Natural Products That Contain Higher Homologated Amino Acids

This review focuses on discussing natural products (NPs) that contain higher homologated amino acids (homoAAs) in the structure as well as the proposed and characterized biosynthesis of these non-proteinogenic amino acids. Homologation of amino acids includes the insertion of a methylene group into its side chain. It is not a very common modification found in NP biosynthesis as approximately 450 homoAA-containing NPs have been isolated from four bacterial phyla (Cyanobacteria, Actinomycetota, Myxococcota, and Pseudomonadota), two fungal phyla (Ascomycota and Basidiomycota), and one animal phylum (Porifera), except for a few examples. Amino acids that are found to be homologated and incorporated in the NP structures include the following ten amino acids: alanine, arginine, cysteine, isoleucine, glutamic acid, leucine, phenylalanine, proline, serine, and tyrosine, where isoleucine, leucine, phenylalanine, and tyrosine share the comparable enzymatic pathway. Other amino acids have their individual homologation pathway (arginine, proline, and glutamic acid for bacteria), likely utilize the primary metabolic pathway (alanine and glutamic acid for fungi), or have not been reported (cysteine and serine). Despite its possible high potential in the drug discovery field, the biosynthesis of homologated amino acids has a large room to explore for future combinatorial biosynthesis and metabolic engineering purpose.

Convenient route to Fmoc-homotyrosine via metallaphotoredox catalysis and its use in the total synthesis of anabaenopeptin cyclic peptides

Herein, we report the first solid-phase total synthesis of the natural cyclic peptide anabaenopeptin F and the use of metallaphotoredox catalysis to overcome the key challenges associated with the preparation of the non-proteinogenic amino acid homotyrosine contained in these peptides. Starting from L-homoserine, enantiopure Fmoc-protected homotyrosine was prepared in a straightforward manner by metallaphotoredox catalysis with N-Fmoc-(S)-2-amino-4-bromobutanoic acid and 4-tert-butoxybromobenzene partners. The prepared protected amino acid was used in solid-phase peptide synthesis to achieve the total synthesis of anabaenopeptin F and establish the stereochemistry of the isoleucine residue. Protease inhibition studies with the synthesized anabaenopeptin F showed inhibitory activities against carboxypeptidase B in the low nanomolar range. The high convergency of the synthetic methodologies paves the way for the rapid access to N-Fmoc-protected non-proteinogenic and unnatural amino acids and the total synthesis of complex bioactive peptides containing these amino acids.

Structural Diversity and Biological Activity of Cyanopeptolins Produced by Nostoc edaphicum CCNP1411

Cyanopeptolins (CPs) are one of the most commonly occurring class of cyanobacterial nonribosomal peptides. For the majority of these compounds, protease inhibition has been reported. In the current work, the structural diversity of cyanopeptolins produced by Nostoc edaphicum CCNP1411 was explored. As a result, 93 CPs, including 79 new variants, were detected and structurally characterized based on their mass fragmentation spectra. CPs isolated in higher amounts were additionally characterized by NMR. To the best of our knowledge, this is the highest number of cyanopeptides found in one strain. The biological assays performed with the 34 isolated CPs confirmed the significance of the amino acid located between Thr and the unique 3-amino-6-hydroxy-2-piperidone (Ahp) on the activity of the compounds against serine protease and HeLa cancer cells.

Hydantoanabaenopeptins from Lake Kinneret Microcystis Bloom, Isolation, and Structure Elucidation of the Possible Intermediates in the Anabaenopeptins Biosynthesis

Anabaenopeptins are common metabolites of cyanobacteria. In the course of reisolation of the known aeruginosins KT608A and KT608B for bioassay studies, we noticed the presence of some unknown anabaenopeptins in the extract of a Microcystis cell mass collected during the 2016 spring bloom event in Lake Kinneret, Israel. The ¹H NMR spectra of some of these compounds presented a significant difference in the appearance of the ureido bridge protons, and their molecular masses did not match any one of the 152 known anabaenopeptins. Analyses of the 1D and 2D NMR, HRMS, and MS/MS spectra of the new compounds revealed their structures as the hydantoin derivatives of anabaenopeptins A, B, F, and ¹[Dht]-anabaenopeptin A and oscillamide Y (1, 2, 3, 6, and 4, respectively) and a new anabaenopeptin, ¹[Dht]-anabaenopeptin A (5). The known anabaenopeptins A, B, and F and oscillamide Y (7, 8, 9, and 10, respectively) were present in the extract as well. We propose that 1-4 and 6 are the possible missing intermediates in the previously proposed partial biosynthesis route to the anabaenopeptins. Compounds 1-6 were tested for inhibition of the serine proteases trypsin and chymotrypsin and found inactive at a final concentration of ca. 54 μM.

Changes in the morphology and cell ultrastructure of a microalgal community exposed to a commercial glyphosate formulation and a toxigenic cyanobacterium

Human activities significantly influence the health of aquatic ecosystems because many noxious chemical wastes are discharged into freshwater bodies. Intensive agriculture contributes to the deterioration by providing indirectly fertilizers, pesticides, and other agrochemicals that affect the aquatic biota. Glyphosate is one of the most used herbicides worldwide, and microalgae are particularly sensitive to its formulation, inducing displacement of some green microalgae from the phytoplankton that leads to alterations in the floristic composition, which fosters the abundance of cyanobacteria, some of which can be toxigenic. The combination of chemical stressors such as glyphosate and biological ones, like cyanotoxins and other secondary metabolites of cyanobacteria, could induce a combined effect potentially more noxious to microalgae, affecting not only their growth but also their physiology and morphology. In this study, we evaluated the combined effect of glyphosate (Faena^®) and a toxigenic cyanobacterium on the morphology and ultrastructure of microalgae in an experimental phytoplankton community. For this purpose, Microcystis aeruginosa (a cosmopolitan cyanobacterium that forms harmful blooms) and the microalgae Ankistrodesmus falcatus, Chlorella vulgaris, Pseudokirchneriella subcapitata, and Scenedesmus incrassatulus were cultivated, individually and jointly, exposing them to sub-inhibitory concentrations of glyphosate (IC₁₀, IC_20, and IC₄₀). Effects were evaluated through scanning electron (SEM) and transmission electron (TEM) microscopy. Exposure to Faena^® produced alterations in the external morphology and ultrastructure of microalgae both individually and in combined cultures. SEM evidenced the loss of the typical shape and integrity of the cell wall and an increase in the biovolume. TEM revealed reduction and disorganization of the chloroplast, variation in starch and polyphosphate granules, formation of vesicles and vacuoles, cytoplasm degradation, and cell wall continuity loss. The presence of M. aeruginosa was, for microalgae, an additional stress factor adding to the chemical stress produced by Faena^®, increasing the damage in their morphology and ultrastructure. These results alert to the effects that can be caused by glyphosate and the presence of toxigenic bacteria on the algal phytoplankton in contaminated and anthropic and eutrophic freshwater ecosystems.

Anabaenopeptins from Nostoc edaphicum CCNP1411

Cyanobacteria of the Nostoc genus belong to the most prolific sources of bioactive metabolites. In our previous study on Nostoc edaphicum strain CCNP1411, the occurrence of cyanopeptolins and nostocyclopeptides was documented. In the current work, the production of anabaenopeptins (APs) by the strain was studied using genetic and chemical methods. Compatibility between the analysis of the apt gene cluster and the structure of the identified APs was found. Three of the APs, including two new variants, were isolated as pure compounds and tested against four serine proteases and carboxypeptidase A (CPA). The in vitro enzymatic assays showed a typical activity of this class of cyanopeptides, i.e., the most pronounced effects were observed in the case of CPA. The activity of the detected compounds against important metabolic enzymes confirms the pharmaceutical potential of anabaenopeptins.

Untargeted Identification of Alkyne-Containing Natural Products Using Ruthenium-Catalyzed Azide Alkyne Cycloaddition Reactions Coupled to LC-MS/MS

Alkyne-containing natural products have been identified from plants, insects, algae, fungi, and bacteria. This class of natural products has been characterized as having a variety of biological activities. Polyynes are a subclass of acetylenic natural products that contain conjugated alkynes and are underrepresented in natural product databases due to the fact that they decompose during purification. Here we report a workflow that utilizes alkyne azide cycloaddition (AAC) reactions followed by LC-MS/MS analysis to identify acetylenic natural products. In this report, we demonstrate that alkyne azide cycloaddition reactions with p-bromobenzyl azide result in p-bromobenzyl-substituted triazole products that fragment to a common brominated tropylium ion. We were able to identify a synthetic alkyne spiked into the extract of Anabaena sp. PCC 7120 at a concentration of 10 μg/mL after optimization of MS/MS conditions. We then successfully identified the known natural product fischerellin A in the extract of Fischerella muscicola PCC 9339. Lastly, we identified the recently identified natural products protegenins A and C from Pseudomonas protegens Pf-5 through a combination of genome mining and RuAAC reactions. This is the first report of RuAAC reactions to detect acetylenic natural products. We also compare CuAAC and RuAAC reactions and find that CuAAC reactions produce fewer byproducts compared to RuAAC but is limited to terminal-alkyne-containing compounds. In contrast, RuAAC is capable of identification of both terminal and internal acetylenic natural products, but byproducts need to be eliminated from analysis by creation of an exclusion list. We believe that both CuAAC and RuAAC reactions coupled to LC-MS/MS represent a method for the untargeted identification of acetylenic natural products, but each method has strengths and weaknesses.

Anabaenopeptins from Cyanobacteria in Freshwater Bodies of Greece

Cyanobacteria are photosynthetic microorganisms that are able to produce a large number of secondary metabolites. In freshwaters, under favorable conditions, they can rapidly multiply, forming blooms, and can release their toxic/bioactive metabolites in water. Among them, anabaenopeptins (APs) are a less studied class of cyclic bioactive cyanopeptides. The occurrence and structural variety of APs in cyanobacterial blooms and cultured strains from Greek freshwaters were investigated. Cyanobacterial extracts were analyzed with LC-qTRAP MS/MS using information-dependent acquisition in enhanced ion product mode in order to obtain the fragmentation mass spectra of APs. Thirteen APs were detected, and their possible structures were annotated based on the elucidation of fragmentation spectra, including three novel ones. APs were present in the majority of bloom samples (91%) collected from nine Greek lakes during different time periods. A large variety of APs was observed, with up to eight congeners co-occurring in the same sample. AP F (87%), Oscillamide Y (87%) and AP B (65%) were the most frequently detected congeners. Thirty cyanobacterial strain cultures were also analyzed. APs were only detected in one strain (Microcystis ichtyoblabe). The results contribute to a better understanding of APs produced by freshwater cyanobacteria and expand the range of structurally characterized APs.

Anabaenopeptins: What We Know So Far

Cyanobacteria are microorganisms with photosynthetic mechanisms capable of colonizing several distinct environments worldwide. They can produce a vast spectrum of bioactive compounds with different properties, resulting in an improved adaptative capacity. Their richness in secondary metabolites is related to their unique and diverse metabolic apparatus, such as Non-Ribosomal Peptide Synthetases (NRPSs). One important class of peptides produced by the non-ribosomal pathway is anabaenopeptins. These cyclic hexapeptides demonstrated inhibitory activity towards phosphatases and proteases, which could be related to their toxicity and adaptiveness against zooplankters and crustaceans. Thus, this review aims to identify key features related to anabaenopeptins, including the diversity of their structure, occurrence, the biosynthetic steps for their production, ecological roles, and biotechnological applications.

From dolastatin 13 to cyanopeptolins, micropeptins, and lyngbyastatins: the chemical biology of Ahp-cyclodepsipeptides

Covering: 1989 up to 2019 Ahp-cyclodepsipeptides (also known as Ahp-containing cyclodepsipeptides, cyanopeptolins, micropeptins, microginines, and lyngbyastatins, and by many other names) are a family of non-ribosomal peptide synthesis (NRPS)-derived natural products with potent serine protease inhibitory properties. Here, we review their isolation and structural elucidation from natural sources as well as studies of their biosynthesis, molecular mode of action, and use in drug discovery efforts. Accordingly, this summary aims to provide a comprehensive overview of the current state-of-the-art Ahp-cyclodepsipeptide research.

Multistressor negative effects on an experimental phytoplankton community. The case of glyphosate and one toxigenic cyanobacterium on Chlorophycean microalgae

Aquatic ecosystems face serious pollution issues. Discharges of toxic substances and eutrophication may lead to changes in the phytoplankton community and foster cyanobacterial blooms. Glyphosate-based herbicides are chemical stressors of microalgae that may affect the structure of phytoplankton communities, and also stimulate the synthesis of cyanotoxins by cyanobacteria. The simultaneous presence of glyphosate and toxigenic cyanobacteria increases the stress on microalgae, jointly affecting their growth and development. This study evaluated the combined effect of a toxigenic cyanobacterium and glyphosate in the development of an experimental microalgal community. We studied the effect of Microcystis aeruginosa on the population growth of the microalgae Ankistrodesmus falcatus, Chlorella vulgaris, Pseudokirchneriella subcapitata, and Scenedesmus incrassatulus. We also evaluated the combined effect of sub-inhibitory glyphosate (Faena®) concentrations on the content of macromolecules and the enzymes superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GPx), as well as on the concentration of TBARS. These effects were evaluated through the integrated biomarker response (IBR). In individual experiments, microalgae showed lower growth rates versus M. aeruginosa. In the mixed bioassays, both M. aeruginosa and microalgae showed reduced growth. IC50 values for Faena® ranged from 1.022 to 2.702 mg L-1. In the microalgae + cyanobacteria bioassays, the herbicide lowered the growth rates of microalgae but stimulated the proliferation of M. aeruginosa. The joint action of both stressors affected growth rate and population dynamics, macromolecule content, and led to increased CAT and GPx levels. Faena® influenced growth rate and caused oxidative stress. On the other hand, the herbicide stimulated the synthesis of cyanotoxins, which further affected microalgal development. The experimental community was not only affected by the herbicide, but the mixed culture with cyanobacteria magnified the effects of chemical stress. These results illustrate the potential damage to phytoplankton expected in anthropically eutrophic water bodies that are also polluted by glyphosate.

Isolation, Identification, and Decomposition of Antibacterial Dialkylresorcinols from a Chinese Pseudomonas aurantiaca Strain

A chemical investigation of a Chinese Pseudomonas aurantiaca strain has yielded a new benzoquinone (4) and furanone (5), in addition to the known dialkylresorcinols 1 and 2. Extensive decomposition studies on the major metabolite 1 produced an additional furanone derivative (6), a hydroxyquinone (7), and two unusual resorcinol and hydroxyquinone dimers (8 and 9). Structures were elucidated by nuclear magnetic resonance spectroscopy in combination with tandem mass spectrometry analysis. These studies illustrate the potential of artifacts as a source of additional chemical diversity. Compounds 1 and 2 showed moderate antibacterial activity against a panel of Gram-positive pathogens, while the antibacterial activities of the artifacts (4-9) were reduced.

Chemistry, bioactivity and biosynthesis of cyanobacterial alkylresorcinols

Covering: up to 2019 Alkylresorcinols are amphiphilic metabolites, well-known for their diverse biological activities, produced by both prokaryotes and eukaryotes. A few classes of alkylresorcinol scaffolds have been reported from the photoautotrophic cyanobacteria, ranging from the relatively simple hierridins to the more intricate cylindrocyclophanes. Recently, it has emerged that cyanobacteria employ two different biosynthetic pathways to produce unique alkylresorcinol scaffolds. However, these convergent pathways intersect by sharing biosynthetic elements which lead to common structural motifs. To obtain a broader view of the biochemical diversity of these compounds in cyanobacteria, we comprehensively cover the isolation, structure, biological activity and biosynthesis of their mono- and dialkylresorcinols. Moreover, we provide an overview of the diversity and distribution of alkylresorcinol-generating biosynthetic gene clusters in this phylum and highlight opportunities for discovery of novel alkylresorcinol scaffolds. Because some of these molecules have inspired notable syntheses, different approaches used to build these molecules in the laboratory are showcased.

The response of Microcystis aeruginosa strain MGK to a single or two consecutive H2 O2 applications

Various approaches have been proposed to control/eliminate toxic Microcystis sp. blooms including H₂ O₂ treatments. Earlier studies showed that pre-exposure of various algae to oxidative stress induced massive cell death when cultures were exposed to an additional H₂ O₂ treatment. We examined the vulnerability of exponential and stationary-phase Microcystis sp. strain MGK cultures to single and double H₂ O₂ applications. Stationary cultures show a much higher ability to decompose H₂ O₂ than younger cultures. Nevertheless, they are more sensitive to an additional H₂ O₂ dose given 1-6 h after the first one. Transcript analyses following H₂ O₂ application showed a fast rise in glutathione peroxidase abundance (227-fold within an hour) followed by a steep decline thereafter. Other genes potentially engaged in oxidative stress were far less affected. Metabolic-related genes were downregulated after H₂ O₂ treatments. Among those examined, the transcript level of prk (encoding phosphoribulose kinase) was the slowest to recover in agreement with the decline in photosynthetic rate revealed by fluorescence measurements. Our findings shed light on the response of Microcystis MGK to oxidative stress suggesting that two consecutive H₂ O₂ applications of low concentrations are far more effective in controlling Microcystis sp. population than a single dose of a higher concentration.

Natural Products from Cyanobacteria: Focus on Beneficial Activities

Cyanobacteria are photosynthetic microorganisms that colonize diverse environments worldwide, ranging from ocean to freshwaters, soils, and extreme environments. Their adaptation capacities and the diversity of natural products that they synthesize, support cyanobacterial success in colonization of their respective ecological niches. Although cyanobacteria are well-known for their toxin production and their relative deleterious consequences, they also produce a large variety of molecules that exhibit beneficial properties with high potential in various fields (e.g., a synthetic analog of dolastatin 10 is used against Hodgkin's lymphoma). The present review focuses on the beneficial activities of cyanobacterial molecules described so far. Based on an analysis of 670 papers, it appears that more than 90 genera of cyanobacteria have been observed to produce compounds with potentially beneficial activities in which most of them belong to the orders Oscillatoriales, Nostocales, Chroococcales, and Synechococcales. The rest of the cyanobacterial orders (i.e., Pleurocapsales, Chroococcidiopsales, and Gloeobacterales) remain poorly explored in terms of their molecular diversity and relative bioactivity. The diverse cyanobacterial metabolites possessing beneficial bioactivities belong to 10 different chemical classes (alkaloids, depsipeptides, lipopeptides, macrolides/lactones, peptides, terpenes, polysaccharides, lipids, polyketides, and others) that exhibit 14 major kinds of bioactivity. However, no direct relationship between the chemical class and the respective bioactivity of these molecules has been demonstrated. We further selected and specifically described 47 molecule families according to their respective bioactivities and their potential uses in pharmacology, cosmetology, agriculture, or other specific fields of interest. With this up-to-date review, we attempt to present new perspectives for the rational discovery of novel cyanobacterial metabolites with beneficial bioactivity.

A novel route towards cycle-tail peptides using oxime resin: teaching an old dog a new trick

In this paper, two anabaenopeptins cycle-tail peptides were synthesized via a novel acid-catalyzed head-to-side-chain concomitant cyclization/cleavage reaction on oxime resin.

Biosynthesis-assisted structural elucidation of the bartolosides, chlorinated aromatic glycolipids from cyanobacteria

The isolation of the bartolosides, unprecedented cyanobacterial glycolipids featuring aliphatic chains with chlorine substituents and C-glycosyl moieties, is reported. Their chlorinated dialkylresorcinol (DAR) core presented a major structural-elucidation challenge. To overcome this, we discovered the bartoloside (brt) biosynthetic gene cluster and linked it to the natural products through in vitro characterization of the DAR-forming ketosynthase and aromatase. Bioinformatic analysis also revealed a novel potential halogenase. Knowledge of the bartoloside biosynthesis constrained the DAR core structure by defining key pathway intermediates, ultimately allowing us to determine the full structures of the bartolosides. This work illustrates the power of genomics to enable the use of biosynthetic information for structure elucidation.

Responses of freshwater mussel (Elliptio complanata) hemocytes exposed in vitro to crude extracts of Microcystis aeruginosa and Lyngbya wollei

Lyngbya wollei is a benthic filamentous cyanobacterium that produces a toxin analogous to the neurotoxic saxitoxin known as lyngbyatoxin (LYNGTX). Microcystis aeruginosa form blooms in the pelagic area of eutrophic lakes and produce a series of potent hepatotoxins-microcystins (MCYST). The aim of this study in vitro study was to examine the difference between the crude extracts of either M. aeruginosa or L. wollei toward the immune system of Elliptio complanata mussels. Freshly isolated hemolymph was plated and exposed to the crude extract of each species at LYNGTX or MCYST equivalent concentrations of 5, 10 and 25 μg/L for 18 h. Immunocompetence was characterized by following changes in hemocyte numbers, metabolic activity (viability), and phagocytosis. Hemocyte counts were not affected, indicating no turnover of hemocytes. Hemocyte metabolic activity was higher in cells exposed to crude extracts of L. wollei. Exposure to L. wollei extracts led to decreased pro-inflammatory precursors such as reactive oxygen species (ROS) and cyclooxygenase (COX) activities. Phagocytosis increased at 25 μg/L for both types of crude extracts. However, hemocytes exposed to crude extracts of M. aeruginosa produced more ROS and COX compared to hemocytes exposed to crude extracts of L. wollei. In conclusion, the data suggest that the crude extract of M. aeruginosa was more toxic than crude extract of L. wollei to mussel hemocytes.

Assessment of microcystin distribution and biomagnification in tissues of aquatic food web compartments from a shallow lake and evaluation of potential risks to public health

The objectives of this study were: (1) to examine the distribution and bioaccumulation of microcystins in the main components of the food web (phytoplankton, zooplankton, crayfish, shrimp, mussel, snail, fish, frog) of Lake Pamvotis (NW Greece), (2) to investigate the possibility of microcystin biomagnification and (3) to evaluate the potential threat of the contaminated aquatic organisms to human health. Significant microcystin concentrations were detected in all the aquatic organisms during two different periods, with the higher concentrations observed in phytoplankton and the lower in fish species and frogs. This is the first study reporting microcystin accumulation in the body of the freshwater shrimp Atyaephyra desmsaresti, in the brain of the fish species common carp (Cyprinus carpio) and in the skin of the frog Rana epirotica. Although there was no evidence for microcystin biomagnification, the fact that microcystins were found in lake water and in the tissues of aquatic organisms, suggests that serious risks to animal and public health are possible to occur. In addition, it is likely to be unsafe to consume aquatic species harvested in Lake Pamvotis due to the high-concentrations of accumulated microcystins.

Lyngbyaureidamides A and B, two anabaenopeptins from the cultured freshwater cyanobacterium Lyngbya sp. (SAG 36.91)

Two anabaenopeptin-type peptides, lyngbyaureidamides A and B, together with two previously reported peptides lyngbyazothrins C and D, were isolated from the cultured freshwater cyanobacterium Lyngbya sp. (SAG 36.91). Their structures were determined by spectroscopic and chemical methods. Lyngbyazothrins C and D were also able to inhibit the 20S proteasome with IC(50) values of 7.1 μM and 19.2 μM, respectively, while lyngbyaureidamides A and B were not active at 50 μM.

Genetic variation of adenylation domains of the anabaenopeptin synthesis operon and evolution of substrate promiscuity

Anabaenopeptins (AP) are bioactive cyclic hexapeptides synthesized nonribosomally in cyanobacteria. APs are characterized by several conserved motifs, including the ureido bond, N-methylation in position 5, and d-Lys in position 2. All other positions of the AP molecule are variable, resulting in numerous structural variants. We have identified a nonribosomal peptide synthetase (NRPS) operon from Planktothrix agardhii strain CYA126/8 consisting of five genes (apnA to apnE) encoding six NRPS modules and have confirmed its role in AP synthesis by the generation of a mutant via insertional inactivation of apnC. In order to correlate the genetic diversity among adenylation domains (A domains) with AP structure variation, we sequenced the A domains of all six NRPS modules from seven Planktothrix strains differing in the production of AP congeners. It is remarkable that single strains coproduce APs bearing either of the chemically divergent amino acids Arg and Tyr in exocyclic position 1. Since the A domain of the initiation module (the ApnA A₁ domain) has been proposed to activate the amino acid incorporated into exocyclic position 1, we decided to analyze this domain both biochemically and phylogenetically. Only ApnA A₁ enzymes from strains producing AP molecules containing Arg or Tyr in position 1 were found to activate these two chemically divergent amino acids in vitro. Phylogenetic analysis of apn A domain sequences revealed that strains with a promiscuous ApnA A₁ domain are derived from an ancestor that activates only Arg. Surprisingly, positive selection appears to affect only three codons within the apnA A₁ gene, suggesting that this remarkable promiscuity has evolved from point mutations only.

Allelopathy in freshwater cyanobacteria

Freshwater cyanobacteria produce several bioactive secondary metabolites with diverse chemical structure, which may achieve high concentrations in the aquatic medium when cyanobacterial blooms occur. Some of the compounds released by cyanobacteria have allelopathic properties, influencing the biological processes of other phytoplankton or aquatic plants. These kinds of interactions are more easily detectable under laboratory studies; however their ecological relevance is often debated. Recent research has discovered new allelopathic properties in some cyanobacteria species, new allelochemicals and elucidated some of the allelopathic mechanisms. Ecosystem-level approaches have shed some light on the factors that influence allelopathic interactions, as well as how cyanobacteria may be able to modulate their surrounding environment by means of allelochemical release. Nevertheless, the role of allelopathy in cyanobacteria ecology is still not well understood, and its clarification should benefit from carefully designed field studies, chemical characterization of allelochemicals and new methodological approaches at the "omics" level.

Synthesis and structure-activity correlation of a brunsvicamide-inspired cyclopeptide collection

Cyanobacterial cyclopeptides: A series of analogues of the cyanobacterial cyclopeptide brunsvicamide A was prepared by effective solid-support-based total synthesis. Variations in stereochemistry revealed the importance of the D-lysine and the L-isoleucine residues for the substrate-competitive inhibitory activity of brunsvicamide A against carboxypeptidase A. The brunsvicamides are modified cyclopeptides from cyanobacteria, cyclised through the epsilon-amino group of a D-lysine unit. They are functionalised with urea groups and show potent carboxypeptidase inhibitory activities. In order to unravel the structural parameters that determine their activities, a collection of brunsvicamide analogues with varied amino acid structures and stereochemistries was synthesised by a combined solution- and solid-phase approach. Biochemical investigation of the compound collection for carboxypeptidase A inhibition revealed that the presence of D-lysine and L-isoleucine in the urea section is important for inhibition. It was found that brunsvicamide A is a substrate-competitive inhibitor of carboxypeptidase A. These findings are in agreement with the substrate specificity of the enzyme and were rationalised by computational studies, which showed the high relevance of the lysine stereochemistry for inhibitory activity.

Homotyrosine-containing cyanopeptolins 880 and 960 and anabaenopeptins 908 and 915 from Planktothrix agardhii CYA 126/8

Two homotyrosine-bearing cyanopeptolins are described from Planktothrix agardhii CYA 126/8. The compounds feature a common homotyrosine-containing cyclohexadepsipeptide and differ by sulfation of an exocyclically located 2-O-methyl-d-glyceric acid residue. In addition we describe two anabaenopeptins, which contain two homotyrosine residues, one of which is N-methylated. The anabaenopeptins have a common cyclopentapeptide portion and differ in the amino acid linked to it via an ureido bond, arginine and tyrosine, respectively.

Towards clarification of the biological role of microcystins, a family of cyanobacterial toxins

Microcystins constitute a serious threat to the quality of drinking water worldwide. These protein phosphatase inhibitors are formed by various cyanobacterial species, including Microcystis sp. Microcystins are produced by a complex microcystin synthetase, composed of peptide synthetases and polyketide synthases, encoded by the mcyA-J gene cluster. Recent phylogenetic analysis suggested that the microcystin synthetase predated the metazoan lineage, thus dismissing the possibility that microcystins emerged as a means of defence against grazing, and their original biological role is not clear. We show that lysis of Microcystis cells, either mechanically or because of various stress conditions, induced massive accumulation of McyB and enhanced the production of microcystins in the remaining Microcystis cells. A rise in McyB content was also observed following exposure to microcystin or the protease inhibitors micropeptin and microginin, also produced by Microcystis. The extent of the stimulation by cell extract was strongly affected by the age of the treated Microcystis culture. Older cultures, or those recently diluted from stock cultures, hardly responded to the components in the cell extract. We propose that lysis of a fraction of the Microcystis population is sensed by the rest of the cells because of the release of non-ribosomal peptides. The remaining cells respond by raising their ability to produce microcystins thereby enhancing their fitness in their ecological niche, because of their toxicity.