Science of Synthesis

Ilari, A.; Bonamore, A.; Boffi, A., Science of Synthesis: Biocatalysis in Organic Synthesis, (2015) 2, 168.

The Pictet–Spengler reaction between tryptamine derivatives and aldehydes yields the tetrahydro-β-carboline moiety, a heterocyclic framework of paramount interest in a wide range of natural products and synthetic pharmaceuticals. At present, three strictosidine synthases obtained from Rauvolfia serpentina (RsSTS), Ophiorrhiza pumila (OpSTS), and Catharantus roseus (CrSTS) have been employed in biocatalysis, and demonstrated to possess diverse catalytic properties.[‌24‌–‌26‌] Recombinant RsSTS is the most highly characterized enzyme and has been shown to display a strict substrate specificity and complete enantioselectivity. Recognition properties of wild-type strictosidine synthase enzymes are limited to a few tryptamine derivatives and secologanin (‌4‌), leading exclusively to 3α-(S)-strictosidine.[‌26‌,‌27‌] RsSTS can be expressed as a recombinant protein in Escherichia coli cells and improved expression methods for His-tagged protein at high yield have been developed.[‌27‌] Kinetic parameters of RsSTS for the natural substrate tryptamine, under saturating secologanin concentrations, indicate a high substrate affinity and relatively high turnover rate (average values: KM = 5 µM and kcat = 100 min−1 in 50 mM phosphate buffer at pH 7.0 and 25 °C).[‌11‌] The properties of CrSTS are almost superimposable with those observed for RsSTS. However, one rationally designed active site RsSTS mutant (namely RsSTSVal208Ala)[‌28‌] and two CrSTS mutants (CrSTSVal214Met and CrSTSPhe232Leu), obtained in a saturation mutagenesis study,[‌29‌] have demonstrated broader substrate specificity and the ability to process A-ring-substituted tryptamines in their reaction with secologanin (‌4‌) (see ‌Scheme 7‌, Section ‌2.1.5.3.1‌). More recently, strictosidine synthase from Ophiorrhiza pumila (OpSTS) has emerged as a most interesting active site natural variant with respect to RsSTS in that it lacks His283 (His277 in RsSTS), a residue that is involved in the hydrogen bonding network including the glucose moiety of secologanin (‌4‌). OpSTS demonstrates a broader, and therefore potentially more useful, aldehyde substrate scope compared to wild-type CrSTS and RsSTS.[‌30‌]

References