Unveiling a Lost Link in Human Tryptophan Metabolism through Comparative Genomics and Enzymatic Characterisation

The kynurenine pathway in animals, including humans, is the main route of tryptophan degradation and also the route of de novo biosynthesis of NAD⁺, which is essential for energy balance and for the presence of important neuroactive and immunomodulatory metabolites. However, the gene coding for the 2-aminomuconate reductase in the pathway has to be assigned yet. Using coevolutionary analysis through phylogenetic profiling and biochemical assays, we identified ASPDH as the missing human 2-aminomuconate reductase enzyme. By performing NMR and MS-MS analyses on the reaction product, we discovered that it was not, as expected by the literature, 2-oxoadipate (2-OA), but a novel metabolite: 2-aminohex-3-enedioic acid (2AH3EA). This finding sheds light on a previously unknown step in tryptophan degradation and enhances our understanding of NAD⁺ biosynthesis in humans.