**6. Bioelectrosynthesis**

β α A variety of oxidoreductases have been used for the asymmetric synthesis of amines, amino acids and alcohols [186–188]. Dong et al. developed a biphasic bioelectrocatalysis system for the synthesis of (R)-CHCN (81.2%), (R)-3-hydroxy-3-Phenylpropanenitrile (96.8% ee), (S)-3-hydroxy-4 phenylbutanenitrile (94.6% ee) using alcohol dehydrogenase and halohydrin dehalogenase. The system showed high selectivity for the synthesis of chiral β-hydroxy nitriles. The NADH cofactor was efficiently regenerated using diaphorase and a cobaltocene-modified poly(allylamine) redox polymer [181]. Combining enzymatic electrosynthesis with biofuel cells can pave the way for the self-powered bioelectrosynthesis of valuable chemicals. Wu et al. designed an electroenzymatic system by combining a bioelectrosynthesis cell and an enzymatic fuel cell to prepare L-3,4-dDihydroxyphenylalanine with a coulombic efficiency of 90% [189]. Chen et al. combined a H2/α-keto acid enzymatic fuel cells with bioelectrosynthesis to synthesis chiral amino acids with high efficiency. An enzymatic cascade

consisting of nitrogenase, diaphorase and L-leucine dehydrogenase was used at the cathode to convert nitrogen to chiral amino acids [190]. The NH<sup>3</sup> and NADH prepared in the reaction were used for the synthesis of L-norleucine from 2-ketohexanoic acid via leucine dehydrogenase (LeuDH). A high NH<sup>3</sup> conversion ratio (92%) and a high faradaic efficiency (87.1%) was observed.

Carbon dioxide emissions are increasing, dramatically bringing about an environmental crisis. Therefore, electrochemical CO<sup>2</sup> reduction to useful chemicals has gained great interest to reduce CO<sup>2</sup> emissions. Cai et al. reduced CO<sup>2</sup> to ethylene and propene via the VFe protein of vanadium nitrogenase that was contacted with the electrode mediated by cobaltocene/cobaltocenium (Figure 16). A study represented a new approach for preparing C–C bonds through a single metalloenzyme [191].

− **Figure 16.** A schematic of C–C bond preparation via bioelectrocatalysts, Amperometric i−t trace carried out for enzymatic electroreduction of CO<sup>2</sup> via VFe nitrogenase [191].

− Yuan et al. reported CO<sup>2</sup> reduction producing formate with a high faradaic efficiency of 99% at a low potential −0.66 V vs. The standard hydrogen electrode (SHE). They used immobilised molybdenum-dependent formate mediated by cobaltocene on the electrode surface [192]. In some studies, CO<sup>2</sup> reduction was carried out via enzymatic cascade, using formate dehydrogenases, formaldehyde dehydrogenase and alcohol dehydrogenase to produce CH3OH [193]. Bioplastics can be prepared from CO<sup>2</sup> reduction via bioelectrocatalysts. For example, Sciarria et al. synthesized polyhydroxyalkanoates (PHA) from CO<sup>2</sup> through a bioelectrochemical reactor. CO<sup>2</sup> was converted into bioplastics successfully in a way that 0.41 kg of carbon as PHA was achieved per 1 kg of CCO2 in the whole system [194].

− − Nitrogenase catalyses the reduction of N<sup>2</sup> to ammonia. Nitrogenase can also be used for the reduction of CO, nitrite, azide and cyanide that can then be used for the synthesis of a range of products [195]. Lee et al. wired nitrogenase to a redox-polymer (neutral red-modified poly(glycidyl methacrylate-co-methylmethacrylate-co-poly(ethyleneglycol)methacrylate)) for the conversion of nitrogen to ammonia. [196]. Milton et al. reported on the use of nitrogenase to prepare ammonia from azide and nitrite using cobaltocene as a mediator. The system operated at a potential of −1.25 V with 70 and 234 nmol of NH<sup>3</sup> produced from the reduction of N<sup>3</sup> <sup>−</sup> and NO<sup>2</sup> <sup>−</sup>, respectively [197]. An ATP-free-mediated electron-transfer system operating at a potential of −0.58 V was developed by Lee et al. for the synthesis of ammonia using cobaltocene-functionalized poly(allylamine) (Cc-PAA) as a mediator. The amount of ammonia prepared was 30 ± 5 and 7 ± 2 nmol from the reduction of N<sup>3</sup> − and NO<sup>2</sup> <sup>−</sup>, respectively [198].
