Giant polyketide synthase enzymes in the biosynthesis of giant marine polyether toxins

Prymnesium parvum are harmful haptophyte algae that cause massive environmental fish kills. Their polyketide polyether toxins, the prymnesins, are among the largest nonpolymeric compounds in nature and have biosynthetic origins that have remained enigmatic for more than 40 years. In this work, we report the "PKZILLAs," massive P. parvum polyketide synthase (PKS) genes that have evaded previous detection. PKZILLA-1 and -2 encode giant protein products of 4.7 and 3.2 megadaltons that have 140 and 99 enzyme domains. Their predicted polyene product matches the proposed pre-prymnesin precursor of the 90-carbon-backbone A-type prymnesins. We further characterize the variant PKZILLA-B1, which is responsible for the shorter B-type analog prymnesin-B1, from P. parvum RCC3426 and thus establish a general model of haptophyte polyether biosynthetic logic. This work expands expectations of genetic and enzymatic size limits in biology.

Theoretical Analysis of Selectivity Differences in Ketoreductases toward Aldehyde and Ketone Carbonyl Groups

Lactobacillus kefir alcohol dehydrogenase (LkADH) and ketoreductase from Chryseobacterium sp. CA49 (ChKRED12) exhibit different chemoselectivity and stereoselectivity toward a substrate with both keto and aldehyde carbonyl groups. LkADH selectively reduces the keto carbonyl group while retaining the aldehyde carbonyl group, producing optically pure R-alcohols. In contrast, ChKRED12 selectively reduces the aldehyde group and exhibits low reactivity toward ketone carbonyls. This study investigated the structural basis for these differences and the role of specific residues in the active site. Molecular dynamics (MD) simulations and quantum chemical calculations were used to investigate the interactions between the substrate and the enzymes and the essential cause of this phenomenon. The present study has revealed that LkADH and ChKRED12 exhibit significant differences in the structure of their respective active pockets, which is a crucial determinant of their distinct chemoselectivity toward the same substrate. Moreover, residues N89, N113, and E144 within LkADH as well as Q151 and D190 within ChKRED12 have been identified as key contributors to substrate stabilization within the active pocket through electrostatic interactions and van der Waals forces, followed by hydride transfer utilizing the coenzyme NADPH. Furthermore, the enantioselectivity mechanism of LkADH has been elucidated using quantum chemical methods. Overall, these findings not only provide fundamental insights into the underlying reasons for the observed differences in selectivity but also offer a detailed mechanistic understanding of the catalytic reaction.

Investigation of chain-length selection by the tenellin iterative highly-reducing polyketide synthase

The programming of widely distributed iterative fungal hr-PKS is mysterious, yet it is central for generating polyketide natural product diversity by controlling the chain length, β-processing level and methylation patterns of fungal polyketides. For the iterative hr-PKS TENS, responsible for producing the pentaketide-tyrosine hybrid pretenellin A 1, the chain length programming is known to be determined by the KR domain. Structure prediction of the KR domain enabled the identification of a relevant substrate binding helix, which was the focus of swap experiments with corresponding sequences from the related hr-PKS DMBS and MILS that produce similar hexa- and heptaketides (2, 3). The investigations of chimeric TENS variants expressed in vivo in the host Aspergillus oryzae NSAR1 revealed the substrate binding helix as a promising target for further investigations, evidenced by observed increase of the chain length during swap experiments. Building on these findings, rational engineering of TENS was applied based on structural analysis and sequence alignment. A minimal set of four simultaneous amino acid mutations achieved the re-programming of TENS by producing hexaketides in minor amounts. To refine our understanding and minimize the number of mutations impacting polyketide chain length, we conducted an alanine scan, pinpointing crucial amino acid positions. Our findings give indications on the intrinsic programming of hr-PKS domains by minimal changes in the amino acid sequence as one influence factor for programming.

Stairway to Stereoisomers: Engineering Short- and Medium-Chain Ketoreductases To Produce Chiral Alcohols

The short- and medium-chain dehydrogenase/reductase superfamilies are responsible for most chiral alcohol production in laboratories and industries. In nature, they participate in diverse roles such as detoxification, housekeeping, secondary metabolite production, and catalysis of several chemicals with commercial and environmental significance. As a result, they are used in industries to create biopolymers, active pharmaceutical intermediates (APIs), and are also used as components of modular enzymes like polyketide synthases for fabricating bioactive molecules. Consequently, random, semi-rational and rational engineering have helped transform these enzymes into product-oriented efficient catalysts. The rise of newer synthetic chemicals and their enantiopure counterparts has proved challenging, and engineering them has been the subject of numerous studies. However, they are frequently limited to the synthesis of a single chiral alcohol. The study attempts to defragment and describe hotspots of engineering short- and medium-chain dehydrogenases/reductases for the production of chiral synthons.

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

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

How cis-Acyltransferase Assembly-Line Ketosynthases Gatekeep for Processed Polyketide Intermediates

With the redefinition of polyketide synthase (PKS) modules, a new appreciation of their most downstream domain, the ketosynthase (KS), is emerging. In addition to performing its well-established role of generating a carbon-carbon bond between an acyl-CoA building block and a growing polyketide, it may gatekeep against incompletely processed intermediates. Here, we investigate 739 KSs from 92 primarily actinomycete, cis-acyltransferase assembly lines. When KSs were separated into 16 families based on the chemistries at the α- and β-carbons of their polyketide substrates, a comparison of 32 substrate tunnel residues revealed unique sequence fingerprints. Surprisingly, additional fingerprints were detected when the chemistry at the γ-carbon was considered. Representative KSs were modeled bound to their natural polyketide substrates to better understand observed patterns, such as the substitution of a tryptophan by a smaller residue to accommodate an l-α-methyl group or the substitution of four smaller residues by larger ones to make better contact with a primer unit or diketide. Mutagenesis of a conserved glutamine in a KS within a model triketide synthase indicates that the substrate tunnel is sensitive to alteration and that engineering this KS to accept unnatural substrates may require several mutations.

A Metagenomic Nanopore Sequence Analysis Combined with Conventional Screening and Spectroscopic Methods for Deciphering the Antimicrobial Metabolites Produced by Alcaligenes faecalis Soil Isolate MZ921504

The continuous development of multidrug resistance pathogens with limited therapeutic options has become a great problem globally that impose sever health hazards. Accordingly, searching for of new antimicrobials became an urgent demand and great challenge. Soil significantly have been associated with several species that are antibiotic producers. In this study, combination of conventional screening methods with Liquid chromatography- Mass spectroscopy (LC/MS) and metagenomic nanopore sequence analysis have been conducted for the deciphering the active metabolites produced by soil isolate(s). Preliminary soil screening resulted in a Gram-negative isolate identified via 16S ribosomal RNA as Alcaligenes faecalis isolate MZ921504 with promising antimicrobial activities against wide range of MDR gram-positive and gram-negative pathogens. The LC/MS analysis of the metabolites of A. faecalis isolate MZ921504 confirmed the presence of ectoine, bacillibactin, quinolobactin and burkholderic acid. Metagenomics sequence analysis of the soil sample (NCBI GenBank accession PRJNA771993) revealed the presence of conserved biosynthetic gene clusters of ectoine, bacteriocin, bacillibactin, quinolobactin, terpene and burkholderic acid of A. faecalis. In conclusion, A. faecalis isolate MZ921504 is a promising source for antimicrobial metabolites. LC/MS spectral analysis and third generation sequencing tools followed by secondary metabolite gene clusters analysis are useful methods to predict the nature of the antimicrobial metabolites.

Molecular basis for acyl carrier protein-ketoreductase interaction in trans-acyltransferase polyketide synthases

The biosynthesis of polyketides by type I modular polyketide synthases (PKS) relies on co-ordinated interactions between acyl carrier protein (ACP) domains and catalytic domains within the megasynthase. Despite the importance of these interactions, and their implications for biosynthetic engineering efforts, they remain poorly understood. Here, we report the molecular details of the interaction interface between an ACP domain and a ketoreductase (KR) domain from a trans-acyltransferase (trans-AT) PKS. Using a high-throughput mass spectrometry (MS)-based assay in combination with scanning alanine mutagenesis, residues contributing to the KR-binding epitope of the ACP domain were identified. Application of carbene footprinting revealed the ACP-binding site on the KR domain surface, and molecular docking simulations driven by experimental data allowed production of an accurate model of the complex. Interactions between ACP and KR domains from trans-AT PKSs were found to be specific for their cognate partner, indicating highly optimised interaction interfaces driven by evolutionary processes. Using detailed knowledge of the ACP:KR interaction epitope, an ACP domain was engineered to interact with a non-cognate KR domain partner. The results provide novel, high resolution insights into the ACP:KR interface and offer valuable rules for future engineering efforts of biosynthetic assembly lines.

The interaction epitope between a cognate KR–ACP domain pairing from a trans-AT polyketide synthase is elucidated in molecular detail, providing unique insights into recognition and specificity of the interface.

Preparative production of an enantiomeric pair by engineered polyketide synthases

Using the updated module boundary of polyketide assembly lines, modules from the pikromycin synthase were recombined into engineered synthases that furnish an enantiomeric pair of 2-stereocenter triketide lactones at >99% ee with yields up to 0.39 g per liter of E. coli K207-3 in shake flasks.

Ketosynthase Domain Constrains the Design of Polyketide Synthases

Modular polyketide synthases (PKSs) produce complex, bioactive secondary metabolites in assembly line-like multistep reactions. Longstanding efforts to produce novel, biologically active compounds by recombining intact modules to new modular PKSs have mostly resulted in poorly active chimeras and decreased product yields. Recent findings demonstrate that the low efficiencies of modular chimeric PKSs also result from rate limitations in the transfer of the growing polyketide chain across the noncognate module:module interface and further processing of the non-native polyketide substrate by the ketosynthase (KS) domain. In this study, we aim at disclosing and understanding the low efficiency of chimeric modular PKSs and at establishing guidelines for modular PKSs engineering. To do so, we work with a bimodular PKS testbed and systematically vary substrate specificity, substrate identity, and domain:domain interfaces of the KS involved reactions. We observe that KS domains employed in our chimeric bimodular PKSs are bottlenecks with regards to both substrate specificity as well as interaction with the acyl carrier protein (ACP). Overall, our systematic study can explain in quantitative terms why early oversimplified engineering strategies based on the plain shuffling of modules mostly failed and why more recent approaches show improved success rates. We moreover identify two mutations of the KS domain that significantly increased turnover rates in chimeric systems and interpret this finding in mechanistic detail.

Site directed mutagenesis as a precision tool to enable synthetic biology with engineered modular polyketide synthases

Modular polyketide synthases (PKSs) are a multidomain megasynthase class of biosynthetic enzymes that have great promise for the development of new compounds, from new pharmaceuticals to high value commodity and specialty chemicals. Their colinear biosynthetic logic has been viewed as a promising platform for synthetic biology for decades. Due to this colinearity, domain swapping has long been used as a strategy to introduce molecular diversity. However, domain swapping often fails because it perturbs critical protein-protein interactions within the PKS. With our increased level of structural elucidation of PKSs, using judicious targeted mutations of individual residues is a more precise way to introduce molecular diversity with less potential for global disruption of the protein architecture. Here we review examples of targeted point mutagenesis to one or a few residues harbored within the PKS that alter domain specificity or selectivity, affect protein stability and interdomain communication, and promote more complex catalytic reactivity.

General chemoenzymatic route to two-stereocenter triketides employing assembly line ketoreductases

Modular polyketide synthases (PKSs) are enzymatic assembly lines that fuse carbon fragments into complex chiral products. Here, their synthetic logic is employed to chemoenzymatically generate two-stereocenter triketides. Each of the four stereoisomers was constructed in a stereocontrolled manner using C-acylation and two PKS ketoreductases possessing opposite stereoselectivities.

ClusterCAD: a computational platform for type I modular polyketide synthase design

ClusterCAD is a web-based toolkit designed to leverage the collinear structure and deterministic logic of type I modular polyketide synthases (PKSs) for synthetic biology applications. The unique organization of these megasynthases, combined with the diversity of their catalytic domain building blocks, has fueled an interest in harnessing the biosynthetic potential of PKSs for the microbial production of both novel natural product analogs and industrially relevant small molecules. However, a limited theoretical understanding of the determinants of PKS fold and function poses a substantial barrier to the design of active variants, and identifying strategies to reliably construct functional PKS chimeras remains an active area of research. In this work, we formalize a paradigm for the design of PKS chimeras and introduce ClusterCAD as a computational platform to streamline and simplify the process of designing experiments to test strategies for engineering PKS variants. ClusterCAD provides chemical structures with stereochemistry for the intermediates generated by each PKS module, as well as sequence- and structure-based search tools that allow users to identify modules based either on amino acid sequence or on the chemical structure of the cognate polyketide intermediate. ClusterCAD can be accessed at https://clustercad.jbei.org and at http://clustercad.igb.uci.edu.

Seven-enzyme in vitro cascade to (3R)-3-hydroxybutyryl-CoA

Economical and environmentally-friendly routes to convert feedstock chemicals like acetate into valuable chiral products such as (R)-3-hydroxybutyrate are in demand. Here, seven enzymes (CoaA, CoaD, CoaE, ACS, BktB, PhaB, and GDH) are employed in a one-pot, in vitro, biocatalytic synthesis of (3R)-3-hydroxybutyryl-CoA, which was readily isolated. This platform generates not only chiral diketide building blocks but also desirable CoA derivatives.

Structural Basis of Polyketide Synthase O-Methylation

Modular type I polyketide synthases (PKSs) produce some of the most chemically complex metabolites in nature through a series of multienzyme modules. Each module contains a variety of catalytic domains to selectively tailor the growing molecule. PKS O-methyltransferases ( O-MTs) are predicted to methylate β-hydroxyl or β-keto groups, but their activity and structure have not been reported. We determined the domain boundaries and characterized the catalytic activity and structure of the StiD and StiE O-MTs, which methylate opposite β-hydroxyl stereocenters in the myxobacterial stigmatellin biosynthetic pathway. Substrate stereospecificity was demonstrated for the StiD O-MT. Key catalytic residues were identified in the crystal structures and investigated in StiE O-MT via site-directed mutagenesis and further validated with the cyanobacterial CurL O-MT from the curacin biosynthetic pathway. Initial structural and biochemical analysis of PKS O-MTs supplies a new chemoenzymatic tool, with the unique ability to selectively modify hydroxyl groups during polyketide biosynthesis.

Stereospecific Formation of Z-Trisubstituted Double Bonds by the Successive Action of Ketoreductase and Dehydratase Domains from trans-AT Polyketide Synthases

Incubation of (±)-2-methyl-3-ketobutyryl-SNAC (3) and (±)-2-methyl-3-ketopentanoyl-SNAC (4) with BonKR2 or OxaKR5, ketoreductase domains from the bongkrekic acid (1) and oxazolomycin (2) polyketide synthases, in the presence of NADPH gave in each case the corresponding (2 R,3 S)-2-methyl-3-hydroxybutyryl-SNAC (5) or (2 R,3 S)-2-methyl-3-hydroxypentanoyl-SNAC (6) products, as established by chiral gas chromatography-mass spectrometry analysis of the derived methyl esters. Identical results were obtained by BonKR2- and OxaKR5-catalyzed reduction of chemoenzymatically prepared (2 R)-2-methyl-3-ketopentanoyl-EryACP6, (2 R)-2-methyl-3-ketobutyryl-BonACP2 (12), and (2 R)-2-methyl-3-ketopentanoyl-BonACP2 (13). The paired dehydratase domains, BonDH2 and OxaDH5, were then shown to catalyze the reversible syn dehydration of (2 R,3 S)-2-methyl-3-hydroxybutyryl-BonACP2 (14) to give the corresponding trisubstituted ( Z)-2-methylbutenoyl-BonACP2 (16).

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.

Dissecting how modular polyketide synthase ketoreductases interact with acyl carrier protein-attached substrates

Interaction studies using fragments excised from the modular mycolactone polyketide synthase show that ketoreductase domains possess a generic binding site for acyl carrier protein domains and provide evidence that the pendant 5'-phosphopantetheine prosthetic group plays a key role in delivering acyl substrates to the active site in the correct orientation.

The Uncommon Enzymology of Cis-Acyltransferase Assembly Lines

The enzymology of 135 assembly lines containing primarily cis-acyltransferase modules is comprehensively analyzed, with greater attention paid to less common phenomena. Diverse online transformations, in which the substrate and/or product of the reaction is an acyl chain bound to an acyl carrier protein, are classified so that unusual reactions can be compared and underlying assembly-line logic can emerge. As a complement to the chemistry surrounding the loading, extension, and offloading of assembly lines that construct primarily polyketide products, structural aspects of the assembly-line machinery itself are considered. This review of assembly-line phenomena, covering the literature up to 2017, should thus be informative to the modular polyketide synthase novice and expert alike.

Trichophycin A, a Cytotoxic Linear Polyketide Isolated from a Trichodesmium thiebautii Bloom

In an effort to isolate and characterize bioactive secondary metabolites from Trichodesmium thiebautii blooms, collected cyanobacteria biomass was subjected to bioassay-guided extraction and fractionation using the human colon cancer cell line HCT-116, resulting in the isolation and subsequent structure characterization of a linear polyketide trichophycin A (1). The planar structure of 1 was completed using 1D and 2D NMR spectroscopy and high-resolution electrospray ionization mass spectrometry (HRESIMS). Trichophycin A was moderately toxic against the murine neuroblastoma cell line Neuro-2A (EC50: 6.5 μM) and HCT-116 cells (EC50: 11.7 μM). Trichophycin A was significantly more cytotoxic than the previously isolated polyketides trichotoxin A and trichotoxin B. These cytotoxicity observations suggest that toxicity may be related to the polyol character of these polyketide compounds.

Coordinated and Iterative Enzyme Catalysis in Fungal Polyketide Biosynthesis

Fungal polyketides are natural products with great chemical diversity that exhibit a wide range of biological activity. This chemical diversity stems from specialized enzymes encoded in the biosynthetic gene cluster responsible for the natural product biosynthesis. Fungal polyketide synthases (PKS) are the megasynthases that produce the carbon scaffolds for the molecules. Subsequent downstream tailoring enzymes such as oxygenases will then further modify the organic framework. In fungi, many of these enzymes have been found to work iteratively-catalyzing multiple reactions on different sites of the substrate. This perspective will analyze several examples of fungal polyketides that are assembled from a scaffold-building iterative PKS and an accompanying iterative tailoring oxygenase. In these examples, the PKS product is designed for downstream iterative oxygenations to generate additional complexity. Together, these iterative enzymes orchestrate the efficient biosynthesis of elaborate natural products such as lovastatin, chaetoglobosin A, cytochalasin E, and aurovertin E.

Stereocontrol within polyketide assembly lines

Most of the stereocenters of polyketide natural products are established during assembly line biosynthesis. The body of knowledge for how stereocenters are set is now large enough to begin constructing physical models of key reactions. Interactions between stereocenter-forming enzymes and polyketide intermediates are examined here at atomic resolution, drawing from the most current structural and functional information of ketosynthases (KSs), ketoreductases (KRs), dehydratases (DHs), enoylreductases (ERs), and related enzymes. While many details remain to be experimentally determined, our understanding of the chemical and physical mechanisms utilized by the chirality-molding enzymes of modular PKSs is rapidly advancing.

Substrate structure-activity relationships guide rational engineering of modular polyketide synthase ketoreductases

Modular polyketide synthase ketoreductases can set two chiral centers through a single reduction. To probe the basis of stereocontrol, a structure-activity relationship study was performed with three α-methyl, β-ketothioester substrates and four ketoreductases. Since interactions with the β-ketoacyl moiety were found to be most critical, residues implicated in contacting this moiety were mutated. Two mutations were sufficient to completely reverse the stereoselectivity of the model ketoreductase EryKR1, converting it from an enzyme that generates (2S,3R)-products into one that yields (2S,3S)-products.

The Phormidolide Biosynthetic Gene Cluster: A trans-AT PKS Pathway Encoding a Toxic Macrocyclic Polyketide

Phormidolide is a polyketide produced by a cultured filamentous marine cyanobacterium and incorporates a 16-membered macrolactone. Its complex structure is recognizably derived from a polyketide synthase pathway, but possesses unique and intriguing structural features that prompted interest in investigating its biosynthetic origin. Stable isotope incorporation experiments confirmed the polyketide nature of this compound. We further characterized the phormidolide gene cluster (phm) through genome sequencing followed by bioinformatic analysis. Two discrete trans-type acyltransferase (trans-AT) ORFs along with KS-AT adaptor regions (ATd) within the polyketide synthase (PKS) megasynthases, suggest that the phormidolide gene cluster is a trans-AT PKS. Insights gained from analysis of the mode of acetate incorporation and ensuing keto reduction prompted our reevaluation of the stereochemistry of phormidolide hydroxy groups located along the linear polyketide chain.

Antimicrobial activity of Alcaligenes sp. HPC 1271 against multidrug resistant bacteria

Alcaligenes sp. HPC 1271 demonstrated antibacterial activity against multidrug resistant bacteria, Enterobacter sp., resistant to sulfamethoxazole, ampicillin, azithromycin, and tetracycline, as well as against Serratia sp. GMX1, resistant to the same antibiotics with the addition of netilmicin. The cell-free culture supernatant was analyzed for possible antibacterials by HPLC, and the active fraction was further identified by LC-MS. Results suggest the production of tunicamycin, a nucleoside antibiotic. The draft genome of this bacterial isolate was analyzed, and the 4.2 Mb sequence data revealed six secondary metabolite-producing clusters, identified using antiSMASH platform as ectoine, butyrolactone, phosphonate, terpene, polyketides, and nonribosomal peptide synthase (NRPS). Additionally, the draft genome demonstrated homology to the tunicamycin-producing gene cluster and also defined 30 ORFs linked to protein secretion that could also play a role in the antibacterial activity observed. Gene expression analysis demonstrated that both NRPS and dTDP-glucose 4,6-dehydratase gene clusters are functional and could be involved in antibacterial biosynthesis.

Polyketide intermediate mimics as probes for revealing cryptic stereochemistry of ketoreductase domains

Among natural product families, polyketides have shown the most promise for combinatorial biosynthesis of natural product-like libraries. Though recent research in the area has provided many mechanistic revelations, a basic-level understanding of kinetic and substrate tolerability is still needed before the full potential of combinatorial biosynthesis can be realized. We have developed a novel set of chemical probes for the study of ketoreductase domains of polyketide synthases. This chemical tool-based approach was validated using the ketoreductase of pikromycin module 2 (PikKR2) as a model system. Triketide substrate mimics 12 and 13 were designed to increase stability (incorporating a nonhydrolyzable thioether linkage) and minimize nonessential functionality (truncating the phosphopantetheinyl arm). PikKR2 reduction product identities as well as steady-state kinetic parameters were determined by a combination of LC-MS/MS analysis of synthetic standards and a NADPH consumption assay. The d-hydroxyl product is consistent with bioinformatic analysis and results from a complementary biochemical and molecular biological approach. When compared to widely employed substrates in previous studies, diketide 63 and trans-decalone 64, substrates 12 and 13 showed 2-10 fold lower K(M) values (2.4 ± 0.8 and 7.8 ± 2.7 mM, respectively), indicating molecular recognition of intermediate-like substrates. Due to an abundance of the nonreducable enol-tautomer, the k(cat) values were attenuated by as much as 15-336 fold relative to known substrates. This study reveals the high stereoselectivity of PikKR2 in the face of gross substrate permutation, highlighting the utility of a chemical probe-based approach in the study of polyketide ketoreductases.

Biocatalysts for natural product biosynthesis

Natural products are important sources of pharmaceuticals, in part owing to their diverse biological activities. Enzymes from natural product biosynthetic pathways have become attractive candidates as biocatalysts for modifying the structures and bioactivities of these complex compounds. Numerous enzymes have been harvested to generate innovative scaffolds, large-scale synthesis of chiral building blocks, and semisynthesis of medicinally relevant natural product derivatives. This review discusses recent examples from three areas: (a) polyketide catalytic domain engineering geared toward synthesis of new polyketides, (b) engineering of tailoring enzymes (other than oxidative enzymes) as biocatalysts, and (c) in vitro total synthesis of natural products using purified enzyme components. With the availability of exponentially increasing genomic information and new genome mining tools, many new and powerful biocatalysts tailored for pharmaceutical synthesis will likely emerge from secondary metabolism.

Structural and functional studies of a trans-acyltransferase polyketide assembly line enzyme that catalyzes stereoselective α- and β-ketoreduction

While the cis-acyltransferase modular polyketide synthase assembly lines have largely been structurally dissected, enzymes from within the recently discovered trans-acyltransferase polyketide synthase assembly lines are just starting to be observed crystallographically. Here we examine the ketoreductase (KR) from the first polyketide synthase module of the bacillaene nonribosomal peptide synthetase/polyketide synthase at 2.35-Å resolution. This KR naturally reduces both α- and β-keto groups and is the only KR known to do so during the biosynthesis of a polyketide. The isolated KR not only reduced an N-acetylcysteamine-bound β-keto substrate to a D-β-hydroxy product, but also an N-acetylcysteamine-bound α-keto substrate to an L-α-hydroxy product. That the substrates must enter the active site from opposite directions to generate these stereochemistries suggests that the acyl-phosphopantetheine moiety is capable of accessing very different conformations despite being anchored to a serine residue of a docked acyl carrier protein. The features enabling stereocontrolled α-ketoreduction may not be extensive since a KR that naturally reduces a β-keto group within a cis-acyltransferase polyketide synthase was identified that performs a completely stereoselective reduction of the same α-keto substrate to generate the D-α-hydroxy product. A sequence analysis of trans-acyltransferase KRs reveals that a single residue, rather than a three-residue motif found in cis-acyltransferase KRs, is predictive of the orientation of the resulting β-hydroxyl group.

Insights into the programmed ketoreduction of partially reducing polyketide synthases: stereo- and substrate-specificity of the ketoreductase domain

Evidence are provided to support that partially reducing polyketide synthases achieve programmed ketoreduction by differential recognition of polyketide intermediates.

Structural studies of an A2-type modular polyketide synthase ketoreductase reveal features controlling α-substituent stereochemistry

Modular polyketide synthase ketoreductases often set two stereocenters when reducing intermediates in the biosynthesis of a complex polyketide. Here we report the 2.55-Å resolution structure of an A2-type ketoreductase from the 11th module of the amphotericin polyketide synthase that sets a combination of l-α-methyl and l-β-hydroxyl stereochemistries and represents the final catalytically competent ketoreductase type to be structurally elucidated. Through structure-guided mutagenesis a double mutant of an A1-type ketoreductase was generated that functions as an A2-type ketoreductase on a diketide substrate analogue, setting an α-alkyl substituent in an l-orientation rather than in the d-orientation set by the unmutated ketoreductase. When the activity of the double mutant was examined in the context of an engineered triketide lactone synthase, the anticipated triketide lactone was not produced even though the ketoreductase-containing module still reduced the diketide substrate analogue as expected. These findings suggest that re-engineered ketoreductases may be catalytically outcompeted within engineered polyketide synthase assembly lines.