The lignin depolymerization and aromatic compound metabolism characteristics of microorganisms are used to realize the utilization of lignin resources

On October 26, 2022, Professor Xie Shangxian's team from the School of Life Sciences, Huazhong University of Science and Technology, published Trends in Biotechnology, an international top journal in the field of biotechnology Published a review paper entitled "Microbial lignin valorization through depolymerization to aromatics conversion", which reviewed the key enzymes in the process of lignin biological depolymerization, the mechanism of microbial lignin conversion and the progress of high-value utilization of lignin by synthetic biology, and discussed the current challenges of lignin biodegradation and transformation research.
The future research strategies and trends of lignin high-value are also prospected
.

Lignin accounts for about 20-30% of lignocellulose, which is mainly formed by three phenylpropane units: guaiac propane, lilacylpropane and p-hydroxyphenylpropane, which are interconnected by ether bonds and carbon carbon bonds.

Due to its heterogeneity and complexity, it becomes very difficult
to use high-value applications.
At present, there are two main strategies for the high-value utilization of lignin: one is to use the polymer characteristics of lignin to synthesize advanced polymer materials (such as carbon fiber, biofoam, etc.
); The second is to depolymerize lignin into aromatic monomers or oligomers, and then further convert them into high-value target chemicals
.
In nature, many microorganisms have evolved very intelligent "biofunnel" models to efficiently convert lignin, not only with natural extracellular synergistic enzymes to efficiently depolymerize lignin, but also to use different aromatic hydrocarbon metabolism pathways to convert complex aromatic mixtures of lignin into relatively single chemicals
.
The research in these areas provides a basis
for the directional transformation of lignin by synthetic biology.

Fig 1.
Overview of the microbial degradation process of lignin

This paper reviews the three main steps of microbial degradation of lignin: first, microorganisms depolymerize lignin polymers into small molecule aromatic compounds through laccase, class II heme-containing peroxidase, dye decolorization peroxidase (DyP) and other lignin degradation modifying enzymes (LDME) under the synergistic action of lignin degradation auxiliary enzyme (LDAE); Second, the depolymerized lignin-derived aromatic mixture is metabolized into the main intermediate through upstream metabolic pathways such as demethylation, hydroxylation and carboxylation; The third is to further cleave the main intermediates through the central pathways of aromatic compounds such as β-ketoadipic acid pathway, urine melanic acid pathway, phenylacetic acid pathway, etc.
, so as to produce central metabolic platform compounds such as acetyl-CoA to provide energy or synthesize various other chemicals
.

Figure 2.
Bacteria degrade the central metabolic pathway and key genes of lignin

Fig.
3 Distribution and co-occurrence correlation analysis of lystic polysaccharide monooxygenase (LPMO) and lignin degradation modified enzymes (LDMEs) in the genome of typical lignocellulose-degrading bacteria

The team demonstrated for the first time the pathway and mechanism
by which lytic polysaccharide monooxygenase LPMO participates in lignin biodegradation as a lignin degradation co-enzyme system by driving peroxidase reactions and enhancing the Fenton reaction.
This paper also analyzes and compares the genomes of different lignin-degrading bacteria for the first time, reveals a strong correlation between LPMO and lignin degradation-modifying enzymes from the genomic level, and proposes the possibility of co-evolution between LPMO and lignin degradation-modifying enzymes, which provides new evidence
for LPMO to participate in lignin biodegradation.

Figure 4.
Multiple high-value conversion pathways of lignin from top to bottom

As nature's richest aromatic polymer, lignin is a green renewable resource
.
In recent years, the high-value conversion of lignin has received continuous and high attention
.
In this paper, the main high-value conversion paths
of lignin are reviewed from two different perspectives: non-ring-opening and ring-opening conversion of aromatic compounds.
Non-ring-opening conversion is based on the structural advantages of aromatic compounds themselves, which has natural potential advantages in the production of aromatic chemicals (such as vanillin, gallic acid), etc.
, which can effectively improve the conversion efficiency
of carbon while saving energy.
The open-loop transformation path relies on the metabolic characteristics of microorganisms, breaking the dependence on substrate structure, and the goal of high-value conversion can be expanded
to more diverse types such as PHA and lipids.
In recent years, many studies have also actively explored the introduction of lignin carbon streams into the TCA cycle to further broaden the variety of high-value products.

Professor Xie Shangxian and Professor Ma Fuying from the School of Life Sciences, Huazhong University of Science and Technology, and Professor Susie Y.
Dai of Texas A&M University are the co-corresponding authors of the paper, and postdoctoral Dr.
Li Fei (now a distinguished professor of the School of Chemistry and Chemical Engineering, Wuhan University of Science and Technology), doctoral student Zhao Yiquan and doctoral student Xue Le are the co-first authors
of the paper.
This study was supported
by the National Natural Science Foundation of China (31970098, 32170122, 32000067).

Professor Xie Shangxian's team has been focusing on the biological depolymerization and high-value transformation of
lignin for many years.
Through the analysis of the lignin degradation mechanism in natural biological systems, based on the innovative rational design of systematic synthetic biology, a multi-transformation platform for lignin to bioenergy, biomaterials, biomedicine and other high-value substances is constructed, and the multi-transformation platform of lignin to bioenergy, biomaterials, biomedicine and other high-value substances is constructed, and the platform for the transformation of lignin to high-value substances such as advanced science, green chemistry, environmental microbiology, applied and environmental microbiology, Journal of Hazardous Materials, International Journal of Biological Macromolecules and other journals have published a series of important results
.

Links to papers: https://doi.
org/10.
1016/j.
tibtech.
2022.
09.
009