1
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Phelan RM, Abrahamson MJ, Brown JTC, Zhang RK, Zwick CR. Development of Scalable Processes with Underutilized Biocatalyst Classes. Org Process Res Dev 2022. [DOI: 10.1021/acs.oprd.1c00467] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Affiliation(s)
- Ryan M. Phelan
- Process Research and Development, AbbVie Inc., North Chicago, Illinois 60064, United States
| | - Michael J. Abrahamson
- Operations Science and Technology, AbbVie Inc., North Chicago, Illinois 60064, United States
| | - Jesse T. C. Brown
- Process Research and Development, AbbVie Inc., North Chicago, Illinois 60064, United States
| | - Ruijie K. Zhang
- Discovery Chemistry and Technology, AbbVie Inc., North Chicago, Illinois 60064, United States
| | - Christian R. Zwick
- Process Research and Development, AbbVie Inc., North Chicago, Illinois 60064, United States
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2
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Brown JTC, Tu NP, Phelan RM. Solid, Noncovalent Formulation of Biocatalysts for Rapid and Accurate Submilligram Dosing to Microtiter Plates. Org Process Res Dev 2021. [DOI: 10.1021/acs.oprd.0c00443] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Affiliation(s)
- Jesse T. C. Brown
- Process Research and Development, AbbVie Inc., 1 North Waukegan Road, North Chicago, Illinois 60064, United States
| | - Noah P. Tu
- Discovery Chemistry and Technology, AbbVie Inc. 1 North Waukegan Road, North Chicago, Illinois 60064, United States
| | - Ryan M. Phelan
- Process Research and Development, AbbVie Inc., 1 North Waukegan Road, North Chicago, Illinois 60064, United States
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3
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Haushalter RW, Phelan RM, Hoh KM, Su C, Wang G, Baidoo EEK, Keasling JD. Production of Odd-Carbon Dicarboxylic Acids in Escherichia coli Using an Engineered Biotin-Fatty Acid Biosynthetic Pathway. J Am Chem Soc 2017; 139:4615-4618. [PMID: 28291347 DOI: 10.1021/jacs.6b11895] [Citation(s) in RCA: 27] [Impact Index Per Article: 3.9] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
Dicarboxylic acids are commodity chemicals used in the production of plastics, polyesters, nylons, fragrances, and medications. Bio-based routes to dicarboxylic acids are gaining attention due to environmental concerns about petroleum-based production of these compounds. Some industrial applications require dicarboxylic acids with specific carbon chain lengths, including odd-carbon species. Biosynthetic pathways involving cytochrome P450-catalyzed oxidation of fatty acids in yeast and bacteria have been reported, but these systems produce almost exclusively even-carbon species. Here we report a novel pathway to odd-carbon dicarboxylic acids directly from glucose in Escherichia coli by employing an engineered pathway combining enzymes from biotin and fatty acid synthesis. Optimization of the pathway will lead to industrial strains for the production of valuable odd-carbon diacids.
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Affiliation(s)
- Robert W Haushalter
- Joint BioEnergy Institute, U.S. Department of Energy , 5885 Hollis Street, Emeryville, California 94608, United States.,Physical Bioscience Division, Lawrence Berkeley National Laboratory , Berkeley, California 94720, United States
| | - Ryan M Phelan
- Joint BioEnergy Institute, U.S. Department of Energy , 5885 Hollis Street, Emeryville, California 94608, United States.,Physical Bioscience Division, Lawrence Berkeley National Laboratory , Berkeley, California 94720, United States
| | - Kristina M Hoh
- Joint BioEnergy Institute, U.S. Department of Energy , 5885 Hollis Street, Emeryville, California 94608, United States.,Physical Bioscience Division, Lawrence Berkeley National Laboratory , Berkeley, California 94720, United States
| | - Cindy Su
- Joint BioEnergy Institute, U.S. Department of Energy , 5885 Hollis Street, Emeryville, California 94608, United States.,Physical Bioscience Division, Lawrence Berkeley National Laboratory , Berkeley, California 94720, United States
| | - George Wang
- Joint BioEnergy Institute, U.S. Department of Energy , 5885 Hollis Street, Emeryville, California 94608, United States.,Physical Bioscience Division, Lawrence Berkeley National Laboratory , Berkeley, California 94720, United States
| | - Edward E K Baidoo
- Joint BioEnergy Institute, U.S. Department of Energy , 5885 Hollis Street, Emeryville, California 94608, United States.,Physical Bioscience Division, Lawrence Berkeley National Laboratory , Berkeley, California 94720, United States
| | - Jay D Keasling
- Joint BioEnergy Institute, U.S. Department of Energy , 5885 Hollis Street, Emeryville, California 94608, United States.,Physical Bioscience Division, Lawrence Berkeley National Laboratory , Berkeley, California 94720, United States
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Phelan RM, Sachs D, Petkiewicz SJ, Barajas JF, Blake-Hedges JM, Thompson MG, Reider Apel A, Rasor BJ, Katz L, Keasling JD. Development of Next Generation Synthetic Biology Tools for Use in Streptomyces venezuelae. ACS Synth Biol 2017; 6:159-166. [PMID: 27605473 DOI: 10.1021/acssynbio.6b00202] [Citation(s) in RCA: 56] [Impact Index Per Article: 8.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
Streptomyces have a rich history as producers of important natural products and this genus of bacteria has recently garnered attention for its potential applications in the broader context of synthetic biology. However, the dearth of genetic tools available to control and monitor protein production precludes rapid and predictable metabolic engineering that is possible in hosts such as Escherichia coli or Saccharomyces cerevisiae. In an effort to improve genetic tools for Streptomyces venezuelae, we developed a suite of standardized, orthogonal integration vectors and an improved method to monitor protein production in this host. These tools were applied to characterize heterologous promoters and various attB chromosomal integration sites. A final study leveraged the characterized toolset to demonstrate its use in producing the biofuel precursor bisabolene using a chromosomally integrated expression system. These tools advance S. venezuelae to be a practical host for future metabolic engineering efforts.
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Affiliation(s)
- Ryan M. Phelan
- Joint Bioenergy Institute, 5885 Hollis Street, Emeryville, California 94608, United States
| | - Daniel Sachs
- Joint Bioenergy Institute, 5885 Hollis Street, Emeryville, California 94608, United States
| | - Shayne J. Petkiewicz
- Joint Bioenergy Institute, 5885 Hollis Street, Emeryville, California 94608, United States
| | - Jesus F. Barajas
- Joint Bioenergy Institute, 5885 Hollis Street, Emeryville, California 94608, United States
| | | | | | - Amanda Reider Apel
- Joint Bioenergy Institute, 5885 Hollis Street, Emeryville, California 94608, United States
| | - Blake J. Rasor
- Department
of Biology, Miami University, 212 Pearson Hall, Oxford, Ohio 45046, United States
| | | | - Jay D. Keasling
- Joint Bioenergy Institute, 5885 Hollis Street, Emeryville, California 94608, United States
- Novo
Nordisk Foundation Center for Biosustainability, Technical University of Denmark, Kogle Allé, DK2970-Hørsholm, Denmark
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5
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Barajas JF, Phelan RM, Schaub AJ, Kliewer JT, Kelly PJ, Jackson DR, Luo R, Keasling JD, Tsai SC. Comprehensive Structural and Biochemical Analysis of the Terminal Myxalamid Reductase Domain for the Engineered Production of Primary Alcohols. ACTA ACUST UNITED AC 2015; 22:1018-29. [PMID: 26235055 DOI: 10.1016/j.chembiol.2015.06.022] [Citation(s) in RCA: 46] [Impact Index Per Article: 5.1] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/23/2015] [Revised: 06/05/2015] [Accepted: 06/22/2015] [Indexed: 01/22/2023]
Abstract
The terminal reductase (R) domain from the non-ribosomal peptide synthetase (NRPS) module MxaA in Stigmatella aurantiaca Sga15 catalyzes a non-processive four-electron reduction to produce the myxalamide family of secondary metabolites. Despite widespread use in nature, a lack of structural and mechanistic information concerning reductive release from polyketide synthase (PKS) and NRPS assembly lines principally limits our ability to redesign R domains with altered or improved activity. Here we report crystal structures for MxaA R, both in the absence and, for the first time, in the presence of the NADPH cofactor. Molecular dynamics simulations were employed to provide a deeper understanding of this domain and further identify residues critical for structural integrity, substrate binding, and catalysis. Aggregate computational and structural findings provided a basis for mechanistic investigations and, in the process, delivered a rationally altered variant with improved activity toward highly reduced substrates.
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Affiliation(s)
- Jesus F Barajas
- Department of Molecular Biology and Biochemistry, Chemistry, and Pharmaceutical Sciences, University of California, Irvine, Irvine, CA 92697, USA
| | - Ryan M Phelan
- Joint Bioenergy Institute, 5885 Hollis Street, Emeryville, CA 94608, USA; QB3 Institute, University of California, Berkeley, Berkeley, CA 94270, USA
| | - Andrew J Schaub
- Department of Molecular Biology and Biochemistry, Chemistry, and Pharmaceutical Sciences, University of California, Irvine, Irvine, CA 92697, USA
| | - Jaclyn T Kliewer
- Department of Molecular Biology and Biochemistry, Chemistry, and Pharmaceutical Sciences, University of California, Irvine, Irvine, CA 92697, USA
| | - Peter J Kelly
- Department of Molecular Biology and Biochemistry, Chemistry, and Pharmaceutical Sciences, University of California, Irvine, Irvine, CA 92697, USA
| | - David R Jackson
- Department of Molecular Biology and Biochemistry, Chemistry, and Pharmaceutical Sciences, University of California, Irvine, Irvine, CA 92697, USA
| | - Ray Luo
- Department of Molecular Biology and Biochemistry, Chemistry, and Pharmaceutical Sciences, University of California, Irvine, Irvine, CA 92697, USA
| | - Jay D Keasling
- Joint Bioenergy Institute, 5885 Hollis Street, Emeryville, CA 94608, USA; QB3 Institute, University of California, Berkeley, Berkeley, CA 94270, USA; Department of Chemical and Biomolecular Engineering and Department of Bioengineering, University of California, Berkeley, Berkeley, CA 94720, USA; Physical Biosciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA.
| | - Shiou-Chuan Tsai
- Department of Molecular Biology and Biochemistry, Chemistry, and Pharmaceutical Sciences, University of California, Irvine, Irvine, CA 92697, USA.
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Phelan RM, Sekurova ON, Keasling JD, Zotchev SB. Engineering terpene biosynthesis in Streptomyces for production of the advanced biofuel precursor bisabolene. ACS Synth Biol 2015; 4:393-9. [PMID: 25006988 DOI: 10.1021/sb5002517] [Citation(s) in RCA: 65] [Impact Index Per Article: 7.2] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
The past decade has witnessed a large influx of research toward the creation of sustainable, biologically derived fuels. While significant effort has been exerted to improve production capacity in common hosts, such as Escherichia coli or Saccharomyces cerevisiae, studies concerning alternate microbes comparatively lag. In an effort to expand the breadth of characterized hosts for fuel production, we map the terpene biosynthetic pathway in a model actinobacterium, Streptomyces venezuelae, and further alter secondary metabolism to afford the advanced biofuel precursor bisabolene. Leveraging information gained from study of the native isoprenoid pathway, we were able to increase bisabolene titer nearly 5-fold over the base production strain, more than 2 orders of magnitude greater than the combined terpene yield in the wild-type host. We also explored production on carbon sources of varying complexity to, notably, define this host as one able to perform consolidated bioprocessing.
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Affiliation(s)
- Ryan M. Phelan
- Joint BioEnergy Institute, 5885 Hollis Avenue, Emeryville, California 94608, United States
| | - Olga N. Sekurova
- Department
of Biotechnology, Norwegian University of Science and Technology, Sem Saelands vei 6/8, 7491 Trondheim, Norway
| | - Jay D. Keasling
- Joint BioEnergy Institute, 5885 Hollis Avenue, Emeryville, California 94608, United States
- Department of Chemical & Biomolecular Engineering, Department of Bioengineering, University of California, Berkeley, California 94720 United States
- Physical
Biosciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720 United States
| | - Sergey B. Zotchev
- Department
of Biotechnology, Norwegian University of Science and Technology, Sem Saelands vei 6/8, 7491 Trondheim, Norway
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Poust S, Phelan RM, Deng K, Katz L, Petzold CJ, Keasling JD. Divergent Mechanistic Routes for the Formation ofgem-Dimethyl Groups in the Biosynthesis of Complex Polyketides. Angew Chem Int Ed Engl 2015; 54:2370-3. [DOI: 10.1002/anie.201410124] [Citation(s) in RCA: 28] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/15/2014] [Revised: 11/18/2014] [Indexed: 11/07/2022]
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Poust S, Phelan RM, Deng K, Katz L, Petzold CJ, Keasling JD. Divergent Mechanistic Routes for the Formation ofgem-Dimethyl Groups in the Biosynthesis of Complex Polyketides. Angew Chem Int Ed Engl 2015. [DOI: 10.1002/ange.201410124] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/19/2023]
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Phelan RM, Townsend CA. Mechanistic insights into the bifunctional non-heme iron oxygenase carbapenem synthase by active site saturation mutagenesis. J Am Chem Soc 2013; 135:7496-502. [PMID: 23611403 DOI: 10.1021/ja311078s] [Citation(s) in RCA: 26] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/07/2023]
Abstract
The carbapenem class of β-lactam antibiotics is known for its remarkable potency, antibacterial spectrum, and resistance to β-lactamase-mediated inactivation. While the biosynthesis of structurally "complex" carbapenems, such as thienamycin, share initial biochemical steps with carbapenem-3-carboxylate ("simple" carbapenem), the requisite inversion at C5 and formation of the characteristic α,β-unsaturated carboxylate are different in origin between the two groups. Here, we consider carbapenem synthase, a mechanistically distinct bifunctional non-heme iron α-ketoglutarate-dependent enzyme responsible for the terminal reactions, C5 epimerization and desaturation, in simple carbapenem production. Interestingly, this enzyme accepts two stereoisomeric substrates and transforms each to a common active antibiotic. Owing both to enzyme and product instability, resorting to saturation mutagenesis of active site and selected second-sphere residues gave clearly differing profiles of CarC tolerance to structural modification. Guided by a crystal structure and the mutational data, in silico docking was used to suggest the positioning of each disastereomeric substrate in the active site. The two orientations relative to the reactive iron-oxo center are manifest in the two distinct reactions, C5-epimerization and C2/3-desaturation. These observations favor a two-step reaction scheme involving two complete oxidative cycles as opposed to a single catalytic cycle in which an active site tyrosine, Tyr67, after hydrogen donation to achieve bicyclic ring inversion, is further hypothesized to serve as a radical carrier.
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Affiliation(s)
- Ryan M Phelan
- Department of Chemistry, Johns Hopkins University, 3400 North Charles Street, Baltimore, Maryland 21218, USA
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Abstract
High-throughput screens and selections have had profound impact on our ability to engineer proteins possessing new, desired properties. These methods are especially useful when applied to the modification of existing enzymes to create natural and unnatural products. In an advance upon existing methods we developed a high-throughput, genetically regulated screen for the in vivo production of β-lactam antibiotics using a green fluorescent protein (gfp) reporter. This assay proved reliable and sensitive and presents a dynamic range under which a wide array of β-lactam architectural subclasses can be detected. Moreover, the graded response elicited in this assay can be used to rank mutant activity. The utility of this development was demonstrated in vivo and then applied to the first experimental investigation of a putative catalytic residue in carbapenem synthase (CarC). Information gained about the mutability of this residue defines one parameter for enzymatic activity and sets boundaries for future mechanistic and engineering efforts.
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Affiliation(s)
- Ryan M. Phelan
- Department
of Chemistry, Johns Hopkins University, 3400 N. Charles Street, Baltimore, Maryland
21218, United States
| | - Benjamin J. DiPardo
- Department
of Chemistry, Johns Hopkins University, 3400 N. Charles Street, Baltimore, Maryland
21218, United States
| | - Craig A. Townsend
- Department
of Chemistry, Johns Hopkins University, 3400 N. Charles Street, Baltimore, Maryland
21218, United States
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Bodner MJ, Li R, Phelan RM, Freeman MF, Moshos KA, Lloyd EP, Townsend CA. Definition of the common and divergent steps in carbapenem β-lactam antibiotic biosynthesis. Chembiochem 2011; 12:2159-65. [PMID: 21913298 PMCID: PMC3281309 DOI: 10.1002/cbic.201100366] [Citation(s) in RCA: 33] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/11/2011] [Indexed: 11/11/2022]
Abstract
Approximately 50 naturally occurring carbapenem β-lactam antibiotics are known. All but one of these have been isolated from Streptomyces species and are disubstituted structural variants of a simple core that is synthesized by Pectobacterium carotovorum (Erwinia carotovora), a phylogenetically distant plant pathogen. While the biosynthesis of the simple carbapenem, (5R)-carbapen-2-em-3-carboxylic acid, is impressively efficient requiring only three enzymes, CarA, CarB and CarC, the formation of thienamycin, one of the former group of metabolites from Streptomyces, is markedly more complex. Despite their phylogenetic separation, bioinformatic analysis of the encoding gene clusters suggests that the two pathways could be related. Here we demonstrate with gene swapping, stereochemical and kinetics experiments that CarB and CarA and their S. cattleya orthologues, ThnE and ThnM, respectively, are functionally and stereochemically equivalent, although their catalytic efficiencies differ. The biosynthetic pathways, therefore, to thienamycin, and likely to the other disubstituted carbapenems, and to the simplest carbapenem, (5R)-carbapen-2-em-3-carboxylic acid, are initiated in the same manner, but share only two common steps before diverging.
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Affiliation(s)
- Micah J. Bodner
- Department of Chemistry, The Johns Hopkins University Baltimore, Maryland 21218 (USA)
| | - Rongfeng Li
- Department of Chemistry, The Johns Hopkins University Baltimore, Maryland 21218 (USA)
| | - Ryan M. Phelan
- Department of Chemistry, The Johns Hopkins University Baltimore, Maryland 21218 (USA)
| | - Michael F. Freeman
- Department of Chemistry, The Johns Hopkins University Baltimore, Maryland 21218 (USA)
| | - Kristos A. Moshos
- Department of Chemistry, The Johns Hopkins University Baltimore, Maryland 21218 (USA)
| | - Evan P. Lloyd
- Department of Chemistry, The Johns Hopkins University Baltimore, Maryland 21218 (USA)
| | - Craig A. Townsend
- Department of Chemistry, The Johns Hopkins University Baltimore, Maryland 21218 (USA)
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Abstract
Carbapenems are a clinically important antibiotic family. More than 50 naturally occurring carbapenam/ems are known and are distinguished primarily by their C-2/C-6 side chains where many are only differentiated by the oxidation states of these substituents. With a limited palette of variations the carbapenem family comprises a natural combinatorial library, and C-2/C-6 oxidation is associated with increased efficacy. We demonstrate that ThnG and ThnQ encoded by the thienamycin gene cluster in Streptomyces cattleya oxidize the C-2 and C-6 moieties of carbapenems, respectively. ThnQ stereospecifically hydroxylates PS-5 (5) giving N-acetyl thienamycin (2). ThnG catalyzes sequential desaturation and sulfoxidation of PS-5 (5), giving PS-7 (7) and its sulfoxide (9). The enzymes are relatively substrate selective but are proposed to give rise to the oxidative diversity of carbapenems produced by S. cattleya, and orthologues likely function similarly in allied streptomyces. Elucidating the roles of ThnG and ThnQ will focus further investigations of carbapenem antibiotic biosynthesis.
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Affiliation(s)
- Micah J. Bodner
- Department of Chemistry, Johns Hopkins University, 3400 N. Charles St. Baltimore, MD 21218
| | - Ryan M. Phelan
- Department of Chemistry, Johns Hopkins University, 3400 N. Charles St. Baltimore, MD 21218
| | - Michael F. Freeman
- Department of Chemistry, Johns Hopkins University, 3400 N. Charles St. Baltimore, MD 21218
| | - Rongfeng Li
- Department of Chemistry, Johns Hopkins University, 3400 N. Charles St. Baltimore, MD 21218
| | - Craig A. Townsend
- Department of Chemistry, Johns Hopkins University, 3400 N. Charles St. Baltimore, MD 21218
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Abstract
Efficient syntheses of N-acetyl thienamycin and epithienamycin A in their readily deprotected form are reported where three contiguous stereocenters are established in a single catalytic asymmetric azetidinone-forming reaction. These examples are a template for synthesizing C-5/C-6 cis or trans carbapenems with independent control of the C-8 stereocenter. A library of oxidatively and sterochemically defined azetidinone precursors to a variety of naturally occurring carbapenems and potential biosynthetic intermediates has been prepared to facilitate studies of carbapenem antibiotic biosynthesis.
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Affiliation(s)
- Micah J Bodner
- Department of Chemistry, Johns Hopkins University, 3400 North Charles Street, Baltimore, Maryland 21218, USA
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Phelan RM, Ostermeier M, Townsend CA. Design and synthesis of a beta-lactamase activated 5-fluorouracil prodrug. Bioorg Med Chem Lett 2009; 19:1261-3. [PMID: 19167216 PMCID: PMC2838426 DOI: 10.1016/j.bmcl.2008.12.057] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/10/2008] [Revised: 12/09/2008] [Accepted: 12/11/2008] [Indexed: 10/21/2022]
Abstract
An efficient synthesis of a 5-fluorouracil-cephalosporin prodrug is described for use against colorectal and other cancers in antibody and gene-directed therapies. The compound shows stability in aqueous media until specifically activated by beta-lactamase (betaL). The kinetic parameters of the 5-fluorouracil-cephalosporin conjugate were determined in the presence of Enterobacter cloacae P99 betaL (ECl betaL) revealing a K(m)=95.4 microM and V(max)=3.21 microMol min(-1) mg(-1). The data compare favorably to related systems that have been reported and enable testing of this prodrug against cancer cell lines in vitro and in vivo.
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Affiliation(s)
- Ryan M. Phelan
- Department of Chemistry, Johns Hopkins University, 3400 N. Charles St., Baltimore, MD 21212, USA
| | - Marc Ostermeier
- Department of Chemical and Biomolecular Engineering, Johns Hopkins University, 3400 N. Charles St., Baltimore, MD 21212, USA
| | - Craig A. Townsend
- Department of Chemistry, Johns Hopkins University, 3400 N. Charles St., Baltimore, MD 21212, USA
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