Inverted Binding of Non-natural Substrates in Strictosidine Synthase Leads to a Switch of Stereochemical Outcome in Enzyme-Catalyzed Pictet-Spengler Reactions.

Elisabeth Eger,Gideon Grogan,Mahima Sharma,Song Yang,Wolfgang Kroutil,Adam Simon,Willem B Breukelaar,K N Houk

doi:10.1021/jacs.9b08704

Abstract

The Pictet–Spengler reaction is a valuable route to 1,2,3,4-tetrahydro-β-carboline (THBC) and isoquinoline scaffolds found in many important pharmaceuticals. Strictosidine synthase (STR) catalyzes the Pictet–Spengler condensation of tryptamine and the aldehyde secologanin to give (S)-strictosidine as a key intermediate in indole alkaloid biosynthesis. STRs also accept short-chain aliphatic aldehydes to give enantioenriched alkaloid products with up to 99% ee STRs are thus valuable asymmetric organocatalysts for applications in organic synthesis. The STR catalysis of reactions of small aldehydes gives an unexpected switch in stereopreference, leading to formation of the (R)-products. Here we report a rationale for the formation of the (R)-configured products by the STR enzyme from Ophiorrhiza pumila (OpSTR) using a combination of X-ray crystallography, mutational, and molecular dynamics (MD) studies. We discovered that short-chain aldehydes bind in an inverted fashion compared to secologanin leading to the inverted stereopreference for the observed (R)-product in those cases. The study demonstrates that the same catalyst can have two different productive binding modes for one substrate but give different absolute configuration of the products by binding the aldehyde substrate differently. These results will guide future engineering of STRs and related enzymes for biocatalytic applications.

Highlights

Isoquinoline and β-carboline derivatives are both widespread structural motifs found in alkaloids isolated from various plants and animals
In a first attempt to elucidate the origin of the (R)stereoselectivity of OpSTR with aldehydes 2b−d (Scheme 1B), X-ray crystallographic studies were performed with OpSTR cocrystallized with racemic THBC products 3a−c as ligands
Two of these helices are connected by a conserved disulfide bridge formed between residues Cys[60] and Cys[72] in OpSTR

Summary

■ INTRODUCTION

Isoquinoline and β-carboline derivatives are both widespread structural motifs found in alkaloids isolated from various plants and animals. Assisted by neighboring tyrosine and histidine.[17] Evaluation of the synthetic potential of this and other STRs revealed a relatively broad applicability of various tryptamine analogues as substrates.17c,18 In addition to secologanin analogues,18c,19 selected aldehydes bearing shorter side chains were accepted by the enzyme, with lower activities.[20] interestingly, we discovered that for small aliphatic aldehydes (e.g., 2a−d) the corresponding (R)-configured THBC products were obtained employing, for example, the STR from Ophiorrhiza pumila (OpSTR) or RsSTR with an ee of >98% for 3a (Scheme 1B), in contrast to the absolute configuration of the natural (S)-configured product (S)strictosidine.20b. We use X-ray crystallography, mutational analysis, and MD simulations to determine the origin of this unexpected phenomenon

■ RESULTS AND DISCUSSION

■ CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES