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Jessen HJ, Dürr-Mayer T, Haas TM, Ripp A, Cummins CC. Lost in Condensation: Poly-, Cyclo-, and Ultraphosphates. Acc Chem Res 2021; 54:4036-4050. [PMID: 34648267 DOI: 10.1021/acs.accounts.1c00370] [Citation(s) in RCA: 6] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/07/2023]
Abstract
Much like linear, branched, and cyclic alkanes, condensed phosphates exist as linear, branched, and cyclic structures. Inasmuch as alkanes are the cornerstone of organic chemistry, generating an inexplorably large chemical space, a comparable richness in structures can be expected for condensed phosphates, as also for them the concepts of isomerism apply. Little of their chemical space has been charted, and only a few different synthesis methods are available to construct isomers of condensed phosphates. Here, we will discuss the application of phosphoramidites with one, two, or three P-N bonds that can be substituted selectively to access different condensed phosphates in a highly controllable manner. Work directed toward the further exploration of this chemical space will contribute to our understanding of the fundamental chemistry of phosphates.In biology, condensed phosphates play important roles in the form of inorganic representatives, such as pyrophosphate, polyphosphate, and cyclophosphate, and also in conjugation with organic molecules, such as esters and amidates. Phosphorus is one of the six biogenic elements; the omnipresence of phosphates in biology points toward their critical involvement in prebiotic chemistry and the emergence of life itself. Indeed, it is hard to imagine any life without phosphate. It is therefore desirable to achieve through synthesis a better understanding of the chemistry of the condensed phosphates to further explore their biology.There is a rich but underexplored chemistry of the family of condensed phosphates per se, which is further diversified by their conjugation to important biomolecules and metabolites. For example, proteins may be polyphosphorylated on lysins, a very recent addition to posttranslational modifications. Adenosine triphosphate, as a representative of the small molecules, on the other hand, is well known as the universal cellular energy currency. In this Account, we will describe our motivations and our approaches to construct, modify, and synthetically apply different representatives of the condensed phosphates. We also describe the generation of hybrids composed of cyclic and linear structures of different oxidation states and develop them into reagents of great utility. A pertinent example is provided in the step-economic synthesis of the magic spot nucleotides (p)ppGpp. Finally, we provide an overview of 31P NMR data collected over the years in our laboratories, helping as a waymarker for not getting lost in condensation.
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Affiliation(s)
- Henning J. Jessen
- Department of Chemistry and Pharmacy, Institute of Organic Chemistry, University of Freiburg, Albertstrasse 21, 79104 Freiburg, Germany
- Cluster of Excellence livMatS @ FIT − Freiburg Center for Interactive Materials and Bioinspired Technologies, University of Freiburg, Georges-Köhler-Allee 105, 79110 Freiburg, Germany
| | - Tobias Dürr-Mayer
- Department of Chemistry and Pharmacy, Institute of Organic Chemistry, University of Freiburg, Albertstrasse 21, 79104 Freiburg, Germany
| | - Thomas M. Haas
- Department of Chemistry and Pharmacy, Institute of Organic Chemistry, University of Freiburg, Albertstrasse 21, 79104 Freiburg, Germany
| | - Alexander Ripp
- Department of Chemistry and Pharmacy, Institute of Organic Chemistry, University of Freiburg, Albertstrasse 21, 79104 Freiburg, Germany
- Cluster of Excellence livMatS @ FIT − Freiburg Center for Interactive Materials and Bioinspired Technologies, University of Freiburg, Georges-Köhler-Allee 105, 79110 Freiburg, Germany
| | - Christopher C. Cummins
- Department of Chemistry, Massachusetts Institute of Technology, Cambridge Massachusetts 02139, United States
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Zong G, Jork N, Hostachy S, Fiedler D, Jessen HJ, Shears SB, Wang H. New structural insights reveal an expanded reaction cycle for inositol pyrophosphate hydrolysis by human DIPP1. FASEB J 2021; 35:e21275. [PMID: 33475202 DOI: 10.1096/fj.202001489r] [Citation(s) in RCA: 11] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/12/2020] [Revised: 10/30/2020] [Accepted: 11/30/2020] [Indexed: 11/11/2022]
Abstract
Nudix hydrolases attract considerable attention for their wide range of specialized activities in all domains of life. One particular group of Nudix phosphohydrolases (DIPPs), through their metabolism of diphosphoinositol polyphosphates (PP-InsPs), regulates the actions of these polyphosphates upon bioenergetic homeostasis. In the current study, we describe, at an atomic level, hitherto unknown properties of human DIPP1.We provide X-ray analysis of the catalytic core of DIPP1 in crystals complexed with either natural PP-InsPs, alternative PP-InsP stereoisomers, or non-hydrolysable methylene bisphosphonate analogs ("PCP-InsPs"). The conclusions that we draw from these data are interrogated by studying the impact upon catalytic activity upon mutagenesis of certain key residues. We present a picture of a V-shaped catalytic furrow with overhanging ridges constructed from flexible positively charged side chains; within this cavity, the labile phosphoanhydride bond is appropriately positioned at the catalytic site by an extensive series of interlocking polar contacts which we analogize as "suspension cables." We demonstrate functionality for a triglycine peptide within a β-strand which represents a non-canonical addition to the standard Nudix catalytic core structure. We describe pre-reaction enzyme/substrate states which we posit to reflect a role for electrostatic steering in substrate capture. Finally, through time-resolved analysis, we uncover a chronological sequence of DIPP1/product post-reaction states, one of which may rationalize a role for InsP6 as an inhibitor of catalytic activity.
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Affiliation(s)
- Guangning Zong
- Signal Transduction Laboratory, National Institute of Environmental Health Sciences, National Institutes of Health, Research Triangle Park, NC, USA
| | - Nikolaus Jork
- Institute of Organic Chemistry, CIBSS - Center for Integrative Biological Signalling Studies, University of Freiburg, Freiburg, Germany
| | - Sarah Hostachy
- Leibniz-Forschungsinstitut für Molekulare Pharmakologie, Berlin, Germany
| | - Dorothea Fiedler
- Leibniz-Forschungsinstitut für Molekulare Pharmakologie, Berlin, Germany
| | - Henning J Jessen
- Institute of Organic Chemistry, CIBSS - Center for Integrative Biological Signalling Studies, University of Freiburg, Freiburg, Germany
| | - Stephen B Shears
- Signal Transduction Laboratory, National Institute of Environmental Health Sciences, National Institutes of Health, Research Triangle Park, NC, USA
| | - Huanchen Wang
- Signal Transduction Laboratory, National Institute of Environmental Health Sciences, National Institutes of Health, Research Triangle Park, NC, USA
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3
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Qiu D, Wilson MS, Eisenbeis VB, Harmel RK, Riemer E, Haas TM, Wittwer C, Jork N, Gu C, Shears SB, Schaaf G, Kammerer B, Fiedler D, Saiardi A, Jessen HJ. Analysis of inositol phosphate metabolism by capillary electrophoresis electrospray ionization mass spectrometry. Nat Commun 2020; 11:6035. [PMID: 33247133 PMCID: PMC7695695 DOI: 10.1038/s41467-020-19928-x] [Citation(s) in RCA: 55] [Impact Index Per Article: 13.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/14/2020] [Accepted: 11/06/2020] [Indexed: 12/22/2022] Open
Abstract
The analysis of myo-inositol phosphates (InsPs) and myo-inositol pyrophosphates (PP-InsPs) is a daunting challenge due to the large number of possible isomers, the absence of a chromophore, the high charge density, the low abundance, and the instability of the esters and anhydrides. Given their importance in biology, an analytical approach to follow and understand this complex signaling hub is desirable. Here, capillary electrophoresis (CE) coupled to electrospray ionization mass spectrometry (ESI-MS) is implemented to analyze complex mixtures of InsPs and PP-InsPs with high sensitivity. Stable isotope labeled (SIL) internal standards allow for matrix-independent quantitative assignment. The method is validated in wild-type and knockout mammalian cell lines and in model organisms. SIL-CE-ESI-MS enables the accurate monitoring of InsPs and PP-InsPs arising from compartmentalized cellular synthesis pathways, by feeding cells with either [13C6]-myo-inositol or [13C6]-D-glucose. In doing so, we provide evidence for the existence of unknown inositol synthesis pathways in mammals, highlighting the potential of this method to dissect inositol phosphate metabolism and signalling. Myo-Inositol phosphates (InsPs) and pyrophosphates (PP-InsPs) are important second messengers but their analysis remains challenging. Here, the authors develop a capillary electrophoresis-mass spectrometry method for the identification and quantitation of InsP and PP-InsP isomers in cells and tissues.
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Affiliation(s)
- Danye Qiu
- Institute of Organic Chemistry, University of Freiburg, Albertstr. 21, 79104, Freiburg, Germany.
| | - Miranda S Wilson
- Medical Research Council, Laboratory for Molecular Cell Biology, University College London, London, WC1E 6BT, UK
| | - Verena B Eisenbeis
- Institute of Organic Chemistry, University of Freiburg, Albertstr. 21, 79104, Freiburg, Germany
| | - Robert K Harmel
- Leibniz-Forschungsinstitut für Molekulare Pharmakologie, Robert-Rössle-Str. 10, 13125, Berlin, Germany
| | - Esther Riemer
- Institute of Crop Science and Resource Conservation, Department of Plant Nutrition, Rheinische Friedrich-Wilhelms-University Bonn, 53115, Bonn, Germany
| | - Thomas M Haas
- Institute of Organic Chemistry, University of Freiburg, Albertstr. 21, 79104, Freiburg, Germany
| | - Christopher Wittwer
- Institute of Organic Chemistry, University of Freiburg, Albertstr. 21, 79104, Freiburg, Germany
| | - Nikolaus Jork
- Institute of Organic Chemistry, University of Freiburg, Albertstr. 21, 79104, Freiburg, Germany
| | - Chunfang Gu
- Signal Transduction Laboratory, National Institute of Environmental Health Sciences, National Institutes of Health, Research Triangle Park, NC, 27709, USA
| | - Stephen B Shears
- Signal Transduction Laboratory, National Institute of Environmental Health Sciences, National Institutes of Health, Research Triangle Park, NC, 27709, USA
| | - Gabriel Schaaf
- Institute of Crop Science and Resource Conservation, Department of Plant Nutrition, Rheinische Friedrich-Wilhelms-University Bonn, 53115, Bonn, Germany
| | - Bernd Kammerer
- Institute of Organic Chemistry, University of Freiburg, Albertstr. 21, 79104, Freiburg, Germany
| | - Dorothea Fiedler
- Leibniz-Forschungsinstitut für Molekulare Pharmakologie, Robert-Rössle-Str. 10, 13125, Berlin, Germany
| | - Adolfo Saiardi
- Medical Research Council, Laboratory for Molecular Cell Biology, University College London, London, WC1E 6BT, UK.
| | - Henning J Jessen
- Institute of Organic Chemistry, University of Freiburg, Albertstr. 21, 79104, Freiburg, Germany. .,CIBSS - Centre for Integrative Biological Signalling Studies, University of Freiburg, 79104, Freiburg, Germany.
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Abstract
The multitudinous inositol phosphate family elicits a wide range of molecular effects that regulate countless biological responses. In this review, I provide a methodological viewpoint of the manner in which key advances in the field of inositol phosphate research were made. I also note some of the considerable challenges that still lie ahead.
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Affiliation(s)
- Stephen B Shears
- Laboratory of Signal Transduction, National Institute of Environmental Health Sciences, National Institutes of Health, Research Triangle Park, NC, USA.
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Laha D, Parvin N, Hofer A, Giehl RFH, Fernandez-Rebollo N, von Wirén N, Saiardi A, Jessen HJ, Schaaf G. Arabidopsis ITPK1 and ITPK2 Have an Evolutionarily Conserved Phytic Acid Kinase Activity. ACS Chem Biol 2019; 14:2127-2133. [PMID: 31525024 DOI: 10.1021/acschembio.9b00423] [Citation(s) in RCA: 33] [Impact Index Per Article: 6.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
Diphospho-myo-inositol polyphosphates, also termed inositol pyrophosphates, are molecular messengers containing at least one high-energy phosphoanhydride bond and regulate a wide range of cellular processes in eukaryotes. While inositol pyrophosphates InsP7 and InsP8 are present in different plant species, both the identity of enzymes responsible for InsP7 synthesis and the isomer identity of plant InsP7 remain unknown. This study demonstrates that Arabidopsis ITPK1 and ITPK2 catalyze the phosphorylation of phytic acid (InsP6) to the symmetric InsP7 isomer 5-InsP7 and that the InsP6 kinase activity of ITPK enzymes is evolutionarily conserved from humans to plants. We also show by 31P nuclear magnetic resonance that plant InsP7 is structurally identical to the in vitro InsP6 kinase products of ITPK1 and ITPK2. Our findings lay the biochemical and genetic basis for uncovering physiological processes regulated by 5-InsP7 in plants.
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Affiliation(s)
- Debabrata Laha
- Institute of Crop Science and Resource Conservation, Department of Plant Nutrition, Rheinische Friedrich-Wilhelms-University Bonn, 53115 Bonn, Germany
- Medical Research Council Laboratory for Molecular Cell Biology (MRC-LMCB), University College London, London WC1E 6BT, United Kingdom
| | - Nargis Parvin
- Institute of Crop Science and Resource Conservation, Department of Plant Nutrition, Rheinische Friedrich-Wilhelms-University Bonn, 53115 Bonn, Germany
| | - Alexandre Hofer
- Department of Chemistry, University of Zurich, 8057 Zurich, Switzerland
| | - Ricardo F. H. Giehl
- Department of Physiology & Cell Biology, Leibniz-Institute of Plant Genetics and Crop Plant Research, 06466 Gatersleben, Germany
| | - Nicolas Fernandez-Rebollo
- Center for Plant Molecular Biology, Department of Plant Physiology, Eberhard Karls University Tübingen, 72076 Tübingen, Germany
| | - Nicolaus von Wirén
- Department of Physiology & Cell Biology, Leibniz-Institute of Plant Genetics and Crop Plant Research, 06466 Gatersleben, Germany
| | - Adolfo Saiardi
- Medical Research Council Laboratory for Molecular Cell Biology (MRC-LMCB), University College London, London WC1E 6BT, United Kingdom
| | - Henning J. Jessen
- Department of Chemistry and Pharmacy, Albert-Ludwigs University Freiburg, 79104 Freiburg, Germany
- CIBSS-Centre for Integrative Biological Signalling Studies, University of Freiburg, 79104 Freiburg, Germany
| | - Gabriel Schaaf
- Institute of Crop Science and Resource Conservation, Department of Plant Nutrition, Rheinische Friedrich-Wilhelms-University Bonn, 53115 Bonn, Germany
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Hauke S, Dutta AK, Eisenbeis VB, Bezold D, Bittner T, Wittwer C, Thakor D, Pavlovic I, Schultz C, Jessen HJ. Photolysis of cell-permeant caged inositol pyrophosphates controls oscillations of cytosolic calcium in a β-cell line. Chem Sci 2019; 10:2687-2692. [PMID: 30996985 PMCID: PMC6419925 DOI: 10.1039/c8sc03479f] [Citation(s) in RCA: 15] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/05/2018] [Accepted: 01/09/2019] [Indexed: 12/16/2022] Open
Abstract
β-Cells respond directly to the intracellular photochemical release of caged inositol pyrophosphate isomers with modulations of oscillations in cytosolic Ca2+.
Among many cellular functions, inositol pyrophosphates (PP-InsPs) are metabolic messengers involved in the regulation of glucose uptake, insulin sensitivity, and weight gain. However, their mechanisms of action are still poorly understood. So far, the influence of PP-InsPs on cellular metabolism has been studied by overexpression or knockout/inhibition of relevant metabolizing kinases (IP6Ks, PPIP5Ks). These approaches are, inter alia, limited by time-resolution and potential compensation mechanisms. Here, we describe the synthesis of cell-permeant caged PP-InsPs as tools to rapidly modulate intracellular levels of defined isomers of PP-InsPs in a genetically non-perturbed cellular environment. We show that caged prometabolites readily enter live cells where they are enzymatically converted into still inactive, metabolically stable, photocaged PP-InsPs. Upon light-triggered release of 5-PP-InsP5, the major cellular inositol pyrophosphate, oscillations of intracellular Ca2+ levels in MIN6 cells were transiently reduced to spontaneously recover again. In contrast, uncaging of 1-PP-InsP5, a minor cellular isomer, was without effect. These results provide evidence that PP-InsPs play an active role in regulating [Ca2+]i oscillations, a key element in triggering exocytosis and secretion in β-cells.
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Affiliation(s)
- S Hauke
- EMBL, Heidelberg , 69117 Heidelberg , Germany .
| | - A K Dutta
- University of Freiburg , Institute of Organic Chemistry , 79104 Freiburg , Germany .
| | - V B Eisenbeis
- University of Freiburg , Institute of Organic Chemistry , 79104 Freiburg , Germany .
| | - D Bezold
- University of Freiburg , Institute of Organic Chemistry , 79104 Freiburg , Germany .
| | - T Bittner
- University of Freiburg , Institute of Organic Chemistry , 79104 Freiburg , Germany .
| | - C Wittwer
- University of Freiburg , Institute of Organic Chemistry , 79104 Freiburg , Germany .
| | - D Thakor
- University of Freiburg , Institute of Organic Chemistry , 79104 Freiburg , Germany .
| | - I Pavlovic
- University of Freiburg , Institute of Organic Chemistry , 79104 Freiburg , Germany .
| | - C Schultz
- EMBL, Heidelberg , 69117 Heidelberg , Germany . .,OHSU , Dept. Physiology & Pharmacology , Portland , OR , USA .
| | - H J Jessen
- University of Freiburg , Institute of Organic Chemistry , 79104 Freiburg , Germany .
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de Souza JM, Brocksom TJ, McQuade DT, de Oliveira KT. Continuous Endoperoxidation of Conjugated Dienes and Subsequent Rearrangements Leading to C-H Oxidized Synthons. J Org Chem 2018; 83:7574-7585. [PMID: 29860826 DOI: 10.1021/acs.joc.8b01307] [Citation(s) in RCA: 25] [Impact Index Per Article: 4.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
We have investigated the continuous flow photooxidation of several conjugated dienes and subsequent rearrangement using a practical and safe continuous-flow homemade engineered setup. End-to-end approaches involving endoperoxidation, Kornblum-DeLaMare rearrangement, and additional rearrangements are comprehensively detailed with optimization, scope, and scale-up to obtain useful hydroxyenones, furans, and 1,4-dicarbonyl building blocks.
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Affiliation(s)
- Juliana M de Souza
- Departamento de Química , Universidade Federal de São Carlos , São Carlos , SP 13565-905 , Brazil
| | - Timothy J Brocksom
- Departamento de Química , Universidade Federal de São Carlos , São Carlos , SP 13565-905 , Brazil.,Centro de Ciências Naturais e Humanas , Universidade Federal do ABC , Santo André , SP 09210-580 , Brazil
| | - D Tyler McQuade
- Department of Chemical and Life Science Engineering , Virginia Commonwealth University , Richmond , Virginia 23284-3068 , United States
| | - Kleber T de Oliveira
- Departamento de Química , Universidade Federal de São Carlos , São Carlos , SP 13565-905 , Brazil
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Brown NW, Marmelstein AM, Fiedler D. Chemical tools for interrogating inositol pyrophosphate structure and function. Chem Soc Rev 2018; 45:6311-6326. [PMID: 27462803 DOI: 10.1039/c6cs00193a] [Citation(s) in RCA: 26] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/08/2023]
Abstract
The inositol pyrophosphates (PP-InsPs) are a unique group of intracellular messengers that represent some of the most highly phosphorylated molecules in nature. Genetic perturbation of the PP-InsP biosynthetic network indicates a central role for these metabolites in maintaining cellular energy homeostasis and in controlling signal transduction networks. However, despite their discovery over two decades ago, elucidating their physiologically relevant isomers, the biochemical pathways connecting these molecules to their associated phenotypes, and their modes of signal transduction has often been stymied by technical challenges. Many of the advances in understanding these molecules to date have been facilitated by the total synthesis of the various PP-InsP isomers and by the development of new methods that are capable of identifying their downstream signalling partners. Chemical tools have also been developed to distinguish between the proposed PP-InsP signal transduction mechanisms: protein binding, and a covalent modification of proteins termed protein pyrophosphorylation. In this article, we review these recent developments, discuss how they have helped to illuminate PP-InsP structure and function, and highlight opportunities for future discovery.
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Affiliation(s)
- Nathaniel W Brown
- Princeton University, Frick Chemistry Laboratory, Washington Road, Princeton, NJ 08544, USA and Leibniz-Institut für Molekulare Pharmakologie, Robert-Rössle-Str 10, 13125 Berlin, Berlin, Germany.
| | - Alan M Marmelstein
- Princeton University, Frick Chemistry Laboratory, Washington Road, Princeton, NJ 08544, USA and Leibniz-Institut für Molekulare Pharmakologie, Robert-Rössle-Str 10, 13125 Berlin, Berlin, Germany.
| | - Dorothea Fiedler
- Princeton University, Frick Chemistry Laboratory, Washington Road, Princeton, NJ 08544, USA and Leibniz-Institut für Molekulare Pharmakologie, Robert-Rössle-Str 10, 13125 Berlin, Berlin, Germany.
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Gerasimaite R, Pavlovic I, Capolicchio S, Hofer A, Schmidt A, Jessen HJ, Mayer A. Inositol Pyrophosphate Specificity of the SPX-Dependent Polyphosphate Polymerase VTC. ACS Chem Biol 2017; 12:648-653. [PMID: 28186404 DOI: 10.1021/acschembio.7b00026] [Citation(s) in RCA: 55] [Impact Index Per Article: 7.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
The free energy of nucleotide hydrolysis depends on phosphate concentration. Cells regulate cytosolic phosphate levels by orchestrating phosphate acquisition and storage through inositol pyrophosphates (PP-InsP) and SPX domains. Here, we report the synthesis of the novel 5-PPP-InsP5 containing a triphosphate subunit. Using this and a series of synthetic PP-InsP, we examined the ligand specificity of the SPX domain in the PP-InsP-controlled yeast polyphosphate polymerase VTC. SPX decodes the relative positioning of the phosphoric anhydrides, their structure (diphosphate vs triphosphate), and the presence of other phosphates on the inositol ring. Despite the higher potency of 1,5-(PP)2-InsP4, 5-PP-InsP5 is the primary activator of VTC in cells, indicating that its higher concentration compensates for its lower potency. 1,5-(PP)2-InsP4 levels rise and could become relevant under stress conditions. Thus, SPX domains may integrate PP-InsP dependent signaling to adapt cytosolic phosphate concentrations to different metabolic situations.
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Affiliation(s)
- Ruta Gerasimaite
- Department
of Biochemistry, University of Lausanne, Chemin de Boveresses 155, 1066 Epalinges, Switzerland
| | - Igor Pavlovic
- Department
of Chemistry, University of Zürich, Winterthurerstrasse 190, 8057 Zürich, Switzerland
| | - Samanta Capolicchio
- Department
of Chemistry, University of Zürich, Winterthurerstrasse 190, 8057 Zürich, Switzerland
| | - Alexandre Hofer
- Department
of Chemistry, University of Zürich, Winterthurerstrasse 190, 8057 Zürich, Switzerland
| | - Andrea Schmidt
- Department
of Biochemistry, University of Lausanne, Chemin de Boveresses 155, 1066 Epalinges, Switzerland
| | - Henning J. Jessen
- Institute
of Organic Chemistry, Albert-Ludwigs University of Freiburg, Albertstrasse 21, 79104 Freiburg, Germany
| | - Andreas Mayer
- Department
of Biochemistry, University of Lausanne, Chemin de Boveresses 155, 1066 Epalinges, Switzerland
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