Whole-cell-mediated

Metabolic pathways containing dioxygenases in wild-type strains are usually related to detoxification processes upon conversion of aromatic xenobiotics to phenols and catechols, which are more readily excreted. Within such pathways, the intermediate chiral cis-diol is rearomatized by a dihydrodiol-dehydrogenase. While this mild route to catechols is also exploited synthetically [221], the chirality is lost. In the context of asymmetric synthesis, such further biotransformations have to be prevented, which was initially realized by using mutant strains deficient in enzymes responsible for the rearomatization. Today, several dioxygenases with complementary substrate profiles are available, as outlined in Table 9.6. Considering the delicate architecture of these enzyme complexes, recombinant whole-cell-mediated biotransformations are the only option for such conversions. E. coli is preferably used as host and fermentation protocols have been optimized [222,223]. 

In contrast to 2,3-dioxygenases, the related ipso/ortho oxygenation of aryl carbox-ylates has received considerable less attention and has hardly been utilized by the synthetic community, so far. Biooxidation of benzoic acid and P-naphthalene carboxylate provide access to corresponding 1,2-dihydroxylated dihydroaryl compounds in excellent stereoselectivity (Scheme 9.35), analogous to TDO- and NDO-mediated ortho/meta oxygenations. Whole-cell-mediated biotransformations were performed with mutant strains of Rahtonia and Pseudomonas and enable access to preparative quantities in >5 gl titers [261,262]. 

Both enzymatic and whole cell-mediated transformation of sulfonamides has been described, usually with high removal efficiencies and relatively short treatments. Although different metabolites have been elucidated, no clear pathways have been defined however, the desulfonated metabolites have been widely identified (with LAC and fungal cells) along with the products of hydroxylation, formylation, and deamination reactions, and combinations of them. 

The book is divided into three sections Enzyme mediated bioprocess, whole cell mediated bioprocess and the engineering principle in bioprocess... 

III. MULTIPLE BAKER S YEAST REDUCTASES AND STEREOSELECTIVITY IN WHOLE CELL-MEDIATED REDUCTIONS... 

Modulating the Whole-Cell Mediated Resolution with Additives... 

Whole cell-mediated reduction of geraniol to citronello Multi-enzymatic fructose-l,6-diphosphate production Bacterial treatment of metal-containing wastewaters Bacterial treatment of chlorinated-wastewaters Cell-free luciferase synthesis... 

Tubular dense membrane/recycle Whole cell-mediated epoxidation of 1,7-octadiene [148]... 

Enzyme-catalysed or whole-cell-mediated oxidation and reduction reactions are generally less easy to perform than, for example, hydrolysis reactions. There are exceptions to this rule, for example the yeast reaction of ketones. 

It seems that enzyme-catalysed reactions and whole-cell-mediated transformations are set to make a major impact in synthetic organic chemistry over the next few years. On the more distant horizons, the complementary held of catalytic antibodies is beginning to demonstrate its potential power but that is another story which may well mushroom so as to necessitate the writing of other specialist text-books devoted to that topic. 

To overcome this obstacle, two different approaches have been exploited, both possessing benefits and disadvantages. For the isolated enzyme, a closed-loop system has been developed, where an auxiliary substrate has been added to regenerate NAD(P)H. The second approach is based on whole-cell-mediated biotransformations. 

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