Biochemical characterization and mutational studies of a chalcone synthase from yellow snapdragon (Antirrhinum majus) flowers

The 4′-O-glucosides of 2′,4,4′,6′- tetrahydroxychalcone (THC) and 2′,3,4,4′,6′- pentahydroxychalcone (PHC) are present in the yellow snapdragon (Antirrhinum majus) flowers and serve as direct precursors for the synthesis of aurones, the yellow pigments of the flowers. Despite the metabolic significance of these chalcones in aurone biosynthesis, the biochemical properties of chalcone synthase (CHS), the enzyme responsible for their synthesis, remains to be clarified. In this study, one known CHS cDNA (niv, also termed AmCHS1) and one CHS-related cDNA, AmCHS2, were isolated from yellow snapdragon buds. AmCHS1 mRNA specifically accumulated in the petals in a coloration-dependent manner, whereas AmCHS2 mRNA accumulated only negligibly in petals and other organs, corroborating the importance of AmCHS1 in chalcone synthesis in the yellow flower. Recombinant AmCHS1 that was heterologously expressed in Escherichia coli cells efficiently utilized both p-coumaroyl-CoA and caffeoyl-CoA (relative activity, 50% of the activity for p-coumaroyl-CoA) as a starting ester to produce THC and PHC, respectively. Previous studies predicted that a single Thr197Ala or the double Val196Met/Thr197Ala substitution in AmCHS1 may provide a wider space for hydroxycinnamoyl-group binding and might enhance the ability of this enzyme to produce PHC [Austin MB, Noel JP (2003) Nat Prod Rep 20: 79-110]. However, these substitutions led to a decreased PHC-producing activity, implying that other factors may also be important for the efficient utilization of caffeoyl-CoA.