Quinate is produced and used by many plants in the biosynthesis of chlorogenic acids (CGAs). Chlorogenic acids are astringent and serve to deter herbivory. They also function as antifungal agents and have potent antioxidant properties. Quinate is produced at a branch point of shikimate biosynthesis by the enzyme quinate dehydrogenase (QDH). However, little information exists on the identity and biochemical properties of plant QDHs. In this study, we utilized structural and bioinformatics approaches to establish a QDH-specific primary sequence motif. Using this motif, we identified QDHs from diverse plants and confirmed their activity by recombinant protein production and kinetic assays. Through a detailed phylogenetic analysis, we show that plant QDHs arose directly from bifunctional dehydroquinate dehydratase-shikimate dehydrogenases (DHQD-SDHs) through different convergent evolutionary events, illustrated by our findings that eudicot and conifer QDHs arose early in vascular plant evolution whereas Brassicaceae QDHs emerged later. This process of recurrent evolution of QDH is further demonstrated by the fact that this family of proteins independently evolved NAD and NADP specificity in eudicots. The acquisition of QDH activity by these proteins was accompanied by the inactivation or functional evolution of the DHQD domain, as verified by enzyme activity assays and as reflected in the loss of key DHQD active site residues. The implications of QDH activity and evolution are discussed in terms of plant growth and development.

Tyrosine biosynthesis via the shikimate pathway is absent in humans and other animals, making it an attractive target for next‐generation antibiotics, which is increasingly important due to the looming proliferation of multidrug‐resistant pathogens. Tyrosine biosynthesis is also of commercial importance for the environmentally friendly production of numerous compounds, such as pharmaceuticals, opioids, aromatic polymers, and petrochemical aromatics. Prephenate dehydrogenase (PDH) catalyzes the penultimate step of tyrosine biosynthesis in bacteria: the oxidative decarboxylation of prephenate to 4‐hydroxyphenylpyruvate. The majority of PDHs are competitively inhibited by tyrosine and consist of a nucleotide‐binding domain and a dimerization domain. Certain PDHs, including several from pathogens on the World Health Organization priority list of antibiotic‐resistant bacteria, possess an additional ACT domain. However, biochemical and structural knowledge was lacking for these enzymes. In this study, we successfully established a recombinant protein expression system for PDH from Bacillus anthracis (BaPDH), the causative agent of anthrax, and determined the structure of a BaPDH ternary complex with NAD+ and tyrosine, a binary complex with tyrosine, and a structure of an isolated ACT domain dimer. We also conducted detailed kinetic and biophysical analyses of the enzyme. We show that BaPDH is allosterically regulated by tyrosine binding to the ACT domains, resulting in an asymmetric conformation of the BaDPH dimer that sterically prevents prephenate binding to either active site. The presented mode of allosteric inhibition is unique compared to both the competitive inhibition established for other PDHs and to the allosteric mechanisms for other ACT‐containing enzymes. This study provides new structural and mechanistic insights that advance our understanding of tyrosine biosynthesis in bacteria. Enzymes Prephenate dehydrogenase from Bacillus anthracis (PDH): EC database ID: https://www.brenda-enzymes.org/enzyme.php?ecno=1.3.1.12. Databases Coordinates and structure factors have been deposited in the Protein Data Bank (PDB) with accession numbers PDB ID: http://www.rcsb.org/pdb/search/structidSearch.do?structureId=6U60 (BaPDH complex with NAD+ and tyrosine), PDB ID: http://www.rcsb.org/pdb/search/structidSearch.do?structureId=5UYY (BaPDH complex with tyrosine), and PDB ID: http://www.rcsb.org/pdb/search/structidSearch.do?structureId=5V0S (BaPDH isolated ACT domain dimer). The diffraction images are available at http://proteindiffraction.org with DOIs: https://doi.org/10.18430/M35USC, https://doi.org/10.18430/M35UYY, and https://doi.org/10.18430/M35V0S.

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