Structural basis for binding specificity between subclasses of modular polyketide synthase docking domains

Unique marine derived cyanobacterial biosynthetic genes for chemical diversity

Cyanobacteria are a prolific source of structurally unique and biologically active natural products that derive from intriguing biochemical pathways. Advancements in genome sequencing have accelerated the identification of unique modular biosynthetic gene clusters in cyanobacteria and reveal a wealth of unusual enzymatic reactions involved in their construction. This article examines several interesting mechanistic transformations involved in cyanobacterial secondary metabolite biosynthesis with a particular focus on marine derived modular polyketide synthases (PKS), nonribosomal peptide synthetases (NRPS) and combinations thereof to form hybrid natural products. Further, we focus on the cyanobacterial genus Moorea and the co-evolution of its enzyme cassettes that create metabolic diversity. Progress in the development of heterologous expression systems for cyanobacterial gene clusters along with chemoenzymatic synthesis makes it possible to create new analogs. Additionally, phylum-wide genome sequencing projects have enhanced the discovery rate of new natural products and their distinctive enzymatic reactions. Summarizing, cyanobacterial biosynthetic gene clusters encode for a large toolbox of novel enzymes that catalyze unique chemical reactions, some of which may be useful in synthetic biology.

Protein-Protein Interactions, Not Substrate Recognition, Dominate the Turnover of Chimeric Assembly Line Polyketide Synthases

The potential for recombining intact polyketide synthase (PKS) modules has been extensively explored. Both enzyme-substrate and protein-protein interactions influence chimeric PKS activity, but their relative contributions are unclear. We now address this issue by studying a library of 11 bimodular and 8 trimodular chimeric PKSs harboring modules from the erythromycin, rifamycin, and rapamycin synthases. Although many chimeras yielded detectable products, nearly all had specific activities below 10% of the reference natural PKSs. Analysis of selected bimodular chimeras, each with the same upstream module, revealed that turnover correlated with the efficiency of intermodular chain translocation. Mutation of the acyl carrier protein (ACP) domain of the upstream module in one chimera at a residue predicted to influence ketosynthase-ACP recognition led to improved turnover. In contrast, replacement of the ketoreductase domain of the upstream module by a paralog that produced the enantiomeric ACP-bound diketide caused no changes in processing rates for each of six heterologous downstream modules compared with those of the native diketide. Taken together, these results demonstrate that protein-protein interactions play a larger role than enzyme-substrate recognition in the evolution or design of catalytically efficient chimeric PKSs.

Genetic engineering of modular PKSs: from combinatorial biosynthesis to synthetic biology

This reviews covers on-going efforts at engineering the gigantic modular polyketide synthases (PKSs), highlighting both notable successes and failures.

Recent advances in the elucidation of enzymatic function in natural product biosynthesis

With the successful production of artemisinic acid in yeast, the promising potential of synthetic biology for natural product biosynthesis is now being realized. The recent total biosynthesis of opioids in microbes is considered to be another landmark in this field. The importance and significance of enzymes in natural product biosynthetic pathways have been re-emphasized by these advancements. Therefore, the characterization and elucidation of enzymatic function in natural product biosynthesis are undoubtedly fundamental for the development of new drugs and the heterologous biosynthesis of active natural products. Here, discoveries regarding enzymatic function in natural product biosynthesis over the past year are briefly reviewed.

Integrating mass spectrometry and genomics for cyanobacterial metabolite discovery

Filamentous marine cyanobacteria produce bioactive natural products with both potential therapeutic value and capacity to be harmful to human health. Genome sequencing has revealed that cyanobacteria have the capacity to produce many more secondary metabolites than have been characterized. The biosynthetic pathways that encode cyanobacterial natural products are mostly uncharacterized, and lack of cyanobacterial genetic tools has largely prevented their heterologous expression. Hence, a combination of cutting edge and traditional techniques has been required to elucidate their secondary metabolite biosynthetic pathways. Here, we review the discovery and refined biochemical understanding of the olefin synthase and fatty acid ACP reductase/aldehyde deformylating oxygenase pathways to hydrocarbons, and the curacin A, jamaicamide A, lyngbyabellin, columbamide, and a trans-acyltransferase macrolactone pathway encoding phormidolide. We integrate into this discussion the use of genomics, mass spectrometric networking, biochemical characterization, and isolation and structure elucidation techniques.

The structural biology of biosynthetic megaenzymes

The modular polyketide synthases (PKSs) and nonribosomal peptide synthetases (NRPSs) are among the largest and most complicated enzymes in nature. In these biosynthetic systems, independently folding protein domains, which are organized into units called 'modules', operate in assembly-line fashion to construct polymeric chains and tailor their functionalities. Products of PKSs and NRPSs include a number of blockbuster medicines, and this has motivated researchers to understand how they operate so that they can be modified by genetic engineering. Beginning in the 1990s, structural biology has provided a number of key insights. The emerging picture is one of remarkable dynamics and conformational programming in which the chemical states of individual catalytic domains are communicated to the others, configuring the modules for the next stage in the biosynthesis. This unexpected level of complexity most likely accounts for the low success rate of empirical genetic engineering experiments and suggests ways forward for productive megaenzyme synthetic biology.

Harnessing natural product assembly lines: structure, promiscuity, and engineering

Many therapeutically relevant natural products are biosynthesized by the action of giant mega-enzyme assembly lines. By leveraging the specificity, promiscuity, and modularity of assembly lines, a variety of strategies has been developed that enables the biosynthesis of modified natural products. This review briefly summarizes recent structural advances related to natural product assembly lines, discusses chemical approaches to probing assembly line structures in the absence of traditional biophysical data, and surveys efforts that harness the inherent or engineered promiscuity of assembly lines for the synthesis of non-natural polyketides and non-ribosomal peptide analogues.

Evolution of Efficient Modular Polyketide Synthases by Homologous Recombination

The structural scaffolds of many complex natural products are produced by multifunctional type I polyketide synthase (PKS) enzymes that operate as biosynthetic assembly lines. The modular nature of these mega-enzymes presents an opportunity to construct custom biocatalysts built in a lego-like fashion by inserting, deleting, or exchanging native or foreign domains to produce targeted variants of natural polyketides. However, previously engineered PKS enzymes are often impaired resulting in limited production compared to native systems. Here, we show a versatile method for generating and identifying functional chimeric PKS enzymes for synthesizing custom macrolactones and macrolides. PKS genes from the pikromycin and erythromycin pathways were hybridized in Saccharomyces cerevisiae to generate hybrid libraries. We used a 96-well plate format for plasmid purification, transformations, sequencing, protein expression, in vitro reactions and analysis of metabolite formation. Active chimeric enzymes were identified with new functionality. Streptomyces venezuelae strains that expressed these PKS chimeras were capable of producing engineered macrolactones. Furthermore, a macrolactone generated from selected PKS chimeras was fully functionalized into a novel macrolide analogue. This method permits the engineering of PKS pathways as modular building blocks for the production of new antibiotic-like molecules.

Architecture of the polyketide synthase module: surprises from electron cryo-microscopy

Modular polyketide synthases (PKS) produce a vast array of bioactive molecules that are the basis of many highly valued pharmaceuticals. The biosynthesis of these compounds is based on ordered assembly lines of multi-domain modules, each extending and modifying a specific chain-elongation intermediate before transfer to the next module for further processing. The first 3D structures of a full polyketide synthase module in different functional states were obtained recently by electron cryo-microscopy. The unexpected module architecture revealed a striking evolutionary divergence of the polyketide synthase compared to its metazoan fatty acid synthase homolog, as well as remarkable conformational rearrangements dependent on its biochemical state during the full catalytic cycle. The design and dynamics of the module are highly optimized for both catalysis and fidelity in the construction of complex, biologically active natural products.

Uncovering the structures of modular polyketide synthases

This review covers a breakthrough in the structural biology of the gigantic modular polyketide synthases (PKS): the structural characterization of intact modules by single-particle cryo-electron microscopy and small-angle X-ray scattering.

Structure of a modular polyketide synthase

Polyketide natural products constitute a broad class of compounds with diverse structural features and biological activities. Their biosynthetic machinery, represented by type I polyketide synthases (PKSs), has an architecture in which successive modules catalyse two-carbon linear extensions and keto-group processing reactions on intermediates covalently tethered to carrier domains. Here we used electron cryo-microscopy to determine sub-nanometre-resolution three-dimensional reconstructions of a full-length PKS module from the bacterium Streptomyces venezuelae that revealed an unexpectedly different architecture compared to the homologous dimeric mammalian fatty acid synthase. A single reaction chamber provides access to all catalytic sites for the intramodule carrier domain. In contrast, the carrier from the preceding module uses a separate entrance outside the reaction chamber to deliver the upstream polyketide intermediate for subsequent extension and modification. This study reveals for the first time, to our knowledge, the structural basis for both intramodule and intermodule substrate transfer in polyketide synthases, and establishes a new model for molecular dissection of these multifunctional enzyme systems.

Insights into the function of trans-acyl transferase polyketide synthases from the SAXS structure of a complete module

Combined analysis by SAXS, NMR and homology modeling reveals the structure of an apo module from a trans-acyltransferase polyketide synthase.

Cyanobacterial polyketide synthase docking domains: a tool for engineering natural product biosynthesis

Modular type I polyketide synthases (PKSs) are versatile biosynthetic systems that initiate, successively elongate, and modify acyl chains. Intermediate transfer between modules is mediated via docking domains, which are attractive targets for PKS pathway engineering to produce natural product analogs. We identified a class 2 docking domain in cyanobacterial PKSs and determined crystal structures for two docking domain pairs, revealing a distinct class 2 docking strategy for promoting intermediate transfer. The selectivity of class 2 docking interactions, demonstrated in binding and biochemical assays, could be altered by mutagenesis. We determined the ideal fusion location for exchanging class 1 and class 2 docking domains and demonstrated effective polyketide chain transfer in heterologous modules. Thus, class 2 docking domains are tools for rational bioengineering of a broad range of PKSs containing either class 1 or 2 docking domains.

Computational Methods for Identification of Novel Secondary Metabolite Biosynthetic Pathways by Genome Analysis

Secondary metabolites belonging to polyketide and nonribosomal peptide families constitute a major class of natural products with diverse biological functions and a variety of pharmaceutically important properties. Experimental studies have shown that the biosynthetic machinery for polyketide and nonribosomal peptides involves multi-functional megasynthases like Polyketide Synthases (PKSs) and nonribosomal peptide synthetases (NRPSs) which utilize a thiotemplate mechanism similar to that for fatty acid biosynthesis. Availability of complete genome sequences for an increasing number of microbial organisms has provided opportunities for using in silico genome mining to decipher the secondary metabolite natural product repertoire encoded by these organisms. Therefore, in recent years there have been major advances in development of computational methods which can analyze genome sequences to identify genes involved in secondary metabolite biosynthesis and help in deciphering the putative chemical structures of their biosynthetic products based on analysis of the sequence and structural features of the proteins encoded by these genes. These computational methods for deciphering the secondary metabolite biosynthetic code essentially involve identification of various catalytic domains present in this PKS/NRPS family of enzymes; a prediction of various reactions in these enzymatic domains and their substrate specificities and also precise identification of the order in which these domains would catalyze various biosynthetic steps. Structural bioinformatics analysis of known secondary metabolite biosynthetic clusters has helped in formulation of predictive rules for deciphering domain organization, substrate specificity, and order of substrate channeling. In this chapter, the progress in development of various computational methods is discussed by different research groups, and specifically, the utility in identification of novel metabolites by genome mining and rational design of natural product analogs by biosynthetic engineering studies.

The missing linker: a dimerization motif located within polyketide synthase modules

The dimerization of multimodular polyketide synthases is essential for their function. Motifs that supplement the contacts made by dimeric polyketide synthase enzymes have previously been characterized outside the boundaries of modules, at the N- and C-terminal ends of polyketide synthase subunits. Here we describe a heretofore uncharacterized dimerization motif located within modules. The dimeric state of this dimerization element was elucidated through the 2.6 Å resolution crystal structure of a fragment containing a dimerization element and a ketoreductase. The solution structure of a standalone dimerization element was revealed by nuclear magnetic resonance spectroscopy to be consistent with that of the crystal structure, and its dimerization constant was measured through analytical ultracentrifugation to be ∼20 μM. The dimer buries ∼990 Å(2) at its interface, and its C-terminal helices rigidly connect to ketoreductase domains to constrain their locations within a module. These structural restraints permitted the construction of a common type of polyketide synthase module.

Functional modular dissection of DEBS1-TE changes triketide lactone ratios and provides insight into Acyl group loading, hydrolysis, and ACP transfer

The DEBS1-TE fusion protein is comprised of the loading module, the first two extension modules, and the terminal TE domain of the Saccharopolyspora erythraea 6-deoxyerythronolide B synthase. DEBS1-TE produces triketide lactones that differ on the basis of the starter unit selected by the loading module. Typical fermentations with plasmid-based expression of DEBS1-TE produce a 6:1 ratio of propionate to isobutyrate-derived triketide lactones. Functional dissection of the loading module from the remainder of DEBS1-TE results in 50% lower titers of triketide lactone and a dramatic shift in the production to a 1:4 ratio of propionate to isobutyrate-derived products. A series of radiolabeling studies of the loading module has shown that transfer from the AT to the ACP occurs much faster for propionate than for isobutyrate. However, the equilibrium occupancy of the AT favors isobutyrate such that propionate is outcompeted for ACP occupancy. Thus, propionyl-ACP is the kinetic product, while isobutyryl-ACP is the thermodynamic product. A slowed transfer from the loading domain ACP to first-extension module KS due to functional dissection of DEBS1-TE allows this isobutyryl-ACP-favored equilibrium to be realized and likely accounts for the observed shift in triketide lactone products.

Adenylating enzymes in Mycobacterium tuberculosis as drug targets

Adenylation or adenylate-forming enzymes (AEs) are widely found in nature and are responsible for the activation of carboxylic acids to intermediate acyladenylates, which are mixed anhydrides of AMP. In a second reaction, AEs catalyze the transfer of the acyl group of the acyladenylate onto a nucleophilic amino, alcohol, or thiol group of an acceptor molecule leading to amide, ester, and thioester products, respectively. Mycobacterium tuberculosis encodes for more than 60 adenylating enzymes, many of which represent potential drug targets due to their confirmed essentiality or requirement for virulence. Several strategies have been used to develop potent and selective AE inhibitors including highthroughput screening, fragment-based screening, and the rationale design of bisubstrate inhibitors that mimic the acyladenylate. In this review, a comprehensive analysis of the mycobacterial adenylating enzymes will be presented with a focus on the identification of small molecule inhibitors. Specifically, this review will cover the aminoacyl tRNAsynthetases (aaRSs), MenE required for menaquinone synthesis, the FadD family of enzymes including the fatty acyl- AMP ligases (FAAL) and the fatty acyl-CoA ligases (FACLs) involved in lipid metabolism, and the nonribosomal peptide synthetase adenylation enzyme MbtA that is necessary for mycobactin synthesis. Additionally, the enzymes NadE, GuaA, PanC, and MshC involved in the respective synthesis of NAD, guanine, pantothenate, and mycothiol will be discussed as well as BirA that is responsible for biotinylation of the acyl CoA-carboxylases.

Characterization of molecular interactions between ACP and halogenase domains in the Curacin A polyketide synthase

Polyketide synthases (PKSs) and non-ribosomal peptide synthetases (NRPSs) are large multidomain proteins present in microorganisms that produce bioactive compounds. Curacin A is such a bioactive compound with potent anti-proliferative activity. During its biosynthesis the growing substrate is bound covalently to an acyl carrier protein (ACP) that is able to access catalytic sites of neighboring domains for chain elongation and modification. While ACP domains usually occur as monomers, the curacin A cluster codes for a triplet ACP (ACP(I)-ACP(II)-ACP(III)) within the CurA PKS module. We have determined the structure of the isolated holo-ACP(I) and show that the ACPs are independent of each other within this tridomain system. In addition, we have determined the structure of the 3-hydroxyl-3-methylglutaryl-loaded holo-ACP(I), which is the substrate for the unique halogenase (Hal) domain embedded within the CurA module. We have identified the interaction surface of both proteins using mutagenesis and MALDI-based identification of product formation. Amino acids affecting product formation are located on helices II and III of ACP(I) and form a contiguous surface. Since the CurA Hal accepts substrate only when presented by one of the ACPs within the ACP(I)-ACP(II)-ACP(III) tridomain, our data provide insight into the specificity of the chlorination reaction.

Acyl-CoA subunit selectivity in the pikromycin polyketide synthase PikAIV: steady-state kinetics and active-site occupancy analysis by FTICR-MS

Polyketide natural products generated by type I modular polyketide synthases (PKSs) are vital components in our drug repertoire. To reprogram these biosynthetic assembly lines, we must first understand the steps that occur within the modular "black boxes." Herein, key steps of acyl-CoA extender unit selection are explored by in vitro biochemical analysis of the PikAIV PKS model system. Two complementary approaches are employed: a fluorescent-probe assay for steady-state kinetic analysis, and Fourier Transform Ion Cyclotron Resonance-mass spectrometry (FTICR-MS) to monitor active-site occupancy. Findings from five enzyme variants and four model substrates have enabled a model to be proposed involving catalysis based upon acyl-CoA substrate loading followed by differential rates of hydrolysis. These efforts suggest a strategy for future pathway engineering efforts using unnatural extender units with slow rates of hydrolytic off-loading from the acyltransferase domain.

A novel docking domain interface model predicting recombination between homoeologous modular biosynthetic gene clusters

An in silico model for homoeologous recombination between gene clusters encoding modular polyketide synthases (PKS) or non-ribosomal peptide synthetases (NRPS) was developed. This model was used to analyze recombination between 12 PKS clusters from Streptomyces species and related genera to predict if new clusters might give rise to new products. In many cases, there were only a limited number of recombination sites (about 13 per cluster pair), suggesting that recombination may pose constraints on the evolution of PKS clusters. Most recombination events occurred between pairs of ketosynthase (KS) domains, allowing the biosynthetic outcome of the recombinant modules to be predicted. About 30% of recombinants were predicted to produce polyketides. Four NRPS clusters from Streptomyces strains were also used for in silico recombination. They yielded a comparable number of recombinants to PKS clusters, but the adenylation (A) domains contained the largest proportion of recombination events; this might be a mechanism for producing new substrate specificities. The extreme G + C-content, the presence of linear chromosomes and plasmids, as well as the lack of a mutSL-mismatch repair system should favor production of recombinants in Streptomyces species.

SBSPKS: structure based sequence analysis of polyketide synthases

Polyketide synthases (PKSs) catalyze biosynthesis of a diverse family of pharmaceutically important secondary metabolites. Bioinformatics analysis of sequence and structural features of PKS proteins plays a crucial role in discovery of new natural products by genome mining, as well as in design of novel secondary metabolites by biosynthetic engineering. The availability of the crystal structures of various PKS catalytic and docking domains, and mammalian fatty acid synthase module prompted us to develop SBSPKS software which consists of three major components. Model_3D_PKS can be used for modeling, visualization and analysis of 3D structure of individual PKS catalytic domains, dimeric structures for complete PKS modules and prediction of substrate specificity. Dock_Dom_Anal identifies the key interacting residue pairs in inter-subunit interfaces based on alignment of inter-polypeptide linker sequences to the docking domain structure. In case of modular PKS with multiple open reading frames (ORFs), it can predict the cognate order of substrate channeling based on combinatorial evaluation of all possible interface contacts. NRPS–PKS provides user friendly tools for identifying various catalytic domains in the sequence of a Type I PKS protein and comparing them with experimentally characterized PKS/NRPS clusters cataloged in the backend databases of SBSPKS. SBSPKS is available at http://www.nii.ac.in/sbspks.html.

Crystal structures of dehydratase domains from the curacin polyketide biosynthetic pathway

Modular polyketide synthases (PKS) make novel natural products through a series of preprogrammed chemical steps catalyzed by an assembly line of multidomain modules. Each assembly-line step involves unique extension and modification reactions, resulting in tremendous diversity of polyketide products. Dehydratase domains catalyze formation of an alpha,beta-double bond in the nascent polyketide intermediate. We present crystal structures of the four dehydratase domains from the curacin A PKS. The catalytic residues and substrate binding site reside in a tunnel within a single monomer. The positions of the catalytic residues and shape of the substrate tunnel explain how chirality of the substrate hydroxyl group may determine the configuration of the product double bond. Access to the active site may require opening the substrate tunnel, forming an open trench. The arrangement of monomers within the dimer is consistent among PKS dehydratases and differs from that seen in the related mammalian fatty acid synthases.

Synthesis and biochemical analysis of complex chain-elongation intermediates for interrogation of molecular specificity in the erythromycin and pikromycin polyketide synthases

The 6-deoxyerythronolide B synthase (DEBS) and pikromycin (Pik) polyketide synthase (PKS) are unique multifunctional enzyme systems that are responsible for the biosynthesis of the erythromycin and pikromycin 14-membered ring aglycones, respectively. Together, these natural product biosynthetic systems provide excellent platforms to examine the fundamental structural and catalytic elements that govern polyketide assembly, processing, and macrocyclization. In these studies, the native pentaketide intermediate for DEBS was synthesized and employed for in vitro chemoenzymatic synthesis of macrolactone products in engineered monomodules Ery5, Ery5-TE, and Ery6. A comparative analysis was performed with the corresponding Pik module 5 (PikAIII) and module 6 (PikAIV), dissecting key similarities and differences between these highly related PKSs. The data revealed that individual modules in the DEBS and Pik PKSs possess distinctive molecular selectivity profiles and suggest that substrate recognition has evolved unique characteristics in each system.

Emergent gene order in a model of modular polyketide synthases

Polyketides are a class of biologically active heteropolymers produced by assembly line-like multiprotein complexes of modular polyketide synthases (PKS). The polyketide product is encoded in the order of the PKS proteins in the assembly line, suggesting that polyketide diversity derives from combinatorial rearrangement of these PKS complexes. Remarkably, the order of PKS genes on the chromosome follows the order of PKS proteins in the assembly line: This fact is commonly referred to as "collinearity". Here we propose an evolutionary origin for collinearity and demonstrate the mechanism by using a computational model of PKS evolution in a population. Assuming continuous evolutionary pressure for novel polyketides, and that new polyketide pathways are formed by horizontal transfer/recombination of PKS-encoding DNA, we demonstrate the existence of a broad range of parameters for which collinearity emerges spontaneously. Collinearity confers no fitness advantage in our model; it is established and maintained through a "secondary selection" mechanism, as a trait which increases the probability of forming long, novel PKS complexes through recombination. Consequently, collinearity hitchhikes on the successful genotypes which periodically sweep through the evolving population. In addition to computer simulation of a simplified model of PKS evolution, we provide a mathematical framework describing the secondary selection mechanism, which generalizes beyond the context of the present model.

Functional dissection of a multimodular polypeptide of the pikromycin polyketide synthase into monomodules by using a matched pair of heterologous docking domains

The pikromyin polyketide synthase (PKS) in Streptomyces venezulae is comprised of a loading module and six extension modules, which generate the corresponding 14-membered macrolactone product. PikAI is a multimodular component of this PKS and houses both the loading domain and the first two extension modules, joined by short intraprotein linkers. We have shown that PikAI can be separated into two proteins at either of these linkers, only when matched pairs of docking domains (DDs) from a heterologous modular phoslactomycin PKS are used in place of the intraprotein linker. In both cases the yields of pikromycin produced by the S. venezuelae mutant were 50% of that of a S. venezuelae strain expressing the native trimodular PikAI. This observation provides the first demonstration that such separations do not dramatically impact the efficiency of the entire in vivo biosynthetic process. Expression of module 2 as a monomodular protein fused to a heterologous N-terminal docking domain was also observed to give almost a tenfold improvement in the in vivo generation of pikromycin from a synthetic diketide intermediate. These results demonstrate the utility of DDs to manipulate biosynthetic processes catalyzed by modular PKSs and the quest to generate novel polyketide products.