Structure-based analysis of the molecular interactions between acyltransferase and acyl carrier protein in vicenistatin biosynthesis

Biosynthesis of macrolactam antibiotics with β-amino acid polyketide starter units

Macrolactam antibiotics incorporating β-amino acid polyketide starter units, isolated primarily from Actinomycetes species, show significant biological activities. This review provides a detailed analysis into the biosynthetic studies of vicenistatin, a macrolactam antibiotic with a 3-aminoisobutyrate starter unit, as well as biosynthetic research on related macrolactam compounds. Firstly, the elucidation of a common mechanism for the incorporation of β-amino acid starter units into the polyketide synthase (PKS) is described. Secondly, the unique biosynthetic mechanisms of the β-amino acids that are used to supply the main macrolactam biosynthetic pathways with starter units are discussed. Thirdly, some distinctive post-PKS modification mechanisms that complete macrolactam antibiotic biosynthesis are summarized. Finally, future directions for creating new macrolactam compounds through engineered biosynthesis pathways are described.

Semi-synthesis and structure-activity relationship study yield antibacterial vicenistatin derivatives with low cytotoxicity

Vicenistatin (1) is a 20-membered polyketide macrocyclic antibiotic with potent antimicrobial and cytotoxic activities. In this study, to further explore the potential of 1 as candidates of antibacterial drug development, 4'-N-demethyl vicenistatin (2), a secondary metabolite obtained from the ∆vicG mutant strain of Monodonata labio-associated Streptomyces parvus SCSIO Mla-L010, was utilized as a starting material for modifications of 4'-amino group of vicenistatin. Six new vicenistatin derivatives (3-8) were semi-synthesized through a concise route of amino modification with various aliphatic and aromatic aldehydes. Our study reveals that the bioactivity of vicenistatin is closely related to amino modification in sugar moiety, which results from the length of alkyl side chain as well as the presence of electron withdrawing/denoting group on the benzene ring. Importantly, compounds 4 with a butyl group and 8 with a 3,5-dihydroxyl-benzyl group at 4'-amino group, respectively, exhibited good antimicrobial activities, with MIC values spanning 0.5-4 μg ml-1 to Gram-positive pathogens, including methicillin-resistant Staphylococcus aureus, methicillin-resistant Staphylococcus epidermidis, Micrococcus luteus and Bacillus subtilis, with low cytotoxicity. This research promotes the further exploration of structure-activity relationships of vicenistatin and provides new vicenistatin derivatives for development of future anti-infectious agents with reduced cytotoxicity.

Architecture of full-length type I modular polyketide synthases revealed by X-ray crystallography, cryo-electron microscopy, and AlphaFold2

Covering: up to the end of 2023Type I modular polyketide synthases construct polyketide natural products in an assembly line-like fashion, where the growing polyketide chain attached to an acyl carrier protein is passed from catalytic domain to catalytic domain. These enzymes have immense potential in drug development since they can be engineered to produce non-natural polyketides by strategically adding, exchanging, and deleting individual catalytic domains. In practice, however, this approach frequently results in complete failures or dramatically reduced product yields. A comprehensive understanding of modular polyketide synthase architecture is expected to resolve these issues. We summarize the three-dimensional structures and the proposed mechanisms of three full-length modular polyketide synthases, Lsd14, DEBS module 1, and PikAIII. We also describe the advantages and limitations of using X-ray crystallography, cryo-electron microscopy, and AlphaFold2 to study intact type I polyketide synthases.

How Acyl Carrier Proteins (ACPs) Direct Fatty Acid and Polyketide Biosynthesis

Megasynthases, such as type I fatty acid and polyketide synthases (FASs and PKSs), are multienzyme complexes responsible for producing primary metabolites and complex natural products. Fatty acids (FAs) and polyketides (PKs) are built by assembling and modifying small acyl moieties in a stepwise manner. A central aspect of FA and PK biosynthesis involves the shuttling of substrates between the domains of the multienzyme complex. This essential process is mediated by small acyl carrier proteins (ACPs). The ACPs must navigate to the different catalytic domains within the multienzyme complex in a particular order to guarantee the fidelity of the biosynthesis pathway. However, the precise mechanisms underlying ACP-mediated substrate shuttling, particularly the factors contributing to the programming of the ACP movement, still need to be fully understood. This Review illustrates the current understanding of substrate shuttling, including concepts of conformational and specificity control, and proposes a confined ACP movement within type I megasynthases.

Exploring the Chain Release Mechanism from an Atypical Apicomplexan Polyketide Synthase

Polyketide synthases (PKSs) are megaenzymes that form chemically diverse polyketides and are found within the genomes of nearly all classes of life. We recently discovered the type I PKS from the apicomplexan parasite Toxoplasma gondii, TgPKS2, which contains a unique putative chain release mechanism that includes ketosynthase (KS) and thioester reductase (TR) domains. Our bioinformatic analysis of the thioester reductase of TgPKS2, TgTR, suggests differences compared to other systems and hints at a possibly conserved release mechanism within the apicomplexan subclass Coccidia. To evaluate this release module, we first isolated TgTR and observed that it is capable of 4 electron (4e-) reduction of octanoyl-CoA to the primary alcohol, octanol, utilizing NADH. TgTR was also capable of generating octanol in the presence of octanal and NADH, but no reactions were observed when NADPH was supplied as a cofactor. To biochemically characterize the protein, we measured the catalytic efficiency of TgTR using a fluorescence assay and determined the TgTR binding affinity for cofactor and substrates using isothermal titration calorimetry (ITC). We additionally show that TgTR is capable of reducing an acyl carrier protein (ACP)-tethered substrate by liquid chromatography mass spectrometry and determine that TgTR binds to holo-TgACP4, its predicted cognate ACP, with a KD of 5.75 ± 0.77 μM. Finally, our transcriptional analysis shows that TgPKS2 is upregulated ∼4-fold in the parasite's cyst-forming bradyzoite stage compared to tachyzoites. Our study identifies features that distinguish TgPKS2 from well-characterized systems in bacteria and fungi and suggests it aids the T. gondii cyst stage.

Molecular basis for short-chain thioester hydrolysis by acyl hydrolase domains in trans -acyltransferase polyketide synthases

Polyketide synthases (PKSs) are multi-domain enzymatic assembly lines that biosynthesise a wide selection of bioactive natural products from simple building blocks. In contrast to their cis -acyltransferase (AT) counterparts, trans -AT PKSs rely on stand-alone AT domains to load extender units onto acyl carrier protein (ACP) domains embedded in the core PKS machinery. Trans -AT PKS gene clusters also encode acyl hydrolase (AH) domains, which are predicted to share the overall fold of AT domains, but hydrolyse aberrant acyl chains from ACP domains, thus ensuring efficient polyketide biosynthesis. How such domains specifically target short acyl chains, in particular acetyl groups, tethered as thioesters to the substrate-shuttling ACP domains, with hydrolytic rather than acyl transfer activity, has remained unclear. To answer these questions, we solved the first structure of an AH domain and performed structure-guided activity assays on active site variants. Our results offer key insights into chain length control and selection against coenzyme A-tethered substrates, and clarify how the interaction interface between AH and ACP domains contributes to recognition of cognate and non-cognate ACP domains. Combining our experimental findings with molecular dynamics simulations allowed for the production of a data-driven model of an AH:ACP domain complex. Our results advance the currently incomplete understanding of polyketide biosynthesis by trans -AT PKSs, and provide foundations for future bioengineering efforts.

Unique recognition of the microalgal plastidial glycerol-3-phosphate acyltransferase for acyl-ACP

Plastidial glycerol-3-phosphate acyltransferases (GPATs) catalyze acyl-ACP and glycerol-3-phosphate to synthesize lysophosphatidic acid in vivo, which initiates the formation of various glycerolipids. Although the physiological substrates of plastidial GPATs are acyl-ACPs, acyl-CoAs have been commonly studied on the GPATs in vitro. However, little is known whether there are any distinct features of GPATs towards acyl-ACP and acyl-CoA. In this study, the results showed that the microalgal plastidial GPATs preferred acyl-ACP to acyl-CoA, while surprisingly, the plant-derived plastidial GPATs showed no obvious preferences towards these two acyl carriers. The key residues responsible for the distinct feature of microalgal plastidial GPATs were compared with plant-derived plastidial GPATs in their efficiency to catalyze acyl-ACP and acyl-CoA. Microalgal plastidial GPATs uniquely recognized acyl-ACP as compared to with other acyltransferases. The structure of the acyltransferases-ACP complex highlights only the involvement of the large structural domain in ACP in microalgal plastidial GPAT while in the other acyltransferases, both large and small structural domains were involved in the recognition process. The interaction sites on the plastidial GPAT from the green alga Myrmecia incisa (MiGPAT1) with ACP turned out to be K204, R212 and R266. A unique recognition between the microalgal plastidial GPAT and ACP was elucidated.

Conformational Changes of Acyl Carrier Protein Switch the Chain Length Preference of Acyl-ACP Thioesterase ChFatB2

Microbial fatty acids are synthesized by Type II fatty acid synthase and could be tailored by acyl-ACP thioesterase. With the prospects of medium-chain fatty-acid-derivative biofuels, the selectivity of thioesterase has been studied to control the fatty acid product chain length. Here, we report an alternative approach by manipulating the acyl carrier protein portion of acyl-ACP to switch the chain length propensity of the thioesterase. It was demonstrated that ChFatB2 from Cuphea hookeriana preferred C10-ACP to C8-ACP with ACP from E. coli, while converting preference to C8-ACP with ACP from Cuphea lanceolate. Circular dichroism (CD) results indicated that the C8-EcACP encountered a 34.4% α-helix increment compared to C10-EcACP, which resulted in an approximate binding affinity decrease in ChFatB2 compared to C10-EcACP. Similarly, the C10-ClACP2 suffered a 45% decrease in helix content compared to C8–ClACP2, and the conformational changes resulted in an 18% binding affinity decline with ChFatB2 compared with C10-ClACP2. In brief, the study demonstrates that the ACP portion of acyl-ACP contributes to the selectivity of acyl-ACP thioesterase, and the conformational changes of EcACP and ClACP2 switch the chain length preference of ChFatB2 between C8 and C10. The result provides fundamentals for the directed synthesis of medium-chain fatty acids based on regulating the conformational changes of ACPs.

Characterization of three succinyl-CoA acyltransferases involved in polyketide chain assembly

Polyketides are a class of natural products with astonishing structural diversities, fascinating biological activities, and a versatile of applications. In polyketides biosynthesis, acyltransferases (ATs) are the 'gatekeeping' enzymes selecting the specific CoA-activated acyl groups as building blocks and transferring them onto the phosphopantetheine arm of acyl carrier proteins (ACPs) to enable the following condensation reactions to assemble the polyketide chain. Herein, the Art2 protein from aurantinins, a group of the antibacterial polyketides, is characterized in vitro as an AT that can load a CoA-activated succinyl unit onto the first ACP domain of Art17 (ACP_Art17-1). In addition, another two proteins, GbnB and EtnB, involved in the biosynthesis of gladiolin and etnangien respectively, were traced by literature mining, homologous searching, and product structure analysis and then identified as functional succinyl-CoA ATs by the ACP_Art17-1 assays. Taken together, by the assay method employing ACP_Art17-1 as an acyl acceptor, we identified three ATs that can introduce a succinyl unit into the polyketide assembly line, which enriches the toolbox of polyketide biosynthetic enzymes and sets a stage for incorporating a succinyl unit into polyketide backbones in synthetic biological manners. KEY POINTS: • Three acyltransferases that are able to load ACP with a succinyl unit were characterized in vitro. • ACP_Art17-1 can be used as an acceptor to assay succinyl-CoA AT from different polyketides. • The succinyl unit can be incorporated into polyketides assembly process.

Acyltransferase Domain Exchange between Two Independent Type I Polyketide Synthases in the Same Producer Strain of Macrolide Antibiotics

Streptomyces graminofaciens A-8890 produces two macrolide antibiotics, FD-891 and virustomycin A, both of which show significant biological activity. In this study, we identified the virustomycin A biosynthetic gene cluster, which encodes type I polyketide synthases (PKSs), ethylmalonyl-CoA biosynthetic enzymes, methoxymalony-acyl carrier protein biosynthetic enzymes, and post-PKS modification enzymes. Next, we demonstrated that the acyltransferase domain can be exchanged between the Vsm PKSs and the PKSs involved in FD-891 biosynthesis (Gfs PKSs), without any supply problems of the unique extender units. We exchanged the malonyltransferase domain in the loading module of Gfs PKS with the ethylmalonyltransferase domain and the methoxymalonyltransferase domain of Vsm PKSs. Consequently, the expected two-carbon-elongated analog 26-ethyl-FD-891 was successfully produced with a titer comparable to FD-891 production by the wild type; however, exchange with the methoxymalonyltransferase domain did not produce any FD-891 analogs. Furthermore, 26-ethyl-FD-891 showed potent cytotoxic activity against HeLa cells, like natural FD-891.

Enzymology of assembly line synthesis by modular polyketide synthases

Modular polyketide synthases (PKSs) run catalytic reactions over dozens of steps in a highly orchestrated manner. To accomplish this synthetic feat, they form megadalton multienzyme complexes that are among the most intricate proteins on earth. Polyketide products are of elaborate chemistry with molecular weights of usually several hundred daltons and include clinically important drugs such as erythromycin (antibiotic), rapamycin (immunosuppressant) and epothilone (anticancer drug). The term 'modular' refers to a hierarchical structuring of modules and domains within an overall assembly line arrangement, in which PKS organization is colinearly translated into the polyketide structure. New structural information obtained during the past few years provides substantial direct insight into the orchestration of catalytic events within a PKS module and leads to plausible models for synthetic progress along assembly lines. In light of these structural insights, the PKS engineering field is poised to enter a new era of engineering.

Characterization of Unexpected Self-Acylation Activity of Acyl Carrier Proteins in a Modular Type I Apicomplexan Polyketide Synthase

Natural products play critical roles as antibiotics, anticancer therapeutics, and biofuels. Polyketides are a distinct natural product class of structurally diverse secondary metabolites that are synthesized by polyketide synthases (PKSs). The biosynthetic gene clusters that encode PKSs have been found across nearly all realms of life, but those from eukaryotic organisms are relatively understudied. A type I PKS from the eukaryotic apicomplexan parasite Toxoplasma gondii,TgPKS2, was recently discovered through genome mining, and the functional acyltransferase (AT) domains were found to be selective for malonyl-CoA substrates. To further characterize TgPKS2, we resolved assembly gaps within the gene cluster, which confirmed that the encoded protein consists of three distinct modules. We subsequently isolated and biochemically characterized the four acyl carrier protein (ACP) domains within this megaenzyme. We observed self-acylation─or substrate acylation without an AT domain─for three of the four TgPKS2 ACP domains with CoA substrates. Furthermore, CoA substrate specificity and kinetic parameters were determined for all four unique ACPs. TgACP2-4 were active with a wide scope of CoA substrates, while TgACP1 from the loading module was found to be inactive for self-acylation. Previously, self-acylation has only been observed in type II systems, which are enzymes that act in-trans with one another, and this represents the first report of this activity in a modular type I PKS whose domains function in-cis. Site-directed mutagenesis of specific TgPKS2 ACP3 acidic residues near the phosphopantetheinyl arm demonstrated that they influence self-acylation activity and substrate specificity, possibly by influencing substrate coordination or phosphopantetheinyl arm activation. Further, the lack of TgPKS2 ACP self-acylation with acetoacetyl-CoA, which is utilized by previously characterized type II PKS systems, suggests that the substrate carboxyl group may be critical for TgPKS2 ACP self-acylation. The unexpected properties observed from T. gondii PKS ACP domains highlight their distinction from well-characterized microbial and fungal systems. This work expands our understanding of ACP self-acylation beyond type II systems and helps pave the way for future studies on biosynthetic enzymes from eukaryotes.

Structural Basis of Transient Interactions of Acyltransferase VinK with the Loading Acyl Carrier Protein of the Vicenistatin Modular Polyketide Synthase

Acyltransferase (AT) recognizes its cognate acyl carrier protein (ACP) for functional transfer of an acyl unit in polyketide biosynthesis. However, structural characterization of AT-ACP complexes is limited because of the weak and transient interactions between them. In the biosynthesis of macrolactam polyketide vicenistatin, the trans-acting loading AT VinK transfers a dipeptidyl unit from the stand-alone ACP VinL to the ACP domain (VinP1ACP_L) of the loading module of modular polyketide synthase VinP1. Although the previously determined structure of the VinK-VinL complex clearly illustrates the VinL recognition mechanism of VinK, how VinK recognizes VinP1ACP_L remains unclear. Here, the crystal structure of a covalent VinK-VinP1ACP_L complex formed with a pantetheine-type cross-linking probe is reported at 3.0 Å resolution. The structure of the VinK-VinP1ACP_L complex provides detailed insights into the transient interactions between VinK and VinP1ACP_L. The importance of residues in the binding interface was confirmed by site-directed mutational analyses. The binding interface between VinK and VinP1ACP_L is similar to that between VinK and VinL, although some of the interface residues are different. However, the ACP orientation and interaction mode observed in the VinK-VinP1ACP_L complex are different from those observed in other AT-ACP complexes such as the disorazole trans-AT-ACP complex and cis-AT-ACP complexes of modular polyketide synthases. Thus, AT-ACP binding interface interactions are different in each type of AT-ACP pair.

Polyene Macrolactams from Marine and Terrestrial Sources: Structure, Production Strategies, Biosynthesis and Bioactivities

Over the past few decades (covering 1972 to 2022), astounding progress has been made in the elucidation of structures, bioactivities and biosynthesis of polyene macrolactams (PMLs), but they have only been partially summarized. PMLs possess a wide range of biological activities, particularly distinctive fungal inhibitory abilities, which render them a promising drug candidate. Moreover, the unique biosynthetic pathways including β-amino acid initiation and pericyclic reactions were presented in PMLs, leading to more attention from inside and outside the natural products community. According to current summation, in this review, the chem- and bio-diversity of PMLs from marine and terrestrial sources are considerably rich. A systematic, critical and comprehensive overview is in great need. This review described the PMLs’ general structural features, production strategies, biosynthetic pathways and the mechanisms of bioactivities. The challenges and opportunities for the research of PMLs are also discussed.

Structural visualization of transient interactions between the cis-acting acyltransferase and acyl carrier protein of the salinomycin modular polyketide synthase

Transient protein–protein interactions between cis-acting acyltransferase (AT) and acyl carrier protein (ACP) domains are critical for the catalysis and processivity of modular polyketide synthases (mPKSs), but are challenging for structural characterization due to the intrinsically weak binding affinity. Here, a stable complex of cis-acting AT and ACP domains from the ninth module of the salinomycin mPKS was obtained using a maleimide cross-linker and the structure of the complex was determined at 2.6 Å resolution. The crystal structure shows that the AT in combination with the ketosynthase (KS)-to-AT linker forms a C-shaped architecture to embrace the ACP. The large hydrolase subdomain of the AT serves as a major binding platform for the ACP, while the small ferredoxin-like subdomain of the AT and the KS-to-AT linker cooperate with each other to constrain binding of the ACP. The importance of interface residues in cis-acting AT–ACP interactions was confirmed by mutagenesis assays. The interaction mode observed in the cis-acting AT–ACP complex is completely different from those observed in trans-acting AT–ACP complexes, where the ACP primarily contacts the small domain of the AT. The complex structure provides detailed mechanistic insights into AT–ACP recognition in cis-AT mPKSs.

Protein-protein Recognition Involved in the Intermodular Transacylation Reaction in Modular Polyketide Synthase in the Biosynthesis of Vicenistatin

The ketosynthase (KS) domain is a core domain found in modular polyketide synthases (PKSs). To maintain the polyketide biosynthetic fidelity, the KS domain must only accept an acyl group from the acyl carrier protein (ACP) domain of the immediate upstream module even when they are separated into different polypeptides. Although it was reported that both the docking domain‐based interactions and KS‐ACP compatibility are important for the interpolypeptide transacylation reaction in 6‐deoxyerythronolide B synthase, it is not clear whether these findings are broadly applied to other modular PKSs. Herein, we describe the importance of protein‐protein recognition in the intermodular transacylation between VinP1 module 3 and VinP2 module 4 in vicenistatin biosynthesis. We compared the transacylation activity and crosslinking efficiency of VinP2 KS₄ against the cognate VinP1 ACP₃ with the noncognate one. As a result, it appeared that VinP2 KS₄ distinguishes the cognate ACP₃ from other ACPs.

Characterization of the Aminosugar Biosynthetic and Regulatory Genes of Vicenistatin in Monodonata labio-Associated Streptomyces parvus SCSIO Mla-L010

Vicenistatin (1) is a potent polyketide antitumor antibiotic composed of a 20-membered macrolactam core appended to a unique aminosugar, vicenisamine. In this study, vicenistatin was isolated and its biosynthetic gene cluster identified from Monodonata labio-associated Streptomyces parvus SCSIO Mla-L010. A set of five genes, vicC, vicD, vicE, vicF, and vicG, was confirmed to be involved in the biosynthesis of the aminosugar by gene inactivations. VicG was characterized as an N-methyltransferase that catalyzes the methylation of the 4'-amino group in the last step of the aminosugar biosynthetic pathway; the N-demethyl intermediate 4'-N-demethylvicenistatin (2) was isolated from the ΔvicG mutant strain. In addition, vicR1 was characterized as a positive pathway-specific regulatory gene. Notably, N-demethyl compound 2 was found to exert impressive antibacterial activities, with MIC values spanning 0.06-4 μg/mL, against a panel of Gram-positive bacteria including methicillin-resistant Staphylococcus aureus, Gram-negative Helicobacter pylori, and mycobacterium Mycobacterium smegmatis and the fungal pathogen Candida albicans. Compound 2 was also found to display reduced cytotoxicities relative to vicenistatin, especially against noncancerous human cell lines.

Structure of putative tumor suppressor ALDH1L1

Putative tumor suppressor ALDH1L1, the product of natural fusion of three unrelated genes, regulates folate metabolism by catalyzing NADP+-dependent conversion of 10-formyltetrahydrofolate to tetrahydrofolate and CO2. Cryo-EM structures of tetrameric rat ALDH1L1 revealed the architecture and functional domain interactions of this complex enzyme. Highly mobile N-terminal domains, which remove formyl from 10-formyltetrahydrofolate, undergo multiple transient inter-domain interactions. The C-terminal aldehyde dehydrogenase domains, which convert formyl to CO2, form unusually large interfaces with the intermediate domains, homologs of acyl/peptidyl carrier proteins (A/PCPs), which transfer the formyl group between the catalytic domains. The 4'-phosphopantetheine arm of the intermediate domain is fully extended and reaches deep into the catalytic pocket of the C-terminal domain. Remarkably, the tetrameric state of ALDH1L1 is indispensable for catalysis because the intermediate domain transfers formyl between the catalytic domains of different protomers. These findings emphasize the versatility of A/PCPs in complex, highly dynamic enzymatic systems.

Solution Structure and Conformational Flexibility of a Polyketide Synthase Module

Polyketide synthases (PKSs) are versatile C-C bond-forming enzymes that are broadly distributed in bacteria and fungi. The polyketide compound family includes many clinically useful drugs such as the antibiotic erythromycin, the antineoplastic epothilone, and the cholesterol-lowering lovastatin. Harnessing PKSs for custom compound synthesis remains an open challenge, largely because of the lack of knowledge about key structural properties. Particularly, the domains-well characterized on their own-are poorly understood in their arrangement, conformational dynamics, and interplay in the intricate quaternary structure of modular PKSs. Here, we characterize module 2 from the 6-deoxyerythronolide B synthase by small-angle X-ray scattering and cross-linking mass spectrometry with coarse-grained structural modeling. The results of this hybrid approach shed light on the solution structure of a cis-AT type PKS module as well as its inherent conformational dynamics. Supported by a directed evolution approach, we also find that acyl carrier protein (ACP)-mediated substrate shuttling appears to be steered by a nonspecific electrostatic interaction network.

Modular polyketide synthase contains two reaction chambers that operate asynchronously

[Figure: see text].

Complex structure of the acyltransferase VinK and the carrier protein VinL with a pantetheine cross-linking probe

Acyltransferases are responsible for the selection and loading of acyl units onto carrier proteins in polyketide and fatty-acid biosynthesis. Despite the importance of protein–protein interactions between the acyltransferase and the carrier protein, structural information on acyltransferase–carrier protein interactions is limited because of the transient interactions between them. In the biosynthesis of the polyketide vicenistatin, the acyltransferase VinK recognizes the carrier protein VinL for the transfer of a dipeptidyl unit. The crystal structure of a VinK–VinL covalent complex formed with a 1,2-bismaleimidoethane cross-linking reagent has been determined previously. Here, the crystal structure of a VinK–VinL covalent complex formed with a pantetheine cross-linking probe is reported at 1.95 Å resolution. In the structure of the VinK–VinL–probe complex, the pantetheine probe that is attached to VinL is covalently connected to the side chain of the mutated Cys106 of VinK. The interaction interface between VinK and VinL is essentially the same in the two VinK–VinL complex structures, although the position of the pantetheine linker slightly differs. This structural observation suggests that interface interactions are not affected by the cross-linking strategy used.

Engineering the acyltransferase domain of epothilone polyketide synthase to alter the substrate specificity

Background

Polyketide synthases (PKSs) include ketone synthase (KS), acyltransferase (AT) and acyl carrier protein (ACP) domains to catalyse the elongation of polyketide chains. Some PKSs also contain ketoreductase (KR), dehydratase (DH) and enoylreductase (ER) domains as modification domains. Insertion, deletion or substitution of the catalytic domains may lead to the production of novel polyketide derivatives or to the accumulation of desired products. Epothilones are 16-membered macrolides that have been used as anticancer drugs. The substrate promiscuity of the module 4 AT domain of the epothilone PKS (EPOAT4) results in production of epothilone mixtures; substitution of this domain may change the ratios of epothilones. In addition, there are two dormant domains in module 9 of the epothilone PKS. Removing these redundant domains to generate a simpler and more efficient assembly line is a desirable goal.

Results

The substitution of module 4 drastically diminished the activity of epothilone PKS. However, with careful design of the KS-AT linker and the post-AT linker, replacing EPOAT4 with EPOAT2, EPOAT6, EPOAT7 or EPOAT8 (specifically incorporating methylmalonyl-CoA (MMCoA)) significantly increased the ratio of epothilone D (4) to epothilone C (3) (the highest ratio of 4:3 = 4.6:1), whereas the ratio of 4:3 in the parental strain Schlegelella brevitalea 104-1 was 1.4:1. We also obtained three strains by swapping EPOAT4 with EPOAT3, EPOAT5, or EPOAT9, which specifically incorporate malonyl-CoA (MCoA). These strains produced only epothilone C, and the yield was increased by a factor of 1.8 compared to that of parental strain 104-1. Furthermore, mutations of five residues in the AT domain identified Ser310 as the critical factor for MMCoA recognition in EPOAT4. Then, the mutation of His308 to valine or tyrosine combined with the mutation of Phe310 to serine further altered the product ratios. At the same time, we successfully deleted the inactive module 9 DH and ER domains and fused the ΨKR domain with the KR domain through an ~ 25-residue linker to generate a productive and simplified epothilone PKS.

Conclusions

These results suggested that the substitution and deletion of catalytic domains effectively produces desirable compounds and that selection of the linkers between domains is crucial for maintaining intact PKS catalytic activity.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12934-021-01578-3.

Elucidation of transient protein-protein interactions within carrier protein-dependent biosynthesis

Fatty acid biosynthesis (FAB) is an essential and highly conserved metabolic pathway. In bacteria, this process is mediated by an elaborate network of protein•protein interactions (PPIs) involving a small, dynamic acyl carrier protein that interacts with dozens of other partner proteins (PPs). These PPIs have remained poorly characterized due to their dynamic and transient nature. Using a combination of solution-phase NMR spectroscopy and protein-protein docking simulations, we report a comprehensive residue-by-residue comparison of the PPIs formed during FAB in Escherichia coli. This technique describes and compares the molecular basis of six discrete binding events responsible for E. coli FAB and offers insights into a method to characterize these events and those in related carrier protein-dependent pathways.

Natural polyenic macrolactams and polycyclic derivatives generated by transannular pericyclic reactions: optimized biogenesis challenging chemical synthesis

Covering from 1992 to the end of 2020-11-20.Genetically-encoded polyenic macrolactams, which are constructed by Nature using hybrid polyketide synthase/nonribosomal peptide synthase (PKSs/NRPSs) assembly lines, are part of the large collection of natural products isolated from bacteria. Activation of cryptic (i.e., silent) gene clusters in these microorganisms has more recently allowed to generate and eventually isolate additional members of the family. Having two unsaturated fragments separated by short saturated chains, the primary macrolactam is posited to undergo transannular reactions and further rearrangements thus leading to the generation of a structurally diverse collection of polycyclic (natural) products and oxidized derivatives. The review will cover the challenges that scientists face on the isolation of these unstable compounds from the cultures of the producing microorganisms, their structural characterization, biological activities, optimized biogenetic routes, as well as the skeletal rearrangements of the primary structures of the natural macrolactams derived from pericyclic reactions of the polyenic fragments. The efforts of the synthetic chemists to emulate Nature on the successful generation and structural confirmation of these natural products will also be reported.

Macrolactam Synthesis via Ring-Closing Alkene-Alkene Cross-Coupling Reactions

Reported herein is a practical method for macrolactam synthesis via a Rh(III)-catalyzed ring closing alkene-alkene cross-coupling reaction. The reaction proceeded via a Rh-catalyzed alkenyl sp² C-H activation process, which allows access to macrocyclic molecules of different ring sizes. Macrolactams containing a conjugated diene framework could be easily prepared in high chemoselectivities and Z,E stereoselectivities.

Interfacial plasticity facilitates high reaction rate of E. coli FAS malonyl-CoA:ACP transacylase, FabD

Fatty acid synthases (FASs) and polyketide synthases (PKSs) iteratively elongate and often reduce two-carbon ketide units in de novo fatty acid and polyketide biosynthesis. Cycles of chain extensions in FAS and PKS are initiated by an acyltransferase (AT), which loads monomer units onto acyl carrier proteins (ACPs), small, flexible proteins that shuttle covalently linked intermediates between catalytic partners. Formation of productive ACP-AT interactions is required for catalysis and specificity within primary and secondary FAS and PKS pathways. Here, we use the Escherichia coli FAS AT, FabD, and its cognate ACP, AcpP, to interrogate type II FAS ACP-AT interactions. We utilize a covalent crosslinking probe to trap transient interactions between AcpP and FabD to elucidate the X-ray crystal structure of a type II ACP-AT complex. Our structural data are supported using a combination of mutational, crosslinking, and kinetic analyses, and long-timescale molecular dynamics (MD) simulations. Together, these complementary approaches reveal key catalytic features of FAS ACP-AT interactions. These mechanistic inferences suggest that AcpP adopts multiple, productive conformations at the AT binding interface, allowing the complex to sustain high transacylation rates. Furthermore, MD simulations support rigid body subdomain motions within the FabD structure that may play a key role in AT activity and substrate selectivity.

Antibody Probes of Module 1 of the 6-Deoxyerythronolide B Synthase Reveal an Extended Conformation During Ketoreduction

The 6-deoxyerythronolide B synthase (DEBS) is a prototypical assembly line polyketide synthase (PKS) that synthesizes the macrocyclic core of the antibiotic erythromycin. Each of its six multidomain modules presumably sample distinct conformations, as biosynthetic intermediates tethered to their acyl carrier proteins interact with multiple active sites during the courses of their catalytic cycles. The spatiotemporal details underlying these protein dynamics remain elusive. Here, we investigate one aspect of this conformational flexibility using two domain-specific monoclonal antibody fragments (F_abs) isolated from a very large naïve human antibody library. Both F_abs, designated 1D10 and 2G10, were bound specifically and with high affinity to the ketoreductase domain of DEBS module 1 (KR1). Comparative kinetic analysis of stand-alone KR1 as well as a truncated bimodular derivative of DEBS revealed that 1D10 inhibited KR1 activity whereas 2G10 did not. Co-crystal structures of each KR1-F_ab complex provided a mechanistic rationale for this difference. A hybrid PKS module harboring KR1 was engineered, whose individual catalytic domains have been crystallographically characterized at high resolution. Size exclusion chromatography coupled to small-angle X-ray scattering (SEC-SAXS) of this hybrid module bound to 1D10 provided further support for the catalytic relevance of the "extended" model of a PKS module. Our findings reinforce the power of monoclonal antibodies as tools to interrogate structure-function relationships of assembly line PKSs.

Structural Characterization of Complex of Adenylation Domain and Carrier Protein by Using Pantetheine Cross-Linking Probe

Adenylation domains (A-domains) are responsible for selective incorporation of carboxylic acid substrates in the biosynthesis of various natural products. Each A-domain must recognize a cognate carrier protein (CP) for functional substrate transfer. The transient interactions between an A-domain and CP have been investigated by using acyl vinylsulfonamide adenosine inhibitors as probes to determine the structures of several A-domain-CP complexes. However, this strategy requires a specific vinylsulfonamide inhibitor that contains an acyl group corresponding to the substrate specificity of a target A-domain in every case. Here, we report an alternative strategy for structural characterization of A-domain-CP complexes. We used a bromoacetamide pantetheine cross-linking probe in combination with a Cys mutation to trap the standalone A-domain-CP complex involved in macrolactam polyketide biosynthesis through a covalent linkage, allowing the determination of the complex structure. This strategy facilitates the structural determination of A-domain-CP complexes.

Comparative structure, dynamics and evolution of acyl-carrier proteins from Borrelia burgdorferi, Brucella melitensis and Rickettsia prowazekii

Acyl carrier proteins (ACPs) are small helical proteins found in all kingdoms of life, primarily involved in fatty acid and polyketide biosynthesis. In eukaryotes, ACPs are part of the fatty acid synthase (FAS) complex, where they act as flexible tethers for the growing lipid chain, enabling access to the distinct active sites in FAS. In the type II synthesis systems found in bacteria and plastids, these proteins exist as monomers and perform various processes, from being a donor for synthesis of various products such as endotoxins, to supplying acyl chains for lipid A and lipoic acid FAS (quorum sensing), but also as signaling molecules, in bioluminescence and activation of toxins. The essential and diverse nature of their functions makes ACP an attractive target for antimicrobial drug discovery. Here, we report the structure, dynamics and evolution of ACPs from three human pathogens: Borrelia burgdorferi, Brucella melitensis and Rickettsia prowazekii, which could facilitate the discovery of new inhibitors of ACP function in pathogenic bacteria.

Repurposing Modular Polyketide Synthases and Non-ribosomal Peptide Synthetases for Novel Chemical Biosynthesis

In nature, various enzymes govern diverse biochemical reactions through their specific three-dimensional structures, which have been harnessed to produce many useful bioactive compounds including clinical agents and commodity chemicals. Polyketide synthases (PKSs) and non-ribosomal peptide synthetases (NRPSs) are particularly unique multifunctional enzymes that display modular organization. Individual modules incorporate their own specific substrates and collaborate to assemble complex polyketides or non-ribosomal polypeptides in a linear fashion. Due to the modular properties of PKSs and NRPSs, they have been attractive rational engineering targets for novel chemical production through the predictable modification of each moiety of the complex chemical through engineering of the cognate module. Thus, individual reactions of each module could be separated as a retro-biosynthetic biopart and repurposed to new biosynthetic pathways for the production of biofuels or commodity chemicals. Despite these potentials, repurposing attempts have often failed owing to impaired catalytic activity or the production of unintended products due to incompatible protein–protein interactions between the modules and structural perturbation of the enzyme. Recent advances in the structural, computational, and synthetic tools provide more opportunities for successful repurposing. In this review, we focused on the representative strategies and examples for the repurposing of modular PKSs and NRPSs, along with their advantages and current limitations. Thereafter, synthetic biology tools and perspectives were suggested for potential further advancement, including the rational and large-scale high-throughput approaches. Ultimately, the potential diverse reactions from modular PKSs and NRPSs would be leveraged to expand the reservoir of useful chemicals.

Structural basis for selectivity in a highly reducing type II polyketide synthase

In type II polyketide synthases (PKSs), the ketosynthase-chain length factor (KS-CLF) complex catalyzes polyketide chain elongation with the acyl carrier protein (ACP). Highly reducing type II PKSs, represented by IgaPKS, produce polyene structures instead of the well-known aromatic skeletons. Here, we report the crystal structures of the Iga11-Iga12 (KS-CLF) heterodimer and the covalently cross-linked Iga10=Iga11-Iga12 (ACP=KS-CLF) tripartite complex. The latter structure revealed the molecular basis of the interaction between Iga10 and Iga11-Iga12, which differs from that between the ACP and KS of Escherichia coli fatty acid synthase. Furthermore, the reaction pocket structure and site-directed mutagenesis revealed that the negative charge of Asp 113 of Iga11 prevents further condensation using a β-ketoacyl product as a substrate, which distinguishes IgaPKS from typical type II PKSs. This work will facilitate the future rational design of PKSs.

Active site labeling of fatty acid and polyketide acyl-carrier protein transacylases

Metabolic engineering of fatty acids and polyketides remains challenging due to unresolved protein-protein interactions that are essential to synthase activity. While several chemical probes have been developed to capture and visualize protein interfaces in these systems, acyl carrier protein (ACP) transacylase (AT) domains remain elusive. Herein, we combine a mutational strategy with fluorescent probe design to expedite the study of AT domains from fatty acid and polyketide synthases. We describe the design and evaluation of inhibitor-inspired and substrate-mimetic reporters containing sulfonyl fluoride and β-lactone warheads. Moreover, specific active-site labeling occurs by optimizing pH, time, and probe concentration, and selective labeling is achieved in the presence of inhibitors of competing domains. These findings provide a panel of AT-targeting probes and set the stage for future combinatorial biosynthetic and drug discovery initiatives.

Mitochondrial acyl carrier protein (ACP) at the interface of metabolic state sensing and mitochondrial function

Acyl carrier protein (ACP) is a principal partner in the cytosolic and mitochondrial fatty acid synthesis (FAS) pathways. The active form holo-ACP serves as FAS platform, using its 4'-phosphopantetheine group to present covalently attached FAS intermediates to the enzymes responsible for the acyl chain elongation process. Mitochondrial unacylated holo-ACP is a component of mammalian mitoribosomes, and acylated ACP species participate as interaction partners in several ACP-LYRM (leucine-tyrosine-arginine motif)-protein heterodimers that act either as assembly factors or subunits of the electron transport chain and Fe-S cluster assembly complexes. Moreover, octanoyl-ACP provides the C8 backbone for endogenous lipoic acid synthesis. Accumulating evidence suggests that mtFAS-generated acyl-ACPs act as signaling molecules in an intramitochondrial metabolic state sensing circuit, coordinating mitochondrial acetyl-CoA levels with mitochondrial respiration, Fe-S cluster biogenesis and protein lipoylation.

Engineering actinomycetes for biosynthesis of macrolactone polyketides

Actinobacteria are characterized as the most prominent producer of natural products (NPs) with pharmaceutical importance. The production of NPs from these actinobacteria is associated with particular biosynthetic gene clusters (BGCs) in these microorganisms. The majority of these BGCs include polyketide synthase (PKS) or non-ribosomal peptide synthase (NRPS) or a combination of both PKS and NRPS. Macrolides compounds contain a core macro-lactone ring (aglycone) decorated with diverse functional groups in their chemical structures. The aglycon is generated by megaenzyme polyketide synthases (PKSs) from diverse acyl-CoA as precursor substrates. Further, post-PKS enzymes are responsible for allocating the structural diversity and functional characteristics for their biological activities. Macrolides are biologically important for their uses in therapeutics as antibiotics, anti-tumor agents, immunosuppressants, anti-parasites and many more. Thus, precise genetic/metabolic engineering of actinobacteria along with the application of various chemical/biological approaches have made it plausible for production of macrolides in industrial scale or generation of their novel derivatives with more effective biological properties. In this review, we have discussed versatile approaches for generating a wide range of macrolide structures by engineering the PKS and post-PKS cascades at either enzyme or cellular level in actinobacteria species, either the native or heterologous producer strains.

Insights into Thiotemplated Pyrrole Biosynthesis Gained from the Crystal Structure of Flavin-Dependent Oxidase in Complex with Carrier Protein

Sequential enzymatic reactions on substrates tethered to carrier proteins (CPs) generate thiotemplated building blocks that are then delivered to nonribosomal peptide synthetases (NRPSs) to generate peptidic natural products. The underlying diversity of these thiotemplated building blocks is the principal driver of the chemical diversity of NRPS-derived natural products. Structural insights into recognition of CPs by tailoring enzymes that generate these building blocks are sparse. Here we present the crystal structure of a flavin-dependent prolyl oxidase that furnishes thiotemplated pyrrole as the product, in complex with its cognate CP in the holo and product-bound states. The thiotemplated pyrrole is an intermediate that is well-represented in natural product biosynthetic pathways. Our results delineate the interactions between the CP and the oxidase while also providing insights into the stereospecificity of the enzymatic oxidation of the prolyl heterocycle to the aromatic pyrrole. Biochemical validation of the interaction between the CP and the oxidase demonstrates that NRPSs recognize and bind to their CPs using interactions quite different from those of fatty acid and polyketide biosynthetic enzymes. Our results posit that structural diversity in natural product biosynthesis can be, and is, derived from subtle modifications of primary metabolic enzymes.

Type II fatty acid and polyketide synthases: deciphering protein-protein and protein-substrate interactions

Covering: up to April 5, 2018 Metabolites from type II fatty acid synthase (FAS) and polyketide synthase (PKS) pathways differ broadly in their identities and functional roles. The former are considered primary metabolites that are linear hydrocarbon acids, while the latter are complex aromatic or polyunsaturated secondary metabolites. Though the study of bacterial FAS has benefitted from decades of biochemical and structural investigations, type II PKSs have remained less understood. Here we review the recent approaches to understanding the protein-protein and protein-substrate interactions in these pathways, with an emphasis on recent chemical biology and structural applications. New approaches to the study of FAS have highlighted the critical role of the acyl carrier protein (ACP) with regard to how it stabilizes intermediates through sequestration and selectively delivers cargo to successive enzymes within these iterative pathways, utilizing protein-protein interactions to guide and organize enzymatic timing and specificity. Recent tools that have shown promise in FAS elucidation should find new approaches to studying type II PKS systems in the coming years.

Protein-protein interactions in "cis-AT" polyketide synthases

Covering: up to the end of 2018 Polyketides are a valuable source of bioactive and clinically important molecules. The biosynthesis of these chemically complex molecules has led to the discovery of equally complex polyketide synthase (PKS) pathways. Crystallography has yielded snapshots of individual catalytic domains, di-domains, and multi-domains from a variety of PKS megasynthases, and cryo-EM studies have provided initial views of a PKS module in a series of defined biochemical states. Here, we review the structural and biochemical results that shed light on the protein-protein interactions critical to catalysis by PKS systems with an embedded acyltransferase. Interactions include those that occur both within and between PKS modules, as well as with accessory enzymes.

Elucidation of the crystal structure of FabD from the multidrug-resistant bacterium Acinetobacter baumannii

Bacterial fatty acid synthesis (FAS) has been extensively studied as a potential target of antimicrobials. In FAS, FabD mediates transacylation of the malonyl group from malonyl-CoA to acyl-carrier protein (ACP). The mounting threat of nosocomial infection by multidrug-resistant Acinetobacter baumannii warrants a deeper understanding of its essential cellular mechanisms, which could lead to effective control of this highly competent pathogen. The molecular mechanisms involved in A. baumannii FAS are poorly understood, and recent research has suggested that Pseudomonas aeruginosa, a closely related nosocomial pathogen of A. baumannii, utilizes FAS to produce virulence factors. In this study, we solved the crystal structure of A. baumannii FabD (AbFabD) to provide a platform for the development of new antibacterial agents. Analysis of the structure of AbFabD confirmed the presence of highly conserved active site residues among bacterial homologs. Binding constants between AbFabD variants and A. baumannii ACP (AbACP) revealed critical conserved residues Lys195 and Lys200 involved in AbACP binding. Computational docking of a potential inhibitor, trifluoperazine, revealed a unique inhibitor-binding pocket near the substrate-binding site. The structural study presented herein will be useful for the structure-based design of potent AbFabD inhibitors.

Acyltransferases as Tools for Polyketide Synthase Engineering

Polyketides belong to the most valuable natural products, including diverse bioactive compounds, such as antibiotics, anticancer drugs, antifungal agents, immunosuppressants and others. Their structures are assembled by polyketide synthases (PKSs). Modular PKSs are composed of modules, which involve sets of domains catalysing the stepwise polyketide biosynthesis. The acyltransferase (AT) domains and their “partners”, the acyl carrier proteins (ACPs), thereby play an essential role. The AT loads the building blocks onto the “substrate acceptor”, the ACP. Thus, the AT dictates which building blocks are incorporated into the polyketide structure. The precursor- and occasionally the ACP-specificity of the ATs differ across the polyketide pathways and therefore, the ATs contribute to the structural diversity within this group of complex natural products. Those features make the AT enzymes one of the most promising tools for manipulation of polyketide assembly lines and generation of new polyketide compounds. However, the AT-based PKS engineering is still not straightforward and thus, rational design of functional PKSs requires detailed understanding of the complex machineries. This review summarizes the attempts of PKS engineering by exploiting the AT attributes for the modification of polyketide structures. The article includes 253 references and covers the most relevant literature published until May 2018.

Biochemical Characterization of β-Amino Acid Incorporation in Fluvirucin B2 Biosynthesis

Naturally occurring lactams, such as the polyketide-derived macrolactams, provide a diverse class of natural products that could enhance existing chemically produced lactams. Although β-amino acid loading in the fluvirucin B₂ polyketide pathway was proposed by a previously identified putative biosynthetic gene cluster, biochemical characterization of the complete loading enzymes has not been described. Here we elucidate the complete biosynthetic pathway of the β-amino acid loading pathway in fluvirucin B₂ biosynthesis. We demonstrate the promiscuity of the loading pathway to utilize a range of amino acids and further illustrate the ability to introduce non-native acyl transferases to selectively transfer β-amino acids onto a polyketide synthase (PKS) loading platform. The results presented here provide a detailed biochemical description of β-amino acid selection and will further aid in future efforts to develop engineered lactam-producing PKS platforms.

Stereospecificity of Enoylreductase Domains from Modular Polyketide Synthases

An enoylreductase (ER) domain of a polyketide synthase module recruiting a methylmalonate extender unit sets the C2 methyl branch to either the S or R configuration during processing of a polyketide intermediate carried by an acyl carrier protein (ACP) domain. In the present study, pantetheine- and ACP-bound trans-2-methylcrotonyl substrate surrogates were used to scrutinize the stereospecificity of the ER domains. The pantetheine-bound thioester was reduced to mixtures of both 2 R and 2 S products, whereas the expected 2 S epimer was almost exclusively generated when the cognate ACP-bound substrate surrogate was utilized. The analogous incubation of an ER with the substrate surrogate carried by a noncognate ACP significantly increased the generation of the unexpected 2 R epimer, highlighting the dependence of stereospecificity on proper protein-protein interactions between ER and ACP domains. The ER mutant assays revealed the involvement of the conserved tyrosine and lysine in stereocontrol. Taken together, these results expand the current understanding of the ER stereochemistry and help in the engineering of modular PKSs.

The modules of trans-acyltransferase assembly lines redefined with a central acyl carrier protein

Here, the term "module" is redefined for trans-acyltransferase (trans-AT) assembly lines to agree with how its domains cooperate and evolutionarily co-migrate. The key domain in both the polyketide synthase (PKS) and nonribosomal peptide synthetase (NRPS) modules of assembly lines is the acyl carrier protein (ACP). ACPs not only relay growing acyl chains through the assembly line but also collaborate with enzymes in modules, both in cis and in trans, to add a specific chemical moiety. A ketosynthase (KS) downstream of ACP often plays the role of gatekeeper, ensuring that only a single intermediate generated by the enzymes of a module is passed downstream. Bioinformatic analysis of 526 ACPs from 33 characterized trans-AT assembly lines reveals ACPs from the same module type generally clade together, reflective of the co-evolution of these domains with their cognate enzymes. While KSs downstream of ACPs from the same module type generally also clade together, KSs upstream of ACPs do not-in disagreement with the traditional definition of a module. Beyond nomenclature, the presented analysis impacts our understanding of module function, the evolution of assembly lines, pathway prediction, and assembly line engineering.

Identification of a biosynthetic gene cluster for the polyene macrolactam sceliphrolactam in a Streptomyces strain isolated from mangrove sediment

Streptomyces are a genus of Actinobacteria capable of producing structurally diverse natural products. Here we report the isolation and characterization of a biosynthetically talented Streptomyces (Streptomyces sp. SD85) from tropical mangrove sediments. Whole-genome sequencing revealed that Streptomyces sp. SD85 harbors at least 52 biosynthetic gene clusters (BGCs), which constitute 21.2% of the 8.6-Mb genome. When cultivated under lab conditions, Streptomyces sp. SD85 produces sceliphrolactam, a 26-membered polyene macrolactam with unknown biosynthetic origin. Genome mining yielded a putative sceliphrolactam BGC (sce) that encodes a type I modular polyketide synthase (PKS) system, several β-amino acid starter biosynthetic enzymes, transporters, and transcriptional regulators. Using the CRISPR/Cas9–based gene knockout method, we demonstrated that the sce BGC is essential for sceliphrolactam biosynthesis. Unexpectedly, the PKS system encoded by sce is short of one module required for assembling the 26-membered macrolactam skeleton according to the collinearity rule. With experimental data disfavoring the involvement of a trans-PKS module, the biosynthesis of sceliphrolactam seems to be best rationalized by invoking a mechanism whereby the PKS system employs an iterative module to catalyze two successive chain extensions with different outcomes. The potential violation of the collinearity rule makes the mechanism distinct from those of other polyene macrolactams.

Structural and Functional Studies of the Daunorubicin Priming Ketosynthase DpsC

Daunorubicin is a type II polyketide, one of a large class of polyaromatic natural products with anticancer, antibiotic, and antiviral activity. Type II polyketides are formed by the assembly of malonyl-CoA building blocks, though in rare cases, biosynthesis is initiated by the incorporation of a nonmalonyl derived starter unit, which adds molecular diversity to the poly-β-ketone backbone. Priming mechanisms for the transfer of novel starter units onto polyketide synthases (PKS) are still poorly understood. Daunorubicin biosynthesis incorporates a unique propionyl starter unit thought to be selected for by a subclass ("DpsC type") of priming ketosynthases (KS III). To date, however, no structural information exists for this subclass of KS III enzymes. Although selectivity for self-acylation with propionyl-CoA has previously been implied, we demonstrate that DpsC shows no discrimination for self-acylation or acyl-transfer to the cognate acyl carrier protein, DpsG with short acyl-CoAs. We present five crystal structures of DpsC, including apo-DpsC, acetyl-DpsC, propionyl-DpsC, butyryl-DpsC, and a cocrystal of DpsC with a nonhydrolyzable phosphopantetheine (PPant) analogue. The DpsC crystal structures reveal the architecture of the active site, the molecular determinants for catalytic activity and homology to O-malonyl transferases, but also indicate distinct differences. These results provide a structural basis for rational engineering of starter unit selection in type II polyketide synthases.

Protein–protein interactions in polyketide synthase–nonribosomal peptide synthetase hybrid assembly lines

The protein–protein interactions in polyketide synthase–nonribosomal peptide synthetase hybrids are summarized and discussed.

Engineering strategies for rational polyketide synthase design

In this review, we highlight strategies in engineering polyketide synthases (PKSs). We focus on important protein–protein interactions that constitute an intact PKS assembly line.