Acyltransferase that catalyses the condensation of polyketide and peptide moieties of goadvionin hybrid lipopeptides.

Fusions of fatty acids and peptides expand the structural diversity of natural products; however, polyketide/ribosomally synthesized and post-translationally modified peptides (PK/RiPPs) hybrid lipopeptides are relatively rare. Here we report a family of PK/RiPPs called goadvionins, which inhibit the growth of Gram-positive bacteria, and an acyltransferase, GdvG, which catalyses the condensation of the PK and RiPP moieties. Goadvionin comprises a trimethylammonio 32-carbon acyl chain and an eight-residue RiPP with an avionin structure. The positions of six hydroxyl groups and one double bond in the very-long acyl chain were determined by radical-induced dissociation tandem mass spectrometry, which collides radical ion species to generate C-C bond cleavage fragments. GdvG belongs to the Gcn5-related N-acetyltransferase superfamily. Unlike conventional acyltransferases, GdvG transfers a very long acyl chain that is tethered to an acyl carrier protein to the N-terminal amino group of the RiPP moiety. gdvG homologues flanked by PK/fatty acid and RiPP biosynthesis genes are widely distributed in microbial species, suggesting that acyltransferase-catalysed condensation of PKs and RiPPs is a general strategy in biosynthesis of similar lipopeptides.

An Unusual Skeletal Rearrangement in the Biosynthesis of the Sesquiterpene Trichobrasilenol from Trichoderma.

The skeletons of some classes of terpenoids are unusual in that they contain a larger number of Me groups (or their biosynthetic equivalents such as olefinic methylene groups, hydroxymethyl groups, aldehydes, or carboxylic acids and their derivatives) than provided by their oligoprenyl diphosphate precursor. This is sometimes the result of an oxidative ring-opening reaction at a terpene-cyclase-derived molecule containing the regular number of Me group equivalents, as observed for picrotoxan sesquiterpenes. In this study a sesquiterpene cyclase from Trichoderma spp. is described that can convert farnesyl diphosphate (FPP) directly via a remarkable skeletal rearrangement into trichobrasilenol, a new brasilane sesquiterpene with one additional Me group equivalent compared to FPP. A mechanistic hypothesis for the formation of the brasilane skeleton is supported by extensive isotopic labelling studies.

Structural Insights into the CotB2-Catalyzed Cyclization of Geranylgeranyl Diphosphate to the Diterpene Cyclooctat-9-en-7-ol.

The diterpene cyclase CotB2 catalyzes the cyclization of geranylgeranyl diphosphate (GGPP) to the tricyclic cyclooctat-9-en-7-ol, which is characterized by a 5-8-5-fused ring skeleton. We have previously proposed a cyclization cascade involving a unique carbon-carbon bond rearrangement combined with multiple hydride shifts, all occurring at a single active site. Here, we report the first high-resolution X-ray crystal structure of CotB2 with bound substrate analog geranylgeranyl thiodiphosphate (GGSPP). In the GGSPP-bound form, GGSPP folds into a unique S-shaped conformation that probably reflects the substrate-bound state prior to ionization of the substrate GGPP. The folded framework of GGSPP is surrounded by hydrophobic residues and several aromatic and asparagine residues that are well-positioned to stabilize a series of reactive carbocation intermediates through a combination of cation-π and dipole charge interactions. The combined crystal structures and mutagenesis-based biochemical assays provide a structural basis for exquisite control of ring formation and stereochemistry during CotB2 catalysis.

"Cation-Stitching Cascade": exquisite control of terpene cyclization in cyclooctatin biosynthesis.

Terpene cyclization is orchestrated by terpene cyclases, which are involved in the biosynthesis of various cyclic natural products, but understanding the origin and mechanism of the selectivity of terpene cyclization is challenging. In this work, we describe an in-depth mechanistic study on cyclooctatin biosynthesis by means of theoretical calculations combined with experimental methods. We show that the main framework of cyclooctatin is formed through domino-type carbocation transportation along the terpene chain, which we call a "cation-stitching cascade", including multiple hydrogen-shifts and a ring rearrangement that elegantly determine the stereoselectivity.

An unusual terpene cyclization mechanism involving a carbon-carbon bond rearrangement.

Terpene cyclization reactions are fascinating owing to the precise control of connectivity and stereochemistry during the catalytic process. Cyclooctat-9-en-7-ol synthase (CotB2) synthesizes an unusual 5-8-5 fused-ring structure with six chiral centers from the universal diterpene precursor, the achiral C20 geranylgeranyl diphosphate substrate. An unusual new mechanism for the exquisite CotB2-catalyzed cyclization that involves a carbon-carbon backbone rearrangement and three long-range hydride shifts is proposed, based on a powerful combination of in vivo studies using uniformly (13)C-labeled glucose and in vitro reactions of regiospecifically deuterium-substituted geranylgeranyl diphosphate substrates. This study shows that CotB2 elegantly demonstrates the synthetic virtuosity and stereochemical control that evolution has conferred on terpene synthases.