The crystal structure of EpHTT, a hydroxycinnamoyl transferase from Echinacea purpurea

Echinacea purpurea hydroxycinnamoyl-CoA:tartaric acid hydroxycinnamoyl transferase (EpHTT) is a cytosolic BAHD acyltransferase that catalyzes the transfer of caffeoyl groups to tartaric acid, a key step in chicoric acid biosynthesis. Understanding the structure of EpHTT is essential to elucidate the molecular basis of substrate recognition and catalytic specificity. Here, we report the crystal structure of apo-form EpHTT at 2.38 Å resolution, revealing a compact, globular architecture typical of the BAHD superfamily. The enzyme adopts a two-domain fold with conserved HXXXD and DFGWG motifs, forming a V-shaped catalytic cleft characteristic of BAHD acyltransferases. Structural comparison with homologous hydroxycinnamoyl transferase enzymes shows high conservation of the overall fold, while EpHTT exhibits unique adaptations that confer specificity for tartaric acid. These results provide a molecular framework for understanding the function and substrate specificity of HTT, offering insights for the metabolic engineering of chicoric acid production.