A catalytic role for histidine 237 in rat mammary gland thioesterase II

Crystal Structure and Substrate Specificity of Human Thioesterase 2: INSIGHTS INTO THE MOLECULAR BASIS FOR THE MODULATION OF FATTY ACID SYNTHASE

The type I fatty acid synthase (FASN) is responsible for the de novo synthesis of palmitate. Chain length selection and release is performed by the C-terminal thioesterase domain (TE1). FASN expression is up-regulated in cancer, and its activity levels are controlled by gene dosage and transcriptional and post-translational mechanisms. In addition, the chain length of fatty acids produced by FASN is controlled by a type II thioesterase called TE2 (E.C. 3.1.2.14). TE2 has been implicated in breast cancer and generates a broad lipid distribution within milk. The molecular basis for the ability of the TE2 to compete with TE1 for the acyl chain attached to the acyl carrier protein (ACP) domain of FASN is unknown. Herein, we show that human TE1 efficiently hydrolyzes acyl-CoA substrate mimetics. In contrast, TE2 prefers an engineered human acyl-ACP substrate and readily releases short chain fatty acids from full-length FASN during turnover. The 2.8 Å crystal structure of TE2 reveals a novel capping domain insert within the α/β hydrolase core. This domain is reminiscent of capping domains of type II thioesterases involved in polyketide synthesis. The structure also reveals that the capping domain had collapsed onto the active site containing the Ser-101-His-237-Asp-212 catalytic triad. This observation suggests that the capping domain opens to enable the ACP domain to dock and to place the acyl chain and 4'-phosphopantetheinyl-linker arm correctly for catalysis. Thus, the ability of TE2 to prematurely release fatty acids from FASN parallels the role of editing thioesterases involved in polyketide and non-ribosomal peptide synthase synthases.

Regulation of lipid synthesis genes and milk fat production in human mammary epithelial cells during secretory activation

Expression of genes for lipid biosynthetic enzymes during initiation of lactation in humans is unknown. Our goal was to study mRNA expression of lipid metabolic enzymes in human mammary epithelial cell (MEC) in conjunction with the measurement of milk fatty acid (FA) composition during secretory activation. Gene expression from mRNA isolated from milk fat globule (MFG) and milk FA composition were measured from 6 h to 42 days postpartum in seven normal women. Over the first 96 h postpartum, daily milk fat output increased severalfold and mirrored expression of genes for all aspects of lipid metabolism and milk FA production, including lipolysis at the MEC membrane, FA uptake from blood, intracellular FA transport, de novo FA synthesis, FA and glycerol activation, FA elongation, FA desaturation, triglyceride synthesis, cholesterol synthesis, and lipid droplet formation. Expression of the gene for a key lipid synthesis regulator, sterol regulatory element-binding transcription factor 1 (SREBF1), increased 2.0-fold by 36 h and remained elevated over the study duration. Expression of genes for estrogen receptor 1, thyroid hormone-responsive protein, and insulin-induced 2 increased progressively to plateau by 96 h. In contrast, mRNA of peroxisome proliferator-activated receptor-γ decreased severalfold. With onset of lactation, increased de novo synthesis of FA was the most prominent change in milk FA composition and mirrored the expression of FA synthesis genes. In conclusion, milk lipid synthesis and secretion in humans is a complex process requiring the orchestration of a wide variety of pathways of which SREBF1 may play a primary role.

Reveromycin A biosynthesis uses RevG and RevJ for stereospecific spiroacetal formation

Spiroacetal compounds are ubiquitous in nature, and their stereospecific structures are responsible for diverse pharmaceutical activities. Elucidation of the biosynthetic mechanisms that are involved in spiroacetal formation will open the door to efficient generation of stereospecific structures that are otherwise hard to synthesize chemically. However, the biosynthesis of these compounds is poorly understood, owing to difficulties in identifying the responsible enzymes and analyzing unstable intermediates. Here we comprehensively describe the spiroacetal formation involved in the biosynthesis of reveromycin A, which inhibits bone resorption and bone metastases of tumor cells by inducing apoptosis in osteoclasts. We performed gene disruption, systematic metabolite analysis, feeding of labeled precursors and conversion studies with recombinant enzymes. We identified two key enzymes, dihydroxy ketone synthase and spiroacetal synthase, and showed in vitro reconstruction of the stereospecific spiroacetal structure from a stable acyclic precursor. Our findings provide insights into the creation of a variety of biologically active spiroacetal compounds for drug leads.

Analysis of a 108-kb Region of theSaccharopolyspora spinosaGenome Covering the Obscurin Polyketide Synthase Locus

A 108-kb genomic DNA region of Saccharopolyspora spinosa NRRL 18395, producer of the agriculturally important insecticidal antibiotics spinosyns, has been cloned, sequenced and analyzed to reveal clustered genes encoding a type I polyketide synthase (PKS) complex. The genes for the PKS are flanked by genes encoding homologs of enzymes that are involved in the urea cycle, valine, leucine and isoleucine biosynthesis and energy metabolism. While the disruption of the PKS genes by insertional inactivation was not expected to abolish the production of spinosyns, no differences were found in the antibacterial, antifungal, or insecticidal activities either of the parental and the knockout mutant strains under the growth conditions tested. Deduction of the most likely structure of the polyketide core of the cryptic metabolite, termed obscurin, from the predicted modules and domains of the PKS suggests the formation of a highly unsaturated substituted C22 carboxylic acid that might undergo further processing after its release from the PKS.

Type II thioesterase ScoT, associated with Streptomyces coelicolor A3(2) modular polyketide synthase Cpk, hydrolyzes acyl residues and has a preference for propionate

Type II thioesterases (TE IIs) were shown to maintain the efficiency of polyketide synthases (PKSs) by removing acyl residues blocking extension modules. However, the substrate specificity and kinetic parameters of these enzymes differ, which may have significant consequences when they are included in engineered hybrid systems for the production of novel compounds. Here we show that thioesterase ScoT associated with polyketide synthase Cpk from Streptomyces coelicolor A3(2) is able to hydrolyze acetyl, propionyl, and butyryl residues, which is consistent with its editing function. This enzyme clearly prefers propionate, in contrast to the TE IIs tested previously, and this indicates that it may have a role in control of the starter unit. We also determined activities of ScoT mutants and concluded that this enzyme is an alpha/beta hydrolase with Ser90 and His224 in its active site.

Effect of modification of the length and flexibility of the acyl carrier protein-thioesterase interdomain linker on functionality of the animal fatty acid synthase

A natural linker of approximately 20 residues connects the acyl carrier protein with the carboxy-terminal thioesterase domain of the animal fatty acid synthase. This study examines the effects of changes in the length and amino acid composition of this linker on catalytic activity, product composition, and segmental motion of the thioesterase domain. Deletion of 10 residues, almost half of the interdomain linker, had no effect on either mobility of the thioesterase domain, estimated from fluorescence polarization of a pyrenebutyl methylphosphono moiety bound covalently to the active site serine residue, or functionality of the fatty acid synthase; further shortening of the linker limited mobility of the thioesterase domain and resulted in reduced fatty acid synthase activity and an increase in product chain length from 16 to 18 and 20 carbon atoms. Surprisingly, however, even when the entire linker region was deleted, the fatty acid synthase retained 28% activity. Lengthening of the linker, by insertion of an unusually long acyl carrier protein-thioesterase linker from a modular polyketide synthase, increased mobility of the thioesterase domain without having any significant effect on catalytic properties of the complex. Interdomain linkers could also be used to tether, to the acyl carrier protein domain of the fatty acid synthase, a thioesterase active toward shorter chain length acyl thioesters generating novel short-chain fatty acid synthases. These studies reveal that although truncation of the interdomain linker partially impacts the ability of the thioesterase domain to terminate growth of the acyl chain, the overall integrity of the fatty acid synthase is quite tolerant to moderate changes in linker length and flexibility. The retention of fatty acid synthesizing activity on deletion of the entire linker region implies that the inherent flexibility of the phosphopantetheine "swinging arm" also contributes significantly to the successful docking of the long-chain acyl moiety in the thioesterase active site.

Mutational analysis of a type II thioesterase associated with nonribosomal peptide synthesis

Recent studies on type II thioesterases (TEIIs) involved in microbial secondary metabolism described a role for these enzymes in the removal of short acyl-S- phosphopantetheine intermediates from misprimed holo-(acyl carrier proteins) and holo-(peptidyl carrier proteins) of polyketide synthases and nonribosomal peptide synthetases. Because of the absence of structural information on this class of enzymes, we performed a mutational analysis on a prototype TEII essential for efficient production of the lipopeptide antibiotic surfactin (TEII(srf)), which led to identification of catalytic and structural residues. On the basis of sequence alignment of 16 TEIIs, 10 single and one double mutant of highly conserved residues of TEII(srf) were constructed and biochemically investigated. We clearly identified a catalytic triad consisting of Ser86, Asp190 and His216, suggesting that TEII(srf) belongs to the alpha/beta-hydrolase superfamily. Exchange of these residues with residues with aliphatic side chains abolished enzyme activity, whereas replacement of the active-site Ser86 with cysteine produced an enzyme with marginally reduced activity. In contrast, exchange of the second strictly conserved asparagine (Asp163) with Ala resulted in an active but unstable enzyme, excluding a role for this residue in catalysis and suggesting a structural function. The results define three catalytic and at least one structural residue in a nonribosomal peptide synthetase TEII.

Characterization of the mupirocin biosynthesis gene cluster from Pseudomonas fluorescens NCIMB 10586

The polyketide antibiotic mupirocin (pseudomonic acid) produced by Pseudomonas fluorescens NCIMB 10586 competitively inhibits bacterial isoleucyl-tRNA synthase and is useful in controlling Staphylococcus aureus, particularly methicillin-resistant Staphylococcus aureus. The 74 kb mupirocin biosynthesis cluster has been sequenced, and putative enzymatic functions of many of the open reading frames (ORFs) have been identified. The mupirocin cluster is a combination of six larger ORFs (mmpA-F), containing several domains resembling the multifunctional proteins of polyketide synthase and fatty acid synthase type I systems, and individual genes (mupA-X and macpA-E), some of which show similarity to type II systems (mupB, mupD, mupG, and mupS). Gene knockout experiments demonstrated the importance of regions in mupirocin production, and complementation of the disrupted gene confirmed that the phenotypes were not due to polar effects. A model for mupirocin biosynthesis is presented based on the sequence and biochemical evidence.

Biochemical evidence for an editing role of thioesterase II in the biosynthesis of the polyketide pikromycin

The pikromycin biosynthetic gene cluster contains the pikAV gene encoding a type II thioesterase (TEII). TEII is not responsible for polyketide termination and cyclization, and its biosynthetic role has been unclear. During polyketide biosynthesis, extender units such as methylmalonyl acyl carrier protein (ACP) may prematurely decarboxylate to generate the corresponding acyl-ACP, which cannot be used as a substrate in the condensing reaction by the corresponding ketosynthase domain, rendering the polyketide synthase module inactive. It has been proposed that TEII may serve as an "editing" enzyme and reactivate these modules by removing acyl moieties attached to ACP domains. Using a purified recombinant TEII we have tested this hypothesis by using in vitro enzyme assays and a range of acyl-ACP, malonyl-ACP, and methylmalonyl-ACP substrates derived from either PikAIII or the loading didomain of DEBS1 (6-deoxyerythronolide B synthase; AT(L)-ACP(L)). The pikromycin TEII exhibited high K(m) values (>100 microm) with all substrates and no apparent ACP specificity, catalyzing cleavage of methylmalonyl-ACP from both AT(L)-ACP(L) (k(cat)/K(m) 3.3 +/- 1.1 m(-1) s(-1)) and PikAIII (k(cat)/K(m) 2.9 +/- 0.9 m(-1) s(-1)). The TEII exhibited some acyl-group specificity, catalyzing hydrolysis of propionyl (k(cat)/K(m) 15.8 +/- 1.8 m(-1) s(-1)) and butyryl (k(cat)/K(m) 17.5 +/- 2.1 m(-1) s(-1)) derivatives of AT(L)-ACP(L) faster than acetyl (k(cat)/K(m) 4.9 +/- 0.7 m(-1) s(-1)), malonyl (k(cat)/K(m) 3.9 +/- 0.5 m(-1) s(-1)), or methylmalonyl derivatives. PikAIV containing a TEI domain catalyzed cleavage of propionyl derivative of AT(L)-ACP(L) at a dramatically lower rate than TEII. These results provide the first unequivocal in vitro evidence that TEII can hydrolyze acyl-ACP thioesters and a model for the action of TEII in which the enzyme remains primarily dissociated from the polyketide synthase, preferentially removing aberrant acyl-ACP species with long half-lives. The lack of rigorous substrate specificity for TEII may explain the surprising observation that high level expression of the protein in Streptomyces venezuelae leads to significant (>50%) titer decreases.

Conserved residues in the putative catalytic triad of human bile acid Coenzyme A:amino acid N-acyltransferase

Human bile acid-CoA:amino acid N-acyltransferase (hBAT), an enzyme catalyzing the conjugation of bile acids with the amino acids glycine or taurine has significant sequence homology with dienelactone hydrolases and other alpha/beta hydrolases. These enzymes have a conserved catalytic triad that maps onto the mammalian BATs at residues Cys-235, Asp-328, and His-362 of the human sequence, albeit that the hydrolases contain a serine instead of a cysteine. In the present study, the function of the putative catalytic triad of hBAT was examined by chemical modification with the cysteine alkylating reagent N-ethylmaleimide (NEM) and by site-directed mutagenesis of the triad residues followed by enzymology studies of mutant and wild-type hBATs. Treatment with NEM caused inactivation of wild-type hBAT. However, preincubation of wild-type hBAT with the substrate cholyl-CoA before NEM treatment prevented loss of N-acyltransferase activity. Substitution of His-362 or Asp-328 with alanine results in inactivation of hBAT. Although substitution of Cys-235 with serine generated an hBAT mutant with lower N-acyltransferase activity, it substantially increased the bile acid-CoA thioesterase activity compared with wild type. In summary, data from this study support the existence of an essential catalytic triad within hBAT consisting of Cys-235, His-362, and Asp-328 with Cys-235 serving as the probable nucleophile and thus the site of covalent attachment of the bile acid molecule.

Characterization of the biosynthetic gene cluster for the antifungal polyketide soraphen A from Sorangium cellulosum So ce26

A genomic DNA region of over 80 kb that contains the complete biosynthetic gene cluster for the synthesis of the antifungal polyketide metabolite soraphen A was cloned from Sorangium cellulosum So ce26. The nucleotide sequence of the soraphen A gene region, including 67,523 bp was determined. Examination of this sequence led to the identification of two adjacent type I polyketide synthase (PKS) genes that encode the soraphen synthase. One of the soraphen A PKS genes includes three biosynthetic modules and the second contains five additional modules for a total of eight. The predicted substrate specificities of the acyltransferase (AT) domains, as well as the reductive loop domains identified within each module, are consistent with expectations from the structure of soraphen A. Genes were identified in the regions flanking the two soraphen synthase genes that are proposed to have roles in the biosynthesis of soraphen A. Downstream of the soraphen PKS genes is an O-methyltransferase (OMT) gene. Upstream of the soraphen PKS genes there is a gene encoding a reductase and a group of genes that are postulated to have roles in the synthesis of methoxymalonyl-acyl carrier protein (ACP). This unusual extender unit is proposed to be incorporated in two positions of the soraphen polyketide chain. One of the genes in this group contains distinct domains for an AT, an ACP, and an OMT.

The peroxisome proliferator-induced cytosolic type I acyl-CoA thioesterase (CTE-I) is a serine-histidine-aspartic acid alpha /beta hydrolase

Long-chain acyl-CoA thioesterases hydrolyze long-chain acyl-CoAs to the corresponding free fatty acid and CoASH and may therefore play important roles in regulation of lipid metabolism. We have recently cloned four members of a highly conserved acyl-CoA thioesterase multigene family expressed in cytosol (CTE-I), mitochondria (MTE-I), and peroxisomes (PTE-Ia and -Ib), all of which are regulated via the peroxisome proliferator-activated receptor alpha (Hunt, M. C., Nousiainen, S. E. B., Huttunen, M. K., Orii, K. E., Svensson, L. T., and Alexson, S. E. H. (1999) J. Biol. Chem. 274, 34317-34326). Sequence comparison revealed the presence of putative active-site serine motifs (GXSXG) in all four acyl-CoA thioesterases. In the present study we have expressed CTE-I in Escherichia coli and characterized the recombinant protein with respect to sensitivity to various amino acid reactive compounds. The recombinant CTE-I was inhibited by phenylmethylsulfonyl fluoride and diethyl pyrocarbonate, suggesting the involvement of serine and histidine residues for the activity. Extensive sequence analysis pinpointed Ser(232), Asp(324), and His(358) as the likely components of a catalytic triad, and site-directed mutagenesis verified the importance of these residues for the catalytic activity. A S232C mutant retained about 2% of the wild type activity and incubation with (14)C-palmitoyl-CoA strongly labeled this mutant protein, in contrast to wild-type enzyme, indicating that deacylation of the acyl-enzyme intermediate becomes rate-limiting in this mutant protein. These data are discussed in relation to the structure/function of acyl-CoA thioesterases versus acyltransferases. Furthermore, kinetic characterization of recombinant CTE-I showed that this enzyme appears to be a true acyl-CoA thioesterase being highly specific for C(12)-C(20) acyl-CoAs.

Hyperactivity and interactions of a chimeric myristoryl-ACP thioesterase from the lux system of luminescent bacteria

A chimeric myristoyl-ACP thioesterase with much higher catalytic efficiency than the parental enzymes has been generated by ligating the N-terminal half of the lux-specific thioesterase (LuxD) from Photobacterium phosphoreum with the C-terminal half of LuxD from Vibrio harveyi. The LuxD chimera had the same rate-limiting step and specificity, but cleaved esters and thioesters over eight times faster than the native enzymes. LuxD, along with acyl-protein synthetase (LuxE) and reductase (LuxC), comprise a multienzyme complex channeling activated fatty acids into the aldehyde substrate for the bacterial bioluminescence reaction. As P. phosphoreum LuxD and LuxE modulate each of their respective activities, the effects of mixing V. harveyi and the chimeric LuxD with P. phosphoreum LuxE were investigated. The chimeric LuxD stimulated acylation of LuxE to the same extent as V. harveyi LuxD, but to a lower level than that caused by P. phosphoreum LuxD. Conversely, P. phosphoreum LuxE stimulated the thioesterase activity of V. harveyi LuxD by 30% and the chimeric LuxD by 20% while the activity of P. phosphoreum LuxD was increased by over 140%. These results show that the stimulatory effects are unrelated to the level of thioesterase activity and indicate that the carboxyl terminal region of LuxD interacts with LuxE and causes a conformational change.

Characterization of an arylesterase from Lactobacillus helveticus CNRZ32

An esterase gene (estA) was isolated from a previously constructed genomic library of Lactobacillus helveticus CNRZ32. The estA gene consisted of a 558 bp open reading frame encoding a putative peptide of 21.3 kDa. Protein sequence homology searches using BLAST revealed that EstA had low amino acid sequence identity with the serine-dependent arylesterases TesI (24%) and EtpA (26%) from Escherichia coli and Vibrio mimicus, respectively. A recombinant EstA fusion protein containing a C-terminal six-histidine tag was constructed and purified to electrophoretic homogeneity. Characterization of EstA revealed that it was a serine-dependent enzyme having a monomeric Mr of 22.6-25.1 kDa. Optimum temperature, NaCl concentration and pH for EstA activity were determined to be 35-40 degrees C, 3.5% NaCl and 7.5-8.0, respectively. EstA had significant activity under conditions simulating those of ripening cheese (10 degrees C, 4% NaCl, pH 5.1). EstA hydrolysed a variety of ester compounds and preferred those with substituted phenyl alcohol and short-chain fatty acid groups. Site-directed mutagenesis suggested that the S10 and H164 residues were essential for EstA activity.

Assembly line enzymology by multimodular nonribosomal peptide synthetases: the thioesterase domain of E. coli EntF catalyzes both elongation and cyclolactonization

BACKGROUND

EntF is a 142 kDa four domain (condensation-adenylation-peptidyl carrier protein-thioesterase) nonribosomal peptide synthetase (NRPS) enzyme that assembles the Escherichia coli N-acyl-serine trilactone siderophore enterobactin from serine, dihydroxybenzoate (DHB) and ATP with three other enzymes (EntB, EntD and EntE). To assess how EntF forms three ester linkages and cyclotrimerizes the covalent acyl enzyme DHB-Ser-S-PCP (peptidyl carrier protein) intermediate, we mutated residues of the proposed catalytic Ser-His-Asp triad of the thioesterase (TE) domain.

RESULTS

The Ser1138-->Cys mutant (kcat decreased 1000-fold compared with wild-type EntF) releases both enterobactin (75%) and linear (DHB-Ser)2 dimer (25%) as products. The His 1271-->Ala mutant (kcat decreased 10,000-fold compared with wild-type EntF) releases only enterobactin, but accumulates both DHB-Ser-O-TE and (DHB-Ser)2-O-TE acyl enzyme intermediates. Electrospray ionization and Fourier transform mass spectrometry of proteolytic digests were used to analyze the intermediates.

CONCLUSIONS

These results establish that the TE domain of EntF is both a cyclotrimerizing lactone synthetase and an elongation catalyst for ester-bond formation between covalently tethered DHB-Ser moieties, a new function for chain-termination TE domains found at the carboxyl termini of multimodular NRPSs and polyketide synthases.

Analysis of genes involved in biosynthesis of coronafacic acid, the polyketide component of the phytotoxin coronatine

Coronafacic acid (CFA) is the polyketide component of coronatine (COR), a phytotoxin produced by the plant-pathogenic bacterium Pseudomonas syringae. The genes involved in CFA biosynthesis are encoded by a single transcript which encompasses 19 kb of the COR gene cluster. In the present study, the nucleotide sequence was determined for a 4-kb region located at the 3' end of the CFA biosynthetic gene cluster. Three open reading frames were identified and designated cfa8, cfa9, and tnp1; the predicted translation products of these genes showed relatedness to oxidoreductases, thioesterases, and transposases, respectively. The translational products of cfa8 and cfa9 were overproduced in Escherichia coli BL21; however, tnp1 was not translated in these experiments. Mutagenesis and complementation analysis indicated that cfa8 is required for the production of CFA and COR. Analysis of a cfa9 mutant indicated that this gene is dispensable for CFA and COR production but may increase the release of enzyme-bound products from the COR pathway; tnp1, however, had no obvious function in CFA or COR biosynthesis. A genetic strategy was used to produce CFA in a P. syringae strain which lacks the COR gene cluster; this approach will be useful in future studies designed to investigate biosynthetic products of the CFA gene cluster.

Reconstitution and characterization of the Escherichia coli enterobactin synthetase from EntB, EntE, and EntF

The siderophore molecule enterobactin, a cyclic trimeric lactone of N-(2,3-dihydroxybenzoyl)serine, is synthesized and secreted by Escherichia coli in response to iron starvation. Here we report the first reconstitution of enterobactin synthetase activity from pure protein components: holo-EntB, EntE, and holo-EntF. Holo-EntB and holo-EntF were obtained by pretreatment of apo-EntB and apo-EntF with coenzyme A and EntD, thereby eliminating the requirement for EntD in the enterobactin synthetase. The holo-EntF monomer acts as the catalyst for the formation of the three amide and three ester bonds in enterobactin using ATP, L-serine, and acyl-holo-EntB, acylated with 2,3-dihydroxybenzoate by EntE, as substrates with a turnover rate of 120-140 min-1. There is no evidence for a stable complex of the enterobactin synthetase components. Mutation of holo-EntF in the thioesterase domain at the putative active site serine residue (Ser1138 to Ala) eliminated enterobactin synthetase activity; however, the mutant holo-EntF retained the ability to adenylate serine and to autoacylate itself by thioester formation between serine and its attached phosphopantetheine cofactor. The mutant holo-EntF also appeared to slowly release N-(2, 3-dihydroxybenzoyl)serine.

Conversion of serine-114 to cysteine-114 and the role of the active site nucleophile in acyl transfer by myristoyl-ACP thioesterase from Vibrio harveyi

The lux-specific myristoyl-ACP thioesterase (LuxD) is responsible for diverting myristic acid into the luminescent system and can function as an esterase and transferase as well as cleave myristoyl-CoA and other thioesters. The recently elucidated crystal structure of the enzyme shows that it belongs to the alpha/beta hydrolase family and that it contains a typical catalytic triad composed of Asp211, His241, and Ser114. What is unusual is that the nucleophilic S114 is not contained within the esterase consensus motif GXSXG although the stereochemistry of the turn involving S114 is almost identical to the nucleophilic elbow found in alpha/beta hydrolases. In contrast to mammalian thioesterases, deacylation of LuxD was the rate-limiting step, with the level of acylated enzyme formed on reaction with myristoyl-CoA and the pre-steady-state burst of p-nitrophenol on cleavage of p-nitrophenyl myristate both being 0.7 mol/mol. Cold chase experiments showed that the deacylation rate of LuxD corresponded closely to the turnover rate of the enzyme with ester or thioester substrates. Replacement of S114 by a cysteine residue generated a mutant (S114C) that was acylated with the same pH dependence as LuxD but had greatly diminished capacity to transfer acyl groups to water or glycerol. The acyl group could be removed from the S114C mutant by neutral hydroxylamine, showing that a cysteine residue had been acylated. Mutation of H241 creating the double mutant, S114C.H241N, decreased acylation of the cysteine residue. These results provide direct kinetic and chemical evidence that S114 is the site of acylation linked to H241 in the charge relay system and have led to the recognition of a more general consensus motif flanking the nucleophilic serine in thioesterases.

Molecular cloning and expression of cDNA encoding rat brain cytosolic acyl-coenzyme A thioester hydrolase

The cDNA encoding rat brain cytosolic acyl-CoA thioester hydrolase (ACT) has been cloned and sequenced, and the primary structure of the enzyme has been deduced. A partial amino acid sequence (38 amino acids) of the enzyme was determined using the peptides generated after CNBr digestion of the purified enzyme. Primers synthesized on the basis of this information were used to isolate two cDNA clones, each encoding the full length of the enzyme. The nucleotide sequences of these clones contained an open reading frame encoding a 358-amino acid polypeptide with a calculated molecular mass of 39.7 kDa, similar to that determined for the purified enzyme (40.9 kDa). The deduced ACT sequence showed no homology to the known sequences of any other thioesterases nor to any other known protein sequence. However, there was a strong homology to a number of expressed sequence tag human brain cDNA clones. The identity of the ACT cDNA was confirmed by the expression of ACT activity in Escherichia coli. There was a 10-15-fold increase in ACT-specific activity in the bacterial extracts after induction with isopropyl thiogalactoside, and the properties of the expressed enzyme (fusion protein) were the same as those of the purified rat brain ACT. Northern blot analysis showed that a 1.65-kilobase ACT transcript was present in rat brain and testis but not in any other rat tissues examined. However, the ACT mRNA was induced in the liver of rats that were fed Wy-14,643, a peroxisome proliferator and inducer of rodent liver cytosolic acyl-CoA thioesterase. These results indicate that the induced rat liver ACT is homologous to the constitutive rat brain ACT.

The catalytic cysteine and histidine in the plant acyl-acyl carrier protein thioesterases

The plant acyl-acyl carrier protein (acyl-ACP) thioesterases (TEs) play an essential role in chain termination during de novo fatty acid synthesis and are of biochemical interest because of their utilities in the genetic engineering of plant seed oils. Biochemical data have shown the possible involvement of an active-site cysteine and a histidine in catalysis, suggesting that these enzymes activate the hydrolysis of the thioester bond using the same basic catalytic machinery as those of proteases and lipases. To identify the cysteine and histidine residues that are critical in catalysis we substituted, in a 12:0 ACP TE (Uc FatB1), a conserved cysteine (Cys-320) to an Ala or a Ser, and three conserved histidines (His-140, His-285, and His-345) to an Ala or an Arg. Each Ala mutation caused a substantial loss of enzyme activity. However, only C320A and H285A completely inactivated the enzyme, indicating that these two residues are essential for catalysis. Considerable activity (>60%) still remained when Cys-320 was converted to a Ser, but this mutant (C320S) displayed a reversed sensitivity toward thiol or serine hydroxyl inhibitors compared with the wild-type enzyme. A pH optimal study demonstrates that while the wild-type enzyme has the highest activity between pH 8.5 and 9.5, the mutant H285A shows a shifted optimum to higher pH and a significant increase of activity around pH 12. This result suggests that Arg-285 (pKa 12) is deprotonated at high pH, thus partially mimicking the role of His-285 for proton abstraction in the wild-type enzyme. We conclude that the Cys-320 of the wild-type enzyme and Ser-320 of the mutant enzyme can attack the thioester bond of the substrate 12:0 ACP, assisted by His-285. Because plant TEs are highly conserved in length and sequence and the residues investigated here are completely conserved in all available TEs, it is reasonable to believe that homologues of Cys-320 and His-285 are present in the active sites of all plant acyl-ACP TEs.

Crystallization and preliminary diffraction studies of thioesterase II from rat mammary gland

Thioesterase II from rat mammary gland has been crystallized in the presence of decanoic acid by the vapor diffusion method. The crystals belong to the orthorhombic space group P2(1)2(1)2(1), and have cell dimensions, a = 52.7 A, b = 78.0 A, and c = 133.6 A. The asymmetric unit likely consists of two protein monomers based on predictions from its calculated Matthews coefficient. Crystals typically diffract to at least 2.5 A resolution and are suitable for X-ray crystallographic analysis.

Analysis of five tylosin biosynthetic genes from the tyllBA region of the Streptomyces fradiae genome

The tyllBA region of the tylosin biosynthetic gene cluster of Streptomyces fradiae contains at least five open reading frames (ORFs). ORF1 (tylI) encodes a cytochrome P450 and mutations in this gene affect macrolide ring hydroxylation. The product of ORF2 (tylB) belongs to a widespread family of proteins whose functions are speculative, although tylB mutants are defective in the biosynthesis or addition of mycaminose during tylosin production. ORFs 3 and 4 (tylA1 and tylA2) encode delta TDP-glucose synthase and delta TDP-glucose dehydratase, respectively, enzymes responsible for the first two steps common to the biosynthesis of all three deoxyhexose sugars of tylosin via the common intermediate, delta TDP-4-keto, 6-deoxyglucose. ORF5 encodes a thioesterase similar to one encoded in the erythromycin gene cluster of Saccharopolyspora erythraea.

Expression, purification and crystallization of the Vibrio harveyi acyltransferase

We have obtained X-ray quality single crystals of Vibrio harveyi acyltransferase. The protein was obtained from V. harveyi by a gene mobilization expression system. The crystals are monoclinic (space group P2(1), a = 89.9 A, b = 83.6 A, c = 47.1 A, beta = 97.3 degrees) with two molecules related by a pronounced non-crystallographic dyad in the asymmetric unit, with a solvent content of approximately 50%. The diffraction pattern from fresh crystals extends beyond 2 A resolution using sealed tube CuK alpha radiation. The elucidation of the three-dimensional structure of this enzyme, believed to contain a proteinase-like catalytic triad, which resembles in many ways other eukaryotic fatty acid chain terminating enzymes, may have important consequences for our understanding of the molecular basis of the final stages of the synthesis of fatty acids.

Organization of the enzymatic domains in the multifunctional polyketide synthase involved in erythromycin formation in Saccharopolyspora erythraea

Localization of the enzymatic domains in the three multifunctional polypeptides from Saccharopolyspora erythraea involved in the formation of the polyketide portion of the macrolide antibiotic erythromycin was determined by computer-assisted analysis. Comparison of the six synthase units (SU) from the eryA genes with each other and with mono- and multifunctional fatty acid and polyketide synthases established the extent of each beta-ketoacyl acyl-carrier protein (ACP) synthase, acyltransferase, beta-ketoreductase, ACP, and thioesterase domain. The extent of the enoyl reductase (ER) domain was established by detecting similarity to other sequences in the database. A segment containing the putative dehydratase (DH) domain in EryAII, with a potential active-site histidine residue, was also found. The finding of conservation of a portion of the DH-ER interdomain region in the other five SU, which lack these two functions, suggests a possible evolutionary path for the generation of the six SU.