Crystal structure of FabZ-ACP complex reveals a dynamic seesaw-like catalytic mechanism of dehydratase in fatty acid biosynthesis

HadBD dehydratase from Mycobacterium tuberculosis fatty acid synthase type II: A singular structure for a unique function

Worldwide, tuberculosis is the second leading infectious killer and multidrug resistance severely hampers disease control. Mycolic acids are a unique category of lipids that are essential for viability, virulence, and persistence of the causative agent, Mycobacterium tuberculosis (Mtb). Therefore, enzymes involved in mycolic acid biosynthesis represent an important class of drug targets. We previously showed that the (3R)-hydroxyacyl-ACP dehydratase (HAD) protein HadD is dedicated mainly to the production of ketomycolic acids and plays a determinant role in Mtb biofilm formation and virulence. Here, we discovered that HAD activity requires the formation of a tight heterotetramer between HadD and HadB, a HAD unit encoded by a distinct chromosomal region. Using biochemical, structural, and cell-based analyses, we showed that HadB is the catalytic subunit, whereas HadD is involved in substrate binding. Based on HadBDMtb crystal structure and substrate-bound models, we identified determinants of the ultra-long-chain lipid substrate specificity and revealed details of structure-function relationship. HadBDMtb unique function is partly due to a wider opening and a higher flexibility of the substrate-binding crevice in HadD, as well as the drastically truncated central α-helix of HadD hotdog fold, a feature described for the first time in a HAD enzyme. Taken together, our study shows that HadBDMtb , and not HadD alone, is the biologically relevant functional unit. These results have important implications for designing innovative antivirulence molecules to fight tuberculosis, as they suggest that the target to consider is not an isolated subunit, but the whole HadBD complex.

Unsaturated fatty acid synthesis in bacteria: Mechanisms and regulation of canonical and remarkably noncanonical pathways

Unsaturated phospholipid acyl chains are required for membrane function in most bacteria. The double bonds of the cis monoenoic chains arise by two distinct pathways depending on whether oxygen is required. The oxygen-independent pathway (traditionally called the anaerobic pathway) introduces the cis double bond by isomerization of the trans double bond intermediate of the fatty acid elongation cycle. Double bond isomerization occurs at an intermediate chain length (e.g., C10) and the isomerization product is elongated to the C16-C18 chains that become phospholipid monoenoic acyl chains. This pathway was first delineated in Escherichia coli and became the paradigm pathway. However, studies of other bacteria show deviations from this paradigm, the most exceptional being reversal of the fatty acid elongation cycle by a reaction paralleling the initial step in the β-oxidative degradation of fatty acids. In the oxygen-dependent pathway diiron enzymes called desaturases introduce a double bond into a saturated acyl chain by regioselective cis dehydrogenation through activation of molecular oxygen with an active-site diiron cluster. This difficult hydrogen abstraction from a methylene group often occurs at the midpoint of a saturated fatty acyl chain. In bacteria the acyl chain is a phospholipid acyl chain, and the desaturase is membrane bound. Both the oxygen-independent oxygen-dependent pathways are transcriptionally regulated by repressor and activator proteins that respond to small molecule ligands such as acyl-CoAs. However, in Bacillus subtilis the desaturase is synthesized only at low growth temperatures, a process controlled by a signal transduction regulatory pathway dependent on membrane lipid properties.

The Molecular Basis of Catalysis by SDR Family Members Ketoacyl-ACP Reductase FabG and Enoyl-ACP Reductase FabI in Type-II Fatty Acid Biosynthesis

The short-chain dehydrogenase/reductase (SDR) superfamily members acyl-ACP reductases FabG and FabI are indispensable core enzymatic modules and catalytic orientation controllers in type-II fatty acid biosynthesis. Herein, we report their distinct substrate allosteric recognition and enantioselective reduction mechanisms. FabG achieves allosteric regulation of ACP and NADPH through ACP binding across two adjacent FabG monomers, while FabI follows an irreversible compulsory order of substrate binding in that NADH binding must precede that of ACP on a discrete FabI monomer. Moreover, FabG and FabI utilize a backdoor residue Phe187 or a "rheostat" α8 helix for acyl chain length selection, and their corresponding triad residues Ser142 or Tyr145 recognize the keto- or enoyl-acyl substrates, respectively, facilitating initiation of nucleophilic attack by NAD(P)H. The other two triad residues (Tyr and Lys) mediate subsequent proton transfer and (R)-3-hydroxyacyl- or saturated acyl-ACP production.

Structures of an unusual 3-hydroxyacyl dehydratase (FabZ) from a ladderane-producing organism with an unexpected substrate preference

The genomes of anaerobic ammonium-oxidizing (anammox) bacteria contain a gene cluster comprising genes of unusual fatty acid biosynthesis enzymes that were suggested to be involved in the synthesis of the unique "ladderane" lipids produced by these organisms. This cluster encodes an acyl carrier protein (denoted as "amxACP") and a variant of FabZ, an ACP-3-hydroxyacyl dehydratase. In this study, we characterize this enzyme, which we call anammox-specific FabZ ("amxFabZ"), to investigate the unresolved biosynthetic pathway of ladderane lipids. We find that amxFabZ displays distinct sequence differences to "canonical" FabZ, such as a bulky, apolar residue on the inside of the substrate binding tunnel, where the canonical enzyme has a glycine. Additionally, substrate screens suggest that amxFabZ efficiently converts substrates with acyl chain lengths of up to eight carbons, whereas longer substrates are converted much more slowly under the conditions used. We also present crystal structures of amxFabZs, mutational studies and the structure of a complex between amxFabZ and amxACP, which show that the structures alone cannot explain the apparent differences from canonical FabZ. Moreover, we find that while amxFabZ does dehydrate substrates bound to amxACP, it does not convert substrates bound to canonical ACP of the same anammox organism. We discuss the possible functional relevance of these observations in the light of proposals for the mechanism for ladderane biosynthesis.

Phospholipid synthesis inside phospholipid membrane vesicles

Construction of living artificial cells from genes and molecules can expand our understanding of life system and establish a new aspect of bioengineering. However, growth and division of cell membrane that are basis of cell proliferation are still difficult to reconstruct because a high-yielding phospholipid synthesis system has not been established. Here, we developed a cell-free phospholipid synthesis system that combines fatty acid synthesis and cell-free gene expression system synthesizing acyltransferases. The synthesized fatty acids were sequentially converted into phosphatidic acids by the cell-free synthesized acyltransferases. Because the system can avoid the accumulation of intermediates inhibiting lipid synthesis, sub-millimolar phospholipids could be synthesized within a single reaction mixture. We also performed phospholipid synthesis inside phospholipid membrane vesicles, which encapsulated all the components, and showed the phospholipids localized onto the mother membrane. Our approach would be a platform for the construction of self-reproducing artificial cells since the membrane can grow sustainably.

Uncovering lipopolysaccharide regulation in bacteria via the critical lipid binding tunnel of YciS/YciM

Gram-negative bacteria contain an asymmetric outer membrane, in which the outer leaflet is composed of lipopolysaccharide (LPS). LPS, a drug target of polymyxin, plays an essential role in drug resistance, biofilm formation, and pathogenesis. An important inner membrane protein, YciM, may be responsible for the regulation of LPS biosynthesis and transport. Here, we report the crystal structure of YciM from Salmonella typhimurium in a complex with a non-specifically bond molecule, an ethylene glycol, which identified a tunnel that could bind lipids. Our in vitro assays showed that YciM could bind lipid molecules with affinity in the micromolar range, while mutagenic and functional studies confirmed that lipid-binding residues are critical for the function of YciM. Additionally, our data also showed that YciM accurately regulates LPS biosynthesis and transport with YciS, which could help to better understand the regulation mechanism of LPS.

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Identifying a critical lipid binding tunnel of YciS/YciM

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The lipid binding tunnel could bind lipid molecules

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Mutants of lipid binding tunnel inhibit cell growth severely

Metabolic and Structural Insights into Hydrogen Sulfide Mis-Regulation in Enterococcus faecalis

Hydrogen sulfide (H2S) is implicated as a cytoprotective agent that bacteria employ in response to host-induced stressors, such as oxidative stress and antibiotics. The physiological benefits often attributed to H2S, however, are likely a result of downstream, more oxidized forms of sulfur, collectively termed reactive sulfur species (RSS) and including the organic persulfide (RSSH). Here, we investigated the metabolic response of the commensal gut microorganism Enterococcus faecalis to exogenous Na2S as a proxy for H2S/RSS toxicity. We found that exogenous sulfide increases protein abundance for enzymes responsible for the biosynthesis of coenzyme A (CoA). Proteome S-sulfuration (persulfidation), a posttranslational modification implicated in H2S signal transduction, is also widespread in this organism and is significantly elevated by exogenous sulfide in CstR, the RSS sensor, coenzyme A persulfide (CoASSH) reductase (CoAPR) and enzymes associated with de novo fatty acid biosynthesis and acetyl-CoA synthesis. Exogenous sulfide significantly impacts the speciation of fatty acids as well as cellular concentrations of acetyl-CoA, suggesting that protein persulfidation may impact flux through these pathways. Indeed, CoASSH is an inhibitor of E. faecalis phosphotransacetylase (Pta), suggesting that an important metabolic consequence of increased levels of H2S/RSS may be over-persulfidation of this key metabolite, which, in turn, inhibits CoA and acyl-CoA-utilizing enzymes. Our 2.05 Å crystallographic structure of CoA-bound CoAPR provides new structural insights into CoASSH clearance in E. faecalis.

Path to Actinorhodin: Regio- and Stereoselective Ketone Reduction by a Type II Polyketide Ketoreductase Revealed in Atomistic Detail

In type II polyketide synthases (PKSs), which typically biosynthesize several antibiotic and antitumor compounds, the substrate is a growing polyketide chain, shuttled between individual PKS enzymes, while covalently tethered to an acyl carrier protein (ACP): this requires the ACP interacting with a series of different enzymes in succession. During biosynthesis of the antibiotic actinorhodin, produced by Streptomyces coelicolor, one such key binding event is between an ACP carrying a 16-carbon octaketide chain (actACP) and a ketoreductase (actKR). Once the octaketide is bound inside actKR, it is likely cyclized between C7 and C12 and regioselective reduction of the ketone at C9 occurs: how these elegant chemical and conformational changes are controlled is not yet known. Here, we perform protein-protein docking, protein NMR, and extensive molecular dynamics simulations to reveal a probable mode of association between actACP and actKR; we obtain and analyze a detailed model of the C7-C12-cyclized octaketide within the actKR active site; and we confirm this model through multiscale (QM/MM) reaction simulations of the key ketoreduction step. Molecular dynamics simulations show that the most thermodynamically stable cyclized octaketide isomer (7R,12R) also gives rise to the most reaction competent conformations for ketoreduction. Subsequent reaction simulations show that ketoreduction is stereoselective as well as regioselective, resulting in an S-alcohol. Our simulations further indicate several conserved residues that may be involved in selectivity of C7-12 cyclization and C9 ketoreduction. Detailed insights obtained on ACP-based substrate presentation in type II PKSs can help design ACP-ketoreductase systems with altered regio- or stereoselectivity.

Helicobacter pylori FabX contains a [4Fe-4S] cluster essential for unsaturated fatty acid synthesis

Unsaturated fatty acids (UFAs) are essential for functional membrane phospholipids in most bacteria. The bifunctional dehydrogenase/isomerase FabX is an essential UFA biosynthesis enzyme in the widespread human pathogen Helicobacter pylori, a bacterium etiologically related to 95% of gastric cancers. Here, we present the crystal structures of FabX alone and in complexes with an octanoyl-acyl carrier protein (ACP) substrate or with holo-ACP. FabX belongs to the nitronate monooxygenase (NMO) flavoprotein family but contains an atypical [4Fe-4S] cluster absent in all other family members characterized to date. FabX binds ACP via its positively charged α7 helix that interacts with the negatively charged α2 and α3 helices of ACP. We demonstrate that the [4Fe-4S] cluster potentiates FMN oxidation during dehydrogenase catalysis, generating superoxide from an oxygen molecule that is locked in an oxyanion hole between the FMN and the active site residue His182. Both the [4Fe-4S] and FMN cofactors are essential for UFA synthesis, and the superoxide is subsequently excreted by H. pylori as a major resource of peroxide which may contribute to its pathogenic function in the corrosion of gastric mucosa.

Activity Mapping the Acyl Carrier Protein: Elongating Ketosynthase Interaction in Fatty Acid Biosynthesis

Elongating ketosynthases (KSs) catalyze carbon-carbon bond-forming reactions during the committed step for each round of chain extension in both fatty acid synthases (FASs) and polyketide synthases (PKSs). A small α-helical acyl carrier protein (ACP) shuttles fatty acyl intermediates between enzyme active sites. To accomplish this task, the ACP relies on a series of dynamic interactions with multiple partner enzymes of FAS and associated FAS-dependent pathways. Recent structures of the Escherichia coli FAS ACP, AcpP, in covalent complexes with its two cognate elongating KSs, FabF and FabB, provide high-resolution details of these interfaces, but a systematic analysis of specific interfacial interactions responsible for stabilizing these complexes has not yet been undertaken. Here, we use site-directed mutagenesis with both in vitro and in vivo activity analyses to quantitatively evaluate these contacting surfaces between AcpP and FabF. We delineate the FabF interface into three interacting regions and demonstrate the effects of point mutants, double mutants, and region deletion variants. Results from these analyses reveal a robust and modular FabF interface capable of tolerating seemingly critical interface mutations with only the deletion of an entire region significantly compromising activity. Structure and sequence analyses of FabF orthologs from related type II FAS pathways indicate significant conservation of type II FAS KS interface residues and, overall, support its delineation into interaction regions. These findings strengthen our mechanistic understanding of molecular recognition events between ACPs and FAS enzymes and provide a blueprint for engineering ACP-dependent biosynthetic pathways.

A kinetic rationale for functional redundancy in fatty acid biosynthesis

Cells build fatty acids with biocatalytic assembly lines in which a subset of enzymes often exhibit overlapping activities (e.g., two enzymes catalyze one or more identical reactions). Although the discrete enzymes that make up fatty acid pathways are well characterized, the importance of catalytic overlap between them is poorly understood. We developed a detailed kinetic model of the fatty acid synthase (FAS) of Escherichia coli and paired that model with a fully reconstituted in vitro system to examine the capabilities afforded by functional redundancy in fatty acid synthesis. The model captures-and helps explain-the effects of experimental perturbations to FAS systems and provides a powerful tool for guiding experimental investigations of fatty acid assembly. Compositional analyses carried out in silico and in vitro indicate that FASs with multiple partially redundant enzymes enable tighter (i.e., more independent and/or broader range) control of distinct biochemical objectives-the total production, unsaturated fraction, and average length of fatty acids-than FASs with only a single multifunctional version of each enzyme (i.e., one enzyme with the catalytic capabilities of two partially redundant enzymes). Maximal production of unsaturated fatty acids, for example, requires a second dehydratase that is not essential for their synthesis. This work provides a kinetic, control-theoretic rationale for the inclusion of partially redundant enzymes in fatty acid pathways and supplies a valuable framework for carrying out detailed studies of FAS kinetics.

Membrane fluidity adjusts the insertion of the transacylase PlsX to regulate phospholipid biosynthesis in Gram-positive bacteria

PlsX plays a central role in the coordination of fatty acid and phospholipid biosynthesis in Gram-positive bacteria. PlsX is a peripheral membrane acyltransferase that catalyzes the conversion of acyl-ACP to acyl-phosphate, which is in turn utilized by the polytopic membrane acyltransferase PlsY on the pathway of bacterial phospholipid biosynthesis. We have recently studied the interaction between PlsX and membrane phospholipids in vivo and in vitro, and observed that membrane association is necessary for the efficient transfer of acyl-phosphate to PlsY. However, understanding the molecular basis of such a channeling mechanism remains a major challenge. Here, we disentangle the binding and insertion events of the enzyme to the membrane, and the subsequent catalysis. We show that PlsX membrane binding is a process mostly mediated by phospholipid charge, whereas fatty acid saturation and membrane fluidity remarkably influence the membrane insertion step. Strikingly, the PlsX^L254E mutant, whose biological functionality was severely compromised in vivo but remains catalytically active in vitro, was able to superficially bind to phospholipid vesicles, nevertheless, it loses the insertion capacity, strongly supporting the importance of membrane insertion in acyl-phosphate delivery. We propose a mechanism in which membrane fluidity governs the insertion of PlsX and thus regulates the biosynthesis of phospholipids in Gram-positive bacteria. This model may be operational in other peripheral membrane proteins with an unprecedented impact in drug discovery/development strategies.

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.

Transcriptomic Analysis Reveals the High-Oleic Acid Feedback Regulating the Homologous Gene Expression of Stearoyl-ACP Desaturase 2 (SAD2) in Peanuts

Peanuts with high oleic acid content are usually considered to be beneficial for human health and edible oil storage. In breeding practice, peanut lines with high monounsaturated fatty acids are selected using fatty acid desaturase 2 (FAD2), which is responsible for the conversion of oleic acid (C18:1) to linoleic acid (C18:2). Here, comparative transcriptomics were used to analyze the global gene expression profile of high- and normal-oleic peanut cultivars at six time points during seed development. First, the mutant type of FAD2 was determined in the high-oleic peanut (H176). The result suggested that early translation termination occurred simultaneously in the coding sequence of FAD2-A and FAD2-B, and the cultivar H176 is capable of utilizing a potential germplasm resource for future high-oleic peanut breeding. Furthermore, transcriptomic analysis identified 74 differentially expressed genes (DEGs) involved in lipid metabolism in high-oleic peanut seed, of which five DEGs encoded the fatty acid desaturase. Aradu.XM2MR belonged to the homologous gene of stearoyl-ACP (acyl carrier protein) desaturase 2 (SAD2) that converted the C18:0 into C18:1. Further subcellular localization studies indicated that FAD2 was located at the endoplasmic reticulum (ER), and Aradu.XM2MR was targeted to the plastid in Arabidopsis protoplast cells. To examine the dynamic mechanism of this finding, we focused on the peroxidase (POD)-mediated fatty acid (FA) degradation pathway. The fad2 mutant significantly increased the POD activity and H₂O₂ concentration at the early stage of seed development, implying that redox signaling likely acted as a messenger to connect the signaling transduction between the high-oleic content and Aradu.XM2MR transcription level. Taken together, transcriptome analysis revealed the feedback mechanism of SAD2 (Aradu.XM2MR) associated with FAD2 mutation during the seed developmental stage, which could provide a potential peanut breeding strategy based on identified candidate genes to improve the content of oleic acid.

Molecular basis for interactions between an acyl carrier protein and a ketosynthase

Fatty acid synthases are dynamic ensembles of enzymes that can biosynthesize long hydrocarbon chains efficiently. Here we visualize the interaction between the Escherichia coli acyl carrier protein (AcpP) and β-ketoacyl-ACP-synthase I (FabB) using X-ray crystallography, NMR, and molecular dynamics simulations. We leveraged this structural information to alter lipid profiles in vivo and provide a molecular basis for how protein-protein interactions can regulate the fatty acid profile in E. coli.

A back-door Phenylalanine coordinates the stepwise hexameric loading of acyl carrier protein by the fatty acid biosynthesis enzyme β-hydroxyacyl-acyl carrier protein dehydratase (FabZ)

The fatty acid biosynthesis pathway (FAS) was a fundamental procedure to generate a diversity of lipid components for cellular metabolism in bacteria, while the mechanism of substrate recognition remains unclear. The β-hydroxyacyl-acyl carrier protein dehydratase hexamer (FabZ) is an essential module in the elongation cycle of type-II FAS, catalyzing the dehydration of β-hydroxyacyl-lipid substrate carried by the holo form acyl carrier protein (holo-ACP). We previously elucidated an alternating seesaw-like ACP loading manner within a FabZ dimer subunits, mediated by a front-door residue Tyrosine (Tyr100). Here, we demonstrated that a back-door residue Phenylalanine (Phe83) of FabZ regulates the stepwise hexameric loading of ACP. Our finding represents clues as to the dynamic ACP recognition and catalysis mechanism of dehydratase in fatty acid biosynthesis, and provides critical information for developing antimicrobials targeting the dehydratase module in fatty acid biosynthesis pathway.

Structural and dynamical rationale for fatty acid unsaturation in Escherichia coli

Fatty acid biosynthesis in α- and γ-proteobacteria requires two functionally distinct dehydratases, FabA and FabZ. Here, mechanistic cross-linking facilitates the structural characterization of a stable hexameric complex of six Escherichia coli FabZ dehydratase subunits with six AcpP acyl carrier proteins. The crystal structure sheds light on the divergent substrate selectivity of FabA and FabZ by revealing distinct architectures of the binding pocket. Molecular dynamics simulations demonstrate differential biasing of substrate orientations and conformations within the active sites of FabA and FabZ such that FabZ is preorganized to catalyze only dehydration, while FabA is primed for both dehydration and isomerization.

Decrypting the oscillating nature of the 4'-phosphopantetheine arm in acyl carrier protein AcpM of Mycobacterium tuberculosis

In Mycobacterium tuberculosis, acyl carrier protein (AcpM)-mediated fatty acid synthase type II is integral for the synthesis of mycolic acids. AcpM, designated as an atypical ACP, comprises of a putative 33 amino acid long C-terminal extension which is distinctive in nature. Here, we aimed at devising an 'easy-to-go' method for the generation of crypto-AcpM loaded with a solvatochromic probe 7-Nitrobenz-2-oxa-1,3-diazol-4-yl, which is linked to the 4'-phosphopantetheine (Ppant) prosthetic group of AcpM. The crypto-AcpM, coupled with fluorescence spectroscopy and molecular dynamics simulation studies, was employed to explore the elusive dynamics of Ppant arm in AcpM. This investigation establishes the role of the flexible C-terminal extension of AcpM in regulating the prosthetic group sequestration ability by modulating the 'Asp-Ser-Leu' motif.

Analysis of Interdependent Kinetic Controls of Fatty Acid Synthases

Biocatalytic systems (e.g., multienzyme pathways or complexes) enable the conversion of simple sugars into complex products under ambient conditions and, thus, represent promising platforms for the synthesis of renewable fuels and chemicals. Unfortunately, to date, many of these systems have proven difficult to engineer without a detailed understanding of the kinetic relationships that regulate the concerted action of their constituent enzymes. This study develops a mechanistic kinetic model of the fatty acid synthase (FAS) of Escherichia coli and uses that model to determine how different FAS components work together to control the production of free fatty acids-precursors to a wide range of oleochemicals. Perturbational analyses indicate that the modification or overexpression of a single FAS component can depress fatty acid production (a commonly observed phenomenon) by sequestering the proteins with which it interacts and/or by depleting common substrate pools. Compositional studies, in turn, suggest that simple changes in the ratios of FAS components can alter the average length of fatty acids but show that specialized enzymes (i.e., highly specific ketoacyl synthases or thioesterases) are required for narrow product profiles. Intriguingly, a sensitivity analysis indicates that two components primarily influence-and, thus, enable fine control over-total production, but suggests that the enzymes that regulate product profile are more broadly influential. Findings thus reveal the general importance of kinetic considerations in efforts to engineer fatty acid biosynthesis and provide strategies-and a kinetic model-for incorporating those considerations into FAS designs.

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.

The structural organization of substrate loading in iterative polyketide synthases

Polyketide synthases (PKSs) are microbial multienzymes for the biosynthesis of biologically potent secondary metabolites. Polyketide production is initiated by the loading of a starter unit onto an integral acyl carrier protein (ACP) and its subsequent transfer to the ketosynthase (KS). Initial substrate loading is achieved either by multidomain loading modules or by the integration of designated loading domains, such as starter unit acyltransferases (SAT), whose structural integration into PKS remains unresolved. A crystal structure of the loading/condensing region of the nonreducing PKS CTB1 demonstrates the ordered insertion of a pseudodimeric SAT into the condensing region, which is aided by the SAT-KS linker. Cryo-electron microscopy of the post-loading state trapped by mechanism-based crosslinking of ACP to KS reveals asymmetry across the CTB1 loading/-condensing region, in accord with preferential 1:2 binding stoichiometry. These results are critical for re-engineering the loading step in polyketide biosynthesis and support functional relevance of asymmetric conformations of PKSs.

Combination of type II fatty acid biosynthesis enzymes and thiolases supports a functional β-oxidation reversal

An engineered reversal of the β-oxidation cycle (r-BOX) and the fatty acid biosynthesis (FAB) pathway are promising biological platforms for advanced fuel and chemical production in part due to their iterative nature supporting the synthesis of various chain length products. While diverging in their carbon-carbon elongation reaction mechanism, iterative operation of each pathway relies on common chemical conversions (reduction, dehydration, and reduction) differing only in the attached moiety (acyl carrier protein (ACP) in FAB vs Coenzyme A in r-BOX). Given this similarity, we sought to determine whether FAB enzymes can be used in the context of r-BOX as a means of expanding available r-BOX components with a ubiquitous set of well characterized enzymes. Using enzymes from the type II FAB pathway (FabG, FabZ, and FabI) in conjunction with a thiolase catalyzing a non-decarboxylative condensation, we demonstrate that FAB enzymes support a functional r-BOX. Pathway operation with FAB enzymes was improved through computationally directed protein design to develop FabZ variants with amino acid substitutions designed to disrupt hydrogen bonding at the FabZ-ACP interface and introduce steric and electrostatic repulsion between the FabZ and ACP. FabZ with R126W and R121E substitutions resulted in improved carboxylic acid and alcohol production from one- and multiple-turn r-BOX compared to the wild-type enzyme. Furthermore, the ability for FAB enzymes to operate on functionalized intermediates was exploited to produce branched chain carboxylic acids through an r-BOX with functionalized priming. These results not only provide an expanded set of enzymes within the modular r-BOX pathway, but can also potentially expand the scope of products targeted through this pathway by operating with CoA intermediates containing various functional groups.

Insights into Substrate Modification by Dehydratases from Type I Polyketide Synthases

Dehydration reactions play a crucial role in the de novo biosynthesis of fatty acids and a wide range of pharmacologically active polyketide natural products with strong emphasis on human medicine. The type I polyketide synthase PpsC from Mycobacterium tuberculosis catalyzes key biosynthetic steps of lipid virulence factors phthiocerol dimycocerosates and phenolic glycolipids. Given the insolubility of the natural C₂₈-C₃₀ fatty acyl substrate of the PpsC dehydratase (DH) domain, we investigated its structure-function relationships in the presence of shorter surrogate substrates. Since most enzymes belonging to the (R)-specific enoyl hydratase/hydroxyacyl dehydratase family conduct the reverse hydration reaction in vitro, we have determined the X-ray structures of the PpsC DH domain, both unliganded (apo) and in complex with trans-but-2-enoyl-CoA or trans-dodec-2-enoyl-CoA derivatives. This study provides for the first time a snapshot of dehydratase-ligand interactions following a hydration reaction. Our structural analysis allowed us to identify residues essential for substrate binding and activity. The structural comparison of the two complexes also sheds light on the need for long acyl chains for this dehydratase to carry out its function, consistent with both its in vitro catalytic behavior and the physiological role of the PpsC enzyme.