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 NAD(2). 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 (K-M) value of 814 +/- 11 mu M and a maximum current density (j(max)) of 27.9 +/- 1.3 mu A cm(-2) for NAD(+) reduction and a K-M value of 47 +/- 2 mu M and j(max) of 0.97 +/- 0.03 mu A 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.