Tyrosine Biosynthesis Revisited: Characterization of novel arogenate dehydrogenases from legumes

Abstract

The aromatic amino acid tyrosine (Tyr) serves the central role as both a building block for proteins and a precursor to a wide variety of derived secondary metabolites. An increasing interest in Tyr‐derived plant natural products for their application to industry and human health has cited the need to revisit and gain a better understanding of the Tyr biosynthetic pathways in plants. It has previously been shown that Tyr synthesis in plants occurs through tightly feedback regulated arogenate dehydrogenase (ADH/TyrAa) enzymes. However, recent research has discovered that some legumes, including Medicago truncatula, have prephenate dehydrogenase (PDH/TyrAp) enzymes, which deviate from this model by acting on a different substrate, prephenate, and do not show sensitivity to Tyr feedback inhibition. A phylogenetic analysis of PDH enzymes discovered a new group of ADH enzymes which show significantly relaxed sensitivity to Tyr inhibition. Phylogeny/structure‐guided site‐directed mutagenesis identified a single active site residue in these ADHs, which could be mutated to confer PDH activity and reduce Tyr sensitivity. The reciprocal mutation of the corresponding residue in M. truncatula PDH enzymes was found to reduce and increase affinity for prephenate and arogenate, respectively, and introduce Tyr sensitivity. These results suggest a biochemical link between PDH activity and sensitivity to Tyr feedback inhibition and represent a potential target for metabolic engineering to optimize Tyr biosynthesis and increase the production of Tyr‐derived metabolites in plants.