S76: Bacterial lignin-degrading enzymes - helping to unlock the potential of biomass

Development of processes that exploit biomass as a renewable source for fuels and other products depends on the deconstruction of biomass. To this end, the microbial degradation of lignin has been well studied in white- and brown-rot fungi. By contrast, the role of bacterial activities in breaking down lignin has been largely overlooked. Recent published work suggests that a range of soil bacteria, often those degrading aromatic compounds, are able to break down lignin. We have been using Rhodococcus jostii RHA1, a soil-dwelling actinobacterium, as a model to characterize the bacterial degradation of lignin. Bioinformatic analysis revealed that RHA1 encodes two predicted dye-decolorizing peroxidases (DyP), which we annotated as DypA and DypB. Deletion of each of these genes, and characterization of the encoded proteins demonstrated that DypB but not DypA is involved in transforming lignin in an H₂O₂–dependent fashion. Unexpectedly, DypB also catalyzed the H₂O₂–dependent oxidation of Mn(II) in a reaction analogous to fungal manganese peroxidases. Structural and mutagenesis studies have enabled us to identify the Mn(II) binding site of DypB and to augment the enzyme’s Mn(II) oxidation activity ~15-fold. On-going work is aimed at harnessing such activities to liberate cellulose and other carbohydrates for biofuel production, as well as to “crack” lignin to produce feedstock aromatics, resins, and carbon fibres.