Two novel diterpenoids from Isodon rubescens var. lushanensis

Two novel diterpenoids, luanchunins A (1) and B (2), along with their precursor, kamebakaurin (3), had been isolated from the stems and leaves of Isodon rubescens var. lushanensis. Their structures were elucidated on the basis of extensive spectroscopic analyses. Compounds 1 and 2 showed potent cytotoxic activity against HL-60 with IC₅₀ values of 4.81 μM and 3.52 μM, respectively. Plausible pathways for the biosynthesis of 1 and 2 were also postulated.     

The biosynthesis of aureothin

Feeding experiments with specifically labelled precursors show that the nitroaromatic, C-6-C-1 unit of the antibiotic aureothin (9) biologically derives by degradation of the C-6-C-3, phenylpropanoid precursor D,L-p-aminophenylalanine (1) through hydroxylation β to the nitrogen to erythro and threop-aminophenylserine (3 and 4). During the biosynthesis there is the loss of the hydrogen originally present in benzylic position in the phenylpropanoid precursor, and, further, the oxidation of the p-amino group to p-nitro takes place very late in the sequence.     

Stereo-controlled synthesis of prelasalocid, a key precursor proposed in the biosynthesis of polyether antibiotic lasalocid A

In the biosynthesis of a polyether ionophore antibiotic, lasalocid A, the cyclic ether skeleton composed of a tetrahydrofuran linked to a tetrahydropyran could be constructed by oxidative cyclization of linear dodecaketide diene precursor. Hence, we hypothesized a prelasalocid having (E,E)-trisubstituted olefins as the dodecaketide biosynthetic precursor. A stereo-controlled synthetic route to the prelasalocid has been devised in a highly convergent manner entailing installation of a variety of substituents at the trisubstituted olefins.     

Importance of the isopropylidene terminal of geranylgeranyl group for the formation of tetraether lipid in methanogenic archaea

Labeling experiments using several deuterated lipids were pursued to study the biosynthesis of macrocyclic isoprenoidal lipids of thermophilic methanogenic archaea, Methanothermobacter thermautotrophicus. The isopropylidene terminal of geranylgeranyl group of monomeric precursor appeared to be important for the CC bond formation at the hydrophobic end in the macrocyclic lipids. A mechanism involving a radical trigger at the allylic methyl group is proposed for this CC bond formation.     

Precursor directed biosynthesis of an orthogonally functional erythromycin analogue: selectivity in the ribosome macrolide binding pocket

The macrolide antibiotic erythromycin A and its semisynthetic analogues have been among the most useful antibacterial agents for the treatment of infectious diseases. Using a recently developed chemical genetic strategy for precursor-directed biosynthesis and colony bioassay of 6-deoxyerythromycin D analogues, we identified a new class of alkynyl- and alkenyl-substituted macrolides with activities comparable to that of the natural product. Further analysis revealed a marked and unexpected dependence of antibiotic activity on the size and degree of unsaturation of the precursor. Based on these leads, we also report the precursor-directed biosynthesis of 15-propargyl erythromycin A, a novel antibiotic that not only is as potent as erythromycin A with respect to its ability to inhibit bacterial growth and cell-free ribosomal protein biosynthesis but also harbors an orthogonal functional group that is capable of facile chemical modification.     

Formal synthesis of 4-diphosphocytidyl-2-C-methyl d-erythritol from d-(+)-arabitol

2-C-Methyl-d-erythritol-4-phosphate (MEP) is a key chemical intermediate of the non-mevalonate pathway for isoprenoid biosynthesis employed by many pathogenic microbes. MEP is also the precursor for the synthesis of 4-diphosphocytidyl-2-C-methyl d-erythritol (CDP-ME), another key intermediate of the non-mevalonate pathway. As this pathway is non-existent in higher animals, including humans, it represents great opportunities for novel antimicrobial development. To facilitate the in-depth studies of this pathway, we reported here a formal synthesis of CDP-ME through a new synthesis of 2-C-methyl-d-erythritol-4-phosphoric acid from d-(+)-arabitol.     

(−)-Wistarin from the marine sponge Ircinia sp.: the first case of enantiomeric sesterterpenes

The sestertepene (−)-wistarin has been isolated from a Red Sea sponge of the genus Ircinia. This metabolite is the enantiomer of (+)-wistarin, a tetracyclic metabolite previously isolated from the sponge Ircinia wistarii. The enantiomeric relationship was proved by comparison of the CD spectrum of (−)-wistarin with that of a synthetic sample of the (+)-isomer. A hypothetical biosynthesis of (−)-wistarin is discussed starting from a putative linear precursor.     

Synthesis of a 2-indolylphosphonamide derivative with inhibitory activity against yersiniabactin biosynthesis

We report the synthesis of an adenosyl-derived indolylphosphonamide analogue of salicyladenosylmonophosphate involved in the plague and tuberculosis siderophore biosyntheses. The compound proved to be a potent inhibitor of the Yersinia pestis salicylate adenylation domain YbtE catalyzing the initial step of yersiniabactin biosynthesis.     

Biosynthesis of the Apoptolidins in Nocardiopsis sp. FU 40

The apoptolidins are 20/21-membered macrolides produced by Nocardiopsis sp. FU40. Several members of this family are potent and remarkably selective inducers of apoptosis in cancer cell lines, likely via a distinct mitochondria associated target. To investigate the biosynthesis of this natural product, the complete genome of the apoptolidin producer Nocardiopsis sp. FU40 was sequenced and a 116 kb region was identified containing a putative apoptolidin biosynthetic gene cluster. The apoptolidin gene cluster comprises a type I polyketide synthase, with 13 homologating modules, apparently initiated in an unprecedented fashion via transfer from a methoxymalonyl-acyl carrier protein loading module. Spanning approximately 39 open reading frames, the gene cluster was cloned into a series of overlapping cosmids and functionally validated by targeted gene disruption experiments in the producing organism. Disruption of putative PKS and P₄₅₀ genes delineated the roles of these genes in apoptolidin biosynthesis and chemical complementation studies demonstrated intact biosynthesis peripheral to the disrupted genes. This work provides insight into details of the biosynthesis of this biologically significant natural product and provides a basis for future mutasynthetic methods for the generation of non-natural apoptolidins.     

Dimroth-type rearrangement of 1-benzyl- and 1-glycosyl-5-aminoimidazoles to 4-(N-substituted amino)imidazoles

An efficient route is described to an unusual exocyclic 4-β-d-ribofuranosyl-aminoimidazole nucleoside, related 4-N-benzylaminoimidazoles and imidazole analogues of precursors in the de novo biosynthesis of purines, via a regiospecific and stereoselective base-catalysed Dimroth-type rearrangement of 1-ribofuranosyl and 1-benzyl-5-aminoimidazoles. Use of a ¹⁵N labelled precursor showed the unequivocal endo- to exocyclic translocation of the nitrogen atom during the rearrangement.     

Antifungal Activities of cis-trans Citral Isomers against Trichophyton rubrum with ERG6 as a Potential Target

Trichophyton rubrum causes ringworm worldwide. Citral (CIT), extracted from Pectis plants, is a monoterpene and naturally composed of geometric isomers neral (cis-citral) and geranial (trans-citral). CIT has promising antifungal activities and ergosterol biosynthesis inhibition effects against several pathogenic fungi. However, no study has focused on neral and geranial against T. rubrum, which hinders the clinical application of CIT. This study aimed to compare antifungal activities of neral and geranial and preliminarily elucidate their ergosterol biosynthesis inhibition mechanism against T. rubrum. Herein, the disc diffusion assays, cellular leakage measurement, flow cytometry, SEM/TEM observation, sterol quantification, and sterol pattern change analyses were employed. The results showed geranial exhibited larger inhibition zones (p < 0.01 or 0.05), higher cellular leakage rates (p < 0.01), increased conidia with damaged membranes (p < 0.01) within 24 h, more distinct shriveled mycelium in SEM, prominent cellular material leakage, membrane damage, and morphological changes in TEM. Furthermore, geranial possessed more promising ergosterol biosynthesis inhibition effects than neral, and both induced the synthesis of 7-Dehydrodesmosterol and Cholesta-5,7,22,24-tetraen-3β-ol, which represented marker sterols when ERG6 was affected. These results suggest geranial is more potent than neral against T. rubrum, and both inhibit ergosterol biosynthesis by affecting ERG6.     

Influence of functional moiety in lupane-type triterpenoids in BACE1 inhibition

Lupane-type triterpenoids have shown a potential effect against neurodegenerative disorders. Alzheimer’s disease, one of the common neurodegenerative disease, is evident by the accumulation of amyloid-beta (Aβ) plaque in the extracellular regions of the brain. β-site amyloid precursor protein cleaving enzyme 1 (BACE1) is a key enzyme for the Aβ formation viathe cleavage of amyloid precursor protein (APP). Therefore, to find the potent BACE1 inhibitors and furthermore to explore the role of the functional group responsible for the strong BACE1 inhibitory activity, we synthesized a series of triterpenoids with lupane skeleton starting from the natural compounds calenduladiol and lupeol. Compound 1 revealed a potent competitive BACE1 inhibitory activity (IC₅₀ = 16.77 ± 1.16 μM; K_i = 19.38). Furthermore, the molecular docking simulation revealed the importance of Tyr198 residue along with the other hydrophobic interactions for the strong affinity of 1‒BACE1 complex. To sum up, our results demonstrated the importance of carbonyl moiety at 3 and 16 position of lupane-type triterpenoid over the hydroxyl group at the same position.     

Structural Insights on Mycobacterium tuberculosis Thiazole Synthase—A Molecular Dynamics/Docking Approach

Tuberculosis (TB), an epidemic disease, affects the world with death rate of two million people every year. The bacterium Mycobacterium tuberculosis was found to be a more potent and disease-prolonged bacterium among the world due to multi-drug resistance. Emergence of new drug targets is needed to overcome the bacterial resistance that leads to control epidemic tuberculosis. The pathway thiamine biosynthesis was targeting M. tuberculosis due to its role in intracellular growth of the bacterium. The screening of enzymes involved in thiamin biosynthesis showed novel target thiazole synthase (ThiG) involved in catalysis of rearrangement of 1-deoxy-d-xylulose 5-phosphate (DXP) to produce the thiazole phosphate moiety of thiamine. We carried out homology modeling for ThiG to understand the structure–function relationship, and the model was refined with MD simulations. The results showed that the model predicted with (α?+?β)₈-fold of synthase family proteins. Molecular docking of ThiG model with substrate DXP showed binding mode and key residues ARG46, ASN69, THR41, and LYS96 involved in the catalysis. First-line anti-tuberculosis drugs were docked with ThiG to identify the inhibition. The report showed the anti-tuberculosis drugs interact well with ThiG which may lead to block thiamin biosynthesis pathway.     

Synthesis of the cyclic nonapeptide of chlorofusin using a convergent [3+3+3]-fragment coupling strategy

A [3+3+3]-fragment coupling strategy was successfully applied in the synthesis of the nonacyclopeptide of chlorofusin, a potent natural antagonist against p53-MDM2 interactions. The accomplished convergent synthesis includes parallel syntheses of three tripeptides and their sequential assembly, and macrocyclization of the linear precursor to the required 27-membered nonacyclopeptide.     

Total synthesis of geralcin A,a representative of a new family of hydrazine-containing natural products

The first total synthesis of geralcin A (2), a naturally occurring α,β-unsaturated γ-lactonohydrazide produced by Streptomyces sp. LMA-545, was achieved according to the hypothetical biosynthesis we have previously published. MH-031 (1), the plausible natural precursor isolated together with geralcin A, was synthesized in 6 steps and in a 25% overall yield. Condensation of glycolic acid with key intermediate Boc-hexylhydrazine afforded the desired geralcin A in 46% overall yield. Spectroscopic data of the synthetic compound were identical to those recorded for the natural one.     

Asymmetrized tris(hydroxymethyl)methane as precursor of iminosugars: application to the synthesis of isofagomine

A new synthetic application of asymmetrized tris(hydroxymethyl)methane (THYM*), readily obtained in both enantiomeric forms through a chemoenzymatic procedure, is reported. In this case THYM* precursor 3 was elaborated by ring closing metathesis into some enantiopure branched tetrahydropyridines, that have been used as precursors of the potent glycosidase inhibitor isofagomine28.     

Effect of Cofactor Folate on the Growth of Corynebacterium glutamicum SYPS-062 and l-Serine Accumulation

The direct fermentative production of l-serine from sugar has attracted increasing attention. Corynebacterium glutamicum SYPS-062 can directly convert sugar to l-serine. In this study, the effects of exogenous and endogenous regulation of cofactor folate on C. glutamicum SYPS-062 growth and l-serine accumulation were investigated. For exogenous regulation, the inhibitor (sulfamethoxazole) or precursor (p-aminobenzoate) of folate biosynthesis was added to the medium, respectively. For endogenous regulation, the gene (pabAB) that encodes the key enzyme of folate biosynthesis was knocked out or overexpressed to obtain the recombinant C. glutamicum SYPS-062 ΔpabAB and SYPS-062(pJC-tac-pabAB), respectively. The results indicated that decreased levels of cofactor folate supported l-serine accumulation, whereas increased levels of cofactor folate aided in cell growth of C. glutamicum SYPS-062. Thus, this study not only elucidated the role of folate in C. glutamicum SYPS-062 growth and l-serine accumulation, but also provided a novel and convenient approach to regulate folate biosynthesis in C. glutamicum.     

Identification of a Prenyl Chalcone as a Competitive Lipoxygenase Inhibitor: Screening,Biochemical Evaluation and Molecular Modeling Studies

Cyclooxygenase (COX) and lipoxygenase (LOX) are key targets for the development of new anti-inflammatory agents. LOX, which is involved in the biosynthesis of mediators in inflammation and allergic reactions, was selected for a biochemical screening campaign to identify LOX inhibitors by employing the main natural product library of Brazilian biodiversity. Two prenyl chalcones were identified as potent inhibitors of LOX-1 in the screening. The most active compound, (E)-2-O-farnesyl chalcone, decreased the rate of oxygen consumption to an extent similar to that of the positive control, nordihydroguaiaretic acid. Additionally, studies on the mechanism of the action indicated that (E)-2-O-farnesyl chalcone is a competitive LOX-1 inhibitor. Molecular modeling studies indicated the importance of the prenyl moieties for the binding of the inhibitors to the LOX binding site, which is related to their pharmacological properties.