2026/04/10 by Clara Morweiser, Sarah Heinicke, Sarah E. O'Connor +1 · 1 voice
Biochemistry, Genetics and Molecular Biology · Pharmacology, Toxicology and Pharmaceutics · #Alkaloids: synthesis and pharmacology #Amino acid #Bioactive natural compounds #Epimer #In silico #Indole test #Plant biochemistry and biosynthesis #Stereoselectivity #Tryptamine
paper · doi:10.1002/anie.6102165
openalex publication_date 2026/04/10 · openalex created_date 2026/04/12 · openalex updated_date 2026/07/21
The class-defining monoterpenoid indole alkaloid (MIA) scaffold strictosidine is generated by condensation of tryptamine and secologanin by the Pictet-Spenglerase strictosidine synthase (STR). All previously characterized STR orthologs are strictly 3S-stereoselective. Here, we report that the medicinal plant species Pogonopus speciosus (Rubiaceae) accumulates the 3R epimer vincosidic acid produced by a an ortholog of STR. This ortholog, named here Epi-STR, exclusively produces the 3R configuration, and is capable of accepting both secologanic acid and the methylester secologanin as aldehyde substrates. Using comparative phylogenetic and structural analyses, we determine the amino acid residues that confer stereoselectivity. Through rational design of reciprocal amino acid substitutions, we achieved switches in stereoselectivity in a canonical STR and in Epi-STR. The stereoselectivity of the engineered mutants is also dependent on the identity of the aldehyde substrate. Notably, previous and extensive engineering efforts have never been shown to switch the stereo-selectivity of STR. Therefore, this discovery now allows cost-effective epimer-pure access to the R-epimer, and offers mechanistic insights into the enzyme stereoselectivity of this important reaction. This work also highlights the importance of phytochemical analyses of poorly described plant species.