3.2.4. NAD(P)+-Dependent Bioelectrosynthesis

Nicotinamide coenzymes (NAD(P)+/NAD(P)H) play essential roles in the function of many NAD(P)+-dependent dehydrogenases, but the dehydrogenases need suitable redox mediators in single-step enzymatic bioelectrocatalysis because the NAD(P)+/NAD(P)H interconversion requires high overpotentials at conventional electrodes due to its hydride ion-transfer-type characteristics that are completely different from two-step single-electron transfer-type electrode reactions. On the other hand, the introduction of dipaphorase (DI) and FNR, which are bioelectrocatalysts for the interconversion of the NAD+/NADH and NADP+/NADPH redox couples, respectively, can realize various NAD(P)+-dependent bioelectrocatalyses with relatively low overpotentials [28,140].

The bienzymatic MET-type system of NAD+-dependent dehydrogenase and DI is kinetically and thermodynamically investigated and often applied to biosensors [40,140,158,159]. In addition, the coulometric electrooxidation of organic substances was achieved in the same multienzymatic system mimicking the tricarboxylic acid cycle [151]. On the other hand, reductive production of chiral compounds is useful in many cases. Minteer's group reported various NAD+-dependent multienzymatic cascade reactions for generating chiral compounds, as shown in Figure 5 [149,152,153]. They also demonstrated the bioelectrosynthesis of polyhydroxybutarate with NADH regeneration by DI [143].

*β* **Figure 5.** Schematic representation of multienzymatic bioelectrosyntheses of (**A**) chiral amine and (**B**) β-hydroxy nitrile. Abbreviations; DH: diaphorase, AdhS: alcohol dehydrogenase, HHDH: halohydrin dehalogenase, COBE: 4-chloroacetoacetate, CHBE: 4-chloro-3-hydroxybutanoate, CHCN: ethyl-4-cyano-3-hydroxybutyrate. Reprinted from Ref. [152,153], Copyright (2019,2020), with permission from ACS Publications.

]. Particularly, Armstrong's group investigated NADP In an NADP <sup>+</sup>-dependent system, on the other hand, FNR showed DET-type bioelectrocatalytic activity [28]. Particularly, Armstrong's group investigated NADP <sup>+</sup>-dependent bienzymatic bioelectrocatalysis using FNR. They reported the reductive carboxylation of pyruvate to malate by malate dehydrogenase [142], reductive amination of 2-oxoglutamate to l-glutamate by glutamate dehydrogenase [28,144], and enantioselective interconversion of the secondary alcohol/ketone couple by engineered alcohol dehydrogenase [141,145].
