Biotransformations redox

These results may be viewed in the wider context of interactions between potential ligands of multifunctional xenobiotics and metal cations in aquatic environments and the subtle effects of the oxidation level of cations such as Fe. The Fe status of a bacterial culture has an important influence on synthesis of the redox systems of the cell since many of the electron transport proteins contain Fe. This is not generally evaluated systematically, although the degradation of tetrachloromethane by a strain of Pseudomonas sp. under denitrifying conditions clearly illustrated the adverse effect of Fe on the biotransformation of the substrate (Lewis and Crawford 1993 Tatara et al. 1993). This possibility should therefore be taken into account in the application of such organisms to bioremediation programs. 

Cervantes FJ (2002) Quinones as electron acceptors and redox mediators for the anaerobic biotransformation of priority pollutants. Ph.D. thesis, Wageningen University, Wageningen... 

Van der Zee FP, Bisschops IAE, Lettinga G et al (2003) Activated carbon as an electron acceptor and redox mediator during the anaerobic biotransformation of azo dyes. Environ Sci Technol 37 402-408... 

Li L, Wang J, Zhou J et al (2008) Enhancement of nitroaromatic compounds anaerobic biotransformation using a novel immobilized redox mediator prepared by electropolymerization. Bioresour Technol 99 6908-6916... 

Both purified laccase as well as the crude enzyme from the WRF Cerrena unicolor were used to convert the dyes in aqueous solution. Biotransformation of the dyes was followed spectrophotometrically and confirmed by high performance liquid chromatography. The results indicate that the decolorization mechanism follows MichaeliseMenten kinetic and that the initial rate of decolorization depends both on the structure of the dye and on the concentration of the dye. Surprisingly, one recalcitrant azo dye (AR 27) was decolorized merely by purified laccase in the absence of any redox mediator [46],... 

The few reports on bioremediation of colored effluents by yeasts usually mention nonenzymatic processes as the major mechanism for azo dye decolorization [5-10]. In a first approximation based on the cellular viability status, these processes can be divided into two different types bioaccumulation and biosorption. Bioaccumulation usually refers to an active uptake mechanism carried out by living microorganisms (actively growing yeasts). The possibility of further dye biotransformation by redox reactions may also occur due to the involvement of... 

The most important coenzymes in synthetic organic chemistry [14] and industrially applied biotransformations [15] are the nicotinamide cofactors NAD/ H (3a/8a, Scheme 43.1) and NAD(P)/H (3b/8b, Scheme 43.1). These pyridine nucleotides are essential components of the cell [16]. In all the reactions where they are involved, they serve solely as hydride donors or acceptors. The oxidized and reduced form of the molecules are shown in Scheme 43.1, the redox reaction taking place at the C-4 atom of the nicotinamide moiety. 

To circumvent the cofactor regeneration problem, redox biotransformations are also carried out in whole cells - for example, baker s yeast [28, 29] or engineered Escherichia coli cells [30] - using the intracellular cofactor pool and inherent or recombinant regeneration systems. 

With the exception of two dehydrogenases, all of the steroidogenic enzymes belong to the cytochrome P-450 (abbreviated as CYP) family of enzymes. The CYP enzymes are often involved with redox or hydroxylation reactions, and are also found in the liver where they are key players in biotransformation reactions (see Section 6.4). Different members of the CYP family are therefore involved with both synthesis in adrenal and gonads and hepatic inactivation of steroid hormones. 

The latter part of the book is dedicated to redox biotransformation application, with Chapter 9 disclosing several methods for the synthesis of chiral secondary alcohols using a range of commercially available ketoreductases (alcohol dehydrogenases) which are now being applied regularly on a large scale. 

New supramolecular compounds mimicking biological cofactors have been proposed which have high redox potentials and, further, can be Hnked covalently onto supports in biotransformation systems [52]. Bioelectrochemical methods available for regeneration of nicotinamide- and flavin-dependent systems are comprehensively reviewed by Kohhnan et al. [53]. 

Multi-Enzyme Systems in Whole-Cell Biotransformations and Expression of Redox Systems 55... 

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