Regeneration of the cofactor NADH

Surface-modified electrodes were used for prevention of high overpotentials with direct oxidation or reduction of the cofactor, electrode fouling, and dimerization of the cofactor [7cj. Membrane electrochemical reactors were designed. The regeneration of the cofactor NADH was ensured electrochemically, using a rhodium complex as electrochemical mediator. A semipermeable membrane (dialysis or ultrafiltration) was integrated in the filter-press electrochemical reactor to confine... 

Fig. 34 Enzymatic reduction in biphasic media. Ketones are reduced enantioselective to the corresponding (S)-alcohols by ADH from Rhodococcus erythropolis. Regeneration of the cofactor NADH is carried out by FDH from C. boidinii. The introduction of a biphasic system allows higher substrate concentrations...

This operation, which needs at least two internal recycles, may be economic for special purposes (e.g., highly prized applications) such as enzyme-catalyst conversions. Indeed, it has been tested for the ADH-catalyzed reduction of hydrophobic ketones coupled with regeneration of the cofactor NADH. Another possibility discussed recently is the use of surface polarity-modified (heterogeneous) catalysts and their distribution between two immiscible solvents which occurs against gravity [8]. 

Regeneration of the cofactor NADH needed for reduction of pyruvate, achieved through the oxidation of ethanol catalyzed by alcohol dehydrogenase (ADH)... 

Some oxidoreductases require expensive cofactors, such as NADH. This requires continuous regeneration of the cofactor in a separate enzymatic reaction using a sacrificial substrate. Nanofiltration membranes have been used to separate the two enzyme systems.256 A sulfonated poly-sulfone nanofiltration was used in the reduction of fructose to mannitol, using glucose to gluconolactone to regenerate the NADH. 

Some oxidoreductases require nicotine adenine dinucleotide (NADH) as a cofactor.146 To use them in organic synthesis, as in the reduction of a ketone to an alcohol, it is necessary to have an efficient system to continuously regenerate them. A common way is to include in the same reaction formic acid and formate dehydrogenase, the byproduct being carbon dioxide.147 The regeneration of the cofactor can also be done electrochemically with or without the addition of a hydrogenase.148 The use of whole organisms eliminates this need. 

Scheme 4.101). The cofactor NADH was regenerated by the simultaneous oxidation of isopropanol to acetone. 

Fig. 22 Electroreduction of ketones or aldehydes using ADH as catalyst. Reduction system A shows the ADH-catalyzed reduction coupled with regeneration of NADPH or NADH by ferre-doxin-NADP+ reductase (FNR) or diaphorase (DP), respectively with assistance of methyl violo-gen as an electron mediator. In system B, ADH is used as sole enzyme which catalyzes both reduction of substrates and regeneration of cofactors...

The indirect electrochemical process for the regeneration of NAD" from NADH is very fast, which is indicated by a strong catalytic current in the presence of glycerol dehydrogenase, the oxidized cofactor, and 7e.so-l,2-cyclohexanediol as substrate [109]. 

A very promising process route is the reductive amination of prochiral a-keto acids to a-amino acids with AADHs and the cofactor NADH and its regeneration by cooxidation of formate to CO2 by formate dehydrogenase (Fig. 15.3-1). 

Many different systems have heen suggested for the regeneration of (mostly) one of the coenzymes NAD, NADH, NADP or NADPH. Only a few publications which appeared after the review articles of the Whitesides group (87-89) shall be mentioned (90,91). For the photochemical and electrochemical regeneration of nicotinamide cofactors see (92,93). 

---