Structure, biosynthesis, and biological activities of cyclooligomer depsipeptides from fungi

Cyclooligomer depsipeptides (CODs) are peptolides, in which their symmetric structure ring structure has two or more ester bonds formed and alternately arranged between amino and α-hydroxy acid. CODs belong to four main groups of cyclotetrapeptides, cyclohexadepsipeptides, cyclooctadepsipeptides and diketomorpholines. These compounds have been mainly isolated from Acremonium, Aspergillus, Beauveria, Cordyceps, Fusarium, Isaria, Nigrospora, Peacilomyces, and Verticillium. The biosynthesis of CODs takes place outside the ribosome by giant multi-domain enzymes called nonribosomal peptide synthetases (NRPSs). Two different models about the formation of these depsipeptides were proposed. Fungal CODs are known to exhibit various biological activities, especially insecticidal, antitumoral, antimicrobial and antiviral activities. Therefore, CODs are considered to be natural substances with extremely potential applications in medicine and agriculture. This review highlights the structures, classification, biosynthesis, and biological activities of the fungal CODs.

Engineering and elucidation of the lipoinitiation process in nonribosomal peptide biosynthesis

Nonribosomal peptide synthetases containing starter condensation domains direct the biosynthesis of nonribosomal lipopeptides, which generally exhibit wide bioactivities. The acyl chain has strong impacts on bioactivity and toxicity, but the lack of an in-depth understanding of starter condensation domain-mediated lipoinitiation limits the bioengineering of NRPSs to obtain novel derivatives with desired acyl chains. Here, we show that the acyl chains of the lipopeptides rhizomide, holrhizin, and glidobactin were modified by engineering the starter condensation domain, suggesting a workable approach to change the acyl chain. Based on the structure of the mutated starter condensation domain of rhizomide biosynthetic enzyme RzmA in complex with octanoyl-CoA and related point mutation experiments, we identify a set of residues responsible for the selectivity of substrate acyl chains and extend the acyl chains from acetyl to palmitoyl. Furthermore, we illustrate three possible conformational states of starter condensation domains during the reaction cycle of the lipoinitiation process. Our studies provide further insights into the mechanism of lipoinitiation and the engineering of nonribosomal peptide synthetases.

Intermediary conformations linked to the directionality of the aminoacylation pathway of nonribosomal peptide synthetases

In-solution analysis of conformational changes of NRPS adenylation and peptidyl-carrier protein domains under catalytic conditions reveals a new intermediary conformation.

Discovery and Biosynthesis of Guanipiperazine from a NRPS-like Pathway

Nonribosomal peptide synthetases (NRPSs) are modular enzymes that use a thiotemplate mechanism to assemble the peptide backbones of structurally diverse and biologically active natural products in bacteria and fungi. Unlike...

Bacterial-like nonribosomal peptide synthetases produce cyclopeptides in the zygomycetous fungus Mortierella alpina

Fungi are traditionally considered as reservoir of biologically active natural products. However, an active secondary metabolism has long not been attributed to early diverging fungi such as Mortierella spec. Here, we report on the biosynthesis of two series of cyclic pentapeptides, the malpicyclins and malpibaldins, as products of Mortierella alpina ATCC32222. The molecular structures of malpicyclins were elucidated by HR-MS/MS, Marfey's method, and 1D and 2D NMR spectroscopy. In addition, malpibaldin biosynthesis was confirmed by HR-MS. Genome mining and comparative qRT-PCR expression analysis pointed at two pentamodular nonribosomal peptide synthetases (NRPS), malpicyclin synthetase MpcA and malpibaldin synthetase MpbA, as candidate biosynthetic enzymes. Heterologous production of the respective adenylation domains and substrate specificity assays proved promiscuous substrate selection and confirmed their respective biosynthetic roles. In stark contrast to known fungal NRPSs, MpbA and MpcA contain bacterial-like dual epimerase/condensation domains allowing the racemization of enzyme-tethered l-amino acids and the subsequent incorporation of d-amino acids into the metabolites. Phylogenetic analyses of both NRPS genes indicate a bacterial origin and a horizontal gene transfer into the fungal genome. We report on the as yet unexplored nonribosomal peptide biosynthesis in basal fungi which highlights this paraphylum as novel and underrated resource of natural products. IMPORTANCE Fungal natural compounds are industrially produced with application in antibiotic treatment, cancer medications and crop plant protection. Traditionally, higher fungi have been intensively investigated concerning their metabolic potential, but re-identification of already known compounds is frequently observed. Hence, alternative strategies to acquire novel bioactive molecules are required. We present the genus Mortierella as representative of the early diverging fungi as an underestimated resource of natural products. Mortierella alpina produces two families of cyclopeptides, denoted malpicyclins and malpibaldins, respectively, via two pentamodular nonribosomal peptide synthetases (NRPSs). These enzymes are much closer related to bacterial than to other fungal NRPSs, suggesting a bacterial origin of these NRPS genes in Mortierella. Both enzymes were biochemically characterized and are involved in as yet unknown biosynthetic pathways of natural products in basal fungi. Hence, this report establishes early diverging fungi as prolific natural compound producers and sheds light on the origin of their biosynthetic capacity.