Ambrose-Dempster, Esther; (2024) Enzymatic breakdown of natural and non-natural polymers. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

There is a growing abundance of waste materials polluting our environment. An overarching need that we hear about today is new methodologies to address the plastic waste crisis. Plastic is particularly persistent in the environment due to the properties that also make it such a useful material. Polyethylene terephthalate (PET) makes up a proportion of the plastic waste problem and in recent years PETase enzymes have been both discovered and heavily engineered to aid the breakdown of this plastic. This work investigates the applicability of whole cell PETases in PET degradation via mechanoenzymatic methodologies. Optimised conditions have been developed for known and novel PETases to increase their degradation activity towards various PET substrates, commercial and post-consumer, when compared with their activity under traditional biocatalytic conditions. A further growing concern is the rise of biodegradable plastics and the common misconception attached to them that they breakdown under any natural condition. Polybutylene adipate terephthalate (PBAT) is a biodegradable plastic structurally similar to PET. Both mechanoenzymatic and solution conditions were tested to see if breakdown could once again be improved. Using known and novel whole cell enzymes, solution degradation was found to be preferable, with breakdown yields surpassing the majority of those reported in the literature. Mechanoenzymatic and solution breakdown techniques were also applied to the natural polymer, lignin, one of the most abundant natural waste products on the planet. The aim was to use tyrosinases and transaminases in breakdown reactions and disseminate complex breakdown product mixtures using lignin itself and two model compounds. Biocatalytic reactions are increasingly more important in sustainable chemistry. Overall, this work highlights the applicability of whole cell enzymes in mechanochemistry for the breakdown of different polymers, importantly producing novel findings and greatly reducing the need for bulk solvents, providing a step towards a greener future.

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