Interrogating the molecular basis for multiple macrolactone ring formation by the pikromycin polyketide synthase

The pikromycin polyketide synthase (PKS) is unique in its ability to generate both 12 and 14 membered ring macrolactones. As such, dissection of the molecular basis for controlling metabolic diversity in this system remains an important objective for understanding modular PKS function and expanding chemical diversity. Here, we describe a series of experiments designed to probe the importance of the protein-protein interaction that occurs between the final two monomodules, PikAIII (module 5) and PikAIV (module 6), for the production of the 12 membered ring macrolactone 10-deoxymethynolide. The results obtained from these in vitro studies demonstrate that PikAIII and PikAIV generate the 12 membered ring macrocycle most efficiently when engaged in their native protein-protein interaction. Accordingly, the data are consistent with PikAIV adopting an alternative conformation that enables the terminal thioesterase domain to directly off-load the PikAIII-bound hexaketide intermediate for macrocyclization.     

Mechanisms of molecular recognition in the pikromycin polyketide synthase

BACKGROUND: Modular polyketide synthases (PKSs) produce a wide range of medically significant compounds. In the case of the pikromycin PKS of Streptomyces venezuelae, four separate polypeptides (PikAI-PikAIV), comprising a total of one loading domain and six extension modules, generate the 14-membered ring macrolactone narbonolide. The polypeptide PikAIV contains a thioesterase (TE) domain and is responsible for catalyzing both the last elongation step with methylmalonyl CoA, and subsequent release of the final polyketide chain elongation intermediate from the PKS. Under certain growth conditions this polypeptide is synthesized from an alternative translational start site, giving rise to an N-terminal truncated form of PikAIV, containing only half of the ketosynthase (KS(6)) domain. The truncated form of PikAIV is unable to catalyze the final elongation step, but is able to cleave a polyketide chain from the preceding module on PikAIII (ACP(5)), giving rise to the 12-membered ring product 10-deoxymethynolide. RESULTS: S. venezuelae mutants expressing hybrid PikAIV polypeptides containing acyl carrier protein (ACP) and malonyl CoA specific acyltransferase (AT) domains from the rapamycin PKS were unable to catalyze production of 12- or 14-membered ring macrolactone products. Plasmid-based expression of a hybrid PikAIV containing the native KS(6) and TE domains, however, restored production of both narbonolide and 10-deoxymethynolide in the S. venezuelae AX912 mutant that generates a TE-deleted form of PikAIV. Use of alternative KS domains or deletion of the KS(6) domain within the hybrid PikAIV resulted in loss of both products. Plasmid-based expression of a TE domain of PikAIV as a separate polypeptide in the AX912 mutant resulted in greater than 50% restoration of 10-deoxymethynolide, but not in mutants expressing a hybrid PikAIV bearing an unnatural AT domain. Mutants expressing hybrid PikAIV polypeptides containing the natural AT(6) domains and different ACP domains efficiently produced polyketide products, but with a significantly higher 10-deoxymethynolide/narbonolide ratio than observed with native PikAIV. CONCLUSIONS: Dimerization of KS(6) modules allows in vivo formation of a PKS heterodimer using PikAIV polypeptides containing different AT and ACP domains. In such heterodimers, the TE domain and the AT(6) domain responsible for formation of the narbonolide product are located on different polypeptide chains. The AT(6) domain of PikAIV plays an important role in facilitating TE-catalyzed chain termination (10-deoxymethynolide formation) at the proceeding module in PikAIII. The pikromycin PKS can tolerate the presence of multiple forms (active and inactive) of PikAIV, and decreased efficiency of elongation by PikAIV can result in increased levels of 10-deoxymethynolide. These results provide new insight into functional molecular interactions and interdomain recognition in modular PKSs.     

Substrate recognition and channeling of monomodules from the pikromycin polyketide synthase

The unique ability of the pikromycin (Pik) polyketide synthase to generate 12- and 14-membered ring macrolactones presents an opportunity to explore the fundamental processes underlying polyketide synthesis, specifically the mechanistic details of the chain extension process. We have overexpressed and purified PikAIII (module 5) and PikAIV (module 6) and assessed the ability of these proteins to generate tri- and tetraketide lactone products using N-acetylcysteamine-activated diketides and (14)C-methylmalonyl-CoA as substrates. Comparison of the stereochemical specificities for PikAIII and PikAIV and the reported values for the DEBS modules reveals significant differences between these systems.     

Redirecting the cyclization steps of fungal polyketide synthase

Regiospecific cyclizations of the nascent poly-beta-ketone backbones dictate the structures of polyketide natural products. The fungal iterative megasynthases use terminal thioesterase/claisen cyclase (TE/CLC) domains to direct the fate of the polyketide chains. In this work, we present two strategies toward redirecting the cyclization steps of fungal PKSs using the Gibberella fujikuroi PKS4. First, inactivation or removal of the TE/CLC domain resulted in the synthesis of the new polyketide SMA93 2. Complementation of the mutant PKS4 with a standalone TE/CLC domain restored the regioselective cyclization steps of PKS4 and led to the synthesis of SMA76 1, demonstrating that cyclization enzymes can interact with the megasynthase in trans. This led to the second approach in which various dissociated, bacterial tailoring enzymes were added to the megasynthase in trans. Addition of the act KR led to the synthesis of mutactin 3, while the addition of first ring and second ring cyclases yielded anthraquinone compounds DMAC 5 and SEK26 6. The cooperative activities of fungal and bacterial PKS components are especially important and enable synthesis of polyketides utilizing enzymes from two distinct families of PKSs.     

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

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

Insights into the stereospecificity of ketoreduction in a modular polyketide synthase

Ketoreductase enzymes are responsible for the generation of hydroxyl stereocentres during the biosynthesis of complex polyketide natural products. Previous studies of isolated polyketide ketoreductases have shown that the stereospecificity of ketoreduction can be switched by mutagenesis of selected active site amino acids. We show here that in the context of the intact polyketide synthase multienzyme the same changes do not alter the stereochemical outcome in the same way. These findings point towards additional factors that govern ketoreductase stereospecificity on intact multienzymes in vivo.     

Stereospecificity of the dehydratase domain of the erythromycin polyketide synthase

The dehydratase (DH) domain of module 4 of the 6-deoxyerythronolide B synthase (DEBS) has been shown to catalyze an exclusive syn elimination/syn addition of water. Incubation of recombinant DH4 with chemoenzymatically prepared anti-(2R,3R)-2-methyl-3-hydroxypentanoyl-ACP (2a-ACP) gave the dehydration product 3-ACP. Similarly, incubation of DH4 with synthetic 3-ACP resulted in the reverse enzyme-catalyzed hydration reaction, giving an ～3:1 equilbrium mixture of 2a-ACP and 3-ACP. Incubation of a mixture of propionyl-SNAC (4), methylmalonyl-CoA, and NADPH with the DEBS β-ketoacyl synthase-acyl transferase [KS6][AT6] didomain, DEBS ACP6, and the ketoreductase domain from tylactone synthase module 1 (TYLS KR1) generated in situ anti-2a-ACP that underwent DH4-catalyzed syn dehydration to give 3-ACP. DH4 did not dehydrate syn-(2S,3R)-2b-ACP, syn-(2R,3S)-2c-ACP, or anti-(2S,3S)-2d-ACP generated in situ by DEBS KR1, DEBS KR6, or the rifamycin synthase KR7 (RIFS KR7), respectively. Similarly, incubation of a mixture of (2S,3R)-2-methyl-3-hydroxypentanoyl-N-acetylcysteamine thioester (2b-SNAC), methylmalonyl-CoA, and NADPH with DEBS [KS6][AT6], DEBS ACP6, and TYLS KR1 gave anti-(2R,3R)-6-ACP that underwent syn dehydration catalyzed by DEBS DH4 to give (4R,5R)-(E)-2,4-dimethyl-5-hydroxy-hept-2-enoyl-ACP (7-ACP). The structure and stereochemistry of 7 were established by GC-MS and LC-MS comparison of the derived methyl ester 7-Me to a synthetic sample of 7-Me.     

Stereospecificity of ketoreductase domains 1 and 2 of the tylactone modular polyketide synthase

Tylactone synthase (TYLS) is a modular polyketide synthase that catalyzes the formation of tylactone (1), the parent aglycone precursor of the macrolide antibiotic tylosin. TYLS modules 1 and 2 are responsible for the generation of antidiketide and triketide intermediates, respectively, each bound to an acyl carrier protein (ACP) domain. Each module harbors a ketoreductase (KR) domain. The stereospecificity of TYLS KR1 and TYLS KR2 has been determined by incubating each of the recombinant ketoreductase domains with reconstituted ketosynthase-acyltransferase [KS][AT] and ACP domains from the 6-deoxyerythronolide B synthase (DEBS) in the presence of the N-acetylcysteamine thioester of syn-(2S,3R)-2-methyl-3-hydroxypentanoate (6), methylmalonyl-CoA, and NADPH resulting in the exclusive formation of the ACP-bound (2R,3R,4S,5R)-2,4-methyl-3,5-dihydroxyhepanoyl triketide, as established by GC-MS analysis of the TMS ether of the derived triketide lactone 7. Both TYLS KR1 and KR2 therefore catalyze the stereospecific reduction of the 2-methyl-3-ketoacyl-ACP substrate from the re-face, with specificity for the reduction of the (2R)-methyl (D) diastereomer. The dehydration that is catalyzed by the dehydratase (DH) domains of TYLS module 2 to give the unsaturated (2E,4S,5R)-2,4-dimethyl-5-hydroxyhept-2-enoyl-ACP2 is therefore a syn elimination of water.     

Enhancing the modularity of the modular polyketide synthases: transacylation in modular polyketide synthases catalyzed by malonyl-CoA:ACP transacylase

Selective incorporation of extender units in modular polyketide synthases is primarily controlled by acyl transferase (AT) domains. The AT domains catalyze transacylation of the extender unit from acyl-CoA to the phosphopantetheine arm of an acyl carrier protein (ACP) domain in the same module. New methods that can modulate the extender unit specificity of individual modules with minimal structural or kinetic perturbations in the engineered module are desirable for the efficient biosynthesis of novel natural product analogues. We have demonstrated that transacylation of malonyl groups onto an AT-null form of a mutant modular polyketide synthase by malonyl-CoA:ACP transacylase is an effective strategy for the engineered biosynthesis of site specifically modified polyketides. Using this strategy, 6-deoxyerythronolide B synthase was engineered to exclusively produce 2-desmethyl-6-deoxyerythronolide B. The productivity of the modified system was comparable to that of the wild-type synthase in vitro and in vivo.     

Macrolactam formation catalyzed by the thioesterase domain of vicenistatin polyketide synthase

The excited thioesterase (TE) domain from the vicenistatin polyketide synthase (PKS) efficiently catalyzed the macrolactam formation of the N-acetylcysteamine thioester of the seco-amino acid of the aglycon vicenilactam. This result indicates that the vicenistatin PKS TE domain cyclizes the extended polyketide chain on the ACP domain in the PKS. Furthermore, the simple ethyl ester of the seco-amino acid was also found to be used as a substrate of the TE domain with similar efficiency.     

Mechanistic analysis of acyl transferase domain exchange in polyketide synthase modules

Many polyketides are synthesized by a class of multifunctional enzymes called type I modular polyketide synthases (PKSs). Several reports have described the power of predictively altering polyketide structure by replacing individual PKS domains with homologues from other PKSs. For example, numerous erythromycin analogues have been generated by replacing individual methylmalonyl-specific acyl transferase (AT) domains of the 6-deoxyerythronolide B synthase (DEBS) with malonyl-, ethylmalonyl-, or methoxymalonyl-specific domains. However, the construction of hybrid PKS modules often attenuates product formation both kinetically and distributively. The molecular basis for this mechanistic imperfection is not understood. We have systematically analyzed the impact of replacing an AT domain of DEBS on acyl-AT formation, acyl-CoA:HS-NAc acyl transferase activity, acyl-CoA:ACP acyl transferase activity (nucleophile charging), acyl-SNAc:ketosynthase acyl transferase activity (electrophile charging), and beta-ketoacyl ACP synthase activity (condensation). As usual, domain junctions were located in interdomain regions flanking the AT domain. Kinetic analysis of hybrid modules containing either malonyl transferase or methylmalonyl transferase domains revealed a 15-20-fold decrease in overall turnover numbers of the hybrid modules as compared to the wild-type module. In contrast, both the activity and the specificity of the heterologous AT domains remained unaffected. Moreover, no defects could be detected in the ability of the heterologous AT domains to catalyze acyl-CoA:ACP acyl transfer. Single turnover studies aimed at directly probing the ketosynthase-catalyzed reaction led to two crucial findings. First, wild-type modules catalyzed chain elongation with comparable efficiency regardless of whether methylmalonyl-ACP or malonyl-ACP were the nucleophilic substrates. Second, chain elongation in all hybrid modules tested was seriously attenuated relative to the wild-type module. Our data suggest that, as currently practiced, the most deleterious impact of AT domain swapping is not on the substrate specificity. Rather, it is due to the impaired ability of the KS and ACP domains in the hybrid module to catalyze chain elongation. Consistent with this proposal, limited proteolysis of wild-type and hybrid modules showed major differences in cleavage patterns, especially in the region between the KR and ACP domains.     

Mechanism and specificity of the terminal thioesterase domain from the erythromycin polyketide synthase

BACKGROUND: Polyketides are important compounds with antibiotic and anticancer activities. Several modular polyketide synthases (PKSs) contain a terminal thioesterase (TE) domain probably responsible for the release and concomitant cyclization of the fully processed polyketide chain. Because the TE domain influences qualitative aspects of product formation by engineered PKSs, its mechanism and specificity are of considerable interest. RESULTS: The TE domain of the 6-deoxyerythronolide B synthase was overexpressed in Escherichia coli. When tested against a set of N-acetyl cysteamine thioesters the TE domain did not act as a cyclase, but showed significant hydrolytic specificity towards substrates that mimic important features of its natural substrate. Also the overall rate of polyketide chain release was strongly enhanced by a covalent connection between the TE domain and the terminal PKS module (by as much as 100-fold compared with separate TE and PKS 'domains'). CONCLUSIONS: The inability of the TE domain alone to catalyze cyclization suggests that macrocycle formation results from the combined action of the TE domain and a PKS module. The chain-length and stereochemical preferences of the TE domain might be relevant in the design and engineered biosynthesis of certain novel polyketides. Our results also suggest that the TE domain might loop back to catalyze the release of polyketide chains from both terminal and pre-terminal modules, which may explain the ability of certain naturally occurring PKSs, such as the picromycin synthase, to generate both 12-membered and 14-membered macrolide antibiotics.     

The structure of docking domains in modular polyketide synthases

Polyketides from actinomycete bacteria provide the basis for many valuable medicines, so engineering genes for their biosynthesis to produce variant molecules holds promise for drug discovery. The modular polyketide synthases are particularly amenable to this approach, because each cycle of chain extension is catalyzed by a different module of enzymes, and the modules are arranged within giant multienzyme subunits in the order in which they act. Protein-protein interactions between terminal docking domains of successive multienzymes promote their correct positioning within the assembly line, but because the overall complex is not stable in vitro, the key interactions have not been identified. We present here the NMR solution structure of a 120 residue polypeptide representing a typical pair of such domains, fused at their respective C and N termini: it adopts a stable dimeric structure which reveals the detailed role of these (predominantly helical) domains in docking and dimerization by modular polyketide synthases.     

Employing modular polyketide synthase ketoreductases as biocatalysts in the preparative chemoenzymatic syntheses of diketide chiral building blocks

Chiral building blocks are valuable intermediates in the syntheses of natural products and pharmaceuticals. A scalable chemoenzymatic route to chiral diketides has been developed that includes the general synthesis of α-substituted, β-ketoacyl N-acetylcysteamine thioesters followed by a biocatalytic cycle in which a glucose-fueled NADPH-regeneration system drives reductions catalyzed by isolated modular polyketide synthase (PKS) ketoreductases (KRs). To identify KRs that operate as active, stereospecific biocatalysts, 11 isolated KRs were incubated with 5 diketides and their products were analyzed by chiral chromatography. KRs that naturally reduce small polyketide intermediates were the most active and stereospecific toward the panel of diketides. Several biocatalytic reactions were scaled up to yield more than 100?mg of product. These syntheses demonstrate the ability of PKS enzymes to economically and greenly generate diverse chiral building blocks on a preparative scale.     

bryA: an unusual modular polyketide synthase gene from the uncultivated bacterial symbiont of the marine bryozoan Bugula neritina

"Candidatus Endobugula sertula," the uncultivated bacterial symbiont of Bugula neritina, is the proposed source of the bryostatin family of anticancer compounds. We cloned a large modular polyketide synthase (PKS) gene complex from "Candidatus Endobugula sertula" and characterized one gene, bryA, which we propose is responsible for the initial steps of bryostatin biosynthesis. Typical PKS domains are present. However, acyltransferase domains are lacking in bryA, and beta-ketoacyl synthase domains of bryA cluster with those of PKSs with discrete, rather than integral, acyltransferases. We propose a model for biosynthesis of the bryostatin D-lactate starter unit by the bryA loading module, utilizing atypical domains homologous to FkbH, KR, and DH. The bryA gene product is proposed to synthesize a portion of the pharmacologically active part of bryostatin and may be useful in semisynthesis of clinically useful bryostatin analogs.     

Evidence from engineered gene fusions for the repeated use of a module in a modular polyketide synthase

Functional evidence for programmed loss of co-linearity on the borrelidin modular polyketide synthase (PKS) is presented.     

Identification of Claisen cyclase domain in fungal polyketide synthase WA, a naphthopyrone synthase of Aspergillus nidulans

BACKGROUND: Based on the homology with fatty acid synthases and bacterial polyketide synthases (PKSs), thioesterase domains have been assigned at the C-terminus regions of fungal iterative type I PKSs. We previously overexpressed Aspergillus nidulans wA PKS gene in a heterologous fungal host and identified it to encode a heptaketide naphthopyrone synthase. In addition, expression of C-terminus-modified WA PKS gave heptaketide isocoumarins suggesting that the C-terminus region of WA PKS is involved in the cyclization of the second aromatic ring of naphthopyrone. To unravel the actual function of the C-terminus region, we carried out functional analysis of WA PKS mutants by C-terminus deletion and site-directed mutagenesis. RESULTS: Only the 32 amino acid deletion from the C-terminus of WA PKS caused product change to heptaketide isocoumarins from heptaketide naphthopyrone, YWA1 1, a product of intact WA PKS. Further C-terminus deletion mutant of WA PKS up to Ser(1967), an active site residue of so far called thioesterase, still produced isocoumarins. Site-directed mutagenesis of amino acid residues in this C-terminus region showed that even a single mutation of S1967A or H2129Q caused production of isocoumarin instead of naphthopyrone. Furthermore, the role of tandem acyl carrier proteins (ACPs), a typical feature of fungal aromatic PKSs, was examined by site-directed mutagenesis and the results indicated that both ACPs can function as ACP independently. CONCLUSIONS: Claisen-type cyclization is assumed to be involved in formation of aromatic compounds by some fungal type I PKSs. These PKSs have a quite identical architecture of active site domain organization, beta-ketoacyl synthase, acyltransferase, tandem ACPs and thioesterase (TE) domains. Since the C-terminus region of WA PKS of this type was determined to be involved in Claisen-type cyclization of the second ring of naphthopyrone, we propose that the so far called TE of these PKSs work not just as TE but as Claisen cyclase.     

Mechanism and stereospecificity of a fully saturating polyketide synthase module: nanchangmycin synthase module 2 and its dehydratase domain

Recombinant nanchangmycin synthase module 2 (NANS module 2), with the thioesterase domain from the 6-deoxyerythronolide B synthase (DEBS TE) appended to the C-terminus, was cloned and expressed in Escherichia coli. Incubation of NANS module 2+TE with (±)-2-methyl-3-keto-butyryl-N-acetylcysteamine thioester (1), the SNAC analog of the natural ACP-bound substrate, with methylmalonyl-CoA (MM-CoA) in the absence of NADPH gave 3,5,6-trimethyl-4-hydroxypyrone (2), identified by direct comparison with synthetic 2 by radio-TLC-phosphorimaging and LC-ESI(+)-MS-MS. The reaction showed k(cat) 0.5 ± 0.1 min(-1) and K(m)(1) 19 ± 5 mM at 0.5 mM MM-CoA and k(cat)(app) 0.26 ± 0.02 min(-1) and K(m)(MM-CoA) 0.11 ± 0.02 mM at 8 mM 1. Incubation in the presence of NADPH generated the fully saturated triketide chain elongation product as a 5:3 mixture of (2S,4R)-2,4-dimethyl-5-ketohexanoic acid (3a) and the diastereomeric (2S,4S)-3b. The structure and stereochemistry of each product was established by comparison with synthetic 3a and 3b by a combination of radio-TLC-phosphorimaging and LC-ESI(-)-MS-MS, as well as chiral capillary GC-MS analysis of the corresponding methyl esters 3a-Me and 3b-Me. The recombinant dehydratase domain from NANS module 2, NANS DH2, was shown to catalyze the formation of an (E)-double bond by syn-dehydration of the ACP-bound substrate anti-(2R,3R,4S,5R)-2,4-dimethyl-3,5-dihydroxyheptanoyl-ACP6 (4), generated in situ by incubation of (2S,3R)-2-methyl-3-hydroxypentanoyl-SNAC (5), methylmalonyl-CoA, and NADPH with the recombinant [KS6][AT6] didomain and ACP6 from DEBS module 6 along with the ketoreductase from the tylactone synthase module 1 (TYLS KR1). These results also indirectly establish the stereochemistry of the reactions catalyzed by the KR and enoylreductase (ER) domains of NANS module 2.     

Molecular basis of Celmer's rules: the role of two ketoreductase domains in the control of chirality by the erythromycin modular polyketide synthase

BACKGROUND: Polyketides are compounds that possess medically significant activities. The modular nature of the polyketide synthase (PKS) multienzymes has generated interest in bioengineering new PKSs. Rational design of novel PKSs, however, requires a greater understanding of the stereocontrol mechanisms that operate in natural PKS modules. RESULTS: The N-acetyl cysteamine (NAC) thioester derivative of the natural beta-keto diketide intermediate was incubated with DEBS1-TE, a derivative of the erythromycin PKS that contains only modules 1 and 2. The reduction products of the two ketoreductase (KR) domains of DEBS1-TE were a mixture of the (2S, 3R) and (2R,3S) isomers of the corresponding beta-hydroxy diketide NAC thioesters. Repeating the incubation using a DEBS1-TE mutant that only contains KR1 produced only the (2S,3R) isomer. CONCLUSIONS: In contrast with earlier results, KR1 selects only the (2S) isomer and reduces it stereospecifically to the (2S, 3R)-3-hydroxy-2-methyl acyl product. The KR domain of module 1 controls the stereochemical outcome at both methyl-and hydroxyl-bearing chiral centres in the hydroxy diketide intermediate. Earlier work showed that the normal enzyme-bound ketoester generated in module 2 is not epimerised, however. The stereochemistry at C-2 is therefore established by a condensation reaction that exclusively gives the (2R)-ketoester, and the stereo-chemistry at C-3 by reduction of the keto group. Two different mechanisms of stereochemical control, therefore, operate in modules 1 and 2 of the erythromycin PKS. These results should provide a more rational basis for designing hybrid PKSs to generate altered stereochemistry in polyketide products.     

Epothilone C macrolactonization and hydrolysis are catalyzed by the isolated thioesterase domain of epothilone polyketide synthase

Epothilone C is produced by the combined action of one nonribosomal peptide synthetase (NRPS) and nine polyketide synthase (PKS) modules in a multienzyme system. The final step in the biosynthesis is the thioesterase (TE)-catalyzed cyclorelease of epothilone from the EpoF protein. It has been unclear whether isolated PKS TE domains could exhibit macrolactonization activity. Here we demonstrate that the excised epothilone TE domain can catalyze the efficient cyclization of the N-acetylcysteamine thioester of seco-epothilone C to generate epothilone C (kcat/KM = 0.41 +/- 0.03 min-1 mM-1). The TE domain also catalyzes the hydrolysis of both the N-acetylcysteamine thioester of seco-epothilone C (kcat = 0.087 +/- 0.005 min-1, KM = 291 +/- 53 muM) and that of the epothilone C (kcat = 0.67 +/- 0.01 min-1, KM = 117 +/- 5 muM) to form seco-epothilone C.