Cofactor regeneration, chemical

Fig. 8.11 Stereoselective synthesis of 14(5), 15(7 )-epoxyeicosatrienoic acid utilizing P450 BM3 F87V and glucose-6-phosphate dehydrogenase (G-6P-DH) for cofactor regeneration. Chemical steps yielded the corresponding antipode...

The principal strategies of cofactor regeneration - namely the enzymatic, chemical and electrochemical approach - are presented in Scheme 43.2 and have been reviewed recently [17, 21-23]. This chapter does not intend to be exhaustive rather, it focuses on the systems where a transition-metal complex and... 

Unlike the whole-cell system, enzymatic reductions require the addition of a hydride donating cofactor to regenerate the reduced form of the enzyme. Depending on the chosen ADH, the cofactor is usually NADH or NADPH, both of which are prohibitively expensive for use in stoichiometric quantities at scale. Given the criticality of cofactor cost, numerous methods of in situ cofactor regeneration, both chemical and biocatalytic, have been investigated. However, only biocatalytic regeneration has so far proven to be sufficiently selective to provide the cofactor total turnover numbers of at least 10 required in production. 

The latest, and most advanced, technique uses an inexpensive flavine-dependent glycerol phosphate oxidase (GPO, EC 1.1.3.21), found in several microorganisms (Scheme 13), for air oxidation of L-glycerol 3-phosphate 65 to generate DHAP practically quantitatively and in high chemical purity [200]. A separate cofactor regeneration step has become obsolete because the reduced... 

Apart from enzymatic cofactor regeneration, both chemical and electrochemical regeneration methods have attracted attention due to their greater flexibility. The electrochemical regeneration of cofactors has a few advantages, especially the use... 

Fig. 29 Chemical cofactor regeneration with a homogeneous reduction system using cationic rhodium complexes. The reducing equivalents are supplied by formate...

The examples presented here are taken from121. Only those biotransformations were chosen where a classical chemical step was replaced. The enzymes involved are mainly from the groups of oxidoreductases (E.C. class 1) and hydrolases (E.C. class 3). There are a few examples of lyases (E.C. class 4) and one example of an isomerase (E. C. class 5). The processes involving oxidoreductases mainly use whole cells because of the problem of cofactor regeneration. The examples are sorted in the order of the main classes of the Enzyme Commission (E. C.). The big letter E denotes the biotransformation in the syntheses schemes. 

Cofactor Regeneration - About 70% of enzymes use nucleoside triphosphates, nlcotlnetmlde derivatives [NAD(P)(H)], or CoA as cofactors. These enzymes include many of those of greatest Interest In the synthesis of fine chemicals. Since these cofactors are too expensive to be used stoichiometrically, it has been necessary to develop recycling systems for them. The problem of recycling the nucleoside triphosphates... 

Another topic related to water and organometallics has been developed by Fish, who is the author of Chapter 10. In particular. Fish has tried to overcome the problem of cofactor regeneration in biocatalysis. At least one-third of all known enzymes require the use of cofactors such as NAD (nicotinamide adenine dinucleotide) and its reduced form, 1,4-NADH, to perform oxidations or reductions. However, these cofactors are expensive and fairly complicated molecules. It has been shown that, in aqueous solution, some organometallic biomimetic models behave chemically like NAD and are able to abstract a hydride ion [249]. A key example is shown below where the hydride 33 reduces 31 into 34 (Scheme 1.27). 

Examples of enzymatic and electrochemical cofactor regeneration as well as chemical and electrochemical cofactor-substitution systems are depicted in Fig. 1. Cofactor regeneration replaces the metabolic cofactor regeneration fotmd in vivo, while cofactor substitution shortcuts the natural system and transfers electrons via the mediator directly to the oxidoreductase or possibly another protein in the electron-transfer chain. The natural monooxygenase system depicted here consists of three proteins, typical for several bacterial P450s, but systems with two or only one component also exist and the principles for the cofactor regeneration and substitution are the same. 

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