Redox biotransformation reactions

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. 

At the same time, engineering rules that apply to whole-cell redox reactions have to be taken into account. In general, aeration and/or carbon dioxide production is involved, and plug flow reactors are not appropriate. Moreover, oxygen transfer to immobilized cells is not very efficient. Consequently, continuous reactors are not very suitable for redox biotransformations with whole cells [12]. These biotransformations can best be carried out in (fed) batch reactors with free cells. Since the production of the cells will also involve a (fed) batch process, these processes may easily be combined. Then, the cells are produced in a fed-batch fer-... 

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. 

It is now well-established that some enzyme families, including various peroxidases and laccases, catalyze the polymerization of vinyl monomers and other redox active species such as phenol-type structures. Vinyl polymerization by these redox catalysts has recently been reviewed 93). These catalysts have been used to prepare polyanilines 94) and polyphenols 95,96). A few examples of related research are included in this book. For example. Smith et al (57) described a novel reaction catalyzed by horseradish peroxidase (HRP). In the presence of HRP and oxygen, D-glucuronic acid was polymerized to a high molecular weight (60,000) polyether. However, the authors have not yet illucidated the polyether structure. Two other oxidative biotransformations were discussed above i) the sono-enzymatic polymerization of catechol via laccase 31), and ii) the oxidation of aryl silanes via aromatic dioxygenases 30). 

The industrial use of bio catalysts has been reviewed in many excellent papers from industrial and academic experts in recent years [6-20]. These publications clearly show that immobilized systems find only limited use in present bioprocesses. Straathof et al. recently investigated 134 industrial biotransformations and came to the conclusion that only 20 confirmed processes rely on immobilized bio catalysts [15]. This is due to the fact that immobilization can be a considerable cost factor and is frequently used in combination with less common continuous reactors. In addition, many transformations belong to the class of redox reactions and require a cofactor for the reaction to occur. Such processes can in many cases be realized perfectly under fermentative conditions by the use of living or resting cells [17,21-25]. 

Whole-cell biocatalysts are basically one-pot cascade reactions, with various enzymatic reactions being carried out concurrently within individual cells. Compared to purified enzymes, whole-cell biocatalysts are inexpensive and easily scalable and can be stably stored indefinitely. Certain enzymes are unstable and lose activity when purified from cells. Living cells also contain and regenerate otherwise expensive redox cofactors and, with metabolic engineering, can produce desired chemicals from inexpensive carbon and nitrogen sources [66, 67]. On the other hand, a major downside of using whole cells for biotransformations is the increased cost of product extrachon and purification from fermentahon broths. One also has to consider the... 

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