Oxidases, reduction

Fe Cytochrome oxidase reduction of oxygen to water Cytochrome P-450 0-insertion from O2, and detoxification Cytochromes b and c electron transport in respiration and photosynthesis Cytochrome f photosynthetic electron transport Ferredoxin electron transport in photosynthesis and nitrogen fixation Iron-sulfur proteins electron transport in respiration and photosynthesis Nitrate and nitrite reductases reduction to ammonium... 

This chapter summarizes some recent developments in the purification of ascorbate oxidase, the number of copper atoms per active molecule, and the stoichiometry of the different copper sites with reference to the classification introduced by Malkin and Malmstrom (5). Furthermore, physical properties of the metal centers are discussed in relation to other simple copper proteins that have been characterized in recent years. Finally, kinetic investigations of ascorbate oxidase reduction are presented as studied by anaerobic stopped-fiow and rapid-freeze techniques. 

Phase I reactions Phase I reactions include oxidation (espeeially by the cytochrome P450 group of enzymes, also called mixed-function oxidases), reduction, deamination, and hydrolysis. Examples are listed in Table 4-2. 

In addition to direct oxygenation, e.g. by aryl hydrocarbon hydroxylase, oxidative N- or 0-dealkylation is another process catalyzed by components of the Cytochrome P-450 System (mixed-function oxidases). Reduction also occurs in this system NADPH-cytochrome P-450 reductase has an activity similar to microsomal nitroreductase, i.e. transformation of aronaatic nitro compounds into the corresponding arylamines takes place. The oxidation may be followed by other enzymic reactions, e.g. epoxides are hydrated to vicinal diols by microsomal epoxide hydratase or they are coupled with glutathione by glutathione-S-epoxide transferase. 

Reduction to H2O takes place when electron transfer in the respiratory chain is completed by terminal oxidases. Reduction to H2O2 is associated with flavoproteins in flavin respiration, and superoxide radicals (O2 ) arise constantly in the respiratory chain and some other biological systems. Hydroperoxide is toxic to the cell and is degraded by catalase and/or peroxidase. Superoxide radicals are highly toxic to the cell, in addition, they promote the formation of more toxic products, namely, singlet oxygen and hydroxyl radicals ... 

The oxygen transporting capacity of SNO-PEG-Hb was evaluated using a hemorrhagic shock model in rats by monitoring the redox state of cytochrome oxidase reduction of cerebral tissues, in which a near-infrared spectroscopy... 

The terminal, non-reducing 2-acetamido-2-deoxy-a-D-galactopyranosyl residue of a Forssman hapten can be oxidized by D-galactose oxidase. Reduction of the oxidized residue with sodium borotritide gave (34). o-Galactosyl- and lactosyl-ceramides, asialo-GMi-ganglioside, and GM,-ganglioside were also labelled... 

Xanthine oxidase, mol wt ca 275,000, present in milk, Hver, and intestinal mucosa (131), is required in the cataboHsm of nucleotides. The free bases guanine and hypoxanthine from the nucleotides are converted to uric acid and xanthine in the intermediate. Xanthine oxidase cataly2es oxidation of hypoxanthine to xanthine and xanthine to uric acid. In these processes and in the oxidations cataly2ed by aldehyde oxidase, molecular oxygen is reduced to H2O2 (133). Xanthine oxidase is also involved in iron metaboHsm. Release of iron from ferritin requires reduction of Fe " to Fe " and reduced xanthine oxidase participates in this conversion (133). 

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. 

In animal metabolism, oxomolybdoenzymes catalyse a number of oxidation processes. These oxidases contain Mo coordinated to terminal O and S atoms, and their action appears to involve loss of an O or S atom along with reduction to Mo or Mo". It is, however, the role of molybdenum in nitrogen fixation which has received most attention. 

In the discussion of benzylamines, we have met medicinal agents that owe their activity to some particular functionality almost without reference to the structure of the rest of the molecule. The hydrazine group is one such function in that it frequently confers monamine oxidase-inhibiting activity to molecules containing that group. Such agents frequently find use as antidepressants. Thus, reduction of the hydrazone of phenyl-acetaldehyde (84) affords the antidepressant phenelzine (85). Similar treatment of the derivative of phenylacetone (86) gives pheniprazine (87). ... 

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). 

Finally in this section on deracemization via cyclic oxidation/reduction methods, there has been some limited work carried out on the deracemization of secondary alcohols. Soda et al. [22] employed lactate oxidase in combination with sodium borohydride to deracemize D/i-lactate (18) via the intermediate pyruvate (19) (Figure 5.12). 

Mo(V) paramagnetic species is also an argument to exclude an interaction between the Mo site and Fe-S center I. These studies were further complemented by detailed study of the observable splitting and its temperature dependence, EPR saturation, and the effect of differential reduction of the Fe-S centers. A magnetic interaction was also seen in xanthine oxidase, between various Mo(V) EPR species and one of the Fe-S centers. A study on the... 

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