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Shi D, Caldovic L, Tuchman M. Sources and Fates of Carbamyl Phosphate: A Labile Energy-Rich Molecule with Multiple Facets. BIOLOGY 2018; 7:biology7020034. [PMID: 29895729 PMCID: PMC6022934 DOI: 10.3390/biology7020034] [Citation(s) in RCA: 19] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 04/24/2018] [Revised: 05/25/2018] [Accepted: 06/07/2018] [Indexed: 11/16/2022]
Abstract
Carbamyl phosphate (CP) is well-known as an essential intermediate of pyrimidine and arginine/urea biosynthesis. Chemically, CP can be easily synthesized from dihydrogen phosphate and cyanate. Enzymatically, CP can be synthesized using three different classes of enzymes: (1) ATP-grasp fold protein based carbamyl phosphate synthetase (CPS); (2) Amino-acid kinase fold carbamate kinase (CK)-like CPS (anabolic CK or aCK); and (3) Catabolic transcarbamylase. The first class of CPS can be further divided into three different types of CPS as CPS I, CPS II, and CPS III depending on the usage of ammonium or glutamine as its nitrogen source, and whether N-acetyl-glutamate is its essential co-factor. CP can donate its carbamyl group to the amino nitrogen of many important molecules including the most well-known ornithine and aspartate in the arginine/urea and pyrimidine biosynthetic pathways. CP can also donate its carbamyl group to the hydroxyl oxygen of a variety of molecules, particularly in many antibiotic biosynthetic pathways. Transfer of the carbamyl group to the nitrogen group is catalyzed by the anabolic transcarbamylase using a direct attack mechanism, while transfer of the carbamyl group to the oxygen group is catalyzed by a different class of enzymes, CmcH/NodU CTase, using a different mechanism involving a three-step reaction, decomposition of CP to carbamate and phosphate, transfer of the carbamyl group from carbamate to ATP to form carbamyladenylate and pyrophosphate, and transfer of the carbamyl group from carbamyladenylate to the oxygen group of the substrate. CP is also involved in transferring its phosphate group to ADP to generate ATP in the fermentation of many microorganisms. The reaction is catalyzed by carbamate kinase, which may be termed as catabolic CK (cCK) in order to distinguish it from CP generating CK. CP is a thermally labile molecule, easily decomposed into phosphate and cyanate, or phosphate and carbamate depending on the pH of the solution, or the presence of enzyme. Biological systems have developed several mechanisms including channeling between enzymes, increased affinity of CP to enzymes, and keeping CP in a specific conformation to protect CP from decomposition. CP is highly important for our health as both a lack of, or decreased, CP production and CP accumulation results in many disease conditions.
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Affiliation(s)
- Dashuang Shi
- Center for Genetic Medicine Research, Children's National Medical Center, Washington, DC 20010, USA.
- Department of Genomics and Precision Medicine, The George Washington University, Washington, DC 20010, USA.
| | - Ljubica Caldovic
- Center for Genetic Medicine Research, Children's National Medical Center, Washington, DC 20010, USA.
- Department of Genomics and Precision Medicine, The George Washington University, Washington, DC 20010, USA.
| | - Mendel Tuchman
- Center for Genetic Medicine Research, Children's National Medical Center, Washington, DC 20010, USA.
- Department of Genomics and Precision Medicine, The George Washington University, Washington, DC 20010, USA.
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Hervé G, Evans HG, Fernado R, Patel C, Hachem F, Evans DR. Activation of Latent Dihydroorotase from Aquifex aeolicus by Pressure. J Biol Chem 2017; 292:629-637. [PMID: 27746403 DOI: 10.1074/jbc.m116.739862] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/25/2016] [Revised: 10/14/2016] [Indexed: 01/12/2023] Open
Abstract
Elevated hydrostatic pressure was used to probe conformational changes of Aquifex aeolicus dihydroorotase (DHO), which catalyzes the third step in de novo pyrimidine biosynthesis. The isolated protein, a 45-kDa monomer, lacks catalytic activity but becomes active upon formation of a dodecameric complex with aspartate transcarbamoylase (ATC). X-ray crystallographic studies of the isolated DHO and of the complex showed that association induces several major conformational changes in the DHO structure. In the isolated DHO, a flexible loop occludes the active site blocking the access of substrates. The loop is mostly disordered but is tethered to the active site region by several electrostatic and hydrogen bonds. This loop becomes ordered and is displaced from the active site upon formation of DHO-ATC complex. The application of pressure to the complex causes its time-dependent dissociation and the loss of both DHO and ATC activities. Pressure induced irreversible dissociation of the obligate ATC trimer, and as a consequence the DHO is also inactivated. However, moderate hydrostatic pressure applied to the isolated DHO subunit mimics the complex formation and reversibly activates the isolated subunit in the absence of ATC, suggesting that the loop has been displaced from the active site. This effect of pressure is explained by the negative volume change associated with the disruption of ionic interactions and exposure of ionized amino acids to the solvent (electrostriction). The interpretation that the loop is relocated by pressure was validated by site-directed mutagenesis and by inhibition by small peptides that mimic the loop residues.
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Affiliation(s)
- Guy Hervé
- From the Laboratoire BIOSIPE, Sorbonne Universités, Institut de Biologie Paris Seine, CNRS, Université Pierre et Marie Curie, 75005 Paris, France,
| | - Hedeel Guy Evans
- the Department of Chemistry, Eastern Michigan University, Ypsilanti, Michigan 48197, and.,the Department of Biochemistry and Molecular Biology, Wayne State University School of Medicine, Detroit, Michigan 48201
| | - Roshini Fernado
- the Department of Chemistry, Eastern Michigan University, Ypsilanti, Michigan 48197, and
| | - Chandni Patel
- the Department of Biochemistry and Molecular Biology, Wayne State University School of Medicine, Detroit, Michigan 48201
| | - Fatme Hachem
- the Department of Biochemistry and Molecular Biology, Wayne State University School of Medicine, Detroit, Michigan 48201
| | - David R Evans
- the Department of Biochemistry and Molecular Biology, Wayne State University School of Medicine, Detroit, Michigan 48201
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Auger C, Appanna ND, Alhasawi A, Appanna VD. Deciphering metabolic networks by blue native polyacrylamide gel electrophoresis: A functional proteomic exploration. EUPA OPEN PROTEOMICS 2015. [DOI: 10.1016/j.euprot.2015.05.003] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/11/2022]
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Braakman R, Smith E. Metabolic evolution of a deep-branching hyperthermophilic chemoautotrophic bacterium. PLoS One 2014; 9:e87950. [PMID: 24516572 PMCID: PMC3917532 DOI: 10.1371/journal.pone.0087950] [Citation(s) in RCA: 30] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/14/2013] [Accepted: 01/05/2014] [Indexed: 11/19/2022] Open
Abstract
Aquifex aeolicus is a deep-branching hyperthermophilic chemoautotrophic bacterium restricted to hydrothermal vents and hot springs. These characteristics make it an excellent model system for studying the early evolution of metabolism. Here we present the whole-genome metabolic network of this organism and examine in detail the driving forces that have shaped it. We make extensive use of phylometabolic analysis, a method we recently introduced that generates trees of metabolic phenotypes by integrating phylogenetic and metabolic constraints. We reconstruct the evolution of a range of metabolic sub-systems, including the reductive citric acid (rTCA) cycle, as well as the biosynthesis and functional roles of several amino acids and cofactors. We show that A. aeolicus uses the reconstructed ancestral pathways within many of these sub-systems, and highlight how the evolutionary interconnections between sub-systems facilitated several key innovations. Our analyses further highlight three general classes of driving forces in metabolic evolution. One is the duplication and divergence of genes for enzymes as these progress from lower to higher substrate specificity, improving the kinetics of certain sub-systems. A second is the kinetic optimization of established pathways through fusion of enzymes, or their organization into larger complexes. The third is the minimization of the ATP unit cost to synthesize biomass, improving thermodynamic efficiency. Quantifying the distribution of these classes of innovations across metabolic sub-systems and across the tree of life will allow us to assess how a tradeoff between maximizing growth rate and growth efficiency has shaped the long-term metabolic evolution of the biosphere.
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Affiliation(s)
- Rogier Braakman
- Krasnow Institute for Advanced Study, George Mason University, Fairfax, Virginia, United States of America
- * E-mail:
| | - Eric Smith
- Santa Fe Institute, Santa Fe, New Mexico, United States of America
- Krasnow Institute for Advanced Study, George Mason University, Fairfax, Virginia, United States of America
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5
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Fang H, Liu H, Chen N, Zhang C, Xie X, Xu Q. Site-directed mutagenesis studies on the uridine monophosphate binding sites of feedback inhibition in carbamoyl phosphate synthetase and effects on cytidine production by Bacillus amyloliquefaciens. Can J Microbiol 2013; 59:374-9. [PMID: 23750951 DOI: 10.1139/cjm-2012-0758] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
A major problem when pyrimidine de novo biosynthesis is used for cytidine production is the existence of many negative regulatory factors. Cytidine biosynthesis in Bacillus amyloliquefaciens proceeds via a pathway that is controlled by uridine monophosphate (UMP) through feedback inhibition of carbamoyl phosphate synthetase (CPS), the enzyme that converts CO2, NH3, and glutamine to carbamoyl phosphate. In this study, the gene carB encoding the large subunit of CPS from B. amyloliquefaciens CYT1 was site directed, and the UMP binding sites of feedback inhibition in Bam-CPS are described. The residues Thr-941, Thr-970, and Lys-986 in CPS from B. amyloliquefaciens were subjected to site-directed mutagenesis to alter UMP's feedback inhibition of CPS. To find feedback-resistant B. amyloliquefaciens, the influence of the T941F, T970A, K986I, T941F/K986I, and T941F/T970A/K986I mutations on CPS enzymatic properties was studied. The recombinant B. amyloliquefaciens with mutated T941F/K986I and T941F/T970A/K986I CPS showed a 3.7- and 5.7-fold increase, respectively, in cytidine production in comparison with the control expressing wild-type CPS, which was more suitable for further application of the cytidine synthesis. To a certain extent, the 5 mutations were found to release the enzyme from UMP inhibition and to improve B. amyloliquefaciens cytidine-producing strains.
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Affiliation(s)
- Haitian Fang
- College of Agriculture, Ningxia University, Yinchuan 750021, People's Republic of China
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Popa E, Perera N, Kibédi-Szabó CZ, Guy-Evans H, Evans DR, Purcarea C. The smallest active carbamoyl phosphate synthetase was identified in the human gut archaeon Methanobrevibacter smithii. J Mol Microbiol Biotechnol 2012; 22:287-99. [PMID: 23107800 PMCID: PMC6158779 DOI: 10.1159/000342520] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/19/2022] Open
Abstract
The genome of the major intestinal archaeon Methanobrevibacter smithii contains a complex gene system coding for carbamoyl phosphate synthetase (CPSase) composed of both full-length and reduced-size synthetase subunits. These ammonia-metabolizing enzymes could play a key role in controlling ammonia assimilation in M. smithii, affecting the metabolism of gut bacterial microbiota, with an impact on host obesity. In this study, we isolated and characterized the small (41 kDa) CPSase homolog from M. smithii. The gene was cloned and overexpressed in Escherichia coli, and the recombinant enzyme was purified in one step. Chemical cross-linking and size exclusion chromatography indicated a homodimeric/tetrameric structure, in accordance with a dimer-based CPSase activity and reaction mechanism. This small enzyme, MS-s, synthesized carbamoyl phosphate from ATP, bicarbonate, and ammonia and catalyzed the same ATP-dependent partial reactions observed for full-length CPSases. Steady-state kinetics revealed a high apparent affinity for ATP and ammonia. Sequence comparisons, molecular modeling, and kinetic studies suggest that this enzyme corresponds to one of the two synthetase domains of the full-length CPSase that catalyze the ATP-dependent phosphorylations involved in the three-step synthesis of carbamoyl phosphate. This protein represents the smallest naturally occurring active CPSase characterized thus far. The small M. smithii CPSase appears to be specialized for carbamoyl phosphate metabolism in methanogens.
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Affiliation(s)
- Elena Popa
- Department of Microbiology, Institute of Biology Bucharest, Romanian Academy, Bucharest 060031, Romania
| | - Nirosha Perera
- Department of Chemistry, Eastern Michigan University, Ypsilanti, MI 48197, USA
| | - Csaba Z. Kibédi-Szabó
- Department of Microbiology, Institute of Biology Bucharest, Romanian Academy, Bucharest 060031, Romania
| | - Hedeel Guy-Evans
- Department of Chemistry, Eastern Michigan University, Ypsilanti, MI 48197, USA
| | - David R. Evans
- Department of Biochemistry and Molecular Biology, Wayne State University School of Medicine, Detroit, MI 48201, USA
| | - Cristina Purcarea
- Department of Microbiology, Institute of Biology Bucharest, Romanian Academy, Bucharest 060031, Romania
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Han S, Auger C, Appanna VP, Lemire J, Castonguay Z, Akbarov E, Appanna VD. A blue native polyacrylamide gel electrophoretic technology to probe the functional proteomics mediating nitrogen homeostasis in Pseudomonas fluorescens. J Microbiol Methods 2012; 90:206-10. [DOI: 10.1016/j.mimet.2012.05.006] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/12/2012] [Revised: 05/08/2012] [Accepted: 05/08/2012] [Indexed: 10/28/2022]
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Popa E, Rusu A, Zamfir M, Dumitru L, Purcarea C. An Ammonia-Metabolizing Enzyme from the Human Archaeon Methanobrevibacter SmithiiMight Represent a Missing Link in the Evolution of Carbamoyl Phosphate Synthetases. BIOTECHNOL BIOTEC EQ 2009. [DOI: 10.1080/13102818.2009.10818480] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/25/2022] Open
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9
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Purcarea C, Fernando R, Evans HG, Evans DR. The sole serine/threonine protein kinase and its cognate phosphatase from Aquifex aeolicus targets pyrimidine biosynthesis. Mol Cell Biochem 2008; 311:199-213. [PMID: 18270660 DOI: 10.1007/s11010-008-9710-z] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/03/2007] [Accepted: 01/10/2008] [Indexed: 10/22/2022]
Abstract
Serine/Threonine kinases participate in complex, interacting signaling pathways in eukaryotes, prokaryotes, and archae. While most organisms contain many different kinases, the extreme hyperthermophile, Aquifex aeolicus encodes a single hypothetical Ser/Thr kinase. A gene homologous to eukaryotic protein phosphatases overlaps the kinase gene by a single base pair. The putative kinase, AaSTPK and phosphatase, AaPPM, were cloned and expressed in E. coli, purified to homogeneity and found to be functional. AaSTPK is a 34-kDa monomer that can use MgATP, MnATP, or MnGTP as co-substrates, although MgATP appears to be the preferred substrate. AaSTPK was autophosphorylated on a threonine residue and was dephosphorylated by AaPPM. AaPPM phosphatase is homologous to the PPM sub-family of Ser/Thr phosphatases and was stimulated by MnCl2 and CoCl2 but not MgCl2. AaSTPK also phosphorylated one threonine residue on the carbamoyl phosphate synthetase, CPS.A subunit. Carbamoyl phosphate synthetase reconstituted with phosphorylated CPS.A had unaltered catalytic activity but allosteric inhibition by UMP and activation by the arginine intermediate, ornithine, were both appreciably attenuated. These changes in allosteric regulation would be expected to activate pyrimidine biosynthesis by releasing the constraints imposed on carbamoyl phosphate synthetase activity by UMP and uncoupling the regulation of pyrimidine and arginine biosynthesis. CPS.A was also dephosphorylated by AaPPM. Aquifex aeolicus occupies the lowest branch on the prokaryotic phylogenetic tree. The Thr/Ser kinase, its cognate phosphatase and a protein substrate may be elements of a simple signaling pathway, perhaps the most primitive example of this mode of regulation described thus far.
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Affiliation(s)
- Cristina Purcarea
- Department of Biochemistry and Molecular Biology, Wayne State University School of Medicine, 540 E. Canfield Street, Detroit, MI 48201, USA
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10
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Peltier JB, Cai Y, Sun Q, Zabrouskov V, Giacomelli L, Rudella A, Ytterberg AJ, Rutschow H, van Wijk KJ. The oligomeric stromal proteome of Arabidopsis thaliana chloroplasts. Mol Cell Proteomics 2005; 5:114-33. [PMID: 16207701 DOI: 10.1074/mcp.m500180-mcp200] [Citation(s) in RCA: 269] [Impact Index Per Article: 14.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/15/2022] Open
Abstract
This study presents an analysis of the stromal proteome in its oligomeric state extracted from highly purified chloroplasts of Arabidopsis thaliana. 241 proteins (88% with predicted cTP), mostly assembled in oligomeric complexes, were identified by mass spectrometry with emphasis on distinguishing between paralogues. This is critical because different paralogues in a gene family often have different subcellular localizations and/or different expression patterns and functions. The native protein masses were determined for all identified proteins. Comparison with the few well characterized stromal complexes from A. thaliana confirmed the accuracy of the native mass determination, and by extension, the usefulness of the native mass data for future in-depth protein interaction studies. Resolved protein interactions are discussed and compared with an extensive collection of native mass data of orthologues in other plants and bacteria. Relative protein expression levels were estimated from spot intensities and also provided estimates of relative concentrations of individual proteins. No such quantification has been reported so far. Surprisingly proteins dedicated to chloroplast protein synthesis, biogenesis, and fate represented nearly 10% of the total stroma protein mass. Oxidative pentose phosphate pathway, glycolysis, and Calvin cycle represented together about 75%, nitrogen assimilation represented 5-7%, and all other pathways such as biosynthesis of e.g. fatty acids, amino acids, nucleotides, tetrapyrroles, and vitamins B(1) and B(2) each represented less than 1% of total protein mass. Several proteins with diverse functions outside primary carbon metabolism, such as the isomerase ROC4, lipoxygenase 2 involved in jasmonic acid biosynthesis, and a carbonic anhydrase (CA1), were surprisingly abundant in the range of 0.75-1.5% of the total stromal mass. Native images with associated information are available via the Plastid Proteome Database.
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Affiliation(s)
- Jean-Benoit Peltier
- Department of Plant Biology, Cornell Theory Center, Cornell University, Ithaca, New York 14853, USA
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11
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Martin PD, Purcarea C, Zhang P, Vaishnav A, Sadecki S, Guy-Evans HI, Evans DR, Edwards BFP. The Crystal Structure of a Novel, Latent Dihydroorotase from Aquifex aeolicus at 1.7Å Resolution. J Mol Biol 2005; 348:535-47. [PMID: 15826652 DOI: 10.1016/j.jmb.2005.03.015] [Citation(s) in RCA: 23] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/06/2004] [Revised: 02/28/2005] [Accepted: 03/03/2005] [Indexed: 11/16/2022]
Abstract
Dihydroorotases (EC 3.5.2.3) catalyze the reversible cyclization of carbamoyl aspartate to form dihydroorotate in de novo pyrimidine biosynthesis. The X-ray structures of Aquifex aeolicus dihydroorotase in two space groups, C222(1) and C2, were determined at a resolution of 1.7A. These are the first structures of a type I dihydroorotase, a class of molecules that includes the dihydroorotase domain of mammalian CAD. The type I enzymes are more ancient and larger, at 45 kDa, than the type II enzymes exemplified by the 38 kDa Escherichia coli dihydroorotase. Both dihydroorotases are members of the metallo-dependent hydrolase superfamily, whose members have a distorted "TIM barrel" domain containing the active site. However, A.aeolicus dihydroorotase has a second, composite domain, which the E.coli enzyme lacks and has only one of the two zinc atoms present in the E.coli enzyme. A.aeolicus dihydroorotase is unique in exhibiting significant activity only when complexed with aspartate transcarbamoylase, whereas the E.coli dihydroorotase and the CAD dihydroorotase domain are active as free proteins. The latency of A.aeolicus dihydroorotase can be related to two differences between its structure and that of E.coli dihydroorotase: (1) the monoclinic structure has a novel cysteine ligand to the zinc that blocks the active site and possibly functions as a "cysteine switch"; and (2) active site residues that bind the substrate in E.coli dihydroorotase are located in disordered loops in both crystal structures of A.aeolicus dihydroorotase and may function as a disorder-to-order "entropy switch".
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Affiliation(s)
- Philip D Martin
- Wayne State University School of Medicine, Department of Biochemistry and Molecular Biology, 540 E. Canfield, Detroit, MI 48201, USA
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Kothe M, Purcarea C, Guy HI, Evans DR, Powers-Lee SG. Direct demonstration of carbamoyl phosphate formation on the C-terminal domain of carbamoyl phosphate synthetase. Protein Sci 2004; 14:37-44. [PMID: 15576558 PMCID: PMC2253338 DOI: 10.1110/ps.041041305] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/26/2022]
Abstract
Carbamoyl phosphate synthetase synchronizes the utilization of two ATP molecules at duplicated ATP-grasp folds to catalyze carbamoyl phosphate formation. To define the dedicated functional role played by each of the two ATP sites, we have carried out pulse/labeling studies using the synthetases from Aquifex aeolicus and Methanococcus jannaschii, hyperthermophilic organisms that encode the two ATP-grasp folds on separate subunits. These studies allowed us to differentially label each active site with [gamma-(32)P]ATP and determine the fate of the labeled gamma-phosphate in the synthetase reaction. Our results provide the first direct demonstration that enzyme-catalyzed transfer of phosphate from ATP to carbamate occurs on the more C-terminal of the two ATP-grasp folds. These findings rule out one mechanism proposed for carbamoyl phosphate synthetase, where one ATP acts as a molecular switch, and provide additional support for a sequential reaction mechanism where the gamma-phosphate groups of both ATP molecules are transferred to reactants. CP synthesis by subunit C in our single turnover pulse/chase assays did not require subunit N, but subunit N was required for detectable CP synthesis in the traditional continuous assay. These findings suggest that cross-talk between domain N and C is required for product release from subunit C.
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Affiliation(s)
- Michael Kothe
- Department of Biology, Northeastern University, Boston, MA 02115, USA
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Ahuja A, Purcarea C, Ebert R, Sadecki S, Guy HI, Evans DR. Aquifex aeolicus dihydroorotase: association with aspartate transcarbamoylase switches on catalytic activity. J Biol Chem 2004; 279:53136-44. [PMID: 15381710 DOI: 10.1074/jbc.m403009200] [Citation(s) in RCA: 25] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Dihydroorotase (DHOase) catalyzes the reversible condensation of carbamoyl aspartate to form dihydroorotate in de novo pyrimidine biosynthesis. The enzyme from Aquifex aeolicus, a hyperthermophilic organism of ancient lineage, was cloned and expressed in Escherichia coli. The purified protein was found to be a 45-kDa monomer containing a single zinc ion. Although there is no other DHOase gene in the A. aeolicus genome, the recombinant protein completely lacked catalytic activity at any temperature tested. However, DHOase formed an active complex with aspartate transcarbamoylase (ATCase) from the same organism. Whereas the k(cat) of 13.8 +/- 0.03 s(-1) was close to the value observed for the mammalian enzyme, the K (m)for dihydroorotate, 3.03 +/- 0.05 mM was 433-fold higher. Gel filtration and chemical cross-linking showed that the complex exists as a 240-kDa hexamer (DHO(3)-ATC(3)) and a 480-kDa duodecamer (DHO(6)-ATC(6)) probably in rapid equilibrium. Complex formation protects both DHOase and ATCase against thermal degradation at temperatures near 100 degrees C where the organism grows optimally. These results lead to the reclassification of both enzymes: ATCase, previously considered a Class C homotrimer, now falls into Class A, whereas the DHOase is a Class 1B enzyme. CD spectroscopy indicated that association with ATCase does not involve a significant perturbation of the DHOase secondary structure, but the visible absorption spectrum of a Co(2+)-substituted DHOase is appreciably altered upon complex formation suggesting a change in the electronic environment of the active site. The association of DHOase with ATCase probably serves as a molecular switch that ensures that free, uncomplexed DHOase in the cell remains inactive. At pH 7.4, the equilibrium ratio of carbamoyl aspartate to dihydroorotate is 17 and complex formation may drive the reaction in the biosynthetic direction.
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Affiliation(s)
- Anupama Ahuja
- Department of Biochemistry and Molecular Biology, Wayne State University School of Medicine, 540 E. Canfield St., Detroit, MI 48201, USA
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Purcarea C, Ahuja A, Lu T, Kovari L, Guy HI, Evans DR. Aquifex aeolicus aspartate transcarbamoylase, an enzyme specialized for the efficient utilization of unstable carbamoyl phosphate at elevated temperature. J Biol Chem 2003; 278:52924-34. [PMID: 14534296 DOI: 10.1074/jbc.m309383200] [Citation(s) in RCA: 32] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Aquifex aeolicus, an organism that flourishes at 95 degrees C, is one of the most thermophilic eubacteria thus far described. The A. aeolicus pyrB gene encoding aspartate transcarbamoylase (ATCase) was cloned, overexpressed in Escherichia coli, and purified by affinity chromatography to a homogeneous form that could be crystallized. Chemical cross-linking and size exclusion chromatography showed that the protein was a homotrimer of 34-kDa catalytic chains. The activity of A. aeolicus ATCase increased dramatically with increasing temperature due to an increase in kcat with little change in the Km for the substrates, carbamoyl phosphate and aspartate. The Km for both substrates was 30-40-fold lower than the corresponding values for the homologous E. coli ATCase catalytic subunit. Although rapidly degraded at high temperature, the carbamoyl phosphate generated in situ by A. aeolicus carbamoyl phosphate synthetase (CPSase) was channeled to ATCase. The transient time for carbamoyl aspartate formation was 26 s, compared with the much longer transient times observed when A. aeolicus CPSase was coupled to E. coli ATCase. Several other approaches provided strong evidence for channeling and transient complex formation between A. aeolicus ATCase and CPSase. The high affinity for substrates combined with channeling ensures the efficient transfer of carbamoyl phosphate from the active site of CPSase to that of ATCase, thus preserving it from degradation and preventing the formation of toxic cyanate.
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Affiliation(s)
- Cristina Purcarea
- Department of Biochemistry and Molecular Biology, Wayne State University School of Medicine, Detroit, Michigan 48201, USA
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15
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Llamas I, Suárez A, Quesada E, Béjar V, del Moral A. Identification and characterization of the carAB genes responsible for encoding carbamoylphosphate synthetase in Halomonas eurihalina. Extremophiles 2003; 7:205-11. [PMID: 12768451 DOI: 10.1007/s00792-002-0311-6] [Citation(s) in RCA: 18] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/05/2002] [Accepted: 12/02/2002] [Indexed: 10/28/2022]
Abstract
Halomonas eurihalina is a moderately halophilic bacterium which produces exopolysaccharides potentially of great use in many fields of industry and ecology. Strain F2-7 of H. eurihalina synthesizes an anionic exopolysaccharide known as polymer V2-7, which not only has emulsifying activity but also becomes viscous under acidic conditions, and therefore we consider it worthwhile making a detailed study of the genetics of this strain. By insertional mutagenesis using the mini-Tn 5 Km2 transposon we isolated and characterized a mutant strain, S36 K, which requires both arginine and uracil for growth and does not excrete EPS. S36 K carries a mutation within the carB gene that encodes the synthesis of the large subunit of the carbamoylphosphate synthetase enzyme, which in turn catalyzes the synthesis of carbamoylphosphate, an important precursor of arginine and pyrimidines. We describe here the cloning and characterization of the carAB genes, which encode carbamoylphosphate synthetase in Halomonas eurihalina, and discuss this enzyme's possible role in the pathways for the synthesis of exopolysaccharides in strain F2-7.
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Affiliation(s)
- Inmaculada Llamas
- Department of Microbiology, Faculty of Pharmacy, University of Granada, Campus Universitario de Cartuja, 18071 Granada, Spain
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