And co-metabolism

The following section discusses the different types and phases of microbial degradation of organic pollutants present at aqueous-solid phase interfaces. This includes a discussion of growth-linked biodegradation, acclimation, detoxification, activation, defusing, threshold, and co-metabolism. 

Secondary biological treatments are considered to be an effective barrier to most PhCs due to the metabolic and co-metabolic processes at work in these systems [58, 77, 78]. 

Figure 8. Oxidation products found in aerobic cultures growing on naphthalene and co-metabolizing benzothiophene. (I) benzothiphene-sulfoxide, (II) 2,3-dihydrobenzothiophene-2,3-diol (III) 2,3-dihydrobenzothiophene-2, 3-dione.

With the exception of mease, which is found in bacteria, fungi, algae and higher plants, the other Ni-mediated processes, such as hydrogen uptake and production, reduction of C02 to CH4 and CO metabolism are restricted to bacteria and archae. In this chapter we will focus on these various metabolic reactions emphasizing the role played by nickel. 

A. W. Norman and co-eds.. Vitamin D Biochemical, Chemical and ClinicalMspects Related to Calcium Metabolism Proceedings of the Third Workshop on Vitamin D, Asilomar, Pacific Grove, Calif., Jan. 1977, Walter de Gmyter, Berlin, 1977, p. 15. 

Stephenson, M., Bacterial Metabolism," London, Longmans, Green, and Co., 1949. 

Hunt, S. and Grotf, J.L. 1990 Advanced Nutrition and Human Metabolism. St Paul, West Publishing Co. 

Romer, S. et al.. Genetic engineering of a zeaxanthin-rich potato by antisense inactivation and co-suppression of carotenoid epoxidation, Metabol. Eng. 4, 263, 2002. 

Horvath RS (1972) Microbial co-metabolism and the degradation of organic compounds in nature. Bacteriol Rev 36 146-155. 

Spokes JR, N Walker (1974) Chlorophenol and chlorobenzoic acid co-metabolism by different genera of soil hacteria. Arch Microbiol 96 125-134. 

The degradation of tetrachloromethane by a strain of Pseudomonas sp. presents a number of exceptional features. Although was a major product from the metabolism of CCI4, a substantial part of the label was retained in nonvolatile water-soluble residues (Lewis and Crawford 1995). The nature of these was revealed by the isolation of adducts with cysteine and A,A -dimethylethylenediamine, when the intermediates that are formally equivalent to COClj and CSClj were trapped—presumably formed by reaction of the substrate with water and a thiol, respectively. Further examination of this strain classified as Pseudomonas stutzeri strain KC has illuminated novel details of the mechanism. The metabolite pyridine-2,6-dithiocarboxylic acid (Lee et al. 1999) plays a key role in the degradation. Its copper complex produces trichloromethyl and thiyl radicals, and thence the formation of CO2, CS2, and COS (Figure 7.64) (Lewis et al. 2001). 

The simple porphyrin category includes macrocycles that are accessible synthetically in one or few steps and are often available commercially. In such metallopor-phyrins, one or both axial coordinahon sites of the metal are occupied by ligands whose identity is often unknown and cannot be controlled, which complicates mechanistic interpretation of the electrocatalytic results. Metal complexes of simple porphyrins and porphyrinoids (phthalocyanines, corroles, etc.) have been studied extensively as electrocatalysts for the ORR since the inihal report by Jasinsky on catalysis of O2 reduction in 25% KOH by Co phthalocyanine [Jasinsky, 1964]. Complexes of all hrst-row transition metals and many from the second and third rows have been examined for ORR catalysis. Of aU simple metalloporphyrins, Ir(OEP) (OEP = octaethylporphyrin Fig. 18.9) appears to be the best catalyst, but it has been little studied and its catalytic behavior appears to be quite distinct from that other metaUoporphyrins [CoUman et al., 1994]. Among the first-row transition metals, Fe and Co porphyrins appear to be most active, followed by Mn [Deronzier and Moutet, 2003] and Cr. Because of the importance of hemes in aerobic metabolism, the mechanism of ORR catalysis by Fe porphyrins is probably understood best among all metalloporphyrin catalysts. 

In some cases, microorganisms can transform a contaminant, but they are not able to use this compound as a source of energy or carbon. This biotransformation is often called co-metabolism. In co-metabolism, the transformation of the compound is an incidental reaction catalyzed by enzymes, which are involved in the normal microbial metabolism.33 A well-known example of co-metabolism is the degradation of (TCE) by methanotrophic bacteria, a group of bacteria that use methane as their source of carbon and energy. When metabolizing methane, methanotrophs produce the enzyme methane monooxygenase, which catalyzes the oxidation of TCE and other chlorinated aliphatics under aerobic conditions.34 In addition to methane, toluene and phenol have been used as primary substrates to stimulate the aerobic co-metabolism of chlorinated solvents. 

The primary metabolism of an organic compound uses a substrate as a source of carbon and energy. For the microorganism, this substrate serves as an electron donor, which results in the growth of the microbial cell. The application of co-metabolism for bioremediation of a xenobiotic is necessary because the compound cannot serve as a source of carbon and energy due to the nature of the molecular structure, which does not induce the required catabolic enzymes. Co-metabolism has been defined as the metabolism of a compound that does not serve as a source of carbon and energy or as an essential nutrient, and can be achieved only in the presence of a primary (enzyme-inducing) substrate. 

Venkataramani, E.S. and Ahlert, R.C., Role of co-metabolism in biological oxidation of synthetic compounds, Biotechnol. Bioeng., 27, 1306-1311, 1985. 

The effect of the skeletal muscle pump is essential during exercise. Although a mass sympathetic discharge and venous vasoconstriction enhance VR, this mechanism alone is insufficient to increase VR and, therefore, CO to meet the metabolic demands of strenuous exercise. The skeletal muscle pump mobilizes the blood stored in these tissues and keeps it flowing toward the heart. As the number of muscles involved in the exercise increases, so does the magnitude of the increase in VR and CO. 

Figure 7.6 Metabolism of MDMA in humans. Abbreviations CYP2D6, cytochrome P450 2D6 CYP3A4, cytochrome P450 3A4 COMT, catechol-O-methyltransferase. (Adapted from de la Torre and co-workers.56)...

Recently, Beatty and Tirrell [201] relied on the simultaneous or sequential addition of two reactive Met analogs, Aha and Hpg, to enable the fluorescent tagging of two protein populations within cells. The first demonstration of two-dye labeling of metabolically tagged cells was described in 2007 by Chang and co-workers [202], who used flow cytometry to show that cells treated with two reactive sugars could be labeled with distinct fluorophores. 

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