An Unprecedented Cyclization Mechanism in the Biosynthesis of Carbazole Alkaloids in Streptomyces

Carquinostatin A (CQS), a potent neuroprotective substance, is a unique carbazole alkaloid with both an ortho‐quinone function and an isoprenoid moiety. We identified the entire gene cluster responsible for CQS biosynthesis in Streptomyces exfoliatus through heterologous production of CQS and gene deletion. Biochemical characterization of seven CQS biosynthetic gene products (CqsB1–7) established the total biosynthetic pathway of CQS. Reconstitution of CqsB1 and CqsB2 showed that the synthesis of the carbazole skeleton involves CqsB1‐catalyzed decarboxylative condensation of an α‐hydroxyl‐β‐keto acid intermediate with 3‐hydroxybutyryl‐ACP followed by CqsB2‐catalyzed oxidative cyclization. Based on crystal structures and mutagenesis‐based biochemical assays, a detailed mechanism for the unique deprotonation‐initiated cyclization catalyzed by CqsB2 is proposed. Finally, analysis of the substrate specificity of the biosynthetic enzymes led to the production of novel carbazoles.     

Two New Degraded Triterpenoids and a Novel seco‐Norabietane Diterpene from the Root Bark of Azadirachta indica

Three new compounds, the degraded ring C‐seco‐tetranortriterpenoid nimbolicidin ( 1 ), the degraded hexanortriterpenoid nimbocin ( 2 ), and the seco‐norabietane diterpene nimbocinin ( 3 ), were isolated from the root bark of Azadirachta indica A.Juss . Compound 1 is O‐bearing both at C(28) and C(29), which has been hitherto unreported in tetranortriterpenoids; 2 represents the first hexanortriterpenoid with a truncated apotirucallane (or apoeuphane) skeleton; 3 is an unprecedented seco‐norabietane. Spectroscopic data and chemical transformations of these compounds provided their complete structures.     

Biosynthesis of Complex Indole Alkaloids: Elucidation of the Concise Pathway of Okaramines

The okaramines are a class of complex indole alkaloids isolated from Penicillium and Aspergillus species. Their potent insecticidal activity arises from selectively activating glutamate‐gated chloride channels (GluCls) in invertebrates, not affecting human ligand‐gated anion channels. Okaramines B ( 1 ) and D ( 2 ) contain a polycyclic skeleton, including an azocine ring and an unprecedented 2‐dimethyl‐3‐methyl‐azetidine ring. Owing to their complex scaffold, okaramines have inspired many total synthesis efforts, but the enzymology of the okaramine biosynthetic pathway remains unexplored. Here, we identified and characterized the biosynthetic gene cluster (oka ) of 1 and 2 , then elucidated the pathway with target gene inactivation, heterologous reconstitution, and biochemical characterization. Notably, we characterized an α‐ketoglutarate‐dependent non‐heme Fe^II dioxygenase that forged the azetidine ring on the okaramine skeleton.     

Early Oxidative Transformations During the Biosynthesis of Terrein and Related Natural Products

The mycotoxin terrein is derived from the C₁₀-precursor 6-hydroxymellein (6-HM) via an oxidative ring contraction. Although the corresponding biosynthetic gene cluster (BGC) has been identified, details of the enzymatic oxidative transformations are lacking. Combining heterologous expression and in vitro studies we show that the flavin-dependent monooxygenase (FMO) TerC catalyzes the initial oxidative decarboxylation of 6-HM. The reactive intermediate is further hydroxylated by the second FMO TerD to yield a highly oxygenated aromatic species, but further reconstitution of the pathway was hampered. A related BGC was identified in the marine-derived Roussoella sp. DLM33 and confirmed by heterologous expression. These studies demonstrate that the biosynthetic pathways of terrein and related (polychlorinated) congeners diverge after oxidative decarboxylation of the lactone precursor that is catalyzed by a conserved FMO and further indicate that early dehydration of the side chain is an essential step.     

Unusual Acetylation‐Dependent Reaction Cascade in the Biosynthesis of the Pyrroloindole Drug Physostigmine

Physostigmine is a parasympathomimetic drug used to treat a variety of neurological disorders, including Alzheimer’s disease and glaucoma. Because of its potent biological activity and unique pyrroloindole skeleton, physostigmine has been the target of many organic syntheses. However, the biosynthesis of physostigmine has been relatively understudied. In this study, we identified a biosynthetic gene cluster for physostigmine by genome mining. The 8.5 kb gene cluster encodes eight proteins (PsmA–H), seven of which are required for the synthesis of physostigmine from 5‐hydroxytryptophan, as shown by in vitro total reconstitution. Further genetic and enzymatic studies enabled us to delineate the biosynthetic pathway for physostigmine. The pathway features an unusual reaction cascade consisting of highly coordinated methylation and acetylation/deacetylation reactions.     

The Biosynthesis of Norsesquiterpene Aculenes Requires Three Cytochrome P450 Enzymes to Catalyze a Stepwise Demethylation Process

Aculenes are a unique class of norsequiterpenes (C₁₄) that are produced by Aspergillus aculeatus. The nordaucane skeleton in aculenes A–D may be derived from an ent‐daucane precursor through demethylation, however, the enzymes involved remain unexplored. We identified the biosynthetic gene cluster and characterized the biosynthetic pathway based on gene inactivation, feeding experiments, and heterologous reconstitution in Saccharomyces cerevisiae and Aspergillus oryzae. We discovered that three cytochrome P450 monoxygenases are required to catalyze the stepwise demethylation process. AneF converts the 12‐methyl group into a carboxylic acid and AneD installs the 10‐hydroxy group for later tautomerization and stabilization. Finally, AneG installs an electron‐withdrawing carbonyl group at the C‐2 position, which triggers C‐12 decarboxylation to yield the nordaucane skeleton. Additionally, a terpene cyclase (AneC) was found that forms a new product (dauca‐4,7‐diene).     

Enzymatic Thioamide Formation in a Bacterial Antimetabolite Pathway

6‐Thioguanine (6TG) is a DNA‐targeting therapeutic used in the treatment of various cancers. While 6TG was rationally designed as a proof of concept for antimetabolite therapy, it is also a rare thioamide‐bearing bacterial natural product and critical virulence factor of Erwinia amylovorans, plant pathogens that cause fire blight. Through gene expression, biochemical assays, and mutational analyses, we identified a specialized bipartite enzyme system, consisting of an ATP‐dependent sulfur transferase (YcfA) and a sulfur‐mobilizing enzyme (YcfC), that is responsible for the peculiar oxygen‐by‐sulfur substitution found in the biosynthesis of 6TG. Mechanistic and phylogenetic studies revealed that YcfA‐mediated 6TG biosynthesis evolved from ancient tRNA modifications that support translational fidelity. The successful in vitro reconstitution of 6TG thioamidation showed that YcfA employs a specialized sulfur shuttle that markedly differs from universal RNA‐related systems. This study sheds light on underexplored enzymatic C?S bond formation in natural product biosynthesis.     

Enantioselective Total Synthesis of Cymoside through a Bioinspired Oxidative Cyclization of a Strictosidine Derivative

The first total synthesis of the caged monoterpene indole alkaloid cymoside is reported. This natural product displays a unique hexacyclic‐fused skeleton whose biosynthesis implies an early oxidative cyclization of strictosidine. Our approach to the furo[3,2‐b]indoline framework relied on an unprecedented biomimetic sequence which started by the diastereoselective oxidation of the indole ring into a hydroxyindolenine which triggered the addition of an enol ether and was followed by the trapping of an oxocarbenium intermediate.     

N-Pyridinium imidates as new sources of 2-aminoimidazole and imidazoline derivatives

New 2-aminoimidazole (2-AI) and imidazoline derivatives were obtained in three steps through the reduction of N-pyridinium imidates into 1,2-dihydropyridine imidates and oxidative addition of guanidine derivatives. Among the possible transformations, imidate substitution allows selectivity in the last deprotection step, leading to an original 2-aminoimidazolo-imidazoline skeleton.     

Analysis of Rhizonin Biosynthesis Reveals Origin of Pharmacophoric Furylalanine Moieties in Diverse Cyclopeptides

Rhizonin A and B are hepatotoxic cyclopeptides produced by bacterial endosymbionts (Mycetohabitans endofungorum) of the fungus Rhizopus microsporus. Their toxicity critically depends on the presence of 3-furylalanine (Fua) residues, which also occur in pharmaceutically relevant cyclopeptides of the endolide and bingchamide families. The biosynthesis and incorporation of Fua by non-ribosomal peptide synthetases (NRPS), however, has remained elusive. By genome sequencing and gene inactivation we elucidated the gene cluster responsible for rhizonin biosynthesis. A suite of isotope labeling experiments identified tyrosine and l -DOPA as Fua precursors and provided the first mechanistic insight. Bioinformatics, mutational analysis and heterologous reconstitution identified dioxygenase RhzB as necessary and sufficient for Fua formation. RhzB is a novel type of heme-dependent aromatic oxygenases (HDAO) that enabled the discovery of the bingchamide biosynthesis gene cluster through genome mining.     

Solving the Puzzle of One‐Carbon Loss in Ripostatin Biosynthesis

Ripostatin is a promising antibiotic that inhibits RNA polymerase by binding to a novel binding site. In this study, the characterization of the biosynthetic gene cluster of ripostatin, which is a peculiar polyketide synthase (PKS) hybrid cluster encoding cis‐ and trans‐acyltransferase PKS genes, is reported. Moreover, an unprecedented mechanism for phenyl acetic acid formation and loading as a starter unit was discovered. This phenyl‐C2 unit is derived from phenylpyruvate (phenyl‐C3) and the mechanism described herein explains the mysterious loss of one carbon atom in ripostatin biosynthesis from the phenyl‐C3 precursor. Through in vitro reconstitution of the whole loading process, a pyruvate dehydrogenase like protein complex was revealed that performs thiamine pyrophosphate dependent decarboxylation of phenylpyruvate to form a phenylacetyl‐S ‐acyl carrier protein species, which is supplied to the subsequent biosynthetic assembly line for chain extension to finally yield ripostatin.     

Sarcaglarols A—D,Lindenane−Monoterpene Heterodimers from Sarcandra glabra Based on Molecular Networks

Sarcaglarols A—D ( 1 — 4 ), two pairs of lindenane?monoterpene heterodimers fused by a 1,2‐dioxane moiety, were discovered and isolated from the leaves of Sarcandra glabra guided by MS/MS molecular networking‐based strategy. Their planar structures, absolute configurations of basic skeleton and flexible polyhydric side chain were established by analysis of HRESIMS, NMR spectroscopic data, ECD spectrum, and the X‐ray diffraction study of isopropylidene derivatives. An intermolecular [2+2+2] cycloaddition may play a key role in the biosynthesis pathway of the 1,2‐dioxane moiety fused lindenane?monoterpene heterodimer skeleton, which can be recognized as the biogenetic precursors of our previous reported lindenane?normonoterpene conjugates. In addition, compounds 1 , 3 and 4 exhibited moderate inhibitory effects of lipid accumulation in free fatty acid‐exposed L02 cells.     

Characterization of Radical SAM Adenosylhopane Synthase,HpnH, which Catalyzes the 5′‐Deoxyadenosyl Radical Addition to Diploptene in the Biosynthesis of C35 Bacteriohopanepolyols

Adenosylhopane is a crucial intermediate in the biosynthesis of bacteriohopanepolyols, which are widespread prokaryotic membrane lipids. Herein, it is demonstrated that reconstituted HpnH, a putative radical S‐adenosyl‐l ‐methionine (SAM) enzyme, commonly encoded in the hopanoid biosynthetic gene cluster, converts diploptene into adenosylhopane in the presence of SAM, flavodoxin, flavodoxin reductase, and NADPH. NMR spectra of the enzymatic reaction product were identical to those of synthetic (22R)‐adenosylhopane, indicating that HpnH catalyzes stereoselective C?C formation between C29 of diploptene and C5′ of 5′‐deoxyadenosine. Further, the HpnH reaction in D₂O‐containing buffer revealed that a D atom was incorporated at the C22 position of adenosylhopane. Based on these results, we propose a radical addition reaction mechanism catalyzed by HpnH for the formation of the C₃₅ bacteriohopane skeleton.     

Total Synthesis of the Hamigerans

The first total synthesis of hamigerans D, G, L, and N–Q has been accomplished. A convergent approach was used to build the basic tricarbocyclic ring system bearing a 5‐6‐6 structure. A sequence of oxidative cleavage, homologation, and ring regeneration provided access to the 5‐7‐6 skeleton of hamigeran G. Based on the biogenetic hypothesis, elegant and highly efficient biomimetic transformations of hamigeran G into hamigerans D, N–Q, and L were achieved.     

Two Novel Triterpenes from the Leaves of Ficus microcarpa

Two novel triterpenes, 29(20→19)abeolupane‐3,20‐dione ( 4 ) and 19,20‐secoursane‐3,19,20‐trione ( 5 ), besides (3β)‐3‐hydroxy‐29(20→19)abeolupan‐20‐one ( 2 ), lupenone, and α‐amyrone ( 6 ), were isolated from the leaves of Ficus microcarpa and were characterized by spectroscopic means, including 2D‐NMR techniques and chemical methods. Compound 4 is the second derivative having the 29(20→19)abeolupane skeleton, and 5 is a novel skeleton. A biosynthetic pathway to 5 is proposed (Scheme).     

Control of the Stereochemical Course of [4+2] Cycloaddition during trans‐Decalin Formation by Fsa2‐Family Enzymes

Enzyme‐catalyzed [4+2] cycloaddition has been proposed to be a key transformation process in various natural product biosynthetic pathways. Recently Fsa2 was found to be involved in stereospecific trans‐decalin formation during the biosynthesis of equisetin, a potent HIV‐1 integrase inhibitor. To understand the mechanisms by which fsa2 determines the stereochemistry of reaction products, we sought an fsa2 homologue that is involved in trans‐decalin formation in the biosynthetic pathway of an enantiomerically opposite analogue, and we found phm7, which is involved in the biosynthesis of phomasetin. A decalin skeleton with an unnatural configuration was successfully constructed by gene replacement of phm7 with fsa2, thus demonstrating enzymatic control of all stereochemistry in the [4+2] cycloaddition. Our findings highlight enzyme‐catalyzed [4+2] cycloaddition as a stereochemically divergent step in natural product biosynthetic pathways and open new avenues for generating derivatives with different stereochemistry.     

Direct Observation of an Oxepin from a Bacterial Cytochrome P450‐Catalyzed Oxidation

The cytochromes P450 are hemoproteins that catalyze a range of oxidative C?H functionalization reactions, including aliphatic and aromatic hydroxylation. These transformations are important in a range of biological contexts, including biosynthesis and xenobiotic biodegradation. Much work has been carried out on the mechanism of aliphatic hydroxylation, implicating hydrogen atom abstraction, but aromatic hydroxylation is postulated to proceed differently. One mechanism invokes as the key intermediate an arene oxide (and/or its oxepin tautomer). Conclusive isolation of this intermediate has remained elusive and, currently, direct formation of phenols from a Meisenheimer intermediate is believed to be favored. We report here the identification of a P450 [P450_cam (CYP101A1) and P450_cin (CYP176A1)]‐generated arene oxide as a product of in vitro oxidation of tert‐butylbenzene. Computations (CBS‐QB3) predict that the arene oxide and oxepin have similar stabilities to other arene oxides/oxepins implicated (but not detected) in P450‐mediated transformations, suggesting that arene oxides can be unstable terminal products of P450‐catalyzed aromatic oxidation that can explain the origin of some observed metabolites.     

N‐Bearing Furanone Derivatives from an Endophytic Fungus in Huperzia serrata

Two new polyketide derivatives, huaspenones C and D ( 1 and 2 , resp.), were isolated from the cultures of an endophytic fungus Peyronellaea sp. HS‐12, derived from the stems of Huperzia serrata. They share N‐bearing furan‐3(2H)‐one backbone, and 2 has an unprecedented furo[3,2‐c]pyridine skeleton. Their structures including the absolute configuration were elucidated by extensive spectroscopic analysis combined with quantum‐chemical calculations. (2E,4E)‐6‐hydroxy‐2‐methylocta‐2,4‐dienoic acid ( 3 ), a key intermediate of the biosynthesis of 1 and 2 , was also obtained from the endophyte.     

Biosynthesis of Phomactin Platelet Activating Factor Antagonist Requires a Two-Enzyme Cascade

Phomactin diterpenoids possess a unique bicyclo[9.3.1]pentadecane skeleton with multiple oxidative modifications, and are good platelet-activating factor (PAF) antagonists that can inhibit PAF-induced platelet aggregation. In this study, we identified the gene cluster (phm) responsible for the biosynthesis of phomactins from a marine fungus, Phoma sp. ATCC 74077. Despite the complexity of their structures, phomactin biosynthesis only requires two enzymes: a type I diterpene cyclase PhmA and a P450 monooxygenase PhmC. PhmA was found to catalyze the formation of the phomactatriene, while PhmC sequentially catalyzes the oxidation of multiple sites, leading to the generation of structurally diverse phomactins. The rearrangement mechanism of the diterpene scaffold was investigated through isotope labeling experiments. Additionally, we obtained the crystal complex of PhmA with its substrate analogue FGGPP and elucidated the novel metal-ion-binding mode and enzymatic mechanism of PhmA through site-directed mutagenesis. This study provides the first insight into the biosynthesis of phomactins, laying the foundation for the efficient production of phomactin natural products using synthetic biology approaches.     

Expeditious Synthesis of Enantiopure,Orthogonally Protected Bis‐α‐Amino Acids (OPBAAs) and their Use in a Study of Nod1 Stimulation

A convenient approach towards the synthesis of orthogonally protected chiral bis‐α‐amino acids (OPBAAs) is described. The key transformations include: (1) a highly stereoselective conjugation (alkylation) of the Schöllkopf bis‐lactim ethers and oxazolidinyl alkyl halides to build a backbone skeleton; and (2) our orthogonal protection strategy. A series of enantiopure OPBAAs bearing a variety of alkyl chain as a spacer; two stereogenic centers; and three protecting groups were prepared as examples. These versatile molecules were applied to the synthesis of biologically interesting di‐ or tri‐peptide analogues, including chiral iE‐meso‐DAP and A‐iE‐meso‐DAP, for the study of Nod1 activation in the innate immune response.