Hideaki Oikawa

Insight into a natural Diels-Alder reaction from the structure of macrophomate synthase.

The Diels–Alder reaction, which forms a six-membered ring from an alkene (dienophile) and a 1,3-diene, is synthetically very useful for construction of cyclic products with high regio- and stereoselectivity under mild conditions. It has been applied to the synthesis of complex pharmaceutical and biologically active compounds. Although evidence on natural Diels–Alderases has been accumulated in the biosynthesis of secondary metabolites, there has been no report on the structural details of the natural Diels–Alderases. The function and catalytic mechanism of the natural Diels–Alderase are of great interest owing to the diversity of molecular skeletons in natural Diels–Alder adducts. Here we present the 1.70 Å resolution crystal structure of the natural Diels–Alderase, fungal macrophomate synthase (MPS), in complex with pyruvate. The active site of the enzyme is large and hydrophobic, contributing amino acid residues that can hydrogen-bond to the substrate 2-pyrone. These data provide information on the catalytic mechanism of MPS, and suggest that the reaction proceeds via a large-scale structural reorganization of the product.

Identification and functional analysis of genes controlling biosynthesis of 2-methylisoborneol

To identify the genes for biosynthesis of the off-flavor terpenoid alcohol, 2-methylisoborneol (2-MIB), the key genes encoding monoterpene cyclase were located in bacterial genome databases by using a combination of hidden Markov models, protein–family search, and the sequence alignment of their gene products. Predicted terpene cyclases were classified into three groups: sesquiterpene, diterpene, and other terpene cyclases. Genes of the terpene cyclase group that form an operon with a gene encoding S-adenosyl-l-methionine (SAM)-dependent methyltransferase were found in genome data of seven microorganisms belonging to actinomycetes, Streptomyces ambofaciens ISP5053, Streptomyces coelicolor A3(2), Streptomyces griseus IFO13350, Streptomyces lasaliensis NRRL3382R, Streptomyces scabies 87.22, Saccharopolyspora erythraea NRRL2338, and Micromonospora olivasterospora KY11048. Among six microorganisms tested, S. ambofaciens, S. coelicolor A3(2), S. griseus, and S. lasaliensis produced 2-MIB but M. olivasterospora produced 2-methylenebornane (2-MB) instead. The regions containing monoterpene cyclase and methyltransferase genes were amplified by PCR from S. ambofaciens, S. lasaliensis, and Saccharopolyspora erythraea, respectively, and their genes were heterologously expressed in Streptomyces avermitilis, which was naturally deficient of 2-MIB biosynthesis by insertion and deletion. All exoconjugants of S. avermitilis produced 2-MIB. Full-length recombinant proteins, monoterpene cyclase and methyltransferase of S. lasaliensis were expressed at high level in Escherichia coli. The recombinant methyltransferase catalyzed methylation at the C2 position of geranyl diphosphate (GPP) in the presence of SAM. 2-MIB was generated by incubation with GPP, SAM, recombinant methyltransferase, and terpene cyclase. We concluded that the biosynthetic pathway involves the methylation of GPP by GPP methyltransferase and its subsequent cyclization by monoterpene cyclase to 2-MIB.

Biogenetically inspired synthesis and skeletal diversification of indole alkaloids

To access architecturally complex natural products, chemists usually devise a customized synthetic strategy for constructing a single target skeleton. In contrast, biosynthetic assembly lines often employ divergent intramolecular cyclizations of a polyunsaturated common intermediate to produce diverse arrays of scaffolds. With the aim of integrating such biogenetic strategies, we show the development of an artificial divergent assembly line generating unprecedented numbers of scaffold variations of terpenoid indole alkaloids. This approach not only allows practical access to multipotent intermediates, but also enables systematic diversification of skeletal, stereochemical and functional group properties without structural simplification of naturally occurring alkaloids. Three distinct modes of [4+2] cyclizations and two types of redox-mediated annulations provided divergent access to five skeletally distinct scaffolds involving iboga-, aspidosperma-, andranginine- and ngouniensine-type skeletons and a non-natural variant within six to nine steps from tryptamine. The efficiency of our approach was demonstrated by successful total syntheses of (±)-vincadifformine, (±)-andranginine and (-)-catharanthine.

Diterpene Cyclases Responsible for the Biosynthesis of Phytoalexins, Momilactones A, B, and Oryzalexins A–F in Rice

Rice (Oryza sativa L.) produces diterpene phytoalexins, such as momilactones, oryzalexins, and phytocassanes. Using rice genome information and in vitro assay with recombinant enzymes, we identified genes (OsKS4 and OsKS10) encoding the type-A diterpene cyclases 9β-pimara-7,15-diene synthase and ent-sandaracopimaradiene synthase which are involved in the biosynthesis of momilactones A, B and oryzalexins A–F respectively. Transcript levels of these two genes increased remarkably after ultraviolet (UV) treatment, which is consistent with elevated production of phytoalexins by UV. These two genes might prove powerful tools for understanding plant defense mechanisms in rice.

Enzymatic activity catalysing exo-selective Diels–Alder reaction in solanapyrone biosynthesis

The crude enzyme from Alternaria solani is able to catalyse the [4 + 2] cycloaddition of prosolanapyrone III 6 to the exo adduct solanapyrone A 1 whose optical purity is estimated as 92 ± 8% e.e. by HPLC analysis monitored using a CD spectrometer; this enzyme also catalyses the oxidation and [4 + 2] cycloaddition of prosolanapyrone II 5 to 1 with 99 ± 4% e.e.

Solanapyrone synthase, a possible Diels-Alderase and iterative type I polyketide synthase encoded in a biosynthetic gene cluster from Alternaria solani.

The solanapyrone biosynthetic gene cluster was cloned from Alternaria solani. It consists of six genes—sol1–6—coding for a polyketide synthase, an O‐methyltransferase, a dehydrogenase, a transcription factor, a flavin‐dependent oxidase, and cytochrome P450. The prosolanapyrone synthase (PSS) encoded by sol1 was expressed in Aspergillus oryzae and its product was identified as desmethylprosolanapyrone I (8). Although PSS is closely related to the PKSs/Diels–Alderases LovB and MlcA of lovastatin and compactin biosynthesis, it did not catalyze cycloaddition. Sol5, encoding a flavin‐dependent oxidase (solanapyrone synthase, SPS), was expressed in Pichia pastoris and purified. The purified recombinant SPS showed activity for the formation of (−)‐solanapyrone A (1) from achiral prosolanapyrone II (2), establishing that this single enzyme catalyzes both the oxidation and the subsequent cycloaddition reaction, possibly as a Diels–Alder enzyme.

Identification of ophiobolin F synthase by a genome mining approach: a sesterterpene synthase from Aspergillus clavatus.

During a screening of putative diterpene synthase genes found in public databases using the Aspergillus oryzae expression system, it was found that a single transformant with the ACLA_76850 gene from A. clavatus produced a sesterterpene alcohol, ophiobolin F, and three minor sesterterpene hydrocarbons. The sesterterpene synthase has two catalytically independent domains (prenyltransferase/terpene cyclase) which are homologous to those of diterpene synthase, fusicoccadiene synthase. Coevolution of both domains and reaction mechanisms of these terpene synthases are discussed.

Reconstruction of the saframycin core scaffold defines dual Pictet-Spengler mechanisms

Saframycin A is a potent antitumor antibiotic with a unique pentacyclic tetrahydroisoquinoline scaffold. We found that the nonribosomal peptide synthetase SfmC catalyzes a seven-step transformation of readily synthesized dipeptidyl substrates with long acyl chains into a complex saframycin scaffold. Based on a series of enzymatic reactions, we propose a detailed mechanism involving the reduction of various peptidyl thioesters by a single R domain followed by iterative C domain-mediated Pictet-Spengler reactions.

ENZYMATIC ACTIVITY AND PARTIAL PURIFICATION OF SOLANAPYRONE SYNTHASE : FIRST ENZYME CATALYZING DIELS-ALDER REACTION

In cell-free extracts of Alternaria solani, an enzymatic activity converting prosolanapyrone II to solanapyrones A and D via oxidation and subsequent Diels-Alder reaction has been found. Chromatography with DEAE-Sepharose provided two active fractions, pools 1 and 2. The former fraction converted prosolanapyrone II to solanapyrones A and D in a ratio of 2.2:1 with optical purities of 99% and 45% ee, respectively. The latter fraction did so in a ratio of 7.6:1 with 99% and nearly 0% ee, respectively. The enzyme partially purified from pool 2 native molecular weight of 40-62 kD and a pl of 4.25. The high reactivity of prosolanapyrone III in aqueous solution and the chromatographic behavior of the enzyme in pool 2 suggest that a single enzyme catalyzes both the oxidation and Diels-Alder reaction.

Genome Mining for Sesterterpenes Using Bifunctional Terpene Synthases Reveals a Unified Intermediate of Di/Sesterterpenes.

Genome mining is a promising method to discover novel secondary metabolites in the postgenomic era. We applied the Aspergillus oryzae heterologous expression system to functionally characterize cryptic bifunctional terpene synthase genes found in fungal genomes and identified the sesterfisherol synthase gene (NfSS) from Neosartorya fischeri. Sesterfisherol contains a characteristic 5-6-8-5 tetracyclic ring system and is modified by cytochrome P450 monooxygenase (NfP450) to sesterfisheric acid. The cyclization mechanism was proposed on the basis of the analysis of in vivo and in vitro enzymatic reactions with isotopically labeled precursors. The mechanism involves C1 cation-olefin IV-olefin V cyclization followed by five hydride shifts, allowing us to propose a unified biogenesis for sesterterpenes branching from bicyclic (5-15), tricyclic (5-12-5), and tetracyclic (5-6-8-5) cation intermediates. Furthermore, the mechanism is distinct from that of a separate class of di/sesterterpenes including fusicoccins and ophiobolins. The difference between mechanisms is consistent with phylogenetic analysis of bifunctional terpene synthases, suggesting that the amino acid sequence reflects the initial cyclization mode, which is most likely related to the initial conformation of a linear prenyl diphosphate.