through screening on natural lignin) should, therefore, be much higher than on the
majority of technical lignins available today. Enzyme and biocatalytic process
engineering could then focus on achieving, e.g., tolerances to depolymerization
conditions that maximize lignin solubility (e.g., high pH, methanol, ethanol) and
minimize re-polymerization, i.e., the tolerance to relevant radical scavengers and
optimal implementation in membrane reactors, recovering LMW compounds in
parallel to the depolymerization reaction. Innovative membrane technology could
thereby through enzyme stabilization, and recycling in addition, serve the need of
minimizing enzyme production costs, which is an often referred to challenge in
rendering biocatalytic processes economically viable [159]. Based on a more native
and bio-accessible lignin substrate through prevention of condensation and other
modifications, it can be expected that complexity of the resulting LMW fraction
will be lowered. This would ultimately increase the portion of individual compounds in the degradation mix that can be isolated and marketed as high-value
chemicals and/or provide a better feedstock for microbial conversion [10] to produce the broad spectrum of chemicals and materials known to be addressable by
industrial biotechnology [8].
5 Microbial Lignin Valorization
As presented above, enzymatic ligninolysis by laccases and peroxidases derives
monolignin products such as ferulate, p-coumarate, caffeate, vanillin, guaiacol,
phenol, benzoate, cresol, catechol, syringate, and gallate. In addition to being
marketable products themselves, they can be used by a broad range of microorganisms for the convergent transformation, the so-called funneling, into a smaller
set of intermediate aromatic metabolites. The current most prominent funneling
pathways comprise those towards protocatechuate, catechol, and gallate/3-Omethyl-gallate, involving enzymes like decarboxylases, O-demethylases, alcohol
and aldehyde dehydrogenases, as well as acyl-CoA synthetases and -
hydratases/lyases. Dependent on the position of ring-cleavage in meta- or ortho
position by a range of different dioxygenases, the resulting muconate, and mesaconate derivatives have different entry points to microbial central carbon metabolism, as recently summarized in excellent review articles [8, 160]. From there, the
whole toolbox of microbial hosts, metabolic engineering, systems biotechnology,
and synthetic biology is available to target efficient production of fuels, chemicals
and materials for a whole range of applications and markets [8]. Relevant fields of
strain development include; (i) improving stress tolerance to process-relevant
parameters, such as the presence of toxic compounds, extreme pH, high temperature, etc., by means of adaptive laboratory evolution (ALE) and/or reverse engineering, (ii) the general improvement of aromatic compound utilization through,
e.g., extending the substrate spectrum and optimizing the conversion routes of
aromatic catabolism and their regulation, and (iii) the establishment and improvement of the production of specific chemicals, materials, and fuels ([8], and
460
G.-S. Nguyen et al.
majority of technical lignins available today. Enzyme and biocatalytic process
engineering could then focus on achieving, e.g., tolerances to depolymerization
conditions that maximize lignin solubility (e.g., high pH, methanol, ethanol) and
minimize re-polymerization, i.e., the tolerance to relevant radical scavengers and
optimal implementation in membrane reactors, recovering LMW compounds in
parallel to the depolymerization reaction. Innovative membrane technology could
thereby through enzyme stabilization, and recycling in addition, serve the need of
minimizing enzyme production costs, which is an often referred to challenge in
rendering biocatalytic processes economically viable [159]. Based on a more native
and bio-accessible lignin substrate through prevention of condensation and other
modifications, it can be expected that complexity of the resulting LMW fraction
will be lowered. This would ultimately increase the portion of individual compounds in the degradation mix that can be isolated and marketed as high-value
chemicals and/or provide a better feedstock for microbial conversion [10] to produce the broad spectrum of chemicals and materials known to be addressable by
industrial biotechnology [8].
5 Microbial Lignin Valorization
As presented above, enzymatic ligninolysis by laccases and peroxidases derives
monolignin products such as ferulate, p-coumarate, caffeate, vanillin, guaiacol,
phenol, benzoate, cresol, catechol, syringate, and gallate. In addition to being
marketable products themselves, they can be used by a broad range of microorganisms for the convergent transformation, the so-called funneling, into a smaller
set of intermediate aromatic metabolites. The current most prominent funneling
pathways comprise those towards protocatechuate, catechol, and gallate/3-Omethyl-gallate, involving enzymes like decarboxylases, O-demethylases, alcohol
and aldehyde dehydrogenases, as well as acyl-CoA synthetases and -
hydratases/lyases. Dependent on the position of ring-cleavage in meta- or ortho
position by a range of different dioxygenases, the resulting muconate, and mesaconate derivatives have different entry points to microbial central carbon metabolism, as recently summarized in excellent review articles [8, 160]. From there, the
whole toolbox of microbial hosts, metabolic engineering, systems biotechnology,
and synthetic biology is available to target efficient production of fuels, chemicals
and materials for a whole range of applications and markets [8]. Relevant fields of
strain development include; (i) improving stress tolerance to process-relevant
parameters, such as the presence of toxic compounds, extreme pH, high temperature, etc., by means of adaptive laboratory evolution (ALE) and/or reverse engineering, (ii) the general improvement of aromatic compound utilization through,
e.g., extending the substrate spectrum and optimizing the conversion routes of
aromatic catabolism and their regulation, and (iii) the establishment and improvement of the production of specific chemicals, materials, and fuels ([8], and
460
G.-S. Nguyen et al.