The exploitation of thermophiles and their enzymes for the construction of multistep enzyme reactions from characterised enzyme parts
Finnigan, William John Andrew
Thesis or dissertation
University of Exeter
Reason for embargo
Chapters in the thesis still need to be published.
Biocatalysis is a field rapidly expanding to meet a demand for green and sustainable chemical processes. As the use of enzymes for synthetic chemistry becomes more common, the construction of multistep enzyme reactions is likely to become more prominent providing excellent cost and productivity benefits. However, the design and optimisation of multistep reactions can be challenging. An enzyme toolbox of well-characterised enzyme parts is critical for the design of novel multistep reactions. Furthermore, while whole-cell biocatalysis offers an excellent platform for multistep reactions, we are limited to the use of mesophilic host organisms such as Escherichia coli. The development of a thermophilic host organism would offer a powerful tool allowing whole-cell biocatalysis at elevated temperatures. This study aimed to investigate the construction of a multistep enzyme reaction from well-characterised enzyme parts, consisting of an esterase, a carboxylic acid reductase and an alcohol dehydrogenase. A novel thermostable esterase Af-Est2 was characterised both biochemically and structurally. The enzyme shows exceptional stability making it attractive for industrial biocatalysis, and features what is likely a structural or regulatory CoA molecule tightly bound near the active site. Five carboxylic acid reductases (CARs) taken from across the known CAR family were thoroughly characterised. Kinetic analysis of these enzymes with various substrates shows they have a broad but similar substrate specificity and that electron rich acids are favoured. The characterisation of these CARs seeks to provide specifications for their use as a biocatalyst. The use of isolated enzymes was investigated as an alternative to whole-cell biocatalysis for the multistep reaction. Additional enzymes for the regeneration of cofactors and removal of by-products were included, resulting in a seven enzyme reaction. Using characterised enzyme parts, a mechanistic mathematical model was constructed to aid in the understanding and optimisation of the reaction, demonstrating the power of this approach. Thermus thermophilus was identified as a promising candidate for use as a thermophilic host organism for whole-cell biocatalysis. Synthetic biology parts including a BioBricks vector, custom ribosome binding sites and characterised promoters were developed for this purpose. The expression of enzymes to complete the multistep enzyme reaction in T. thermophilus was successful, but native T. thermophilus enzymes prevented the biotransformation from being completed. In summary, this work makes a number of contributions to the enzyme toolbox of well-characterised enzymes, and investigates their combination into a multistep enzyme reaction both in vitro and in vivo using a novel thermophilic host organism.
PhD in Biological Sciences