb the orientation of phosphoHpids in the membranes, which produces leaks (13). 

RL Cutler, AM Davies, S Creighton, A Warshel, GR Moore, M Smith, AG Mauk. Role of arginine-38 in regulation of the cytochrome c oxidation-reduction equilibrium. Biochemistry 28 3188-3197, 1989. 

Complex rV is called, cytochrome c oxidase because it accepts electrons from cytochrome c and directs them to the four-electron reduction of O2 to form H2O ... 

Cytochrome c oxidase contains two heme centers (cytochromes a and %) as well as two copper atoms (Figure 21.17). The copper sites, Cu and Cug, are associated with cytochromes a and respectively. The copper sites participate in electron transfer by cycling between the reduced (cuprous) Cu state and the oxidized (cupric) Cu state. (Remember, the cytochromes and copper sites are one-electron transfer agents.) Reduction of one oxygen molecule requires passage of four electrons through these carriers—one at a time (Figure... 

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

Write a balanced equation for the reduction of molecular oxygen by reduced cytochrome e as carried out by complex IV (cytochrome oxidase) of the electron transport pathway. 

This impressive reaction is catalyzed by stearoyl-CoA desaturase, a 53-kD enzyme containing a nonheme iron center. NADH and oxygen (Og) are required, as are two other proteins cytochrome 65 reductase (a 43-kD flavo-protein) and cytochrome 65 (16.7 kD). All three proteins are associated with the endoplasmic reticulum membrane. Cytochrome reductase transfers a pair of electrons from NADH through FAD to cytochrome (Figure 25.14). Oxidation of reduced cytochrome be, is coupled to reduction of nonheme Fe to Fe in the desaturase. The Fe accepts a pair of electrons (one at a time in a cycle) from cytochrome b and creates a cis double bond at the 9,10-posi-tion of the stearoyl-CoA substrate. Og is the terminal electron acceptor in this fatty acyl desaturation cycle. Note that two water molecules are made, which means that four electrons are transferred overall. Two of these come through the reaction sequence from NADH, and two come from the fatty acyl substrate that is being dehydrogenated. 

Heme d,6 another isobacteriochlorin, occurs as one of two cofactors in the reductase cytochrome cdj which mediates the nitrite reduction to nitrogen monoxide (NO) and from there to dinitrogen oxide (N20) in denitrifying bacteria.7... 

The overall hydroxylation or epoxidation reaction catalyzed by cytochrome P450s involves the insertion of one oxygen atom, derived from molecular oxygen, into a C-H bond or into the Jt-system of an olefin, with the concomitant reduction of the... 

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