New-to-Nature Processes in Biocatalysis

Identifier:  TDX:4460

Handle: https://hdl.handle.net/20.500.11797/TDX4460

Authors:  Kqiku, Laura

Abstract:
My doctoral thesis focused on repurposing enzymes to achieve novel enantioselective transformations, encompassing both ground-state and excited-state reactivity. By incorporating principles from organocatalysis into enzymes through protein engineering, we enhanced their synthetic capabilities. Specifically, by combining enzymes with organocatalysis and photochemistry, we unlocked entirely new catalytic abilities to create chiral molecules with high stereocontrol. Chapter II explores structural modifications of 4-OT tautomerase enzymes, previously shown to activate aldehydes and enals via enamine or iminium ion formation, respectively. We identified two novel enzyme variants that sequentially perform both activation modes in a cascade process, enabling one-pot synthesis of complex cyclohexene carbaldehydes with high efficiency, diastereomeric ratios and enantiomeric excesses. This demonstrated that biocatalysis could match or surpass traditional organocatalytic methods in efficiency and stereoselectivity. Chapter III presents a novel strategy for designing a non-natural photodecarboxylase using visible light to excite enzyme-bound iminium ion intermediates. This approach enabled asymmetric radical coupling through a unique photoactivation mechanism. We used an engineered DERA class I aldolase enzyme to activate chiral enantiopure carboxylic acids via single-electron transfer oxidation. The resulting radicals formed two stereogenic centers with excellent diastereomeric control, preserving the stereochemical information encoded in the chiral carboxylic acid substrates. This work showcased a unique case of ‘memory of chirality’ in enantioselective radical chemistry.