Exploring biocatalytic alternatives for challenging chemical reactions

Abstract

Catalysts are often used in challenging chemical reactions to accelerate the reaction rate, increase reaction efficiency, reduce energy consumption, and minimise waste production. In biocatalysis, enzymes or whole cells are used as catalysts with the advantage of reactivity, selectivity and mild reaction condition over chemocatalysis. Nowadays, with the increasing variety of enzymes, biocatalysis exhibits more and more applicability potential as an alternative tool for chemical reactions.

This thesis focuses on two categories of challenging chemical reactions: oxyfunctionalisation and decarboxylation reactions, where two enzyme families have been investigated. Unspecific peroxygenases (UPOs) exhibit remarkable catalytic activity by facilitating the specific incorporation of oxygen atoms into both C-H and C=C bonds through hydroxylation and epoxidation reactions, respectively. This biocatalytic ability occurs under mild reaction conditions, rendering UPOs highly versatile and attractive for various synthetic applications. Fatty acid photodecarboxylases (FAPs) demonstrate the capacity to effectively catalyse the cleavage of carboxylic groups from substrates, leading to the formation of the corresponding alka(e)nes when subjected to illumination. This photoenzymatic reaction offers a sustainable and environmentally friendly pathway for the conversion of fatty acids into valuable hydrocarbon products by harnessing light as an energy source. In chapter 1, we show a critical and quantitative comparison between chemocatalysis and biocatalysis in oxyfunctionalisation reactions and an overview of decarboxylation reactions.

For oxyfunctionalisation reactions, this thesis is focusing on both classic hydroxylation and epoxidation reactions. For instance, further derivatisation fatty acids generally relies on pre-existing functional groups such as the carboxylate group or C=C-double bonds. However, the enzymatic conversions of saturated, non-activated fatty acids remain relatively underdeveloped, primarily owing to the inherent difficulty of C-H activation. In chapter 2, we demonstrate the application of a peroxygenase mutant AaeUPO-Fett for selective fatty acid hydroxylation. The primary products (i.e. hydroxy fatty acids) are interesting building blocks for lactone and polyester synthesis. Besides, when the produced w-1 hydroxy fatty acid (esters) are transformed, further synthetic possibilities arise as demonstrated by the fatty acid decarboxylation, Baeyer-Villiger oxidation and reductive amination reactions. Thereby, the utilisation of peroxygenase-promoted enzymatic cascades has emerged as a versatile toolbox for the conversion of recalcitrant saturated fatty acids into valuable products and essential building blocks...