Exploring Sources, Biological Functions, and Potential Applications of the Ubiquitous Marine Cyclic Dipeptide: A Concise Review of Cyclic Glycine-Proline

Cyclic glycine-proline (cGP), a prevalent marine cyclic dipeptide, possesses a distinct pyrrolidine-2,5-dione scaffold, which contributes to the chemical diversity and broad bioactivities of cGP. The diverse sources from marine-related, endogenous biological, and synthetic pathways and the in vitro and in vivo activities of cGP are reviewed. The potential applications for cGP are also explored. In particular, the pivotal roles of cGP in regulating insulin-like growth factor-1 homeostasis, enhancing neuroprotective effects, and improving neurotrophic function in central nervous system diseases are described. The potential roles of this endogenous cyclic peptide in drug development and healthcare initiatives are also highlighted. This review underscores the significance of cGP as a fundamental building block in drug discovery with exceptional drug-like properties and safety. By elucidating the considerable value of cGP, this review aims to reignite interest in cGP-related research within marine medicinal chemistry and synthetic biology.

A benzaldehyde derivative obtained from Hypoxylon truncatum NBRC 32353 treated with hygromycin B

The ribosome-targeted antifungal agent hygromycin B (HygB) alters the secondary metabolite profiles of fungi. Hypoxylon truncatum NBRC 32353 fermented in the presence of hygromycin B in barley medium activated secondary metabolite synthesis. A new benzaldehyde derivative truncaaldehyde (1) was obtained, along with thirteen known compounds (2-14). The structures of the new compounds were revealed using NMR and single-crystal X-ray crystallography. The total synthesis of (±)-1 was achieved using a four-step sequence, and chiral separation was accomplished. The isolated compounds were tested for their monoamine oxidase (MAO) -A and -B inhibitory activities, with six compounds ((±)-1, 4, 5, 7, 8, and 10) showing inhibitory activity.

New Chlorinated 2,5-Diketopiperazines from Marine-Derived Bacteria Isolated from Sediments of the Eastern Mediterranean Sea

From the organic extracts of five bacterial strains isolated from marine sediments collected in the East Mediterranean Sea, three new (15, 16, 31) and twenty-nine previously reported (1–14, 17–30, 32) metabolites bearing the 2,5-diketopiperazine skeleton were isolated. The structures of the chlorinated compounds 15, 16, and 31 were elucidated by extensive analysis of their spectroscopic data (NMR, MS, UV, IR). Compounds 15 and 16 were evaluated for their antifungal activity against Candida albicans and Aspergillus niger but were proven inactive. The relevant literature is supplemented with complete NMR assignments and revisions for the 29 previously reported compounds.

Microindolinone A, a Novel 4,5,6,7-Tetrahydroindole, from the Deep-Sea-Derived Actinomycete Microbacterium sp. MCCC 1A11207

A novel indole, microindolinone A (1), was isolated from a deep-sea-derived actinomycete Microbacterium sp. MCCC 1A11207, together with 18 known compounds (2-19). By detailed analysis of the ¹H, ¹³C, HSQC, COSY, HMBC, high resolution electron spray ionization mass spectrum (HRESIMS), and circular dichroism (CD) data, the absolute configuration of 1 was elucidated as 5R-hydroxy-4,5,6,7-tetrahydroindole-4-one. It is noteworthy that 1 is the second example of a saturated indole isolated from nature.

A subdomain swap strategy for reengineering nonribosomal peptides

Nonribosomal peptide synthetases (NRPSs) protect microorganisms from environmental threats by producing diverse siderophores, antibiotics, and other peptide natural products. Their modular molecular structure is also attractive from the standpoint of biosynthetic engineering. Here we evaluate a methodology for swapping module specificities of these mega-enzymes that takes advantage of flavodoxin-like subdomains involved in substrate recognition. Nine subdomains encoding diverse specificities were transplanted into the Phe-specific GrsA initiation module of gramicidin S synthetase. All chimeras could be purified as soluble protein. One construct based on a Val-specific subdomain showed sizable adenylation activity and functioned as a Val-Pro diketopiperazine synthetase upon addition of the proline-specific GrsB1 module. These results suggest that subdomain swapping could be a viable alternative to previous NRPS design approaches targeting binding pockets, domains, or entire modules. The short length of the swapped sequence stretch may facilitate straightforward exploitation of the wealth of existing NRPS modules for combinatorial biosynthesis.

Characterisation and determination of metabolites formed by microbial and enzymatic degradation of ergot alkaloids

Ergot alkaloids are frequent contaminants of cereal crops. Strategies for their inactivation include the use of microorganisms or enzymes as feed additives capable of degrading ergot alkaloids. Recently, an ergopeptine-degrading Rhodococcus erythropolis strain MTHt3 (DSM 25948) has been isolated from soil and the involved enzymes ErgA and ErgB have been identified. The aim of the current study was to characterise the metabolites formed by degradation of various ergopeptines with the MTHt3 strain, its lysate and the purified enzyme ErgA. Using preparative HPLC, 1H-, 13C- and 2D-NMR as well as HR-MS measurements, two main groups of metabolites formed during microbial and enzymatic degradation of ergopeptines were identified: diketopiperazines (cyclic dipeptides, DKPs) and unstable ergine hydroxy carboxylic acids. However, degradation by strain, lysate and enzyme yielded different end-products. Whereas DKPs were transient and lysergic acid the only final product upon incubation of ergopeptines with the R. erythropolis strain, incubation with the lysate resulted in formation of lysergic acid and two isomeric DKPs at different ratios. Enzymatic degradation by ErgA yielded only one DKP isomer and ended at the stage of the ergine hydroxy carboxylic acids which then spontaneously degraded to ergine. In conclusion, we succeeded in identification of metabolites formed by microbial and enzymatic degradation of ergot alkaloids which is a crucial step in the future development of feed additives for gastro intestinal detoxification of ergopeptines in farm animals.

Nigrosphaerin A a new isochromene derivative from the endophytic fungus Nigrospora sphaerica

Nigrosphaerin A, a new isochromene derivative (1), was isolated from the endophytic fungus Nigrospora sphaerica and chemically identified as 3-(3,4-dihydroxyphenyl)-4,6,8-trihydroxy-1H-isochromen-1-one-6-O-β-d-glucopyranoside. In addition nineteen known compounds (2-20) were isolated from the same fungus and chemically identified. Compounds (1-3, 5, and 7-16) were isolated for the first time from this fungus. In vitro antileukemic, antileishmanial, antifungal, antibacterial and antimalarial activities of (1-20) were examined. Compounds 5, 7, 9 and 10 showed good antileukemic activity against HL60 cells with IC₅₀ values of 0.03, 0.39, 0.2 and 0.4 μg/mL, respectively and against K562 cells with IC₅₀ values of 0.35, 0.35, 0.49 and 0.01 μg/mL, respectively. Compounds 3, 4 and 6 showed moderate antileishmanial activity with IC₅₀ values of 30.2, 26.4 and 36.4 μg/ml, respectively. Compound 7 showed moderate antifungal activity against Cryptococcus neoformans with IC₅₀ value of 14.8 μg/mL.

Minor constituents from the tubers of Gymnadenia conopsea

Four new minor constituents including two cyclodipeptides (1 and 2) and two cyclopentene derivatives (3 and 4), together with four known cyclodipeptides, have been isolated from an ethanolic extract of the tubers of Gymnadenia conopsea. Their structures including absolute configurations were determined by spectroscopic data interpretation combined with chemical methods. Among them, compound 1 contains an abnormal S-(4''-hydroxybenzyl)cysteine residue, 3 and 4 possess [(4-methylcyclopentyl)methyl]benzene and (4-hydroxymethylcyclopentyl)benzene carbon skeletons, respectively, both of which are first found from the natural source.

(3R,8aS)-3-Ethyl-perhydro-pyrrolo[1,2-a]pyrazine-1,4-dione

In the title compound, C(9)H(14)N(2)O(2), the pyrrolidine and piperazine rings adopt envelope and boat conformations, respectively. The chiral centers were assigned on the basis of the known stereogenic center of an enanti-omerically pure starting material and the trans relationship between the H atoms attached to these centers. The crystal packing is stabilized by an inter-molecular hydrogen bond between the N-H group and a carbonyl O atom of the diketopiperazine group, forming zigzag C(5) chains along [010].

GPR12 selections of the metabolites from an endophytic Streptomyces sp. associated with Cistanches deserticola

An endophytic Streptomyces sp. (AC-2) was isolated from the root of Cistanches deserticola Y.C.Ma.. Chemical investigations of the culture broth of AC-2 afforded fifteen compounds including K1115 A (1), tyrosol (2), phenylethylamine derivatives (3, 4), cyclic dipeptides (5-8), nucleosides and their aglycones (9-13), N-acetyltryptamine (14), and pyrrole-2-carboxylic acid (15). Only tyrosol can promote an increase of intracellular cAMP special on GPR12 transfected cells, such as CHO and HEK293, which means it may be a possible ligand for GPR12.