In vitro and in vivo oxidation of diastereomers of lignin models of arylgylcerol b-aryl ether type and heterologous expression of laccase

Abstract: Enzymic and non-enzymic oxidation of lignin models of the arylglycerol ?-aryl ether type was investigated in experiments aimed at providing a better understanding of the initial phase of biological lignin degradation. Product profiles and diasteromer selectivity were determined to gain knowledge of the properties of the oxidants and pave the way for evaluation of the presence of the oxidants in ligninolytic fungal cultures.Diastereomers of the non-phenolic lignin model 1-(3,4-dimethoxyphenyl)-2-(2-methoxyphenoxy)-1,3-propanediol (1) were oxidized in vitro with lignin peroxidase (LP), laccase-mediator systems, Fenton's reagent, cerium(IV) ammonium nitrate (CAN) and lead(IV) tetraacetate. The product profiles were found to be distinctive for the different oxidants, except for LP and CAN, which generated the same products in similar proportions. The reactions resulted in preferential degradation of the threo isomer, except for the Fenton's reagent reaction, which showed no stereo-preference, and the laccase reaction mediated by ABTS, which preferentially degraded the erythro isomer. Cultures of the white-rot fungi Trametes versicolor and Phanerochaete chrysosporium preferentially degraded the threo isomer of 1. This is not in agreement with the action of Fenton's reagent, since this oxidant does not exhibit any stereo-preference.Laccase from T. versicolor was expressed in Pichia pastoris and Aspergillus niger. Production of catalytically active T. versicolor laccase was achieved in both P. pastoris and A. niger using glyceraldehyde-3-phosphate dehydrogenase promoters. The level of laccase activity obtained with P. pastoris was significantly lower than that obtained with A. niger, but speed and convenience make the P. pastoris system attractive for screening purposes. A. niger produced high yields of heterologous laccase with properties similar to those of the native enzyme. A method was devised to purify laccase from cultures of A. niger. The method gave a 250-fold purification and a yield of >50%.