Discovery And Engineering Of Non-Native Photoenzymatic Asymmetric Transformations

Abstract

Biocatalysis, the use of biological systems or their components to catalyze synthetic processes, is a powerful tool in organic chemistry for the synthesis of valuable compounds. Enzymes are attractive catalysts due to their exceptional selectivities, high turnover numbers and mild reaction conditions. However, enzymes are frequently perceived as highly specific catalysts with a limited scope of reactivity, precluding their general use in organic synthesis. In the first chapter, I outline my work in using external photocatalysts to activate non-natural radical-mediated ketoreductase reactivity in flavin-dependent ene-reductases, demonstrating that biocatalysts have potential for reactivity beyond their natural mechanisms.Among all of their catalytic advantages, it is their inherent evolvability that distinguishes enzymes from all other catalysts. The dramatic decrease in the price of DNA sequencing, novel tools in bioinformatics, high-throughput screening processes, and computer modeling have created boundless opportunities in biocatalysis by enabling researchers to develop enzymes that catalyze unnatural reactions efficiently. Enzymes that exhibit substrate promiscuity, the ability to accept an unnatural substrate, or catalytic promiscuity, the ability to catalyze an unnatural reaction, can be optimized for the desired transformation by employing an accelerated, in vitro version of Darwinian evolution, called directed evolution. By introducing mutations via random mutagenesis, site-directed mutagenesis or site-saturation mutagenesis, an enzyme can be engineered to improve its thermostability, increase tolerance to organic solvents, expand substrate specificity, increase enantioselectivity and catalyze unnatural transformations. While protein-engineering methods are well established, there is currently no general method for engineering enzymes that require photoirradiation. The second chapter of this dissertation outlines my work in developing a method for engineering photoenzymes to improve quantum yield efficiency in a radical-mediated asymmetric hydroalkylation.