Cyclization of polyketides and non-ribosomal peptides on and off their assembly lines

Modifications of Protein-Bound Substrates by Trans-Acting Enzymes in Natural Products Biosynthesis

Enzymatic modifications of small molecules are a common phenomenon in natural product biosynthesis, leading to the production of diverse bioactive compounds. In polyketide biosynthesis, modifications commonly take place after the completion of the polyketide backbone assembly by the polyketide synthases and the mature products are released from the acyl-carrier protein (ACP). However, exceptions to this rule appear to be widespread, as on-line hydroxylation, methyl transfer, and cyclization during polyketide assembly process are common, particularly in trans-AT PKS systems. Many of these modifications are catalyzed by specific domains within the modular PKS systems. However, several of the on-line modifications are catalyzed by stand-alone proteins. Those include the on-line Baeyer-Villiger oxidation, α-hydroxylation, halogenation, epoxidation, and methyl esterification during polyketide assembly, dehydrogenation of ACP-bound short fatty acids by acyl-CoA dehydrogenase-like enzymes, and glycosylation of ACP-bound intermediates by discrete glycosyltransferase enzymes. This review article highlights some of these trans-acting proteins that catalyze enzymatic modifications of ACP-bound small molecules in natural product biosynthesis.

Mutasynthesis generates nine new pyrroindomycins

Pyrroindomycins (PYRs) represent the only spirotetramate natural products discovered in nature, and possess potent activities against methicillin-resistant Staphylococcus aureus and vancomycin-resistant Enterococcus faecium. Their unique structure and impressive biological activities make them attractive targets for synthesis and biosynthesis; however, the discovery and generation of new PYRs remains challenging. To date, only the initial components A and B have been reported. Herein, we report a mutasynthesis approach for the generation of nine new PYRs with varying acyl modifications on their deoxy-trisaccharide moieties. This was achieved by blocking the formation of the acyl group 1,8-dihydropyrrolo[2,3-b]indole (DHPI) via gene pyrK1 inactivation and supplying chemical acyl precursors. The gene pyrK1 encodes a DUF1864 family protein that probably catalyzes the oxidative transformation of L-tryptophan to DHPI, and its deletion results in the abolishment of DHPI-containing PYRs and the accumulation of three new PYRs either without acyl modification or with DHPI replaced by benzoic acid and pyrrole-2-carboxylic acid. Capitalizing on the capacity of the ΔpyrK1 mutant to produce new PYRs, we have successfully developed a mutasynthesis strategy for the generation of six novel PYR analogs with various aromatic acid modifications on their deoxy-trisaccharide moieties, showcasing the potential for generating structurally diverse PYRs. Overall, this research contributes significantly to understanding the biosynthesis of PYRs and offers valuable perspectives on their structural diversity.

Enzymatic α-Ketothioester Decarbonylation Occurs in the Assembly Line of Barbamide for Skeleton Editing

The decarbonylation reaction has been developed significantly in organic chemistry as an effective approach to various synthetic applications, but enzymatic precedents for this reaction are rare. Based on investigations into the hybrid nonribosomal peptide synthetase (NRPS)-polyketide synthase (PKS) assembly line of barbamide, we report an on-line α-ketothioester decarbonylation reaction that leads to one-carbon truncation of the elongating skeleton. This enzymatic editing reaction occurs in the first round of lipopeptide extension and modification involving the multienzymes BarE and BarF, which successively house an NRPS module to initiate the biosynthesis and a PKS module to catalyze the first round of chain extension. Starting with processing a leucine-derived α-ketoacyl starter, the ketosynthase domain in BarE displays an unusual dual activity that results in net one-carbon chain elongation. It extrudes carbon monoxide from α-keto-isocaproyl thioester and then mediates decarboxylative condenses of the resultant isovaleryl thioester with malonyl thioester to form a diketide intermediate, followed by BarF-based O-methylation to stabilize the enol form of the β-carbonyl and afford an unusual E-double bond. Biochemical characterization, chemical synthesis, computational analysis, and the experimental outcome of site-directed mutagenesis illustrate the extraordinary catalytic capability of this ketosynthase domain. This work furthers the appreciation of assembly line chemistry and opens the door to new approaches for skeleton editing/engineering of related molecules using synthetic biology approaches.

Enzyme-Catalyzed Spiroacetal Formation in Polyketide Antibiotic Biosynthesis

A key step in the biosynthesis of numerous polyketides is the stereospecific formation of a spiroacetal (spiroketal). We report here that spiroacetal formation in the biosynthesis of the macrocyclic polyketides ossamycin and oligomycin involves catalysis by a novel spiroacetal cyclase. OssO from the ossamycin biosynthetic gene cluster (BGC) is homologous to OlmO, the product of an unannotated gene from the oligomycin BGC. The deletion of olmO abolished oligomycin production and led to the isolation of oligomycin-like metabolites lacking the spiroacetal structure. Purified OlmO catalyzed complete conversion of the major metabolite into oligomycin C. Crystal structures of OssO and OlmO reveal an unusual 10-strand β-barrel. Three conserved polar residues are clustered together in the β-barrel cavity, and site-specific mutation of any of these residues either abolished or substantially diminished OlmO activity, supporting a role for general acid/general base catalysis in spiroacetal formation.

Caerulomycin and collismycin antibiotics share a trans-acting flavoprotein-dependent assembly line for 2,2'-bipyridine formation

Linear nonribosomal peptide synthetases (NRPSs) and polyketide synthases (PKSs) template the modular biosynthesis of numerous nonribosomal peptides, polyketides and their hybrids through assembly line chemistry. This chemistry can be complex and highly varied, and thus challenges our understanding in NRPS and PKS-programmed, diverse biosynthetic processes using amino acid and carboxylate building blocks. Here, we report that caerulomycin and collismycin peptide-polyketide hybrid antibiotics share an assembly line that involves unusual NRPS activity to engage a trans-acting flavoprotein in C-C bond formation and heterocyclization during 2,2'-bipyridine formation. Simultaneously, this assembly line provides dethiolated and thiolated 2,2'-bipyridine intermediates through differential treatment of the sulfhydryl group arising from L-cysteine incorporation. Subsequent L-leucine extension, which does not contribute any atoms to either caerulomycins or collismycins, plays a key role in sulfur fate determination by selectively advancing one of the two 2,2'-bipyridine intermediates down a path to the final products with or without sulfur decoration. These findings further the appreciation of assembly line chemistry and will facilitate the development of related molecules using synthetic biology approaches.

Investigation of Indigoidine Synthetase Reveals a Conserved Active-Site Base Residue of Nonribosomal Peptide Synthetase Oxidases

Nonribosomal peptide synthetase (NRPS) oxidase (Ox) domains oxidize protein-bound intermediates to install crucial structural motifs in bioactive natural products. The mechanism of this domain remains elusive. Here, by studying indigoidine synthetase, a single-module NRPS involved in the biosynthesis of indigoidine and several other bacterial secondary metabolites, we demonstrate that its Ox domain utilizes an active-site base residue, tyrosine 665, to deprotonate a protein-bound l-glutaminyl residue. We further validate the generality of this active-site residue among NRPS Ox domains. These findings not only resolve the biosynthetic pathway mediated by indigoidine synthetase but enable mechanistic insight into NRPS Ox domains.

On-Line Polyketide Cyclization into Diverse Medium-Sized Lactones by a Specialized Ketosynthase Domain

Ketosynthase (KS) domains of modular type I polyketide synthases (PKSs) typically catalyze the Claisen condensation of acyl and malonyl units to form linear chains. In stark contrast, the KS of the rhizoxin PKS branching module mediates a Michael addition, which sets the basis for a pharmacophoric δ-lactone moiety. The precise role of the KS was evaluated by site-directed mutagenesis, chemical probes, and biotransformations. Biochemical and kinetic analyses helped to dissect branching and lactonization reactions and unequivocally assign the entire sequence to the KS. Probing the range of accepted substrates with diverse synthetic surrogates in vitro, we found that the KS tolerates defined acyl chain lengths to produce five- to seven-membered lactones. These results show that the KS is multifunctional, as it catalyzes β-branching and lactonization. Information on the increased product portfolio of the unusual, TE-independent on-line cyclization is relevant for synthetic biology approaches.

Characterization of the metallo-dependent amidohydrolases responsible for "auxiliary" leucinyl removal in the biosynthesis of 2,2'-bipyridine antibiotics

2,2'-Bipyridine (2,2'-BiPy) is an attractive core structure present in a number of biologically active natural products, including the structurally related antibiotics caerulomycins (CAEs) and collismycins (COLs). Their biosynthetic pathways share a similar key 2,2'-BiPy-l-leucine intermediate, which is desulfurated or sulfurated at C5, arises from a polyketide synthase/nonribosomal peptide synthetase hybrid assembly line. Focusing on the common off-line modification steps, we here report that the removal of the "auxiliary" l-leucine residue relies on the metallo-dependent amidohydrolase activity of CaeD or ColD. This activity leads to the production of similar 2,2'-BiPy carboxylate products that then receive an oxime functionality that is characteristic for both CAEs and COLs. Unlike many metallo-dependent amidohydrolase superfamily proteins that have been previously reported, these proteins (particularly CaeD) exhibited a strong zinc ion-binding capacity that was proven by site-specific mutagenesis studies to be essential to proteolytic activity. The kinetics of the conversions that respectively involve CaeD and ColD were analyzed, showing the differences in the efficiency and substrate specificity of these two proteins. These findings would generate interest in the metallo-dependent amidohydrolase superfamily proteins that are involved in the biosynthesis of bioactive natural products.

Spatial Organization of Metabolic Enzyme Complexes in Cells

The organization of metabolic multienzyme complexes has been hypothesized to benefit metabolic processes and provide a coordinated way for the cell to regulate metabolism. Historically, their existence has been supported by various in vitro techniques. However, it is only recently that the existence of metabolic complexes inside living cells has come to light to corroborate this long-standing hypothesis. Indeed, subcellular compartmentalization of metabolic enzymes appears to be widespread and highly regulated. On the other hand, it is still challenging to demonstrate the functional significance of these enzyme complexes in the context of the cellular milieu. In this review, we discuss the current understanding of metabolic enzyme complexes by primarily focusing on central carbon metabolism and closely associated metabolic pathways in a variety of organisms, as well as their regulation and functional contributions to cells.

Minimization of the Thiolactomycin Biosynthetic Pathway Reveals that the Cytochrome P450 Enzyme TlmF Is Required for Five-Membered Thiolactone Ring Formation

Thiolactomycin (TLM) belongs to a class of rare and unique thiotetronate antibiotics that inhibit bacterial fatty acid synthesis. Although this group of natural product antibiotics was first discovered over 30 years ago, the study of TLM biosynthesis remains in its infancy. We recently discovered the biosynthetic gene cluster (BGC) for TLM from the marine bacterium Salinispora pacifica CNS-863. Here, we report the investigation of TLM biosynthetic logic through mutagenesis and comparative metabolic analyses. Our results revealed that only four genes (tlmF, tlmG, tlmH, and tlmI) are required for the construction of the characteristic γ-thiolactone skeleton of this class of antibiotics. We further showed that the cytochrome P450 TlmF does not directly participate in sulfur insertion and C-S bond formation chemistry but rather in the construction of the five-membered thiolactone ring as, upon its deletion, we observed the alternative production of the six-membered δ-thiolactomycin. Our findings pave the way for future biochemical investigation of the biosynthesis of this structurally unique group of thiotetronic acid natural products.

Towards Precision Engineering of Canonical Polyketide Synthase Domains: Recent Advances and Future Prospects

Modular polyketide synthases (mPKSs) build functionalized polymeric chains, some of which have become blockbuster therapeutics. Organized into repeating clusters (modules) of independently-folding domains, these assembly-line-like megasynthases can be engineered by introducing non-native components. However, poor introduction points and incompatible domain combinations can cause both unintended products and dramatically reduced activity. This limits the engineering and combinatorial potential of mPKSs, precluding access to further potential therapeutics. Different regions on a given mPKS domain determine how it interacts both with its substrate and with other domains. Within the assembly line, these interactions are crucial to the proper ordering of reactions and efficient polyketide construction. Achieving control over these domain functions, through precision engineering at key regions, would greatly expand our catalogue of accessible polyketide products. Canonical mPKS domains, given that they are among the most well-characterized, are excellent candidates for such fine-tuning. The current minireview summarizes recent advances in the mechanistic understanding and subsequent precision engineering of canonical mPKS domains, focusing largely on developments in the past year.

Biosynthesis and molecular engineering of templated natural products

Bioactive small molecules that are produced by living organisms, often referred to as natural products (NPs), historically play a critical role in the context of both medicinal chemistry and chemical biology. How nature creates these chemical entities with stunning structural complexity and diversity using a limited range of simple substrates has not been fully understood. Focusing on two types of NPs that share a highly evolvable ‘template’-biosynthetic logic, we here provide specific examples to highlight the conceptual and technological leaps in NP biosynthesis and witness the area of progress since the beginning of the twenty-first century. The biosynthesis of polyketides, non-ribosomal peptides and their hybrids that share an assembly-line enzymology of modular multifunctional proteins exemplifies an extended ‘central dogma’ that correlates the genotype of catalysts with the chemotype of products; in parallel, post-translational modifications of ribosomally synthesized peptides involve a number of unusual biochemical mechanisms for molecular maturation. Understanding the biosynthetic processes of these templated NPs would largely facilitate the design, development and utilization of compatible biosynthetic machineries to address the challenge that often arises from structural complexity to the accessibility and efficiency of current chemical synthesis.

Enzymatic [4+2] Cycloadditions in the Biosynthesis of Spirotetramates and Spirotetronates

The Diels-Alder reaction is a quintessential type of [4+2] cycloaddition that remains one of the most intriguing transformations in synthetic chemistry. This reaction has long been envisaged to participate in the biosynthesis of a number of cyclohexene-containing natural products, although the question of whether a bona fide Diels-Alderase exists remains unsolved. In nature, there are remarkably few enzymes known to have the activity of [4+2] cycloaddition. These enzymes are phylogenetically distinct and are often classified according to the specific chemical structures. The variation of protein ancestors and in many cases the instability/complexity of the substrates and products pose a significant challenge in identification of the [4+2] cycloaddition catalysts using general homology-based mining approaches. We here provide the detailed description of the multiple comparison-based strategy and methods for the characterization of two distinct types of dedicated [4+2] cyclases (eg, PyrE3 and PyrI4) in the biosynthesis of spirotetramates and spirotetronates, where they act in tandem for coordinated cross-bridging of a linear polyene intermediate into a enantiomerically pure pentacyclic core. The search of new protein scaffolds with the [4+2] cycloaddition activity could enrich the pool of the candidates for mechanistic examination of a true enzymatic Diels-Alder reaction. The protocols presented in this study would also be applicable to the study of other functionally similar but phylogenetically different proteins, eg, the spiroketal cyclases.

Catalytic Enantioselective Synthesis of Amino Skipped Diynes

The Cu-catalyzed synthesis of nonracemic 3-amino skipped diynes via an enantiodetermining C-C bond formation is described using StackPhos as ligand. Despite challenging issues of reactivity and stereoselectivity inherent to these chiral skipped diynes, the reaction tolerates an extremely broad substrate scope with respect to all components and provides the title compounds in excellent enantiomeric excess. The alkyne moieties are demonstrated here to be useful synthetic handles, and 3-amino skipped diynes are convenient building blocks for enantioselective synthesis.