references therein). Particularly promising microbial platforms for lignin biotechnology, readily providing relevant aromatics catabolic pathways, natural robustness
towards toxic substrates and proven suitability for industrial-scale biotechnology
applications, are P. putida and C. glutamicum. Other platforms of natural degraders
of aromatics with good chemical tolerance, e.g., from the clade of the Actinobacteria currently being explored, may also provide viable options in the future. As a
key feature towards aromatic tolerance, optimizing the operation of aromatic catabolism at a high level has been suggested [8]. Important chemicals and materials
currently being targeted by microbial conversion for production from lignin-derived
mono-lignins include cis,cis-muconic acid, organic acids like pyruvate, lactate, and
succinate, pyridine-related organic acids, the polymers polyhydroxyalkanoates
(PHA) and nylon, lipids with prospects for biofuel application, as well as diverse
flavor and fragrance compounds ([8], and references therein).
A common aspect to be considered in all these approaches, however, is the
challenge of as of yet comparably low product titers from lignin-based compounds
than from glucose or other common substrates. Rendering such processes competitive will, therefore, require a combination of strain engineering and evolution
for optimal conversion into products and tolerances to inhibitors, as well as bioprocess development that minimizes inhibition by substrates, inhibitors, and
products [160]. In the meantime, and with respect to the large bulk of
water-insoluble, condensed lignin from current large-scale industrial lignocellulose
processes that also in the future will be hardly usable for enzymatic and microbial
utilization, the path of gasification followed by syngas fermentation might develop
into a viable alternative for higher value creation form lignin using industrial
biotechnology.
6 Conclusions and Outlook
The economic success of future lignocellulosic biorefineries is highly dependent on
an efficient valorization of lignin, comprising a major share of its feedstock that is
too valuable only to be used for low-value energy applications. This is, however,
still challenged by the structural complexity of lignin and its heterogeneity due to its
diverse sources and recovery processes. Much progress has lately been made
towards better-exploiting lignin through depolymerization into a wide range of
value-added products and their subsequent funneling into microbial production
processes as documented by recent comprehensive reviews [8, 9, 160, 161].
However, despite of the considerable progress also on discovering and engineering
new enzymes active on lignin and the microbial utilization of lignin-derived aromatic compounds into a broad spectrum of valuable products, biotechnological
approaches meet great competition in thermochemical and chemo-catalytic
approaches. The latter are rapidly developing in parallel, with the advantage of
being more adaptable to the current mix of low-quality lignins available from
established industrial processes. One important field of development is, therefore,
Recent Advances in Enzymatic Conversion of Lignin …
461
towards toxic substrates and proven suitability for industrial-scale biotechnology
applications, are P. putida and C. glutamicum. Other platforms of natural degraders
of aromatics with good chemical tolerance, e.g., from the clade of the Actinobacteria currently being explored, may also provide viable options in the future. As a
key feature towards aromatic tolerance, optimizing the operation of aromatic catabolism at a high level has been suggested [8]. Important chemicals and materials
currently being targeted by microbial conversion for production from lignin-derived
mono-lignins include cis,cis-muconic acid, organic acids like pyruvate, lactate, and
succinate, pyridine-related organic acids, the polymers polyhydroxyalkanoates
(PHA) and nylon, lipids with prospects for biofuel application, as well as diverse
flavor and fragrance compounds ([8], and references therein).
A common aspect to be considered in all these approaches, however, is the
challenge of as of yet comparably low product titers from lignin-based compounds
than from glucose or other common substrates. Rendering such processes competitive will, therefore, require a combination of strain engineering and evolution
for optimal conversion into products and tolerances to inhibitors, as well as bioprocess development that minimizes inhibition by substrates, inhibitors, and
products [160]. In the meantime, and with respect to the large bulk of
water-insoluble, condensed lignin from current large-scale industrial lignocellulose
processes that also in the future will be hardly usable for enzymatic and microbial
utilization, the path of gasification followed by syngas fermentation might develop
into a viable alternative for higher value creation form lignin using industrial
biotechnology.
6 Conclusions and Outlook
The economic success of future lignocellulosic biorefineries is highly dependent on
an efficient valorization of lignin, comprising a major share of its feedstock that is
too valuable only to be used for low-value energy applications. This is, however,
still challenged by the structural complexity of lignin and its heterogeneity due to its
diverse sources and recovery processes. Much progress has lately been made
towards better-exploiting lignin through depolymerization into a wide range of
value-added products and their subsequent funneling into microbial production
processes as documented by recent comprehensive reviews [8, 9, 160, 161].
However, despite of the considerable progress also on discovering and engineering
new enzymes active on lignin and the microbial utilization of lignin-derived aromatic compounds into a broad spectrum of valuable products, biotechnological
approaches meet great competition in thermochemical and chemo-catalytic
approaches. The latter are rapidly developing in parallel, with the advantage of
being more adaptable to the current mix of low-quality lignins available from
established industrial processes. One important field of development is, therefore,
Recent Advances in Enzymatic Conversion of Lignin …
461