Oxidation enzyme-initiated

Fermentation. The term fermentation arose from the misconception that black tea production is a microbial process (73). The conversion of green leaf to black tea was recognized as an oxidative process initiated by tea—enzyme catalysis circa 1901 (74). The process, which starts at the onset of maceration, is allowed to continue under ambient conditions. Leaf temperature is maintained at less than 25—30°C as lower (15—25°C) temperatures improve flavor (75). Temperature control and air diffusion are faciUtated by distributing macerated leaf in layers 5—8 cm deep on the factory floor, but more often on racked trays in a fermentation room maintained at a high rh and at the lowest feasible temperature. Depending on the nature of the leaf, the maceration techniques, the ambient temperature, and the style of tea desired, the fermentation time can vary from 45 min to 3 h. More highly controlled systems depend on the timed conveyance of macerated leaf on mesh belts for forced-air circulation. If the system is enclosed, humidity and temperature control are improved (76). 

An enzyme-initiated hydroxylation-oxidation carbo-Diels-Alder domino reaction [82]... 

Oxidation is initiated by formation of radicals which may be the result of enzyme catalysed reactions like oxygen activation by xanthine oxidase in... 

Selenoprotein A is remarkably heat stable, as seen by the loss of only 20% of activity on boiling at pH 8.0 for lOmin (Thrner and Stadtman 1973). Although selenoprotein A contains one tyrosine and no tryptophan residues, it contains six phenylalanine residues and thus has an unusual absorbance spectrum (Cone et al. 1977). Upon reduction, a unique absorption peak emerges at 238 nm, presumably due to the ionized selenol of selenocysteine, which is not present in the oxidized enzyme. The activity of selenoprotein A was initially measured as its ability to complement fractions B and C for production of acetate from glycine, in the presence of reducing equivalents (e.g., dithiothreitol). Numerous purification schemes were adopted for isolation of selenoprotein A, all of which employed the use of an anion exchange column to exploit the strongly acidic character of the protein. 

Suzuki I, Silver M. 1966. The initial product and properties of the sulfur-oxidizing enzyme of thiobacilli. Biochim Biophys Acta 122 22-33. 

The classical type A enzymes show marked differences in their abilities to oxidize two-electron donors. All the enzymes initially examined, whether eukaryotic or prokaryotic in origin, were members of clade III. These are rather effective as two-electron peroxidases. But the oxidations of ethanol and formate by the paradigmatic clade II enzyme from E. coli, HPII, are much slower. There is little information concerning the activity of type A enzymes in clade I. 

Most of the pro-oxidative enzymes of bacteria are stabilized inside the cell, but are very fragile outside the cell. Therefore, the view that analysis may be carried out via isolated enzymes for aromatic processing, perhaps coupled to an electrode of some kind, appears quite impractical. MCA takes advantage of what bacterial cells can actually do, namely to stabilize and protect enzymes, besides the initial synthesis. Hence, MCA is likely to be far more practical than any bioelectrode method for analysis. 

When O2 is the electron acceptor, the reduction can occur in either two-electron steps with FADH2 as reductant and H2O2 as the product or in a one-electron manner with 02 as the product. In the latter case, the reduced form of the flavin could be either FADH2 or FAD Recent studies on the reaction of O2 with reduced xanthine oxidase has shown that reoxidation of the six-electron reduced enzyme by O2 proceeds initially with two sequential two-electron steps to form two moles of H2O2 and the two-electron reduced form of the enzyme. Oxidation of the two-electron reduced form by O2 then proceeds via two-sequential one-electron steps to form two moles of O2 and oxidized enzyme. The differential rate of O2 release is suggestive of one mole arising from the one-electron... 

Two ascorbate radicals can react with each other in a disproportionation reaction to give ascorbate plus dehydroascorbate. However, most cells can reduce the radicals more directly. In many plants this is accomplished by NADH + H+ using a flavoprotein monodehydroascorbate reductase.0 Animal cells may also utilize NADH or may reduce dehydroascorbate with reduced glutathione.CC/ff Plant cells also contain a very active blue copper ascorbate oxidase (Chapter 16, Section D,5), which catalyzes the opposite reaction, formation of dehydroascorbate. A heme ascorbate oxidase has been purified from a fungus. 11 1 Action of these enzymes initiates an oxidative degradation of ascorbate, perhaps through the pathway of Fig. 20-2. 

Most likely the changes in IEP value, as well as in PCD potential, during the enzymatic treatment of wool are the result of enzyme-initiated oxidation reactions. As can be seen from the XPS results specified below (Table 1), a slight increase in SO2, SO3, SO4 groups concentration, from 0.248% (0.248%= 11.8% of 2.1% of total elemental concentration) for untreated sample to 0.314% (0.314% = 19.6% of 1.6% of total elemental concentration) can be observed. 

P Oxidation focuses on the ft carbon of the fatty acid. In some instances, it is impossible to form a ketone on the P carbon, for example, if the P carbon is methylated. In such cases, the a carbon may be oxidized to initiate the oxidation process, a Oxidation is useful in the degradation of certain plant materials, such as phytol. The degradation of this compound is illustrated in Figure 19.9. In Refsum disease, caused by a genetic lesion, the enzyme hydroxylating phytanic acid is absent and phytanic acid accumulates in tissues. 

Each of the other families of eicosanoids—thromboxanes and leukotrienes—has interesting biosynthetic pathways too, but we will mention only one small detail. A completely different oxidation enzyme, lipoxygenase, initiates a separate pathway leading to the leukotrienes, but the first steps are very similar. They just occur elsewhere in the arachidonic acid molecule. 

Several findings in the above results are not consistent with earlier reports (Yoshikawa et al., 1995 Van Gelder, 1966 Tiesjema et al., 1973 Schroedl and Hartzell, 1977 Babcock et al., 1978 Blair et al., 1986 Steffens et al., 1993). It has been widely accepted that four electron equivalents are sufficient for complete reduction of the fuUy oxidized enzyme as prepared. However, most of the previous titrations were performed in the presence of electron transfer mediators. In the presence of electron transfer mediators, such as phenazine methosulfate (PMS) under anaerobic conditions, the bovine heart enzyme purified with crystallization also showed a four-electron reduction without the initial lag phase as observed in Fig. 9. A catalytic amount of PMS induced a small spectral change corresponding to the initial lag phase. These results suggest that electron transfer mediators in other titration experiments also induce autoreductions to provide the enzyme form that receives four electrons for the complete reduction. 

A more recent examination of the kinetics of this enzyme by initial rate measurements has included product inhibition patterns and has led to the conclusion that at least under some conditions an ordered bi-bi mechanism applies which involves a ternary complex of enzyme, NAD, and dihydrolipoamide (157). Clear spectral evidence is presented for the existence of a complex between NAD and the oxidized enzyme and this will be discussed in Section III,E. The product inhibition pattern for NAD tended toward that expected for this mechanism only at high NAD concentration. 

The pH dependenee of both the oxidative and reductive half-reactions of sulfite oxidase has reeently been examined (Brody and Hille, 1999). From a comparison of the pH dependence of kcat with the limiting rate constants for the two half-reactions, kred and kox, respectively, it is evident that k d i s principally rate-limiting above pH 7, but at lower pH kox becomes increasingly important, kred is essentially pH-independent, consistent with a reaction mechanism in which nucleophilic attack by the substrate lone pair on a Mo=0 group initiates the eatalytie sequence. The pH dependence of kred/ d " " indieates an aetive site group having a p a of 9.3 must be depro-tonated for reaetion of oxidized enzyme with free sulfite, possibly Tyr 322 whieh from the erystal strueture of the enzyme constitutes part of the... 

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