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1.  Generation of food-grade recombinant Lactobacillus casei delivering Myxococcus xanthus prolyl endopeptidase 
Applied microbiology and biotechnology  2014;98(15):6689-6700.
Prolyl endopeptidases (PEP), a family of serine proteases with the ability to hydrolyze the peptide bond on the carboxyl side of an internal proline residue, are able to degrade immunotoxic peptides responsible for celiac disease (CD), such as a 33-residue gluten peptide (33-mer). Oral administration of PEP has been suggested as a potential therapeutic approach for CD, although delivery of the enzyme to the small intestine requires intrinsic gastric stability or advanced formulation technologies. We have engineered two food-grade Lactobacillus casei strains to deliver PEP in an in vitro model of small intestine environment. One strain secretes PEP into the extracellular medium, whereas the other retains PEP in the intracellular environment. The strain that secretes PEP into the extracellular medium is the most effective to degrade the 33-mer and is resistant to simulated gastrointestinal stress. Our results suggest that in a future, after more studies and clinical trials, an engineered food-grade Lactobacillus strain may be useful as a vector for in situ production of PEP in the upper small intestine of CD patients.
doi:10.1007/s00253-014-5730-7
PMCID: PMC4393947  PMID: 24752841
Celiac disease; gluten; prolyl endopeptidase; Myxococcus xanthus; heterologous expression; Lactobacillus casei
2.  Elucidation of the Cryptic Epimerase Activity of Redox-Inactive Ketoreductase Domains from Modular Polyketide Synthases by Tandem Equilibrium Isotope Exchange 
Journal of the American Chemical Society  2014;136(29):10190-10193.
Many modular polyketide synthases harbor one or more redox-inactive domains of unknown function that are highly homologous to ketoreductase (KR) domains. A newly developed tandem equilibrium isotope exchange (EIX) assay has now established that such “KR0” domains catalyze the biosynthetically essential epimerization of transient (2R)-2-methyl-3-ketoacyl-ACP intermediates to the corresponding (2S)-2-methyl-3-ketoacyl-ACP diastereomers. Incubation of [2-2H]-(2R,3S)-2-methyl-3-hydroxypentanoyl-SACP ([2-2H]-3b) with the EryKR30 domain from module 3 of the 6-deoxyerythronolide B synthase, and the redox-active, nonepimerizing EryKR6 domain and NADP+ resulted in time- and cofactor-dependent washout of deuterium from 3b, as a result of EryKR30-catalyzed epimerization of transiently generated [2-2H]-2-methyl-3-ketopentanoyl-ACP (4). Similar results were obtained with redox-inactive PicKR30 from module 3 of the picromycin synthase. Four redox-inactive mutants of epimerase-active EryKR1 were engineered by mutagenesis of the NADPH binding site of this enzyme. Tandem EIX established that these EryKR10 mutants retained the intrinsic epimerase activity of the parent EryKR1 domain. These results establish the intrinsic epimerase activity of redox-inactive KR0 domains, rule out any role for the NADPH cofactor in epimerization, and provide a general experimental basis for decoupling the epimerase and reductase activities of a large class of PKS domains.
doi:10.1021/ja5056998
PMCID: PMC4111212  PMID: 25004372
3.  Architectures of Whole-Module and Bimodular Proteins from the 6-Deoxyerythronolide B Synthase 
Journal of molecular biology  2014;426(11):2229-2245.
The 6-deoxyerythronolide B synthase (DEBS) is a prototypical assembly line polyketide synthase (PKS) produced by the actinomycete Saccharopolyspora erythraea that synthesizes the macrocyclic core of the antibiotic erythromycin, 6-deoxyerythronolide B (6-dEB). The megasynthase is a 2 MDa trimeric complex comprised of three unique homodimers assembled from the gene products DEBS1, DEBS2, and DEBS3, which are housed within the erythromycin biosynthetic gene cluster. Each homodimer contains two clusters of catalytically independent enzymatic domains, each referred to as a module, which catalyzes one round of polyketide chain extension and modification. Modules are named sequentially to indicate the order in which they are utilized during synthesis of 6-dEB. We report small angle X-ray scattering (SAXS) analyses of a whole module and bimodule from DEBS as well as a set of domains for which high-resolution structures are available. In all cases, the solution state was probed under previously established conditions that ensure each protein is catalytically active. SAXS data are consistent with atomic-resolution structures of DEBS fragments. Therefore, we used the available high-resolution structures of DEBS domains to model the architectures of the larger protein assemblies using rigid body refinement. Our data supports a model in which, the third module of DEBS forms a disc-shaped structure capable of caging the acyl carrier protein domain proximal to each active site. The molecular envelope of DEBS3 is a thin, elongated ellipsoid, and the results of rigid body modeling suggest that modules 5 and 6 stack colinearly along the 2-fold axis of symmetry.
doi:10.1016/j.jmb.2014.03.015
PMCID: PMC4284093  PMID: 24704088
4.  Comparative Analysis of the Substrate Specificity of trans- versus cis-Acyltransferases of Assembly Line Polyketide Synthases 
Biochemistry  2014;53(23):3796-3806.
Due to their pivotal role in extender unit selection during polyketide biosynthesis, acyltransferase (AT) domains are important engineering targets. A subset of assembly line polyketide synthases (PKSs) are serviced by discrete, trans-acting ATs. Theoretically, these trans-ATs can complement an inactivated cis-AT, promoting introduction of a noncognate extender unit. This approach requires a better understanding of the substrate specificity and catalytic mechanism of naturally occurring trans-ATs. We kinetically analyzed trans-ATs from the disorazole and kirromycin synthases and compared them to a representative cis-AT from the 6-deoxyerythronolide B synthase (DEBS). During transacylation, the disorazole AT favored malonyl-CoA over methylmalonyl-CoA by >40000-fold, whereas the kirromycin AT favored ethylmalonyl-CoA over methylmalonyl-CoA by 20-fold. Conversely, the disorazole AT had broader specificity than its kirromycin counterpart for acyl carrier protein (ACP) substrates. The presence of the ACP had little effect on the specificity (kcat/KM) of the cis-AT domain for carboxyacyl-CoA substrates but had a marked influence on the corresponding specificity parameters for the trans-ATs, suggesting that these enzymes do not act strictly by a canonical ping-pong mechanism. To investigate the relevance of the kinetic analysis of isolated ATs in the context of intact PKSs, we complemented an in vitro AT-null DEBS assembly line with either trans-AT. Whereas the disorazole AT efficiently complemented the mutant PKS at substoichiometric protein ratios, the kirromycin AT was considerably less effective. Our findings suggest that knowledge of both carboxyacyl-CoA and ACP specificity is critical to the choice of a trans-AT in combination with a mutant PKS to generate novel polyketides.
doi:10.1021/bi5004316
PMCID: PMC4067149  PMID: 24871074
5.  Assembly Line Polyketide Synthases: Mechanistic Insights and Unsolved Problems 
Biochemistry  2014;53(18):2875-2883.
Two hallmarks of assembly line polyketide synthases have motivated an interest in these unusual multienzyme systems, their stereospecificity and their capacity for directional biosynthesis. In this review, we summarize the state of knowledge regarding the mechanistic origins of these two remarkable features, using the 6-deoxyerythronolide B synthase as a prototype. Of the 10 stereocenters in 6-deoxyerythronolide B, the stereochemistry of nine carbon atoms is directly set by ketoreductase domains, which catalyze epimerization and/or diastereospecific reduction reactions. The 10th stereocenter is established by the sequential action of three enzymatic domains. Thus, the problem has been reduced to a challenge in mainstream enzymology, where fundamental gaps remain in our understanding of the structural basis for this exquisite stereochemical control by relatively well-defined active sites. In contrast, testable mechanistic hypotheses for the phenomenon of vectorial biosynthesis are only just beginning to emerge. Starting from an elegant theoretical framework for understanding coupled vectorial processes in biology [Jencks, W. P. (1980) Adv. Enzymol. Relat. Areas Mol. Biol. 51, 75–106], we present a simple model that can explain assembly line polyketide biosynthesis as a coupled vectorial process. Our model, which highlights the important role of domain–domain interactions, not only is consistent with recent observations but also is amenable to further experimental verification and refinement. Ultimately, a definitive view of the coordinated motions within and between polyketide synthase modules will require a combination of structural, kinetic, spectroscopic, and computational tools and could be one of the most exciting frontiers in 21st Century enzymology.
doi:10.1021/bi500290t
PMCID: PMC4020578  PMID: 24779441
6.  The initiation ketosynthase (FabH) is the sole rate-limiting enzyme of the fatty acid synthase of Synechococcus sp. PCC 7002 
Metabolic engineering  2014;22:53-59.
Cyanobacteria are Gram-negative bacteria that are desirable hosts for biodiesel production, because they are photosynthetic, relatively fast growing, and can secrete products. We have reconstituted the fatty acid synthase (FAS) of the cyanobacterium Synechococcus sp. PCC 7002 and subjected it to in vitro kinetic analysis. Our data revealed that the overall rate of this metabolic pathway is exclusively limited by the FabH ketosynthase, which initiates product synthesis by condensing malonyl-ACP with acetyl-CoA to form acetoacetyl-ACP. This finding sharply contrasts with our previous findings that the E. coli FAS is predominantly limited by its dehydratase (FabZ) and enoyl reductase (FabI) activities and that FabH activity is not limiting. We therefore reconstituted and analyzed a set of “hybrid” FASs. When the Synechococcus FabH was used to replace its counterpart in the reconstituted E. coli FAS, the resulting synthase was strongly limited by FabH activity. Conversely, replacement of the E. coli FabZ with its Synechococcus homolog dramatically alleviated the dependence of E. coli FAS activity on FabZ. In agreement with this finding, introduction of the E. coli FabH in the Synechococcus FAS virtually eliminated its dependence on this subunit, whereas substitution of the Synechococcus FabZ with its E. coli homolog shifted a substantial fraction of the overall flux control in the Synechococcus FAS to FabZ. Our findings demonstrate that the rate-limiting steps can differ dramatically between closely related bacterial fatty acid synthases, and that such regulatory behavior is fundamentally the property of the controlling enzyme(s).
doi:10.1016/j.ymben.2013.12.008
PMCID: PMC4040180  PMID: 24395007
Fatty acid biosynthesis; FabI; FabZ; E. coli; cyanobacteria; biofuel
7.  Elevated Transglutaminase 2 Activity is Associated with Hypoxia-Induced Experimental Pulmonary Hypertension in Mice 
ACS chemical biology  2013;9(1):266-275.
Previous studies in human patients and animal models have suggested that transglutaminase 2 (TG2) is upregulated in pulmonary hypertension (PH), a phenomenon that appears to be associated with the effects of serotonin (5-hydroxytryptamine; 5-HT) in this disease. Using chemical tools to interrogate and inhibit TG2 activity in vivo, we have shown that pulmonary TG2 undergoes marked post-translational activation in a mouse model of hypoxia-induced PH. We have also identified irreversible fluorinated TG2 inhibitors that may find use as non-invasive positron emission tomography probes for diagnosis and management of this debilitating, lifelong disorder. Pharmacological inhibition of TG2 attenuated the elevated right ventricular pressure but had no effect on hypertrophy of the right ventricle of the heart. A longitudinal study of pulmonary TG2 activity in PH patients is warranted.
doi:10.1021/cb4006408
PMCID: PMC3947056  PMID: 24152195
8.  Metabolic Flux Between Unsaturated and Saturated Fatty Acids is Controlled by the FabA:FabB Ratio in the Fully Reconstituted Fatty Acid Biosynthetic Pathway of E. coli# 
Biochemistry  2013;52(46):10.1021/bi401116n.
The entire fatty acid biosynthetic pathway from Escherichia coli, starting from the acetyl-CoA carboxylase, has been reconstituted in vitro from fourteen purified protein components. Radiotracer analysis verified stoichiometric conversion of acetyl-CoA and NAD(P)H into the free fatty acid product, allowing implementation of a facile spectrophotometric assay for kinetic analysis of this multi-enzyme system. At steady state, a maximum turnover rate of 0.5 s−1 was achieved. Under optimal turnover conditions, the predominant products were C16 and C18 saturated as well as monounsaturated fatty acids. The reconstituted system allowed us to quantitatively interrogate the factors that influence metabolic flux toward unsaturated versus saturated fatty acids. In particular, the concentrations of the dehydratase FabA and the β-ketoacyl synthase FabB were found to be crucial for controlling this property. By altering these variables, the percentage of unsaturated fatty acid produced could be adjusted between 10 and 50% without significantly affecting the maximum turnover rate of the pathway. Our reconstituted system provides a powerful tool to understand and engineer rate-limiting and regulatory steps in this complex and practically significant metabolic pathway.
doi:10.1021/bi401116n
PMCID: PMC3858588  PMID: 24147979
9.  In vitro Reconstitution and Analysis of the 6-Deoxyerythronolide B Synthase 
Journal of the American Chemical Society  2013;135(45):10.1021/ja409048k.
Notwithstanding an extensive literature on assembly line polyketide synthases such as the 6-deoxyerythronolide B synthase (DEBS), a complete naturally occurring synthase has never been reconstituted in vitro from purified protein components. Here, we describe the fully reconstituted DEBS and quantitatively characterize some of the properties of the assembled system that have never been explored previously. The maximum turnover rate of the complete hexamodular system is 1.1 min−1, comparable to the turnover rate of a truncated trimodular derivative (2.5 min−1) but slower than a bimodular derivative (21 min−1). In the presence of similar concentrations of methylmalonyl- and ethylmalonyl-CoA substrates, DEBS synthesizes multiple regiospecifically modified analogs, one of which we have analyzed in detail. Our studies lay the foundation for biochemically interrogating and rationally engineering polyketide assembly lines in an unprecedented manner.
doi:10.1021/ja409048k
PMCID: PMC3858836  PMID: 24161212
10.  Rationale for Using Social Media to Collect Patient-Reported Outcomes in Patients with Celiac Disease 
Patients with celiac disease (CD) are increasingly interconnected through social media, exchanging patient experiences and health-tracking information between individuals through various web-based platforms. Social media represents potentially unique communication interface between gastroenterologists and active social media users – especially young adults and adolescents with celiac disease-regarding adherence to the strict gluten-free diet, gastrointestinal symptoms, and meaningful discussion about disease management. Yet, various social media platforms may be underutilized for research purposes to collect patient-reported outcomes data. In this commentary, we summarize the scientific rationale and potential for future growth of social media in patient-reported outcomes research, focusing on college freshmen with celiac disease as a case study and provide overview of the methodological approach. Finally, we discuss how social media may impact patient care in the future through increasing mobile technology use.
doi:10.4172/2161-069X.1000166
PMCID: PMC4226063  PMID: 25392743
Social media; Social networking; Facebook; Patient-reported outcomes; Healthcare; Mobile technology; Quality-of-life
11.  Coupled Methyl-Group Epimerization and Reduction by Polyketide Synthase Ketoreductase Domains. Ketoreductase-Catalyzed Equilibrium Isotope Exchange 
Journal of the American Chemical Society  2013;135(44):10.1021/ja408944s.
Incubation of [2-2H]-(2S,3R)-2-methyl-3-hydroxypentanoyl-SACP ([2-2H]-1a) with the epimerizing ketoreductase domain EryKR1 in the presence of a catalytic amount NADP+ (0.05 equiv) resulted in time-and cofactor-dependent washout of deuterium from 1a, as a result of equilibrium isotope exchange of transiently generated [2-2H]-2-methyl-3-ketopentanoyl-ACP (2). Incubations of [2-2H]-(2S,3S)-2-methyl-3-hydroxypentanoyl-SACP ([2-2H]-1b) with RifKR7 and with NysKR1 also resulted in time-dependent loss of deuterium. By contrast, incubations of [2-2H]-(2R,3S)-2-methyl-3-hydroxypentanoyl-SACP ([2-2H]-1c) and [2-2H]-(2R,3R)-2-methyl-3-hydroxypentanoyl-SACP ([2-2H]-1d) with the non-epimerizing ketoreductase domains EryKR6 and TylKR1, respectively, did not result in any significant washout of deuterium. The isotope exchange assay directly establishes that specific polyketide synthase ketoreductase domains also have an intrinsic epimerase activity, thus enabling mechanistic analysis of a key determinant of polyketide stereocomplexity.
doi:10.1021/ja408944s
PMCID: PMC3865775  PMID: 24161343
12.  Computational identification and analysis of orphan assembly-line polyketide synthases 
The Journal of antibiotics  2013;67(1):89-97.
The increasing availability of DNA sequence data offers an opportunity for identifying new assembly-line polyketide synthases (PKSs) that produce biologically active natural products. We developed an automated method to extract and consolidate all multimodular PKS sequences (including hybrid PKS/non-ribosomal peptide synthetases) in the National Center for Biotechnology Information (NCBI) database, generating a non-redundant catalog of 885 distinct assembly-line PKSs, the majority of which were orphans associated with no known polyketide product. Two in silico experiments highlight the value of this search method and resulting catalog. First, we identified an orphan that could be engineered to produce an analog of albocycline, an interesting antibiotic whose gene cluster has not yet been sequenced. Second, we identified and analyzed a hitherto overlooked family of metazoan multimodular PKSs, including one from Caenorhabditis elegans. We also developed a comparative analysis method that identified sequence relationships among known and orphan PKSs. As expected, PKS sequences clustered according to structural similarities between their polyketide products. The utility of this method was illustrated by highlighting an interesting orphan from the genus Burkholderia that has no close relatives. Our search method and catalog provide a community resource for the discovery of new families of assembly-line PKSs and their antibiotic products.
doi:10.1038/ja.2013.125
PMCID: PMC4028714  PMID: 24301183
assembly line; orphan antibiotics; polyketide synthase
13.  Expanding the fluorine chemistry of living systems using engineered polyketide synthase pathways 
Science (New York, N.Y.)  2013;341(6150):1089-1094.
Organofluorines represent a rapidly expanding proportion of molecules used in pharmaceuticals, diagnostics, agrochemicals, and materials. Despite the prevalence of fluorine in synthetic compounds, the known biological scope is limited to a single pathway that produces fluoroacetate. Here, we demonstrate that this pathway can be exploited as a source of fluorinated building blocks for introduction of fluorine into natural product scaffolds. Specifically, we have constructed pathways involving two polyketide synthase systems and show that fluoroacetate can be used to incorporate fluorine into the polyketide backbone in vitro. We further show that fluorine can be introduced site-selectively and introduced into polyketide products in vivo. These results highlight the prospects for the production of complex fluorinated natural products using synthetic biology.
doi:10.1126/science.1242345
PMCID: PMC4057101  PMID: 24009388
14.  Dihydroisoxazole inhibitors of Anopheles gambiae seminal transglutaminase AgTG3 
Malaria Journal  2014;13:210.
Background
Current vector-based malaria control strategies are threatened by the rise of biochemical and behavioural resistance in mosquitoes. Researching mosquito traits of immunity and fertility is required to find potential targets for new vector control strategies. The seminal transglutaminase AgTG3 coagulates male Anopheles gambiae seminal fluids, forming a ‘mating plug’ that is required for male reproductive success. Inhibitors of AgTG3 can be useful both as chemical probes of A. gambiae reproductive biology and may further the development of new chemosterilants for mosquito population control.
Methods
A targeted library of 3-bromo-4,5-dihydroxoisoxazole inhibitors were synthesized and screened for inhibition of AgTG3 in a fluorescent, plate-based assay. Positive hits were tested for in vitro activity using cross-linking and mass spectrometry, and in vivo efficacy in laboratory mating assays.
Results
A targeted chemical library was screened for inhibition of AgTG3 in a fluorescent plate-based assay using its native substrate, plugin. Several inhibitors were identified with IC50 < 10 μM. Preliminary structure-activity relationships within the library support the stereo-specificity and preference for aromatic substituents in the chemical scaffold. Both inhibition of plugin cross-linking and covalent modification of the active site cysteine of AgTG3 were verified. Administration of an AgTG3 inhibitor to A. gambiae males by intrathoracic injection led to a 15% reduction in mating plug transfer in laboratory mating assays.
Conclusions
A targeted screen has identified chemical inhibitors of A. gambiae transglutaminase 3 (AgTG3). The most potent inhibitors are known inhibitors of human transglutaminase 2, suggesting a common binding pose may exist within the active site of both enzymes. Future efforts to develop additional inhibitors will provide chemical tools to address important biological questions regarding the role of the A. gambiae mating plug. A second use for transglutaminase inhibitors exists for the study of haemolymph coagulation and immune responses to wound healing in insects.
doi:10.1186/1475-2875-13-210
PMCID: PMC4113009  PMID: 24888439
15.  Stereochemistry of Reductions Catalyzed by Methyl-Epimerizing Ketoreductase Domains of Polyketide Synthases 
Ketoreductase (KR) domains from modular polyketide synthases catalyze reduction of 2-methyl-3-ketoacyl-ACP substrates and in certain cases epimerization of the 2-methyl group as well. The structural and mechanistic basis of epimerization is poorly understood, and only a small number of such KRs been studied. We have studied three recombinant KR domains with putative epimerase activity – NysKR1 from module 1 of the nystatin PKS whose stereospecificity can be predicted from both protein sequence and product structure, RifKR7 from module 7 of the rifamycin PKS whose stereospecificity cannot be predicted from protein sequence, and RifKR10 from module 10 of the rifamycin PKS whose specificity is unclear from both sequence and structure. Each KR was individually incubated with NADPH and (2R)- or (2RS)-2-methyl-3-ketopentanoyl-ACP generated enzymatically in situ or via chemo-enzymatic synthesis, respectively. Chiral GC-MS analysis revealed that each KR stereospecifically produced the corresponding (2S,3S)-2-methyl-3-hydroxypentanoyl-ACP in which the 2-methyl substituent had undergone KR-catalyzed epimerization. Thus, our results have led to the identification of a prototypical set of KR domains that generate (2S,3S)-2-methyl-3-hydroxyacyl products in the course of polyketide biosynthesis.
doi:10.1021/ja4014776
PMCID: PMC3699853  PMID: 23659177
16.  Gluten-sensitive enteropathy coincides with decreased capability of intestinal T cells to secrete IL-17 and IL-22 in a macaque model for celiac disease 
Celiac disease (CD) is an autoimmune disorder caused by intolerance to dietary gluten. The interleukin (IL)-17 and IL-22 function as innate regulators of mucosal integrity. Impaired but not well-understood kinetics of the IL-17/22 secretion was described in celiac patients. Here, the IL-17 and IL-22-producing intestinal cells were studied upon their in vitro stimulation with mitogens in class II major histocompatibility complex-defined, gluten-sensitive rhesus macaques. Pediatric biopsies were collected from distal duodenum during the stages of disease remission and relapse. Regardless of dietary gluten content, IL-17 and IL-22-producing cells consisted of CD4+ and CD8+ T lymphocytes as well as of lineage-negative (Lin−) cells. Upon introduction of dietary gluten, capability of intestinal T cells to secrete IL-17/22 started to decline (p < 0.05), which was paralleled with gradual disruption of epithelial integrity. These data indicate that IL-17/22-producing cells play an important role in maintenance of intestinal mucosa in gluten-sensitive primates.
doi:10.1016/j.clim.2013.02.012
PMCID: PMC3732447  PMID: 23518597
Celiac disease; Th17; Rhesus; Tissue transglutaminase; Autoimmunity
17.  Mechanism and Specificity of an Acyltransferase Domain from a Modular Polyketide Synthase 
Biochemistry  2013;52(11):1839-1841.
Acyltransferase (AT) domains of modular polyketide synthases exercise tight control over the choice of α-carboxyacyl-CoA substrates, but the mechanistic basis for this specificity is unknown. We show that whereas the specificity for the electrophilic malonyl or methylmalonyl component is primarily expressed in the first half-reaction (formation of the acyl enzyme intermediate), the second half-reaction shows comparable specificity for the acyl carrier protein that carries the nucleophilic pantetheine arm. We also show that currently used approaches for engineering AT domain specificity work mainly by degrading specificity for the natural substrate rather than by enhancing specificity for alternative substrates.
doi:10.1021/bi400185v
PMCID: PMC3612939  PMID: 23452124
18.  Analysis and Refactoring of the A-74528 Biosynthetic Pathway 
A-74528 is a C-30 polyketide natural product that functions as an inhibitor of 2′,5′-oligoadenylate phosphodiesterase (2′-PDE), a key regulatory enzyme of the interferon pathway. Modulation of 2′-PDE represents a unique therapeutic approach to regulate viral infections. The gene cluster responsible for biosynthesis of A-74528 yields minute amounts of this natural product together with considerably larger quantities of a structurally dissimilar C-30 cytotoxic agent, fredericamycin. Through construction and analysis of a series of knockout mutants, we identified the necessary genes for A-74528 biosynthesis. Remarkably, the formation of six stereocenters and the regiospecific formation of six rings in A- 74528 appears to be catalyzed by only two tailoring enzymes, a cyclase and an oxygenase, in addition to the core polyketide synthase. The inferred pathway was genetically refactored in a heterologous host, Streptomyces coelicolor CH999, to produce 3 mg/L A-74528 in the absence of fredericamycin.
doi:10.1021/ja311579s
PMCID: PMC3601660  PMID: 23442197
19.  Selective Inhibition of Extracellular Thioredoxin by Asymmetric Disulfides 
Journal of medicinal chemistry  2013;56(3):1301-1310.
Whereas the role of mammalian thioredoxin (Trx) as an intracellular protein cofactor is widely appreciated, its function in the extracellular environment is not well understood. Only few extracellular targets of Trx-mediated thiol-disulfide exchange are known. For example, Trx activates extracellular transglutaminase 2 (TG2) via reduction of an intramolecular disulfide bond. Because hyperactive TG2 is thought to play a role in various diseases, understanding the biological role of extracellular Trx may provide critical insight into the pathogenesis of these disorders. Starting from a clinical-stage asymmetric disulfide lead, we have identified analogs with >100-fold specificity for Trx. Structure-activity relationship and computational docking model analyses have provided insights into the features important for enhancing potency and specificity. The most active compound identified had an IC50 below 0.1µM in cell culture, and may be appropriate for in vivo use to interrogate the role of extracellular Trx in health and disease.
doi:10.1021/jm301775s
PMCID: PMC3574193  PMID: 23327656
20.  Molecular Insights into the Biosynthesis of Guadinomine, a Type III Secretion System Inhibitor 
Journal of the American Chemical Society  2012;134(42):17797-17806.
Guadinomines are a recently discovered family of anti-infective compounds produced by Streptomyces sp. K01-0509 with a novel mode of action. With an IC50 of 14 nM, guadinomine B is the most potent known inhibitor of the Type III Secretion System (TTSS) of Gram-negative bacteria. TTSS activity is required for the virulence of many pathogenic Gram-negative bacteria including Escherichia coli, Salmonella spp., Yersinia spp., Chlamydia spp., Vibrio spp., and Pseudomonas spp. The guadinomine (gdn) biosynthetic gene cluster has been cloned and sequenced, and includes 26 open reading frames spanning 51.2 kb. It encodes a chimeric multimodular polyketide synthase – nonribosomal peptide synthetase, along with enzymes responsible for the biosynthesis of the unusual aminomalonyl-ACP extender unit and the signature carbamoylated cyclic guanidine. Its identity was established by targeted disruption of the gene cluster, as well as by heterologous expression and analysis of key enzymes in the biosynthetic pathway. Identifying the guadinomine gene cluster provides critical insight into the biosynthesis of these scarce but potentially important natural products.
doi:10.1021/ja308622d
PMCID: PMC3483642  PMID: 23030602
21.  Engineering the acyltransferase substrate specificity of assembly line polyketide synthases 
Polyketide natural products act as a broad range of therapeutics, including antibiotics, immunosuppressants and anti-cancer agents. This therapeutic diversity stems from the structural diversity of these small molecules, many of which are produced in an assembly line manner by modular polyketide synthases. The acyltransferase (AT) domains of these megasynthases are responsible for selection and incorporation of simple monomeric building blocks, and are thus responsible for a large amount of the resulting polyketide structural diversity. The substrate specificity of these domains is often targeted for engineering in the generation of novel, therapeutically active natural products. This review outlines recent developments that can be used in the successful engineering of these domains, including AT sequence and structural data, mechanistic insights and the production of a diverse pool of extender units. It also provides an overview of previous AT domain engineering attempts, and concludes with proposed engineering approaches that take advantage of current knowledge. These approaches may lead to successful production of biologically active ‘unnatural’ natural products.
doi:10.1098/rsif.2013.0297
PMCID: PMC3971720  PMID: 23720536
polyketide; acyltransferase; antibiotics; enzyme engineering
22.  Novel chemo-sensitizing agent, ERW1227B, impairs cellular motility and enhances cell death in glioblastomas 
Journal of neuro-oncology  2010;103(2):207-219.
Glioblastomas display variable phenotypes that include increased drug-resistance associated with enhanced migratory and anti-apoptotic characteristics. These shared characteristics contribute to failure of clinical treatment regimens. Identification of novel compounds that promote cell death and impair cellular motility is a logical strategy to develop more effective clinical protocols. We recently described the ability of the small molecule, KCC009, a tissue transglutaminase (TG2) inhibitor, to sensitize glioblastoma cells to chemotherapy. In the current study, we synthesized a series of related compounds that show variable ability to promote cell death and impair motility in glioblastomas, irrespective of their ability to inhibit TG2. Each compound has a 3-bromo-4,5-dihydroisoxazole component that presumably reacts with nucleophilic cysteine thiol residues in the active sites of proteins that have an affinity to the small molecule.
Our studies focused on the effects of the compound, ERW1227B. Treatment of glioblastoma cells with ERW1227B was associated with both down-regulation of the PI-3 kinase/Akt pathway, which enhanced cell death; as well as disruption of focal adhesive complexes and intracellular actin fibers, which impaired cellular mobility. Bioassays as well as time-lapse photography of glioblastoma cells treated with ERW1227B showed cell death and rapid loss of cellular motility. Mice studies with in vivo glioblastoma models demonstrated the ability of ERW1227B to sensitize tumor cells to cell death after treatment with either chemotherapy or radiation. The above findings identify ERW1227B as a potential novel therapeutic agent in the treatment of glioblastomas.
doi:10.1007/s11060-010-0379-2
PMCID: PMC3732656  PMID: 20824305
Focal adhesive complexes; Glioblastomas; ERW1227B; Cell death; Drug-resistance; Cellular motility
23.  Tissue transgluaminase 2 expression in meningiomas 
Journal of neuro-oncology  2008;90(2):125-132.
Meningiomas are common intracranial tumors that occur in extra-axial locations, most often over the cerebral convexities or along the skull-base. Although often histologically benign these tumors frequently present challenging clinical problems. Primary clinical management of patients with symptomatic tumors is surgical resection. Radiation treatment may arrest growth or delay recurrence of these tumors, however, meningioma cells are generally resistant to apoptosis after treatment with radiation. Tumor cells are known to alter their expression of proteins that interact in the ECM to provide signals important in tumor progression. One such protein, fibronectin, is expressed in elevated levels in the ECM in a number of tumors including meningiomas. We recently reported that levels of both extracellular fibronectin and tissue transglutaminase 2 (TG2) were increased in glioblastomas. We examined the expression of fibronectin and its association TG2 in meningiomas. Both fibronectin and TG2 were strongly expressed in all meningiomas studied. TG2 activity was markedly elevated in meningiomas, and TG2 was found to co-localize with fibronectin. Treatment of meningiomas with the small molecule TG2 inhibitor, KCC009, inhibited the binding of TG2 to fibronectin and blocked disposition of linear strands of fibronectin in the ECM. KCC009 treatment promoted apoptosis and enhanced radiation sensitivity both in cultured IOMM-Lee meningioma cells and in meningioma tumor explants. These findings support a potential protective role for TG2 in meningiomas.
doi:10.1007/s11060-008-9642-1
PMCID: PMC3732188  PMID: 18587533
Brain tumor; Cell death; tissue transglutaminase inhibitor; Radiation
24.  Precursor Directed Biosynthesis of an Orthogonally Functional Erythromycin Analogue: Selectivity in the Ribosome Macrolide Binding Pocket 
Journal of the American Chemical Society  2012;134(29):12259-12265.
The macrolide antibiotic erythromycin A and its semisynthetic analogues have been among the most useful antibacterial agents for the treatment of infectious diseases. Using a recently developed chemical genetic strategy for precursor-directed biosynthesis and colony bioassay of 6-deoxyerythromycin D analogues, we identified a new class of alkynyl- and alkenyl-substituted macrolides with activity comparable to that of the natural product. Further analysis revealed a marked and unexpected dependence of antibiotic activity on the size and degree of unsaturation of the precursor. Based on these leads, we also report the precursor-directed biosynthesis of 15-propargyl erythromycin A, a novel antibiotic that is not only as potent as erythromycin A with respect to its ability to inhibit bacterial growth and cell-free ribosomal protein biosynthesis, but also harbors an orthogonal functional group that is capable of facile chemical modification.
doi:10.1021/ja304682q
PMCID: PMC3405186  PMID: 22741553
25.  Noninflammatory Gluten Peptide Analogs as Biomarkers for Celiac Sprue 
Chemistry & biology  2009;16(8):868-881.
SUMMARY
New tools are needed for managing celiac sprue, a lifelong immune disease of the small intestine. Ongoing drug trials are also prompting a search for noninvasive biomarkers of gluten-induced intestinal change. We have synthesized and characterized noninflammatory gluten peptide analogs in which key Gln residues are replaced by Asn or His. Like their proinflammatory counterparts, these biomarkers are resistant to gastrointestinal proteases, susceptible to glutenases, and permeable across enterocyte barriers. Unlike gluten peptides, however, they are not appreciably recognized by transglutaminase, HLA-DQ2, or disease-specific T cells. In vitro and animal studies show that the biomarkers can detect intestinal permeability changes as well as glutenase-catalyzed gastric detoxification of gluten. Accordingly, controlled clinical studies are warranted to evaluate the use of these peptides as probes for abnormal intestinal permeability in celiac patients and for glutenase efficacy in clinical trials and practice.
doi:10.1016/j.chembiol.2009.07.009
PMCID: PMC3721637  PMID: 19716477

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