S1: Bacterial Enzymes for Lignin Degradation and Production of Aromatic Chemicals from Lignocellulose

The lignin content of lignocellulose and lignin-containing wastes represents a possible resource for production of aromatic chemicals, if efficient biocatalytic routes for lignin degradation can be found. The enzymology of fungal lignin degradation is well studied, but the enzymology of bacterial lignin degradation is much less well known.

Using a colorimetric assay for lignin degradation, we have identified a number of bacterial aromatic degraders that have activity for lignin breakdown, including Rhodococcus jostii RHA1, whose genome has been sequenced. Using a bioinformatics approach, we have identified peroxidase DypB in R. jostii RHA1 as a lignin peroxidase that is activated by Mn²⁺, and shows activity with a beta-aryl ether lignin model compound, with Kraft lignin, and with lignocellulose. Stopped flow kinetic analysis establishes that DypB is able to oxidise either Mn²⁺ or beta-aryl ether, at similar rates. Using the colorimetric assay as a screen, we have also identified a number of novel lignin-degrading bacteria from soil samples, which show higher activity for lignin degradation. The majority of lignin-degrading bacteria identified to date fall into three classes: actinobacteria, alpha-proteobacteria, and gamma-proteobacteria. A number of low molecular weight metabolites have been identified from cultures of these bacteria with lignocellulose, which can be rationalised in terms of lignin breakdown pathways. The seminar will also describe gene deletion studies carried out on genes involved in vanillic acid breakdown, in which we have observed much higher yields of aromatic breakdown products.