Structural basis of the Cope rearrangement and C–C bond-forming cascade in hapalindole/fischerindole biogenesis

Abstract

STRUCTURESThe atomic coordinates and structure factors for:HpiC1 W73M/K132M SeMet (P212121) –1.7 ÅHpiC1 native (C2) –1.5 ÅHpiC1 native (P42) –2.1 ÅHpiC1 Y101F (C2) –1.4 ÅHpiC1 Y101S (C2) –1.4 ÅHpiC1 F138S (P21) –1.7 ÅHpiC1 Y101F/F138S (P21 –1.65 Å have been deposited with the Research Collaboratory for Structural Bioinformatics as Protein Data Bank entries 5WPP, 5WPR, 6AL6, 5WPR, 5WPU, 6AL7, and 6AL8 (www.rcsb.org).GRANTSThis work was supported by: The authors thank the National Science Foundation under the CCI Center for Selective C-H Functionalization (CHE-1205646), the National Institutes of Health (CA70375 to RMW and DHS), R35 GM118101, R01 GM076477 and the Hans W. Vahlteich Professorship (to DHS) for financial support. M.G-B. thanks the Ramón Areces Foundation for a postdoctoral fellowship. J.N.S. acknowledges the support of the National Institute of General Medical Sciences of the National Institutes of Health under Award Number F32GM122218. Computational resources were provided by the UCLA Institute for Digital Research and Education (IDRE) and the Extreme Science and Engineering Discovery Environment (XSEDE), which is supported by the NSF (OCI-1053575). The content does not necessarily represent the official views of the National Institutes of Health.ABSTRACTHapalindole alkaloids are a structurally diverse class of cyanobacterial natural products defined by their varied polycyclic ring systems and diverse biological activities. These polycyclic scaffolds are generated from a common biosynthetic intermediate by the Stig cyclases in three mechanistic steps, including a rare Cope-rearrangement, 6-exo-trig cyclization, and electrophilic aromatic substitution. Here we report the structure of HpiC1, a Stig cyclase that catalyzes the formation of 12-epi-hapalindole U in vitro. The 1.5 Å structure reveals a dimeric assembly with two calcium ions per monomer and the active sites located at the distal ends of the protein dimer. Mutational analysis and computational methods uncovered key residues for an acid catalyzed [3,3]-sigmatropic rearrangement and specific determinants that control the position of terminal electrophilic aromatic substitution leading to a switch from hapalindole to fischerindole alkaloids.

Total synthesis, biological evaluation and biosynthetic re-evaluation of Illicium-derived neolignans

“…Although the calculated barrier of the Cope rearrangement of 54 is relatively high (t 1/2 6 days at 25 °C), it is possible that this could be accelerated through enzyme catalysis. Examples of enzymes catalyzing pericyclic reactions are rare; 39 however, there is evidence to support biosynthetic Cope rearrangements in alkaloid biosynthesis [40][41][42] and there are biogenetically inspired syntheses of other Illicium natural products which feature Claisen and Cope rearrangements. 43 Biological investigations: axonal growth assay Having completed our total synthesis studies, we sought to investigate the potential neuronal functional properties of tetrahydrodibenzofuran-containing Illicium neolignans.…”

The expanding world of biosynthetic pericyclases: cooperation of experiment and theory for discovery

“…Theoretical studies by our group indicate that the pericyclic Cope rearrangement mechanism is accelerated by hydrogen-bonding groups, 71 while full protonation of the indoline will cause the mechanism to change to a stepwise dissociative process involving an allyl cation intermediate ( Figure 16B). 69 The mechanism of this reaction has not been established experimentally, so it is not certain if this enzyme is a true pericyclase or catalyzes a dissociation and recombination of a protonated intermediate.…”

Hot off the Press

“…The crystal structure of HpiC1, a Stig cyclase from Stigonematales, an enzyme responsible for three mechanistic steps, a Cope rearrangement, a 6-exo-trig cyclisation and an electrophilic aromatic substitution during the biosynthesis of 12-epi-hapalindole U 28 has been solved. 24 A combination of mutational analysis that has identified the enzyme active site and, DFT calculations have generated detailed insight of the enzymatic processes that control the stereochemical outcome and product formation. A new approach for the biosynthesis of unnatural variants of thaxtomin phytotoxins, potential herbicides from Streptomyces scabies, has been developed.…”