It is widely recognised that there is a growing global plastic waste crisis which has been challenging to address because it has many dimensions: social, technical, and economic. The polymers we use as plastics were designed to be durable and stable, and do not degrade under conditions typically encountered in the natural environment or during standard waste processing. Conventional methods of recycling plastic waste are costly and can produce harmful greenhouse gases that are detrimental to the environment. Therefore, there is a pressing need for alternative methods of recycling plastic waste that are more economical and environmentally sustainable, such as the use of microorganisms as molecular biocatalysts. It has been known for many years that microbes that decompose wood, secrete oxidative enzymes to degrade lignin, which is a heavily crosslinked aromatic polymer that is one of the major constituents of wood. This enzymatic activity is significant since lignin consists of unreactive carbon-carbon and carbon-oxygen bonds that are generally resistant to chemical breakdown, yet these enzymes catalyse their decomposition under ambient conditions without any hazardous chemical reagents. The fact that many synthetic polymers share the same chemical structural motifs that are present in lignin, offers the intriguing possibility of applying these enzymes for the degradation of plastic materials. Hence, it was thought that the cleavage pathways for lignin degradation could potentially be used for the degradation of synthetic polymers. This would provide a greener alternative recycling method for catalysing the degradation of synthetic polymers. This work describes the biocatalytic activity of the enzymes; laccases CotA from Bacillus sp, the dye decolourising peroxidases DypB from Rhodococcus jostii, Dyp1B from Pseudomonase fluorescens, and TfuDyp from Thermobifida fusca, and the Lignin peroxidase 14 H8 (LipH8) from Phanerochaete chrysosporium, for the biodegradation of smaller lignin model compounds. The biocatalytic activity of the enzymes were further tested for the oxidation of various smaller synthetic plastic model compounds of phenol formaldehyde resin, polyurethane, polycarbonate and polystyrene. Degradation pathways were inferred and enabled the prediction of various oxidative cleavage pathways and products induced by the enzymes. Therefore, this study intended to provide experimental evidence for the degradation of smaller synthetic mimics of plastics, which could further enhance plastic degradation studies of larger polymers. Finally, saturation mutagenesis studies with CotA from Bacillus sp. was shown to alter the biocatalytic activity for the oxidation of a trimer model compound of phenol formaldehyde resin (Bakelite). This analysis allowed the degradation pathways to be inferred and enabled us to predict various oxidative cleavage products induced by the CotA mutants. This study intended to improve the biocatalytic oxidative cleavage activity of CotA for the trimer compound, which could further enhance biodegradation studies with larger polymers of phenol formaldehyde resins.
Synthetic and Mechanistic Studies into Enzymatic Degradation of Small Lignin and Plastic Model Compounds
Lee, H. (Author). 31 Dec 2023
Student thesis: Phd