Acyl carrier protein (ACP) synthase (AcpS) catalyzes the transfer of the 4′-phosphopantetheine moiety from coenzyme A (CoA) onto a serine residue of apo-ACP, resulting in the conversion of apo-ACP to the functional holo-ACP. The holo form of bacterial ACP plays an essential role in mediating the transfer of acyl fatty acid intermediates during the biosynthesis of fatty acids and phospholipids. AcpS is therefore an attractive target for therapeutic intervention. In this study, we have purified and characterized the AcpS enzymes from Escherichia coli, Streptococcus pneumoniae, and Mycoplasma pneumoniae, which exemplify gram-negative, gram-positive, and atypical bacteria, respectively. Our gel filtration column chromatography and cross-linking studies demonstrate that the AcpS enzyme from M. pneumoniae, like E. coli enzyme, exhibits a homodimeric structure, but the enzyme from S. pneumoniae exhibits a trimeric structure. Our biochemical studies show that the AcpS enzymes from M. pneumoniae and S. pneumoniae can utilize both short- and long-chain acyl CoA derivatives but prefer long-chain CoA derivatives as substrates. On the other hand, the AcpS enzyme from E. coli can utilize short-chain CoA derivatives but not the long-chain CoA derivatives tested. Finally, our biochemical studies show that M. pneumoniae AcpS is kinetically a very sluggish enzyme compared with those from E. coli and S. pneumoniae. Together, the results of these studies show that the AcpS enzymes from different bacterial species exhibit different native structures and substrate specificities with regard to the utilization of CoA and its derivatives. These findings suggest that AcpS from different microorganisms plays a different role in cellular physiology.
Recently, two types of fatty acid synthases (FASs) have been discovered from apicomplexan parasites. Although significant progress has been made in characterizing these apicomplexan FASs, virtually nothing was previously known about the activation and regulation of these enzymes. In this study, we report the discovery and characterization of two distinct types of phosphopantetheinyl transferase (PPTase) that are responsible for synthesizing holo-acyl carrier protein (ACP) from three apicomplexan parasites: surfactin production element (SFP) type in Cryptosporidium parvum (CpSFP-PPT), holo-ACP synthase (ACPS)-type in Plasmodium falciparum (PfACPS-PPT), and both SFP and ACPS types in Toxoplasma gondii (TgSFP-PPT and TgACPS-PPT). CpSFP-PPT and TgSFP-PPT are monofunctional, cytosolic, and phylogenetically related to animal PPTases. However, PfACPS-PPT and TgACPS-PPT are bifunctional (fused with a metal-dependent hydrolase), likely targeted to the apicoplast, and more closely related to proteobacterial PPTases. The function of apicomplexan PPTases has been confirmed by detailed functional analysis using recombinant CpSFP-PPT expressed from an artificially synthesized gene with codon usage optimized for Escherichia coli. The recombinant CpSFP-PPT was able to activate the ACP domains from the C. parvum type I FAS in vitro using either CoA or acetyl-CoA as a substrate, or in vivo when coexpressed in bacteria, with kinetic characteristics typical of PPTases. These observations suggest that the two types of fatty acid synthases in the Apicomplexa are activated and regulated by two evolutionarily distinct PPTases.
Acyl carrier protein (ACP) is a required cofactor for fatty acid synthesis in Escherichia coli. Mutants lacking beta-ketoacyl-ACP synthase II activity (fabF1 or fabF3) possessed a different molecular species of ACP (F-ACP) that was separated from the normal form of the protein by conformationally sensitive gel electrophoresis. Synthase I mutants contained the normal protein. Complementation of fabF1 mutants with an F' factor harboring the wild-type synthase II allele resulted in the appearance of normal ACP, whereas complementation with an F' possessing the fabF2 allele (a mutation that produces a synthase II enzyme with altered catalytic activity) resulted in the production of both forms of ACP. The structural difference between F-ACP and ACP persisted after the removal of the 4'-phosphopantetheine prosthetic group, and both forms of the protein had identical properties in an in vitro fatty acid synthase assay. Both ACP and F-ACP were purified to homogeneity, and their primary amino acid sequences were determined. The two ACP species were identical but differed from the sequence reported for E. coli E-15 ACP in that an Asn instead of an Asp was at position 24 and an Ile instead of a Val was at position 43. Therefore, F-ACP appears to be a modification of ACP that is detected when beta-ketoacyl-ACP synthase II activity is impaired.
Bacterial acyl carrier protein synthase plays an essential role in the synthesis of fatty acids, nonribosomal peptides and polyketides. In Mycobacterium tuberculosis, AcpS or group I phosphopentatheine transferase exhibits two different structural conformations depending upon the pH.
The crystal structures of acyl carrier protein synthase (AcpS) from Mycobacterium tuberculosis (Mtb) and Corynebacterium ammoniagenes determined at pH 5.3 and pH 6.5, respectively, are reported. Comparison of the Mtb apo-AcpS structure with the recently reported structure of the Mtb AcpS–ADP complex revealed that AcpS adopts two different conformations: the orthorhombic and trigonal space-group structures show structural differences in the α2 helix and in the conformation of the α3–α4 connecting loop, which is in a closed conformation. The apo-AcpS structure shows electron density for the entire model and was obtained at lower pH values (4.4–6.0). In contrast, at a higher pH value (6.5) AcpS undergoes significant conformational changes, resulting in disordered regions that show no electron density in the AcpS model. The solved structures also reveal that C. ammoniagenes AcpS undergoes structural rearrangement in two regions, similar to the recently reported Mtb AcpS–ADP complex structure. In vitro reconstitution experiments show that AcpS has a higher post-translational modification activity between pH 4.4 and 6.0 than at pH values above 6.5, where the activity drops owing to the change in conformation. The results show that apo-AcpS and AcpS–ADP adopt different conformations depending upon the pH conditions of the crystallization solution.
acyl carrier protein synthases; Mycobacterium tuberculosis; Corynebacterium ammoniagenes; acyl carrier proteins; fatty-acid synthases
Phosphopantetheinyl transferases (PPTases) are essential to the activities of type I/II polyketide synthases (PKSs) and nonribosomal peptide synthetases (NRPSs) through converting acyl carrier proteins (ACPs) in PKSs and peptidyl carrier proteins (PCPs) in NRPSs from inactive apo-forms into active holo-forms, leading to biosynthesis of polyketides and nonribosomal peptides. The industrial natamycin (NTM) producer, Streptomyces chattanoogensis L10, contains two PPTases (SchPPT and SchACPS) and five PKSs. Biochemical characterization of these two PPTases shows that SchPPT catalyzes the phosphopantetheinylation of ACPs in both type I PKSs and type II PKSs, SchACPS catalyzes the phosphopantetheinylation of ACPs in type II PKSs and fatty acid synthases (FASs), and the specificity of SchPPT is possibly controlled by its C terminus. Inactivation of SchPPT in S. chattanoogensis L10 abolished production of NTM but not the spore pigment, while overexpression of the SchPPT gene not only increased NTM production by about 40% but also accelerated productions of both NTM and the spore pigment. Thus, we elucidated a comprehensive phosphopantetheinylation network of PKSs and improved polyketide production by engineering the cognate PPTase in bacteria.
We have characterized an acyl carrier protein (ACP) presumed to be involved in the synthesis of fatty acids in Streptomyces coelicolor A3(2). This is the third ACP to have been identified in S. coelicolor; the two previously characterized ACPs are involved in the synthesis of two aromatic polyketides: the blue-pigmented antibiotic actinorhodin and a grey pigment associated with the spore walls. The three ACPs are clearly related. The presumed fatty acid synthase (FAS) ACP was partially purified, and the N-terminal amino acid sequence was obtained. The corresponding gene (acpP) was cloned and sequenced and found to lie within 1 kb of a previously characterized gene (fabD) encoding another subunit of the S. coelicolor FAS, malonyl coenzyme A:ACP acyl-transferase. Expression of S. coelicolor acpP in Escherichia coli yielded several different forms, whose masses corresponded to the active (holo) form of the protein carrying various acyl substituents. To test the mechanisms that normally prevent the FAS ACP from substituting for the actinorhodin ACP, acpP was cloned in place of actI-open reading frame 3 (encoding the actinorhodin ACP) to allow coexpression of acpP with the act polyketide synthase (PKS) genes. Pigmented polyketide production was observed, but only at a small fraction of its former level. This suggests that the FAS and PKS ACPs may be biochemically incompatible and that this could prevent functional complementation between the FAS and PKSs that potentially coexist within the same cells.
GmACP3 from Geobacter metallireducens is a specialized acyl carrier protein (ACP) whose gene, gmet_2339, is located near genes encoding many proteins involved in lipopolysaccharide (LPS) biosynthesis, indicating a likely function for GmACP3 in LPS production. By overexpression in Escherichia coli, about 50% holo-GmACP3 and 50% apo-GmACP3 were obtained. Apo-GmACP3 exhibited slow precipitation and non-monomeric behavior by 15N NMR relaxation measurements. Addition of 4′-phosphopantetheine (4′-PP) via enzymatic conversion by E. coli holo-ACP synthase, resulted in stable >95% holo-GmACP3 that was characterized as monomeric by 15N relaxation measurements and had no indication of conformational exchange. We have determined a high-resolution solution structure of holo-GmACP3 by standard NMR methods, including refinement with two sets of NH residual dipolar couplings, allowing for a detailed structural analysis of the interactions between 4′-PP and GmACP3. Whereas the overall four helix bundle topology is similar to previously solved ACP structures, this structure has unique characteristics, including an ordered 4′-PP conformation that places the thiol at the entrance to a central hydrophobic cavity near a conserved hydrogen-bonded Trp-His pair. These residues are part of a conserved WDSLxH/N motif found in GmACP3 and it’s orthologs. The helix locations and the large hydrophobic cavity are more similar to medium- and long-chain acyl-ACPs than to other apo- and holo-ACP structures. Taken together, structural characterization along with bioinformatic analysis of nearby genes suggest that GmACP3 is involved in lipid A acylation, possibly by atypical long-chain hydroxy fatty acids, and potentially involved in synthesis of secondary metabolites.
Modular polyketide synthases are multifunctional megasynthases which biosynthesize a variety of secondary metabolites using various combinations of dehydratase (DH), ketoreductase (KR) and enoyl-reductase (ER) domains. During the catalysis of various reductive steps these domains act on a substrate moiety which is covalently attached to the phosphopantetheine (P-pant) group of the holo-Acyl Carrier Protein (holo-ACP) domain, thus necessitating the formation of holo-ACP:DH and holo-ACP:KR complexes. Even though three dimensional structures are available for DH, KR and ACP domains, no structures are available for DH or KR domains in complex with ACP or substrate moieties. Since Ser of holo-ACP is covalently attached to a large phosphopantetheine group, obtaining complexes involving holo-ACP by standard protein-protein docking has been a difficult task.
We have modeled the holo-ACP:DH and holo-ACP:KR complexes for identifying specific residues on DH and KR domains which are involved in interaction with ACP, phosphopantetheine and substrate moiety. A novel combination of protein-protein and protein-ligand docking has been used to first model complexes involving apo-ACP and then dock the phosphopantetheine and substrate moieties using covalent connectivity between ACP, phosphopantetheine and substrate moiety as constraints. The holo-ACP:DH and holo-ACP:KR complexes obtained from docking have been further refined by restraint free explicit solvent MD simulations to incorporate effects of ligand and receptor flexibilities. The results from 50 ns MD simulations reveal that substrate enters into a deep tunnel in DH domain while in case of KR domain the substrate binds a shallow surface exposed cavity. Interestingly, in case of DH domain the predicted binding site overlapped with the binding site in the inhibitor bound crystal structure of FabZ, the DH domain from E.Coli FAS. In case of KR domain, the substrate binding site identified by our simulations was in proximity of the known stereo-specificity determining residues.
We have modeled the holo-ACP:DH and holo-ACP:KR complexes and identified the specific residues on DH and KR domains which are involved in interaction with ACP, phosphopantetheine and substrate moiety. Analysis of the conservation profile of binding pocket residues in homologous sequences of DH and KR domains indicated that, these results can also be extrapolated to reductive domains of other modular PKS clusters.
Molecular dynamics; Protein-ligand docking; Protein-protein interaction; Substrate binding site; Evolutionary conservation; Modular polyketide synthase; Dehydratase domain; Ketoreductase domain
Acyl carrier protein (ACP) is a cofactor in a variety of biosynthetic pathways, including fatty acid metabolism. Thus it is of interest to determine structures of physiologically relevant ACP-fatty acid complexes. We report here the NMR solution structures of spinach ACP with decanoate (10:0-ACP) and stearate (18:0-ACP) attached to the 4′ phosphopantetheine prosthetic group. The protein in the fatty acid complexes adopts a single conformer, unlike apo- and holo-ACP, which interconvert in solution between two major conformers. The protein component of both 10:0- and 18:0-ACP adopts the four-helix bundle topology characteristic of ACP, and a fatty acid binding cavity was identified in both structures. Portions of the protein close in space to the fatty acid and the 4′ phosphopantetheine were identified using filtered/edited NOESY experiments. A docking protocol was used to generate protein structures containing bound fatty acid for 10:0- and 18:0-ACP. In both cases, the predominant structure contained fatty acid bound down the center of the helical bundle, in agreement with the location of the fatty acid binding pockets. These structures demonstrate the conformational flexibility of spinach-ACP and suggest how the protein changes to accommodate its myriad binding partners.
Polyketide synthases elongate a polyketide backbone by condensing carboxylic acid precursors that are thioesterified to either coenzyme A or an acyl carrier protein (ACP). Two of the three known ACP-linked extender units, (2S)-aminomalonyl-ACP and (2R)-hydroxymalonyl-ACP, are found in the biosynthesis of the agriculturally important antibiotic zwittermicin A. We previously reconstituted the formation of (2S)-aminomalonyl-ACP and (2R)-hydroxymalonyl-ACP from the primary metabolites L-serine and 1,3-bisphospho-D-glycerate. In this report we characterize the two acyltransferases involved in the specific transfer of the (2S)-aminomalonyl- and (2R)-hydroxymalonyl moieties from the ACPs associated with extender unit formation to the ACPs integrated into the polyketide synthase. This work establishes which acyltransferases recognize each extender unit and also provides insight into the substrate selectivity of these enzymes. These are important step towards harnessing these rare polyketide synthase extender units for combinatorial biosynthesis.
Polyketide synthase; zwittermicin A; extender unit; aminomalonyl-ACP; hydroxymalonyl-ACP
Although the Escherichia coli fatty acid synthesis (FAS) pathway is the best studied type II fatty acid synthesis system, a major experimental limitation has been the inability to feed intermediates into the pathway in vivo because exogenously-supplied free fatty acids are not efficiently converted to the acyl-acyl carrier protein (ACP) thioesters required by the pathway. We report that expression of Vibrio harveyi acyl-ACP synthetase (AasS), a soluble cytosolic enzyme that ligates free fatty acids to ACP to form acyl-ACPs, allows exogenous fatty acids to enter the E. coli fatty acid synthesis pathway. The free fatty acids are incorporated intact and can be elongated or directly incorporated into complex lipids by acyltransferases specific for acyl-ACPs. Moreover, expression of AasS strains and supplementation with the appropriate fatty acid restored growth to E. coli mutant strains that lack essential fatty acid synthesis enzymes. Thus, this strategy provides a new tool for circumventing the loss of enzymes essential for FAS function.
Acyl-acyl carrier protein thioesterases (acyl-ACP TEs) catalyze the hydrolysis of the thioester bond that links the acyl chain to the sulfhydryl group of the phosphopantetheine prosthetic group of ACP. This reaction terminates acyl chain elongation of fatty acid biosynthesis, and in plant seeds it is the biochemical determinant of the fatty acid compositions of storage lipids.
To explore acyl-ACP TE diversity and to identify novel acyl ACP-TEs, 31 acyl-ACP TEs from wide-ranging phylogenetic sources were characterized to ascertain their in vivo activities and substrate specificities. These acyl-ACP TEs were chosen by two different approaches: 1) 24 TEs were selected from public databases on the basis of phylogenetic analysis and fatty acid profile knowledge of their source organisms; and 2) seven TEs were molecularly cloned from oil palm (Elaeis guineensis), coconut (Cocos nucifera) and Cuphea viscosissima, organisms that produce medium-chain and short-chain fatty acids in their seeds. The in vivo substrate specificities of the acyl-ACP TEs were determined in E. coli. Based on their specificities, these enzymes were clustered into three classes: 1) Class I acyl-ACP TEs act primarily on 14- and 16-carbon acyl-ACP substrates; 2) Class II acyl-ACP TEs have broad substrate specificities, with major activities toward 8- and 14-carbon acyl-ACP substrates; and 3) Class III acyl-ACP TEs act predominantly on 8-carbon acyl-ACPs. Several novel acyl-ACP TEs act on short-chain and unsaturated acyl-ACP or 3-ketoacyl-ACP substrates, indicating the diversity of enzymatic specificity in this enzyme family.
These acyl-ACP TEs can potentially be used to diversify the fatty acid biosynthesis pathway to produce novel fatty acids.
β-Ketoacyl-acyl carrier protein (ACP) synthetase II (KAS II) is one of three Escherichia coli isozymes that catalyze the elongation of growing fatty acid chains by condensation of acyl-ACP with malonyl-ACP. Overexpression of this enzyme has been found to be extremely toxic to E. coli, much more so than overproduction of either of the other KAS isozymes, KAS I or KAS III. The immediate effect of KAS II overproduction is the cessation of phospholipid synthesis, and this inhibition is specifically due to the blockage of fatty acid synthesis. To determine the cause of this inhibition, we examined the intracellular pools of ACP, coenzyme A (CoA), and their acyl thioesters. Although no significant changes were detected in the acyl-ACP pools, the CoA pools were dramatically altered by KAS II overproduction. Malonyl-CoA increased to about 40% of the total cellular CoA pool upon KAS II overproduction from a steady-state level of around 0.5% in the absence of KAS II overproduction. This finding indicated that the conversion of malonyl-CoA to fatty acids had been blocked and could be explained if either the conversion of malonyl-CoA to malonyl-ACP and/or the elongation reactions of fatty acid synthesis had been blocked. Overproduction of malonyl-CoA:ACP transacylase, the enzyme catalyzing the conversion of malonyl-CoA to malonyl-ACP, partially relieved the toxicity of KAS II overproduction, consistent with a model in which high levels of KAS II blocks access of the other KAS isozymes to malonyl-CoA:ACP transacylase.
Germicidin synthase (Gcs) from Streptomyces coelicolor is a type III polyketide synthase (PKS) with broad substrate flexibility for acyl groups linked through a thioester bond to either coenzyme A (CoA) or acyl carrier protein (ACP). Germicidin synthesis was reconstituted in vitro by coupling Gcs with fatty acid biosynthesis. Since Gcs has broad substrate flexibility, we directly compared the kinetic properties of Gcs with both acyl-ACP and acyl-CoA. The catalytic efficiency of Gcs for acyl-ACP was 10-fold higher than for acyl-CoA suggesting a strong preference towards carrier protein starter unit transfer. The 2.9 Å germicidin synthase crystal structure revealed canonical type III PKS architecture along with an unusual helical bundle of unknown function that appears to extend the dimerization interface. A pair of arginine residues adjacent to the active site affect catalytic activity but not ACP binding. This investigation provides new and surprising information about the interactions between type III PKSs and ACPs that will facilitate the construction of engineered systems for production of novel polyketides.
Polyketide synthase; germicidin; acyl carrier protein
The erythromycin A-producing polyketide synthase from the gram-positive bacterium Saccharopolyspora erythraea (formerly Streptomyces erythraeus) has evident structural similarity to fatty acid synthases, particularly to the multifunctional fatty acid synthases found in eukaryotic cells. Fatty acid synthesis in S. erythraea has previously been proposed to involve a discrete acyl carrier protein (ACP), as in most prokaryotic fatty acid synthases. We have cloned and sequenced the structural gene for this ACP and find that it does encode a discrete small protein. The gene lies immediately adjacent to an open reading frame whose gene product shows sequence homology to known beta-ketoacyl-ACP synthases. A convenient expression system for the S. erythraea ACP was obtained by placing the gene in the expression vector pT7-7 in Escherichia coli. In this system the ACP was efficiently expressed at levels 10 to 20% of total cell protein. The recombinant ACP was active in promoting the synthesis of branched-chain acyl-ACP species by extracts of S. erythraea. Electrospray mass spectrometry is shown to be an excellent method for monitoring the efficiency of in vivo posttranslational modification of ACPs.
The 1.51 Å resolution X-ray crystal structure of the trans-acyltransferase (AT) from the “AT-less” disorazole synthase (DSZS), and its acetate complex at 1.35 Å resolution, are reported. Separately, comprehensive alanine scanning mutagenesis of one of its acyl carrier protein substrates (ACP1 from DSZS) led to the identification of a conserved Asp45 residue on the ACP, which contributes to the substrate specificity of this unusual enzyme. Together, these experimental findings were used to derive a model for the selective association of the DSZS AT and its ACP substrate. Towards the goal of structurally characterizing the AT-ACP interface, a strategy was developed for covalently cross-linking active site Ser→Cys mutant of the DSZS AT to its ACP substrate, and for purifying the resulting AT-ACP complex to homogeneity. The S86C DSZS AT mutant was found to be functional, albeit with a 200-fold lower transacylation efficiency than its wild-type counterpart. Our findings provide new insights as well as new opportunities for high-resolution analysis of an important protein-protein interface in polyketide synthases.
In an effort to better understand the control of the formation of branched fatty acids in Micrococcus luteus, the structure of β-ketoacyl-ACP synthase III, which catalyzes the initial step of fatty-acid biosynthesis, has been determined.
Micrococcus luteus is a Gram-positive bacterium that produces iso- and anteiso-branched alkenes by the head-to-head condensation of fatty-acid thioesters [coenzyme A (CoA) or acyl carrier protein (ACP)]; this activity is of interest for the production of advanced biofuels. In an effort to better understand the control of the formation of branched fatty acids in M. luteus, the structure of FabH (MlFabH) was determined. FabH, or β-ketoacyl-ACP synthase III, catalyzes the initial step of fatty-acid biosynthesis: the condensation of malonyl-ACP with an acyl-CoA. Analysis of the MlFabH structure provides insights into its substrate selectivity with regard to length and branching of the acyl-CoA. The most structurally divergent region of FabH is the L9 loop region located at the dimer interface, which is involved in the formation of the acyl-binding channel and thus limits the substrate-channel size. The residue Phe336, which is positioned near the catalytic triad, appears to play a major role in branched-substrate selectivity. In addition to structural studies of MlFabH, transcriptional studies of M. luteus were also performed, focusing on the increase in the ratio of anteiso:iso-branched alkenes that was observed during the transition from early to late stationary phase. Gene-expression microarray analysis identified two genes involved in leucine and isoleucine metabolism that may explain this transition.
biofuels; β-ketoacyl-ACP synthase III; iso- and anteiso-branched alkenes; microarray
A DNA fragment containing the Pseudomonas aeruginosa fabD (encoding malonyl-coenzyme A [CoA]:acyl carrier protein [ACP] transacylase), fabG (encoding β-ketoacyl-ACP reductase), acpP (encoding ACP), and fabF (encoding β-ketoacyl-ACP synthase II) genes was cloned and sequenced. This fab gene cluster is delimited by the plsX (encoding a poorly understood enzyme of phospholipid metabolism) and pabC (encoding 4-amino-4-deoxychorismate lyase) genes; the fabF and pabC genes seem to be translationally coupled. The fabH gene (encoding β-ketoacyl-ACP synthase III), which in most gram-negative bacteria is located between plsX and fabD, is absent from this gene cluster. A chromosomal temperature-sensitive fabD mutant was obtained by site-directed mutagenesis that resulted in a W258Q change. A chromosomal fabF insertion mutant was generated, and the resulting mutant strain contained substantially reduced levels of cis-vaccenic acid. Multiple attempts aimed at disruption of the chromosomal fabG gene were unsuccessful. We purified FabD as a hexahistidine fusion protein (H6-FabD) and ACP in its native form via an ACP-intein-chitin binding domain fusion protein, using a novel expression and purification scheme that should be applicable to ACP from other bacteria. Matrix-assisted laser desorption–ionization spectroscopy, native polyacrylamide electrophoresis, and amino-terminal sequencing revealed that (i) most of the purified ACP was properly modified with its 4′-phosphopantetheine functional group, (ii) it was not acylated, and (iii) the amino-terminal methionine was removed. In an in vitro system, purified ACP functioned as acyl acceptor and H6-FabD exhibited malonyl-CoA:ACP transacylase activity.
The polyunsaturated fatty acid (PUFA) synthases from deep-sea bacteria invariably contain multiple acyl carrier protein (ACP) domains in tandem. This conserved tandem arrangement has been implicated in both amplification of fatty acid production (additive effect) and in structural stabilization of the multidomain protein (synergistic effect). While the more accepted model is one in which domains act independently, recent reports suggest that ACP domains may form higher oligomers. Elucidating the three-dimensional structure of tandem arrangements may therefore give important insights into the functional relevance of these structures, and hence guide bioengineering strategies. In an effort to elucidate the three-dimensional structure of tandem repeats from deep-sea anaerobic bacteria, we have expressed and purified a fragment consisting of five tandem ACP domains from the PUFA synthase from Photobacterium profundum. Analysis of the tandem ACP fragment by analytical gel filtration chromatography showed a retention time suggestive of a multimeric protein. However, small angle X-ray scattering (SAXS) revealed that the multi-ACP fragment is an elongated monomer which does not form a globular unit. Stokes radii calculated from atomic monomeric SAXS models were comparable to those measured by analytical gel filtration chromatography, showing that in the gel filtration experiment, the molecular weight was overestimated due to the elongated protein shape. Thermal denaturation monitored by circular dichroism showed that unfolding of the tandem construct was not cooperative, and that the tandem arrangement did not stabilize the protein. Taken together, these data are consistent with an elongated beads-on-a-string arrangement of the tandem ACP domains in PUFA synthases, and speak against synergistic biocatalytic effects promoted by quaternary structuring. Thus, it is possible to envision bioengineering strategies which simply involve the artificial linking of multiple ACP domains for increasing the yield of fatty acids in bacterial cultures.
Acyltransferase (AT) domains of multimodular polyketide synthases are the primary gatekeepers for stepwise incorporation of building blocks into a growing polyketide chain. Each AT domain has two substrates, an α-carboxylated CoA thioester (e.g. malonyl-CoA or methylmalonyl-CoA) and an acyl carrier protein (ACP). Whereas the acyl-CoA specificity of AT domains has been extensively investigated, little is known about their ACP specificity. Guided by recent high-resolution structural insights, we have systematically probed the protein-protein interactions between AT domains, ACP domains and the linkers that flank AT domains. Representative AT domains of the 6-deoxyerythronolide B synthase (DEBS) have greater than 10-fold specificity for their cognate ACP substrates as compared to other ACP domains from the same synthase. Both the flanking (N- and C-terminal) linkers of an AT domain contributed to the efficiency and specificity of transacylation. As a frame of reference, the activity and specificity of a stand-alone AT domain from the “AT-less” disorazole synthase (DSZS) were also quantified. The activity (kcat/KM) of this AT was >250-fold higher than the corresponding values for DEBS AT domains. Although the AT from DSZS discriminated modestly against ACP domains from DEBS, it exhibited >40-fold higher activity in trans in the presence of these heterologous substrates than their natural AT domains. Our results highlight the opportunity for regioselective modification of a polyketide backbone by in trans complementation of inactivated AT domains. They also reinforce the need for more careful consideration of protein-protein interactions in the engineering of these assembly line enzymes.
The phosphopantetheinyl transferases (PPTs) are a superfamily of essential enzymes required for the synthesis of a wide range of compounds, including fatty acids, polyketides, and nonribosomal peptide metabolites. These enzymes activate carrier proteins in specific biosynthetic pathways by transfer of a phosphopantetheinyl moiety. The diverse PPT superfamily can be divided into two families based on specificity and conserved sequence motifs. The first family is typified by the Escherichia coli acyl carrier protein synthase (AcpS), which is involved in fatty acid synthesis. The prototype of the second family is the broad-substrate-range PPT Sfp, which is required for surfactin biosynthesis in Bacillus subtilis. Most cyanobacteria do not encode an AcpS-like PPT, and furthermore, some of their Sfp-like PPTs belong to a unique phylogenetic subgroup defined by the PPTs involved in heterocyst differentiation. Here, we describe the first functional characterization of a cyanobacterial PPT based on a structural analysis and subsequent functional analysis of the Nodularia spumigena NSOR10 PPT. Southern hybridizations suggested that this enzyme may be the only PPT encoded in the N. spumigena NSOR10 genome. Expression and enzyme characterization showed that this PPT was capable of modifying carrier proteins resulting from both heterocyst glycoplipid synthesis and nodularin toxin synthesis. Cyanobacteria are a unique and vast source of bioactive metabolites; therefore, an understanding of cyanobacterial PPTs is important in order to harness the biotechnological potential of cyanobacterial natural products.
Recent advances in the structural study of fatty acid synthase (FAS) and polyketide synthase (PKS) biosynthetic enzymes have illuminated our understanding of modular enzymes of the acetate pathway. However, one significant and persistent challenge in such analyses is resolution of the acyl carrier protein (ACP), a small (~9 kDa) protein to which biosynthetic intermediates are tethered throughout the biosynthetic cycle. Here we report a chemoenzymatic crosslinking strategy in which the installation of a historical suicide substrate scaffold upon the 4′-phosphopantetheine (PPant) arm of the ACP is used to capture the active site of acyl carrier protein dehydratase (DH) domains in FAS. Through the synthesis of a small panel of related probes we identify structural features essential for ACP–DH crosslinking, and apply gel-based assays to demonstrate the stability as well as purification strategies for isolation of the chemoenzymatically modified ACP. Applying these carrier protein crosslinking techniques to the structural analysis of FAS and PKS complexes has the potential to provide snapshots of these biosynthetic assembly lines at work.
Mechanism-based inhibition; Polyketide biosynthesis; Fatty acid biosynthesis; Dehydratase; Protein; protein interaction
The Streptomyces glaucescens β-ketoacyl-acyl carrier protein (ACP) synthase III (KASIII) initiates straight- and branched-chain fatty acid biosynthesis by catalyzing the decarboxylative condensation of malonyl-ACP with different acyl-coenzyme A (CoA) primers. This KASIII has one cysteine residue, which is critical for forming an acyl-enzyme intermediate in the first step of the process. Three mutants (Cys122Ala, Cys122Ser, Cys122Gln) were created by site-directed mutagenesis. Plasmid-based expression of these mutants in S. glaucescens resulted in strains which generated 75 (Cys122Ala) to 500% (Cys122Gln) more straight-chain fatty acids (SCFA) than the corresponding wild-type strain. In contrast, plasmid-based expression of wild-type KASIII had no effect on fatty acid profiles. These observations are attributed to an uncoupling of the condensation and decarboxylation activities in these mutants (malonyl-ACP is thus converted to acetyl-ACP, a SCFA precursor). Incorporation experiments with perdeuterated acetic acid demonstrated that 9% of the palmitate pool of the wild-type strain was generated from an intact D3 acetyl-CoA starter unit, compared to 3% in a strain expressing the Cys122Gln KASIII. These observations support the intermediacy of malonyl-ACP in generating the SCFA precursor in a strain expressing this mutant. To study malonyl-ACP decarboxylase activity in vitro, the KASIII mutants were expressed and purified as His-tagged proteins in Escherichia coli and assayed. In the absence of the acyl-CoA substrate the Cys122Gln mutant and wild-type KASIII were shown to have comparable decarboxylase activities in vitro. The Cys122Ala mutant exhibited higher activity. This activity was inhibited for all enzymes by the presence of high concentrations of isobutyryl-CoA (>100 μM), a branched-chain fatty acid biosynthetic precursor. Under these conditions the mutant enzymes had no activity, while the wild-type enzyme functioned as a ketoacyl synthase. These observations indicate the likely upper and lower limits of isobutyryl-CoA and related acyl-CoA concentrations within S. glaucescens.
Escherichia coli acetyl coenzyme A carboxylase (ACC), the first enzyme of the fatty acid biosynthetic pathway, is inhibited by acylated derivatives of acyl carrier protein (ACP). ACP lacking an acyl moiety does not inhibit ACC. Acylated derivatives of ACP having chain lengths of 6 to 20 carbon atoms were similarly inhibitory at physiologically relevant concentrations. The observed feedback inhibition was specific to the protein moiety, as shown by the inability of the palmitoyl thioester of spinach ACP I to inhibit ACC.
Sulfated molecules with diverse functions are common in biology, but sulfonation as a method to activate a metabolite for chemical catalysis is rare. Catalytic activity was characterized and crystal structures were determined for two such “activating” sulfotransferases (STs) that sulfonate β-hydroxyacyl thioester substrates. The CurM polyketide synthase (PKS) ST domain from the curacin A biosynthetic pathway of Moorea producens and the olefin synthase (OLS) ST from a hydrocarbon-producing system of Synechococcus PCC 7002 both occur as a unique acyl carrier protein (ACP), ST and thioesterase (TE) tridomain within a larger polypeptide. During pathway termination, these cyanobacterial systems introduce a terminal double bond into the β-hydroxyacyl-ACP-linked substrate by the combined action of the ST and TE. Under in vitro conditions, CurM PKS ST and OLS ST acted on β-hydroxy fatty acyl-ACP substrates; however, OLS ST was not reactive toward analogs of the natural PKS ST substrate bearing a C5-methoxy substituent. The crystal structures of CurM ST and OLS ST revealed that they are members of a distinct protein family relative to other prokaryotic and eukaryotic sulfotransferases. A common binding site for the sulfonate donor 3'-phosphoadenosine-5'-phosphosulfate was visualized in complexes with the product 3'-phosphoadenosine-5'-phosphate. Critical functions for several conserved amino acids in the active site were confirmed by site-directed mutagenesis, including a proposed glutamate catalytic base. A dynamic active-site flap unique to the “activating” ST family affects substrate selectivity and product formation, based on the activities of chimeras of the PKS and OLS STs with exchanged active-site flaps.