Sarpagan-Ajmalan-Type Indoles: Biosynthesis, Structural Biology, and Chemo-Enzymatic Significance.

The biosynthetic pathway of the monoterpenoid indole alkaloid ajmaline in the genus Rauvolfia, in particular Rauvolfia serpentina Benth. ex Kurz, is one of the few pathways that have been comprehensively uncovered. Every step in the progress of plant alkaloid biosynthesis research is due to the endeavors of several generations of scientists and the advancement of technologies. The tissue and cell suspension cultures developed in the 1970s by M.H. Zenk enabled the extraction of alkaloids and crude enzymes for use as experimental materials, thus establishing the foundation for further research on enzymatic reaction networks. In vivo NMR technology was first used in biosynthetic investigations in the 1990s following the invention of high-field cryo-NMR, which allowed the rapid and reliable detection of bioconversion processes within living plant cells. Shortly before, in 1988, a milestone was reached with the heterologous expression of the strictosidine synthase cDNA, which paved the way for the application of "reverse genetics" and "macromolecular crystallography." Both methods allowed the structural analysis of several Rauvolfia enzymes involved in ajmaline biosynthesis and expanded our knowledge of the enzyme mechanisms, substrate specificities, and structure-activity relationships. It also opened the door for rational enzyme engineering and metabolic steering. Today, the research focus of ajmaline biosynthesis is shifting from "delineation" to "utilization." The Pictet-Spenglerase strictosidine synthase, strictosidine glucosidase, together with raucaffricine glucosidase, as pioneers in this area, have become useful tools to generate "privileged structures" and "diversity oriented" syntheses, which may help to construct novel scaffolds and to set up libraries of sarpagan-ajmalan-type alkaloids in chemo-enzymatic approaches.

Using Strictosidine Synthase to Prepare Novel Alkaloids.

The Pictet-Spenglerasestrictosidine synthase (STR) has been characterized as the central enzyme in the biosynthesis of around 2000 monoterpenoid indole alkaloids in plants. In the light of a high therapeutic value and huge scaffold diversity these alkaloids represent, STR as an enzyme has attracted great attentions in recent years, intending to be utilized in the formation of new interesting alkaloids with unusual substitution pattern or even with novel scaffolds. For outlining the application potential that STR possesses, together with insight into the reaction mechanism catalyzed by STR, strategies and methods for exploring the applicability of STR have been updated in this article by taking R. serpertina STR(RS-STR) and C. roseus.STR (CR-STR) as representative models, followed by introducing the latest released complex structures of RS-STR with new substrates. Examples provided here, including substrate scaffold tailoring, X-ray crystal complex structure comparison, protein engineering and biosynthetic pathway reprogramming, pave the way to finally construct novel alkaloids libraries by chemo-enzymatic approaches.