Engineering New Branches of the Kynurenine Pathway To Produce Oxo-(2-aminophenyl) and Quinoline Scaffolds in Yeast.

The kynurenine pathway, named after its nonproteinogenic amino acid precursor l-kynurenine, is responsible for the de novo biosynthesis of nicotinamide adenine dinucleotide (NAD+) in eukaryotes. Oxo-(2-aminophenyl) and quinoline molecules downstream from l-kynurenine also serve as antagonists of several receptors of the central nervous system in mammals. In this study, we engineered new biosynthetic routes in yeast Saccharomyces cerevisiae to produce a suite of l-kynurenine-derived natural products. Overexpression of Homo sapiens l-tryptophan 2,3-dioxygenase (HsTDO2) in S. cerevisiae led to a marked increase in the production of l-kynurenine and downstream metabolites. Using this background, new branch points to the kynurenine pathway were added through the incorporation of a Psilocybe cubensis noncanonical L-aromatic amino acid decarboxylase (PcncAAAD) capable of catalyzing both decarboxylation and decarboxylation-dependent oxidative-deamination reactions of l-kynurenine and 3-hydroxy-l-kynurenine to yield their corresponding monoamines, aldehydes, and downstream nonenzymatically cyclized quinolines. The PcncAAAD-catalyzed decarboxylation products, kynuramine and 3-hydroxykynuramine, could further be converted to quinoline scaffolds through the addition of H. sapiens monoamine oxidase A (HsMAO-A). Finally, by incorporating upstream regiospecific l-tryptophan halogenases into the engineering scheme, we produced a number of halogenated oxo-(2-aminophenyl) and quinoline compounds. This work illustrates a synthetic biology approach to expand primary metabolic pathways in the production of novel natural-product-like scaffolds amenable for downstream functionalization.