From gene to biorefinery: microbial ß-etherases as promising biocatalysts for lignin valorization

The set-up of biorefineries for the valorization of lignocellulosic biomass will be core in the future to reach sustainability targets. In this area, biomass-degrading enzymes are attracting significant research interest for their potential in the production of chemicals and biofuels from renewable feedstock. Glutathione-dependent ß-etherases are emerging enzymes for the biocatalytic depolymerization of lignin, a heterogeneous aromatic polymer abundant in nature. They selectively catalyze the reductive cleavage of ß-O-4 aryl-ether bonds which account for 45–60% of linkages present in lignin. Hence, application of ß-etherases in lignin depolymerization would enable a specific lignin breakdown, selectively yielding (valuable) low-molecular-mass aromatics. Albeit ß-etherases have been biochemically known for decades, only very recently novel ß-etherases have been identified and thoroughly characterized for lignin valorization, expanding the enzyme toolbox for efficient ß-O-4 aryl-ether bond cleavage. Given their emerging importance and potential, this mini-review discusses recent developments in the field of ß-etherase biocatalysis covering all aspects from enzyme identification to biocatalytic applications with real lignin samples.