Studies on new bioactive non-ribosomal peptides and polyketides from marine-derived Streptomyces spp.

Abstract

During my doctoral course, my research works were concentrated to secondary metabolites derived from marine-bacteria, which have been regarded as a prolific sources for drug discovery. Especially, my targeted compounds were divided into non-ribosomal peptides (Part A) and polyketides (Part B) from marine bacteria.

Part A. Non-ribosomal peptides from marine Streptomyces spp.

Nature creates a vast diversity of secondary metabolites by assembling acetate building blocks to polyketides using polyketide synthases (PKSs), by assembling amino acids to peptides using non-ribosomal peptides synthases (NRPSs). Peptide natural products are generally biosynthesized through the NRPSs with amino acids less than 40, showing characteristic UV spectra originated from aromatic amino acids and large molecular ions. Peptide natural products are structurally and biologically diverse due to incorporation of regular and non-proteinogenic amino acids As part of my efforts to search for new bioactive peptides, actinomycete strains were selectively isolated from uncommon marine environments, intertidal mudflat. The application of a chemical analysis-based discovery strategy using LC/MS chemical profiles allowed us to discover marine Streptomyces sp., producing three kinds of structurally novel peptides. Mohangamides A and B, new dilactone-tethered pseudo-dimeric peptides, were discovered from an intertidal mud flat in Buan, Republic of Korea. WS9326F-I, which are new members of WS9326 class, were also obtained from strain SNM55. Additional variation of cultural condition resulted in the discovery of new antibiotic cyclic depsipeptides, hormaomycins B and C. The planar structures of those compounds were determined by 1D and 2D NMR spectroscopy, UV spectroscopy, and mass spectrometry. Mohangamides A and B are first dimeric/pseudo-dimeric peptides of WS9326s class bearing two unusual acyl chains and 14 amino acid residues. WS9326F-I were elucidated as new analogues of WS9326A bearing an unusual acyl chain and 6~7 amino units. Especially, WS9326I incorporates an unprecedented 4-amino-2,4-dihydro-3H-pyarzol-3-one and a sugar-derived moiety. Hormaomycins B and C are the first natural analogues belonging to the hormaomycin peptide class bearing structurally unique units, such as 4-(Z)-propenylproline, 3-(2-nitrocyclopropyl)alanine, 5-chloro-1-hydroxypyrrol-2-carboxylic acid, and -methylphenylalanine. Determination of absolute configurations of amino acids were established by derivatization of Marfeys reagents and 2,3,4,6-tetra-O-acetyl-β-d-glucopyranosyl isothiocyanatederivatiztion (GITC), followed by LC-MS analysis. The absolute configuration of additional chiral centers of mohangamide A were determined by chemical derivatizations including methanolysis, acetylation, OsO4 oxidation, and bis-benzoyl esterification followed by chromatographic and spectroscopic analyses. The absolute configuration of WS9326I were established by four of chemical steps including acetonide protection, acetylation, acetonide deprotection, and MTPA esterifications. The absolute configurations of hormaomycins B and C were determined by the comparison of ECD spectrum of hormaomycin. Mohangamide A displayed displayed strong inhibitory activity against Candida albicans isocitrate lyase. WS9326F-I exhibited remarkable antiangiogenic activity. Hormaomycins B and C showed significant antibacterial activity.

Part B. Polyketides from marine Streptomyces spp.

Polyketides are any of a large class of diverse compounds that are characterized by more than two carbonyl groups connected by a intervening carbon atoms and generally biosynthesized by PKSs. Especially, NRPS-PKSs hybrid synthase are mega-synthases involved in the biosynthesis of pharmaceutically important natural products such as cyclosporin, rifamycin and erythromycin. Focusing on NRPS and PKS compounds provides better understanding on their biosynthetic mechanisms, contributing massive production and diverse derivatives by combinatorial biosynthesis. As part of my efforts to search for PKS compounds, I collected sediments from uninvestigated environments such as mudflat and volcanic Islands. Mohangic acids were isolated from a Streptomyces sp. SNM31 collected from mud flat in Buan, Korea. Comprehensive analysis of spectroscopic data revealed that mohangic acids A-E are new members of p-aminoacetophenonic acid class. The relative and absolute configurations of mohangic acids were established by chemical derivatization, application of chiral NMR solvent, and circular dichroism spectroscopy. Mohangic acid E displayed significant quinone reductase induction activity. Further investigation of Streptomyces sp. strain isolated from Jeju Island discovered new polyene polyols compounds, succinilenes A-D. Determination of geometry of succinilenes were performed by quantum-mechanics-driven 1H iterative full spin analysis (QM-HiFSA). The absolute configuration of succinilenes were established by chemical derivatizations and chiroptical spectroscopy. Succinilenes showed significant anti-inflammatory activity except succinilene D. Ullengacenes A and B, new benz[a]anthracene dimer compounds linked by mono-sulfide bond, were isolated from sediment in Ul-leungdo, Korea. The exact components of compounds were identified by energy dispersive X-ray experiments. The absolute configuration of ullengacenes were determined by combination of ECD calculations and DP4 calculations. Ullengacene A exhibited quinone reductase induction activity while ullengacene B displayed remarkable antiangiogenic activity.