NAD-Regenerating Biocathode Applied to Electroenzymatic Conversion of Lignin

Abstract

Non-renewable and limited fossil resources (i.e. petroleum, coal, and natural gas) have been used over the last years for the production of energy, commodity chemicals, and polymer materials. Nonetheless, the extraction and consumption of these resources leads to deleterious and irreversible environmental impacts. As a sustainable alternative, lignin, one of the major components of lignocellulosic biomass, represents the most promising renewable raw material to produce aromatic-based chemicals and high value-added products. However, the use of lignin as a source of valuable molecules similar to the petroleum-derived chemicals is hindered due to its recalcitrance and structural heterogeneity. Biological approaches have been studied to develop selective and ecofriendly pathways for lignin valorization. It has been reported that some microorganisms can cleave selectively β-ether bonds representing more than a half of lignin bonding pattern.[1] The etherolytic pathway of the soil probacterium Sphingobium paucimobilis, which involves NAD-dependent dehydrogenases and β-etherases, has been well characterized for this purpose.[2] Here we focus on the design of a bioanode combining NAD-regenerating properties and the immobilization of lignin degrading enzymatic cascade to cleave such bonds. We explored the possibility of combing in situ electrochemical regeneration of NAD+ cofactor with a surface immobilized biocatalysts on a single interface. First, we have demonstrated that the electrodes based on multiwalled carbon nanotubes modified with toluidine blue are capable to regenerate NAD+ cofactor at low potential from its reduced form. This process is essential for dehydrogenases activities. Secondly, we used cross-linked hydrogel of pyrene-modified linear poly(ethyleneimine) to entrap the enzymes on the electrode surface.[3] The obtained bioanode modified with NAD-regenerating catalyst and the multi-enzymatic cascade of Lig enzymes was tested in the presence of guaiacylglycerol-β-guaiacylether (GGE), a lignin dimer substrate with β-ether bond, to produce γ-hydroxypropiovanillone (HPV). We believe that such electrochemical systems employing the enzymatic cascade acting like a metabolic funnel with the simultaneous regeneration of the cofactor would permit to refine the heterogeneous mixture of aromatics from lignin depolymerization products into a uniform distribution of value-added compounds.[1] Renewable Sustainable Energy Rev., 2022, 157, 112025.[2] Catal. Sci. Technol., 2016,6, 2195-2205.[3] Chem. Sci., 2018,9, 5172-5177. Figure 1