Biocatalytic reaction oxidoreductases

Together with enantioselective hydrolysis/acylation reactions, enantioselective ketone reductions dominate biocatalytic reactions in the pharma industry [10], In addition, oxidases [11] have found synthetic applications, such as in enantioselective Baeyer-Villiger reactions [12] catalyzed by, for example, cyclohexanone monooxygenase (EC 1.14.13) or in the TEMPO-mediated oxidation of primary alcohols to aldehydes, catalyzed by laccases [13]. Hence, the class of oxidoreductases is receiving increased attention in the field of biocatalysis. Traditionally they have been perceived as difficult due to cofactor requirements etc, but recent examples with immobilization and cofactor regeneration seem to prove the opposite. 

Biocatalytic ledox reactions offer great synthetic utility to organic chemists. The majority of oxidase-catalyzed preparative bioconversions are still performed using a whole-ceU technique, despite the fact that the presence of more than one oxidoreductase in cells often leads to product degradation and lower selectivity. Fortunately, several efficient cofactor regeneration systems have been developed (160), making some cell-free enzymatic bioconversions economically feasible (161,162). 

Since the beginning of enzyme catalysis in microemulsions in the late 1970s, several biocatalytic transformations of various hydrophilic and hydrophobic substrates have been demonstrated. Examples include reverse hydrolytic reactions such as peptide synthesis [44], synthesis of esters through esterification and transesterification reactions [42,45-48], resolution of racemic amino acids [49], oxidation and reduction of steroids and terpenes [50,51], electron-transfer reactions, [52], production of hydrogen [53], and synthesis of phenolic and aromatic amine polymers [54]. Isolated enzymes including various hydrolytic enzymes (proteases, lipases, esterases, glucosidases), oxidoreductases, as well as multienzyme systems [52], were anployed. 

More recently, the focus has been put on formal nucleophilic substitution of —OH or —NH2 groups. To perform this biocatalytic variant of the Mitsunobu reaction, an oxidation-nucleophilic addition-reduction sequence is necessary, for which linked NAD-dependent oxidoreductases are ideally suited. The early contributions from the Forschungszentrum Jiilich [79] have been recently rediscovered by Kroutil and coworkers [80]. By combining a mandelate racemase (MR) with a mandelate dehydrogenase and an L-amino acid dehydrogenase, the authors could completely transform racemic mandelic acid into enantiopure (S)-phenyl-glycine (Scheme 8.16). 

However, the most extensively investigated class of ERs is members of the OYE family of flavin oxidoreductases (EC 1.6.99.1). There is detailed information known about OYEs, such as their structure, reaction mechanism, substrate scope, kinetic properties, and biocatalytic approaches. Therefore, this chapter will focus on this latter class of enzymes. They have been intensively studied over the past decade in view of their applicability in preparative-scale biotransformations [1, 8-12]. These FMN-containing enzymes catalyze the asymmetric reduction of a,p-unsaturated... 

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