Diterpene Biosynthesis from Geranylgeranyl Diphosphate Analogues with Changed Reactivities Expands Skeletal Diversity

Engineering terpene synthases and their substrates for the biocatalytic production of terpene natural products and analogues

Terpene synthases produce a wide number of hydrocarbon skeletons by controlling intramolecular rearrangements of allylic pyrophosphate subtrates via reactive carbocation intermediates. Here we review recent research focused on engineering terpene synthases and modifying their substrates to rationally manipulate terpene catalyisis. Molecular dynamic simulations and solid state X-ray crystallography are powerful techniques to identify substrate binding modes, key active site residues for substrate folding, and the location of active site water. Variants in specific 'hotspots' of terpene synthases including the G1/2, K/H and Hα-1 helices have been targeted to modify active site water management and yield new products. We discuss the potential of exploiting substrate analogues to synthesise novel compounds and briefly outline biphasic flow systems for biocatalysis of terpenes. We forsee greater applications for terpenes as the field converges on effective methods for engineering of terpene synthases by new computational and high throughput experimental methods and for high-yield production. It is crucial when engineering terpene synthases that both product distribution and enzyme activity are simultaneously optimised.

On the Role of Hydrogen Migrations in the Taxadiene System

Taxa-4,11-diene is made by the taxa-4,11-diene synthase (TxS) from Taxus brevifolia. The unique reactivity of the taxane system is characterised by long distance hydrogen migrations in the biosynthesis. This study demonstrates that selective long range hydrogen migrations also play a role in the high energy process of the EI-MS fragmentation of taxa-4,11-diene. A TxS enzyme variant was generated that produces cyclophomactene, a compound that is formed through a concerted process involving a long range proton shift and a ring closure that can also be described as the addition of a methylcarbinyl cation to an olefin. Based on a previous computational study the cyclisation mechanism towards taxa-4,11-diene was suggested to involve two long distance proton migrations instead of one direct transfer. A substrate analog with a shifted double bond was converted with TxS to obtain experimental evidence for this proposal.

Beyond Essential Oils: Diterpenes, Lignans, and Biflavonoids from Juniperus communis L. as a Source of Multi-Target Lead Compounds

Common Juniper (Juniperus communis L.) is a gymnosperm that stands out through its fleshy, spherical female cones, often termed simply "berries". The cone berries and various vegetative parts (leaves, twigs and even roots) are used in traditional phytotherapy, based on the beneficial effects exerted by a variety of secondary metabolites. While the volatile compounds of Juniperus communis are known for their aromatic properties and have been well-researched for their antimicrobial effects, this review shifts focus to non-volatile secondary metabolites-specifically diterpenes, lignans, and biflavonoids. These compounds are of significant biomedical interest due to their notable pharmacological activities, including antioxidant, anti-inflammatory, antimicrobial, and anticancer effects. The aim of this review is to offer an up-to-date account of chemical composition of Juniperus communis and related species, with a primary emphasis on the bioactivities of diterpenes, lignans, and biflavonoids. By examining recent preclinical and clinical data, this work assesses the therapeutic potential of these metabolites and their mechanisms of action, underscoring their value in developing new therapeutic options. Additionally, this review addresses the pharmacological efficacy and possible therapeutic applications of Juniperus communis in treating various human diseases, thus supporting its potential role in evidence-based phytotherapy.

Telescoping a Prenyltransferase and a Diterpene Synthase to Transform Unnatural FPP Derivatives to Diterpenoids

New diterpenoids are accessible from non-natural FPP derivatives as substrates for an enzymatic elongation cyclization cascade using the geranylgeranyl pyrophosphate synthase (GGPPS) from Streptomyces cyaneofuscatus and the spata-13,17-diene synthase (SpS) from Streptomyces xinghaiensis. This approach led to four new biotransformation products including three new cyclododecane cores and a macrocyclic ether. For the first time, a 1,12-terpene cyclization was observed when shifting the central olefinic double bond toward the geminial methyl groups creating a nonconjugated 1,4-diene.

Biosynthesis of fungal terpenoids

Covering: up to August 2023Terpenoids, which are widely distributed in animals, plants, and microorganisms, are a large group of natural products with diverse structures and various biological activities. They have made great contributions to human health as therapeutic agents, such as the anti-cancer drug paclitaxel and anti-malarial agent artemisinin. Accordingly, the biosynthesis of this important class of natural products has been extensively studied, which generally involves two major steps: hydrocarbon skeleton construction by terpenoid cyclases and skeleton modification by tailoring enzymes. Additionally, fungi (Ascomycota and Basidiomycota) serve as an important source for the discovery of terpenoids. With the rapid development of sequencing technology and bioinformatics approaches, genome mining has emerged as one of the most effective strategies to discover novel terpenoids from fungi. To date, numerous terpenoid cyclases, including typical class I and class II terpenoid cyclases as well as emerging UbiA-type terpenoid cyclases, have been identified, together with a variety of tailoring enzymes, including cytochrome P450 enzymes, flavin-dependent monooxygenases, and acyltransferases. In this review, our aim is to comprehensively present all fungal terpenoid cyclases identified up to August 2023, with a focus on newly discovered terpenoid cyclases, especially the emerging UbiA-type terpenoid cyclases, and their related tailoring enzymes from 2015 to August 2023.

Enhancing structural diversity of terpenoids by multisubstrate terpene synthases

Terpenoids are one of the largest class of natural products with diverse structures and activities. This enormous diversity is embedded in enzymes called terpene synthases (TSs), which generate diverse terpene skeletons via sophisticated cyclization cascades. In addition to the many highly selective TSs, there are many promiscuous TSs that accept multiple prenyl substrates, or even noncanonical ones, with 6, 7, 8, 11, and 16 carbon atoms, synthesized via chemical approaches, C-methyltransferases, or engineered lepidopteran mevalonate pathways. The substrate promiscuity of TSs not only expands the structural diversity of terpenes but also highlights their potential for the discovery of novel terpenoids via combinatorial biosynthesis. In this review, we focus on the current knowledge on multisubstrate terpene synthases (MSTSs) and highlight their potential applications.

Boswellianols A-I, Structurally Diverse Diterpenoids from the Oleo-Gum Resin of Boswellia carterii and Their TGF-β Inhibition Activity

Olibanum, a golden oleo-gum resin from species in the Boswellia genus (Burseraceae family), is a famous traditional herbal medicine widely used around the world. Previous phytochemical studies mainly focused on the non-polar fractions of olibanum. In this study, nine novel diterpenoids, boswellianols A-I (1-9), and three known compounds were isolated from the polar methanolic fraction of the oleo-gum resin of Boswellia carterii. Their structures were determined through comprehensive spectroscopic analysis as well as experimental and calculated electronic circular dichroism (ECD) data comparison. Compound 1 is a novel diterpenoid possessing an undescribed prenylmaaliane-type skeleton with a 6/6/3 tricyclic system. Compounds 2-4 were unusual prenylaromadendrane-type diterpenoids, and compounds 5-9 were new highly oxidized cembrane-type diterpenoids. Compounds 1 and 5 showed significant transforming growth factor β (TGF-β) inhibitory activity via inhibiting the TGF-β-induced phosphorylation of Smad3 and the expression of fibronectin and N-cadherin (the biomarker of the epithelial-mesenchymal transition) in a dose-dependent manner in LX-2 human hepatic stellate cells, indicating that compounds 1 and 5 should be potential anti-fibrosis agents. These findings give a new insight into the chemical constituents of the polar fraction of olibanum and their inhibitory activities on the TGF-β/Smad signaling pathway.

Enzymatic Synthesis of Diterpenoids from iso-GGPP III: A Geranylgeranyl Diphosphate Analog with a Shifted Double Bond

The analog of the diterpene precursor geranylgeranyl diphosphate with a double bond shifted from C14=C15 to C15=C16 (named iso-GGPP III) has been synthesized and enzymatically converted with six bacterial diterpene synthases; this allowed the isolation of nine unnatural diterpenes. For some of the enzyme-substrate combinations, the different reactivity implemented in the substrate analog iso-GGPP III opened reaction pathways that are not observed with natural GGPP, resulting in the formation of diterpenes with novel skeletons. A stereoselective deuteration strategy was used to assign the absolute configurations of the isolated diterpenes.

Sesquiterpene Backbones Generated by Sesquiterpene Cyclases: Formation of iso-Caryolan-1-ol and an Isoclovane

New sesquiterpene backbones are accessible after incubation of caryolan-synthase (GcoA) and presilphiperfolan-8-β-ol synthase (BcBOT2) with a non-natural farnesyldiphosphate in which the central olefinic double bond is isomerized toward the methyl group. Two newly formed sesquiterpenoids are reported, a constitutional isomer of caryolan-1-ol (3), which we name iso-caryolan-1-ol (17), and the first terpenoid based on the isoclovane ring skeleton generated enzymatically thus far.

Subrutilane-A Hexacyclic Sesterterpene from Streptomyces subrutilus

Mining of a terpene synthase from Streptomyces subrutilus resulted in the identification of the hexacyclic sesterterpene subrutilane, besides eight pentacyclic side products. Subrutilane represents the first case of a saturated sesterterpene hydrocarbon. Its structure, including the absolute configuration, was unambiguously determined through X-ray crystallographic analysis and stereoselective deuteration. The cyclisation mechanism to subrutilane and its side products was investigated in all detail by isotopic labelling experiments and DFT calculations. The subrutilane synthase (SrS) also converted (2Z)-GFPP into one major product. Additional compounds were obtained from the substrate analogues (7R)-6,7-dihydro-GFPP and (2Z,7R)-6,7-dihydro-GFPP with blocked reactivity at the C6-C7 bond. Interestingly, the early steps of the cyclisation cascade with (2Z)-GFPP and the saturated substrate analogues were analogous to those of GFPP, but then deviations from the natural cyclisation mode occur.

Unraveling the role of prenyl side-chain interactions in stabilizing the secondary carbocation in the biosynthesis of variexenol B

Terpene cyclization reactions involve a number of carbocation intermediates. In some cases, these carbocations are stabilized by through-space interactions with π orbitals. Several terpene/terpenoids, such as sativene, santalene, bergamotene, ophiobolin and mangicol, possess prenyl side chains that do not participate in the cyclization reaction. The role of these prenyl side chains has been partially investigated, but remains elusive in the cyclization cascade. In this study, we focus on variexenol B that is synthesized from iso-GGPP, as recently reported by Dickschat and co-workers, and investigate the possibility of through-space interactions with prenyl side chains using DFT calculations. Our calculations show that (i) the unstable secondary carbocation is stabilized by the cation-π interaction from prenyl side chains, thereby lowering the activation energy, (ii) the four-membered ring formation is completed through bridging from the exomethylene group, and (iii) the annulation from the exomethylene group proceeds in a barrier-free manner.