Biosynthesis of fungal polyketides by collaborating and trans-acting enzymes

The chemical ecology and physiological functions of type I polyketide natural products: the emerging picture

Covering: up to 2024.For many years, the value of complex polyketides lay in their medical properties, including their antibiotic and antifungal activities, with little consideration paid to their native functions. However, more recent evidence gathered from the study of inter-organismal interactions has revealed the influence of these metabolites upon the ecological adaptation and distribution of their hosts, as well as their modes of communication. The increasing number of sequenced genomes and associated transcriptomes has also unveiled the widespread occurrence of the underlying biosynthetic enzymes across all kingdoms of life, and the important contributions they make to physiological events specific to each organism. This review depicts the diversity of roles fulfilled by type I polyketides, particularly in light of studies carried out during the last decade, providing an initial overall picture of their diverse functions.

Cupin-domain containing protein is not essential for the alkyl salicylaldehyde formation in Aspergillus ustus

Previous studies demonstrated the requirement of four enzymes including a cupin-domain containing protein for the formation of alkyl salicylaldehydes and derivatives. Heterologous expression of three biosynthetic genes from Aspergillus ustus resulted in the formation of such compounds in high-yields without involvement of a cupin analogue.

Combinatorial biosynthesis for the engineering of novel fungal natural products

Natural products are small molecules synthesized by fungi, bacteria and plants, which historically have had a profound effect on human health and quality of life. These natural products have evolved over millions of years resulting in specific biological functions that may be of interest for pharmaceutical, agricultural, or nutraceutical use. Often natural products need to be structurally modified to make them suitable for specific applications. Combinatorial biosynthesis is a method to alter the composition of enzymes needed to synthesize a specific natural product resulting in structurally diversified molecules. In this review we discuss different approaches for combinatorial biosynthesis of natural products via engineering fungal enzymes and biosynthetic pathways. We highlight the biosynthetic knowledge gained from these studies and provide examples of new-to-nature bioactive molecules, including molecules synthesized using combinations of fungal and non-fungal enzymes.

Biosynthesis of Cytosporones in Leotiomycetous Filamentous Fungi

Polyketides with the isochroman-3-one pharmacophore are rare among fungal natural products as their biosynthesis requires an unorthodox S-type aromatic ring cyclization. Genome mining uncovered a conserved gene cluster in select leotiomycetous fungi that encodes the biosynthesis of cytosporones, including isochroman-3-one congeners. Combinatorial biosynthesis in total biosynthetic and biocatalytic formats in Saccharomyces cerevisiae and in vitro reconstitution of key reactions with purified enzymes revealed how cytosporone structural and bioactivity diversity is generated. The S-type acyl dihydroxyphenylacetic acid (ADA) core of cytosporones is assembled by a collaborating polyketide synthase pair. Thioesterase domain-catalyzed transesterification releases ADA esters, some of which are known Nur77 modulators. Alternatively, hydrolytic release allows C6 hydroxylation by a flavin-dependent monooxygenase, yielding a trihydroxybenzene moiety. Reduction of the C9 carbonyl by a short chain dehydrogenase/reductase initiates isochroman-3-one formation, affording cytosporones with cytotoxic and antimicrobial activity. Enoyl di- or trihydroxyphenylacetic acids are generated as shunt products, while isocroman-3,4-diones are formed by autoxidation. The cytosporone pathway offers novel polyketide biosynthetic enzymes for combinatorial synthetic biology to advance the production of "unnatural" natural products for drug discovery.

Structural diversification of natural substrates modified by the O-methyltransferase AurJ from Fusarium Graminearum

Aromatic polyketide and phenylpropanoid derivatives are a large class of natural products produced by bacteria, fungi, and plants. The O-methylation is a unique decoration that can increase structural diversity of aromatic compounds and improve their pharmacological properties, but the substrate specificity of O-methyltransferase hinders the discovery of more natural products with O-methylation through biosynthesis. Here, we reported that the O-methyltransferase AurJ from plant pathogenic fungus Fusarium graminearum could methylate a broad range of natural substrates of monocyclic, bicyclic, and tricyclic aromatic precursors, exhibiting excellent substrate tolerance. This finding will partly change our stereotype about the specificity of traditional methyltransferases, and urge us to mine more O-methyltransferases with good substrate tolerance and discover more methylated natural products for drug discovery and development through directed evolution and combinatorial biosynthesis.

Structural enzymology of iterative type I polyketide synthases: various routes to catalytic programming

Time span of literature covered: up to mid-2023Iterative type I polyketide synthases (iPKSs) are outstanding natural chemists: megaenzymes that repeatedly utilize their catalytic domains to synthesize complex natural products with diverse bioactivities. Perhaps the most fascinating but least understood question about type I iPKSs is how they perform the iterative yet programmed reactions in which the usage of domain combinations varies during the synthetic cycle. The programmed patterns are fulfilled by multiple factors, and strongly influence the complexity of the resulting natural products. This article reviews selected reports on the structural enzymology of iPKSs, focusing on the individual domain structures followed by highlighting the representative programming activities that each domain may contribute.

Curiouser and curiouser: progress in understanding the programming of iterative highly-reducing polyketide synthases

Covering: 1996-2022Investigations over the last 2 decades have begun to reveal how fungal iterative highly-reducing polyketide synthases are programmed. Both in vitro and in vivo experiments have revealed the interplay of intrinsic and extrinsic selectivity of the component catalytic domains of these systems. Structural biology has begun to provide high resolution structures of hr-PKS that can be used as the basis for their engineering and reprogramming, but progress to-date remains rudimentary. However, significant opportunities exist for translating the current level of understanding into the ability to rationally re-engineer these highly efficient systems for the production of important biologically active compounds through biotechnology.

Fungal polyketides from a rhizospheric soil-derived Penicillium sp. YUD17004 associated with Gastrodia elata

Five unprecedented polyketide metabolites were isolated and characterized from a rhizospheric soil-derived Penicillium sp. YUD17004. Their diverse structures included two indanone-type polyketides penicillyketides A and B, a phthalide-like polyketides penicillyketide C, a symmetrical chromone dimer penicillyketide D, along with a pyrone derivative pyranlyketide, which were elucidated by spectroscopic data interpretation and quantum chemical electronic circular dichroism calculation. Notably, the structures of penicillyketides A and B feature a highly functionalized indanone ring nucleus, but differ from other indanone-containing polyketides by the alkyl substitution pattern. The structure of penicillyketide C comprises a furanone ring instead of the hydroxycyclopentenone ring characteristic for penicillyketides A and B, and represents an undescribed arrangement within C₁₇ polyketides. Penicillyketide D represented the first example of a chromone homodimer with the bridge at C-2/2'. Penicillyketide B exhibited weak anti-inflammatory activity with an IC₅₀ value of 32 ± 1.0 μM. Penicillyketide D displayed weak cytotoxicity against MCF-7 cell line with an IC₅₀ value of 25 ± 0.9 μM.

Depside Bond Formation by the Starter-Unit Acyltransferase Domain of a Fungal Polyketide Synthase

Depsides are polyphenolic molecules comprising two or more phenolic acid derivatives linked by an ester bond, which is called a depside bond in these molecules. Despite more than a century of intensive research on depsides, the biosynthetic mechanism of depside bond formation remains unclear. In this study, we discovered a polyketide synthase, DrcA, from the fungus Aspergillus duricaulis CBS 481.65 and found that DrcA synthesizes CJ-20,557 (1), a heterodimeric depside composed of 3-methylorsellinic acid and 3,5-dimethylorsellinic acid. Moreover, we determined that depside bond formation is catalyzed by the starter-unit acyltransferase (SAT) domain of DrcA. Remarkably, this is a previously undescribed form of SAT domain chemistry. Further investigation revealed that 1 is transformed into duricamidepside (2), a depside-amino acid conjugate, by the single-module nonribosomal peptide synthetase DrcB.

Genome-Based Analysis of Verticillium Polyketide Synthase Gene Clusters

Simple Summary

Fungi can produce many types of secondary metabolites, including mycotoxins. Poisonous mushrooms and mycotoxins that cause food spoilage have been known for a very long time. For example, Aspergillus flavus, which can grow on grains and nuts, produces highly toxic substances called Aflatoxins. Despite their menace to other living organisms, mycotoxins can be used for medicinal purposes, i.e., as antibiotics, growth-promoting compounds, and other kinds of drugs. These and other secondary metabolites produced by plant-pathogenic fungi may cause host plants to display disease symptoms and may play a substantial role in disease progression. Therefore, the identification and characterization of the genes involved in their biosynthesis are essential for understanding the molecular mechanism involved in their biosynthetic pathways and further promoting sustainable knowledge-based crop production.

Abstract

Polyketides are structurally diverse and physiologically active secondary metabolites produced by many organisms, including fungi. The biosynthesis of polyketides from acyl-CoA thioesters is catalyzed by polyketide synthases, PKSs. Polyketides play roles including in cell protection against oxidative stress, non-constitutive (toxic) roles in cell membranes, and promoting the survival of the host organisms. The genus Verticillium comprises many species that affect a wide range of organisms including plants, insects, and other fungi. Many are known as causal agents of Verticillium wilt diseases in plants. In this study, a comparative genomics approach involving several Verticillium species led us to evaluate the potential of Verticillium species for producing polyketides and to identify putative polyketide biosynthesis gene clusters. The next step was to characterize them and predict the types of polyketide compounds they might produce. We used publicly available sequences from ten species of Verticillium including V. dahliae, V. longisporum, V. nonalfalfae, V. alfalfae, V. nubilum, V. zaregamsianum, V. klebahnii, V. tricorpus, V. isaacii, and V. albo-atrum to identify and characterize PKS gene clusters by utilizing a range of bioinformatic and phylogenetic approaches. We found 32 putative PKS genes and possible clusters in the genomes of Verticillium species. All the clusters appear to be complete and functional. In addition, at least five clusters including putative DHN-melanin-, cytochalasin-, fusarielien-, fujikurin-, and lijiquinone-like compounds may belong to the active PKS repertoire of Verticillium. These results will pave the way for further functional studies to understand the role of these clusters.

Subcellular localization of fungal specialized metabolites

Fungal specialized metabolites play an important role in the environment and have impacted human health and survival significantly. These specialized metabolites are often the end product of a series of sequential and collaborating biosynthetic enzymes that reside within different subcellular compartments. A wide variety of methods have been developed to understand fungal specialized metabolite biosynthesis in terms of the chemical conversions and the biosynthetic enzymes required, however there are far fewer studies elucidating the compartmentalization of the same enzymes. This review illustrates the biosynthesis of specialized metabolites where the localization of all, or some, of the biosynthetic enzymes have been determined and describes the methods used to identify the sub-cellular localization.

Formation of 3-Orsellinoxypropanoic Acid in Penicillum crustosum is Catalyzed by a Bifunctional Nonreducing Polyketide Synthase

The heterologous expression of a nonreducing polyketide synthase gene oesA from Penicillium crustosum led to the identification of orsellinoylpropanoic acid (1). Domain deletion and recombination proved that OesA catalyzes not only the formation of orsellinic acid but also its transfer to 3-hydroxypropanoic acid. Both ACP domains contribute independently and complementarily to the product formation. Feeding experiments provided evidence that only the orsellinyl residue is derived from acetate.