Sustainable Bioelectrosynthesis of the Bioplastic Polyhydroxybutyrate: Overcoming Substrate Requirement for NADH Regeneration

One of the main limitations to achieve sustainable synthesis of polyhydroxybutyrate (PHB) is the cost of NADH regeneration, as it requires a side enzymatic reaction usually including a NAD-dependent dehydrogenase enzyme with its substrate or other photo- and electrochemical approaches that create unwanted byproducts and the enzymatically inactive dimer NAD2. Herein, a bioelectrocatalytic method combining both enzymatic and electrochemical approaches was used to regenerate enzymatically active NADH. The method employed a modified glassy carbon electrode that possesses both NADH regeneration and acetoacetyl-CoA (AcAcCoA) reduction features. The modified electrode exhibited an apparent Michaelis constant (KM) value of 814 ± 11 μM and a maximum current density (jmax) of 27.9 ± 1.3 μA cm-2 for NAD+ reduction and a KM value of 47 ± 2 μM and jmax of 0.97 ± 0.03 μA cm-2 for AcAcCoA reduction. The modified electrode was subsequently employed in the bioelectrosynthesis of the bioplastic PHB and yielded 1.6 mg in a 5 mL reaction mixture, indicating that the NADH was regenerated at least 8 times during the 16 h reaction.