*3.2. Bioelectrosynthesis*

Bioelectrosynthesis is a coined term for the generation of renewable energy resources and useful organic compounds [132]. Particularly, stereoselective enzymatic bioelectrocatalysis is desirable for pharmaceutical use because pharmaceutical precursors are required to have high enantiomeric purity [133,134].

Some useful compounds are reduced forms of redox couples, and reductive conversion is often the opposite reaction in metabolisms in vivo. In bioelectrocatalysis, on the other hand, several enzymes can proceed bidirectional electroenzymatic reactions (both the oxidation and reduction of substrate redox couples), in which the driving forces are reversed depending on pH. For example, H2ase [135,136], formate dehydrogenase (FoDH) [75,136–139], carbon monoxide dehydrogenase (CODH) [136], diaphorase (DI) [136,140], ferredoxine-NADP<sup>+</sup> reductase (FNR) [28], and some NAD(P)+-dependent dehydrogenases [28,140–143] show bidirectional bioelectrocatalytic activities. The characteristic of these enzymes is that the formal potentials of the substrate redox couples and the cofactors in the enzymes are relatively close to each other, and thus, the small uphill intramolecular electron transfer proceeds in acceptable velocity [34]. Bidirectional bioelectrocatalysis is a key reaction for constructing bioelectrochemical energy/compound conversion systems.

There are mainly two types of bioelectrosynthetic systems: a fuel-cell-type [142,144–149] and an electrolysis-type [28,53,141,143,150–154]. The former realizes a spontaneous production of compounds without any external power supplies, whereas the latter proceeds relatively rapid reactions due to an optimally controlled electrode potential to realize diffusion-controlled conditions. We will show examples of bioelectrosyntheses in the following sections.
