No description available.

Les cyclodipeptides et leurs dérivés, les dicétopipérazines (DKP), constituent une large classe de métabolites secondaires aux activités biologiques remarquables qui sont essentiellement synthétisés par des microorganismes. Les voies de biosynthèse de certaines DKP contiennent des synthases de cyclodipeptides (CDPS), une famille d’enzymes récemment identifiée. Les CDPS ont la particularité de détourner les ARNt aminoacylés de leur rôle essentiel dans la synthèse protéique ribosomale pour les utiliser comme substrats et ainsi catalyser la formation des deux liaisons peptidiques de différents cyclodipeptides. Le travail de thèse présenté dans ce manuscrit a pour objectif de caractériser la nouvelle famille des CDPS. Dans un premier temps, la caractérisation tant structurale que mécanistique de la première CDPS identifiée, AlbC de Streptomyces noursei, est présentée. Puis, les résultats obtenus avec trois autres CDPS, chacune de ces enzymes ayant des caractéristiques adéquates pour approfondir l’étude de la famille des CDPS, sont décrits. Ainsi, la CDPS Ndas_1148 de Nocardiopsis dassonvillei a permis d’étendre nos connaissances sur les bases moléculaires de la spécificité des CDPS. La CDPS AlbC-IMI de S. sp. IMI 351155 est un bon modèle pour analyser l’interaction de chacun des deux substrats nécessaires à la formation d’un cyclodipeptide. Enfin, la caractérisation de la CDPS Nvec-CDPS2 chez l’animal Nematostella vectensis a permis de fournir le premier exemple d’enzyme d’origine animale impliquée dans la synthèse peptidique non ribosomale. / Cyclodipeptides and their derivatives, the diketopiperazines (DKPs), constitute a large class of secondary metabolites with noteworthy biological activities that are mainly synthesized by microorganisms. The biosynthetic pathways of some DKPs contain cyclodipeptide synthases (CDPSs), a newly defined family of enzymes. CDPSs hijack aminoacyl-tRNAs from their essential role in ribosomal protein synthesis to catalyze the formation of the two peptide bonds of various cyclodipeptides. The aim of the work presented in this thesis manuscript is to characterize the CDPS family. At first, the structural and mechanistic characterization of the first identified CDPS, AlbC of Streptomyces noursei, is presented. Then, the results obtained with three other CDPSs, each of which having suitable properties to increase our understanding of the CDPS family, are described. The CDPS Ndas_1148 of Nocardiopsis dassonvillei extends our knowledge of the molecular bases of the CDPS specificity. The CDPS AlbC-IMI of S. sp. IMI 351155 is a good model to analyze the interaction of each of the two substrates required for the formation of a cyclodipeptide. Finally, the characterization of the CDPS Nvec-CDPS2 from Nematostella vectensis provides the first example of enzymes of animal origin involved in nonribosomal peptide synthesis.

Les aminoacyl-ARNt synthétases mitochondriales humaines (aaRS mt) sont des enzymes clés de la traduction mitochondriale. Elles catalysent l'aminoacylation des ARNt par les acides aminés correspondent. Des mutations dans leurs gènes sont corrélées à des pathologies avec un large spectre de phénotypes cliniques, mais aux mécanismes moléculaires sous-jacents encore incompris. L'objectif de ce travail de thèse s'intègre dans les axes scientifiques du laboratoire, mais élargit l'intérêt et les connaissance à un système encore peu exploré: l'arginyl-ARNt synthétase mitochondriale (ArgRS mt). Des mutations dans la ArgRS sont liées à une hypoplasie Pontocérébelleuse (PCH6), une pathologie neurodéveloppementale sévère. Le travail de cette thèse s’articule autour de 3 axes : (I) L’analyse des phénotypes cliniques des pathologies liées aux mutations dans les aaRS mt, (II) La caractérisation des propriétéscellulaires de l’ArgRS mt, et (III) L'étude de l’impact de mutations « pathologiques » sur diverses propriétés de l’ArgRS mt. Combinés avec les travaux précédents, les résultats obtenus sont une contribution importante à l'élargissement des connaissances fondamentales des mt aaRSs, et apportent un nouvel éclairage sur le lien entre les mt-aaRSs-mutations et la maladie. / Human mitochondrial aminoacyl-tRNA synthetases (mt-aaRSs) are housekeeping enzymes involved in the mitochondrial translation. They catalyze the aminoacylation of tRNAs with their cognate amino acids. Mutations in their nuclear genes are correlated with pathologies with a broad spectrum of clinical phenotypes, but with so far no clear explanations about the underlying molecular mechanism(s). The aim of this PhD work follows the long-standing efforts of the host laboratory but expand the interest and knowledge to an unexplored system: the human mitochondrial arginyl-tRNA synthetase (mt-ArgRS). Mutations in the mt-ArgRS lead to Pontocebellar hypoplasia type 6, a severe neuro-developmental pathology. I thus contributed to i) comprehensively analyze the clinical data reported in pathologies related to mutations on mt-aaRSs, resulting in a categorization according to the affected anatomical system; ii) decipher some cellular properties of the mt-ArgRS; and iii) investigate to impact of disease-associated mutations on mt-aaRSs properties. Combined with previous works, the present results expand the knowledge of the mt-aaRSs, shedding new light on the link between mt-aaRSs-mutations and disease.

La traduction implique l’utilisation d’aminoacyl-ARNt produits par les aminoacyl-ARNt synthétases (aaRS). Il devrait exister 20 aaRS, une spécifique de chaque acide aminé. Or, les données actuelles montrent qu’une grande majorité des organismes ne possèdent pas l’asparaginyl- (AsnRS) et/ou la glutaminyl-ARNt synthétase (GlnRS). Ils ne peuvent synthétiser l’Asn-ARNtAsn et le Gln-ARNtGln que par l’utilisation de voies impliquant la formation préalable d’aspartyl-ARNtAsn et/ou de glutamyl-ARNtGln. Ces précurseurs « mésacylés » sont synthétisés par une aspartyl-ARNt synthétase et/ou une glutamyl-ARNt synthétase non-discriminantes (AspRS-ND ou GluRS-ND). Ils sont ensuite amidés par une amidotransférase (AdT), pour fournir à la cellule l’Asn-ARNtAsn et/ou le Gln-ARNtGln nécessaires à la traduction des codons Asn et Gln.Ce travail de thèse, effectué dans le contexte biologique de deux organismes différents, Thermus thermophilus et Helicobacter pylori, a permis de montrer que les étapes enzymatiques – formation du précurseur, et amidation par l’AdT – sont réalisées au sein de complexes ribonucléoprotéiques, réunissant l’aaRS-ND, l’ARNtAsn ou l’ARNtGln, et l’AdT : l’Asn-transamidosome ou le Gln-transamidosome. Selon leur origine ou la voie à laquelle ils appartiennent (asparaginylation ou glutaminylation), ces complexes possèdent des particularités mécanistiques et structurales très différentes, mais sont tous adaptés pour éviter la libération des intermédiaires mésacylés toxiques par des stratégies spécifiques. Ce travail permet de mieux comprendre les mécanismes évolutifs qui ont conduit à l’incorporation de l’Asn et de la Gln dans le code génétique. / Protein synthesis requires the biosynthesis of aminoacyl-tRNAs by aminoacyl-tRNA synthétases (aaRS). Since 20 amino acids are présent within the genetic code, 20 aaRS should be used by a single organism. However, the vast majority of organisms found today are deprived of asparaginyl- and/or glutaminyl-tRNA synthetases (Asn- or GlnRS). They can only synthesize Asn-tRNAAsn and/or Gln-tRNAGln through biosynthesis pathways involving the preliminary formation of aspartyl-tRNAAsn and /or glutamyl-tRNAGln. Those « misacylated » precursors are synthesized by so called non-discriminating aspartyl- or glutamyl-tRNA synthetases (ND-AspRS or –GluRS). Then, they are transferred to an amidotransferase (AdT) to provide the Asn-tRNAAsn and/or Gln-tRNAGln species (necessary to fuel protein synthesis) through amidation.This work was performed in the context of two organisms – Thermus thermophilus and Helicobacter pylori. It showed that the two enzymatic steps of asparaginylation and glutaminylation – biosynthesis of the misacylated precursor and amidation by AdT – are carried out within a single ribonucleoprotein complex, namely the (Asn- or Gln-) transamidosome, gathering the ND-aaRS necessary for the misacylation, the tRNA substrate (Asn or Gln) and the AdT. According to their origin or the pathway they originate from (asparaginylation or glutaminylation), those complexes display significant mechanistical and structural peculiarities, but they are all adapted to prevent libération of the toxic misacylated species through specific strategies. This work shed new light on the évolutive mechanisms that led to the incorporation of Asn or Gln into the genetic code.

No description available.

No description available.

Proteins—polymers of amino acids—are a major class of biomolecules whose myriad functions facilitate many crucial biological processes. Accordingly, human control over these biological processes depends upon the ability to study, produce, and modify proteins. One innovative tool for accomplishing these aims is cell-free protein synthesis (CFPS). This technique, rather than using living cells to make protein, simply extracts the cells' natural protein-making machinery and then uses it to produce protein in vitro. Because living cells are no longer involved, scientists can freely adapt the protein production environment in ways not otherwise possible. However, improved versatility and yield of CFPS protein production is still the subject of considerable research. This work focuses on two ideas for furthering that research.The first idea is the adaptation of CFPS to make proteins containing unnatural amino acids. Unnatural amino acids are not found in natural biological proteins; they are synthesized artificially to possess useful properties which are then conferred upon any protein made with them. However, current methods for incorporating unnatural amino acids do not allow incorporation of more than one type of unnatural amino acid into a single protein. This work helps lay the groundwork for the incorporation of different unnatural amino acid types into proteins. It does this by using modified aminoacyl-tRNA synthetases (aaRSs), which are key components in CFPS, to be compatible with unnatural amino acids. The second idea is the preservation of DNA templates from enzyme degradation in CFPS. Among the advantages of CFPS is the option of using linear expression templates (LETs) in place of plasmids as the DNA template for protein production. Because LETs can be produced more quickly than plasmids can, using LETs greatly reduces the time required to obtain a DNA template for protein production. This renders CFPS a better candidate for high-throughput testing of proteins. However, LETs are more susceptible to enzyme-mediated degradation than plasmids are, which means that LET-based CFPS protein yields are lower than plasmid-based CFPS yields. This work explores the possibility of increasing the protein yield of LET-based CFPS by addition of sacrificial DNA, DNA which is not used as a protein-making template but which is degraded by the enzymes in place of the LETs.

No description available.

Aminoacyl-tRNA synthetases (aaRSs) are at the center of the question of the origin of life and are essential proteins found in all living organisms. AARSs arose early in evolution to interpret genetic code and are believed to be a group of ancient proteins. They constitute a family of enzymes integrating the two levels of cellular organization: nucleic acids and proteins. These enzymes ensure the fidelity of transfer of genetic information from the DNA to the protein. They are responsible for attaching amino acid residues to their cognate tRNA molecules by virtue of matching the nucleotide triplet, which is the first step in the protein synthesis. The translation of genetic code into protein sequence is mediated by tRNA, which accurately picks up the cognate amino acids. The attachment of the cognate amino acid to tRNA is catalyzed by aaRSs, which have binding sites for the anticodon region of tRNA and for the amino acid to be attached. The two binding sites are separated by ≈ 76 Å and experiments have shown that the communication does not go through tRNA (Gale et al., 1996). The problem addressed here is how the information of binding of tRNA anticodon near the anticodon binding site is communicated to the active site through the protein structure. These enzymes are modular with distinct domains on which extensive kinetic and mutational experiments and supported by structural data are available, highlighting the role of inter-domain communication (Alexander and Schimmel, 2001). Hence these proteins present themselves as excellent systems for in-silico studies. Various methods involved for the construction of protein structure networks are well established and analyzed in a variety of ways to gain insights into different aspects of protein structure, stability and function (Kannan and Vishveshwara, 1999; Brinda and Vishveshwara, 2005). In the present study, we have incorporated network parameters for the analysis of molecular dynamics (MD) simulation data, representing the global dynamic behavior of protein in a more elegant way. MD simulations have been performed on the available (and modeled) structures of aaRSs bound to a variety of ligands, and the protein structure networks (PSN) of non-covalent interactions have been characterized in dynamical equilibrium. The changes in the structure networks are used to understand the mode of communication, and the paths between the two sites of interest identified by the analysis of the shortest path. The allosteric concept has played a key role in understanding the biological functions of aaRSs. The rigidity/plasticity and the conformational population are the two important ideas invoked in explaining the allosteric effect. We have explored the conformational changes in the complexes of aaRSs through novel parameters such as cliques and communities (Palla et al., 2005), which identify the rigid regions in the protein structure networks (PSNs) constructed from the non-covalent interactions of amino acid side chains. The thesis consists of 7 chapters. The first chapter constitutes the survey of the literature and also provides suitable background for this study. The aims of the thesis are presented in this chapter. Chapter 2 describes various techniques employed and the new techniques developed for the analysis of PSNs. It includes a brief description of well -known methods of molecular dynamics simulations, essential dynamics, and cross correlation maps. The method used for the construction of graphs and networks is also described in detail. The incorporation of network parameters for the analysis of MD simulation data are done for the first time and has been applied on a well studied protein lysozyme, as described in chapter 3. Chapter 3 focuses on the dynamical behavior of protein structure networks, examined by considering the example of T4-lysozyme. The equilibrium dynamics and the process of unfolding are followed by simulating the protein with explicit water molecules at 300K and at higher temperatures (400K, 500K) respectively. Three simulations of 10ns duration have been performed at 500K to ensure the validity of the results. The snapshots of the protein structure from the simulations are represented as Protein Structure Networks (PSN) of non-covalent interactions. The strength of the non-covalent interaction is evaluated and used as an important criterion in the construction of edges. The profiles of the network parameters such as the degree distribution and the size of the largest cluster (giant component) have been examined as a function of interaction strength (Ghosh et al., 2007). We observe a critical strength of interaction (Icritical) at which there is a transition in the size of the largest cluster. Although the transition profiles at all temperatures show behavior similar to those found in the crystal structures, the 500K simulations show that the non-native structures have lower Icritical values. Based on the interactions evaluated at Icritical value, the folding/unfolding transition region has been identified from the 500K simulation trajectories. Furthermore, the residues in the largest cluster obtained at interaction strength higher than Icritical have been identified to be important for folding. Thus, the compositions of the top largest clusters in the 500K simulations have been monitored to understand the dynamical processes such as folding/unfolding and domain formation/disruption. The results correlate well with experimental findings. In addition, the highly connected residues in the network have been identified from the 300K and 400K simulations and have been correlated with the protein stability as determined from mutation experiments. Based on these analyses, certain residues, on which experimental data is not available, have been predicted to be important for the folding and the stability of the protein. The method can also be employed as a valuable tool in the analysis of MD simulation data, since it captures the details at a global level, which may elude conventional pair-wise interaction analysis. After standardizing the concept of dynamical network analysis using Lysozyme, it was applied to our system of interest, the aaRSs. The investigations carried out on Methionyl-tRNA synthetases (MetRS) are presented in chapter 4. This chapter is divided into three parts: Chapter 4A deals with the introduction to aminoacyl tRNA synthetases (aaRS). Classification and functional insights of aaRSs obtained through various studies are presented. Chapter 4B is again divided into parts: BI and BII. Chapter 4BI elucidates a new technique developed for finding communication pathways essential for proper functioning of aaRS. The enzymes of the family of tRNA synthetases perform their functions with high precision, by synchronously recognizing the anticodon region and the amino acylation region, which is separated by about 70Å in space. This precision in function is brought about by establishing good communication paths between the two regions. We have modelled the structure of E.coli Methionyl tRNA synthetase, which is complexed with tRNA and activated methionine. Molecular dynamics simulations have been performed on the modeled structure to obtain the equilibrated structure of the complex and the cross correlations between the residues in MetRS. Furthermore, the network analysis on these structures has been carried out to elucidate the paths of communication between the aminoacyl activation site and the anticodon recognition site (Ghosh and Vishveshwara, 2007). This study has provided the detailed paths of communication, which are consistent with experimental results. A similar study on the (MetRS + activated methionine) and (MetRS+tRNA) complexes along with ligand free-native enzyme has also been carried out. A comparison of the paths derived from the four simulations has clearly shown that the communication path is strongly correlated and unique to the enzyme complex, which is bound to both the tRNA and the activated methionine. The method developed here could also be utilized to investigate any protein system where the function takes place through long distance communication. The details of the method of our investigation and the biological implications of the results are presented in this chapter. In chapter 4BII, we have explored the conformational changes in the complexes of E.coli Methionyl tRNA synthetase (MetRS) through novel parameters such as cliques and communities, which identify the rigid regions in the protein structure networks (PSNs). The rigidity/plasticity and the conformational population are the two important ideas invoked in explaining the allosteric effect. MetRS belongs to the aminoacyl tRNA Synthetases (aaRSs) family that play a crucial role in initiating the protein synthesis process. The network parameters evaluated here on the conformational ensembles of MetRS complexes, generated from molecular dynamics simulations, have enabled us to understand the inter-domain communication in detail. Additionally, the characterization of conformational changes in terms of cliques/communities has also become possible, which had eluded conventional analyses. Furthermore, we find that most of the residues participating in clique/communities are strikingly different from those that take part in long-range communication. The cliques/communities evaluated here for the first time on PSNs have beautifully captured the local geometries in their detail within the framework of global topology. Here the allosteric effect is revealed at the residue level by identifying the important residues specific for structural rigidity and functional flexibility in MetRS. Chapter 4C focuses on MD simulations of Methionyl tRNA synthetase (AmetRS) from a thermophilic bacterium, Aquifex aeolicus. As describe in Chapter 4B, we have explored the communication pathways between the anticodon binding region and the aminoacylation site, and the conformational changes in the complexes through cliques and communities. The two MetRSs from E.coli and Aquifex aeolicus are structurally and sequentially very close to each other. But the communication pathways between anticodon binding region and the aminoacylation site from A. aeolicus have differed significantly with the communication paths obtained from E.coli. The residue composition and cliques/communities structure participating in communication are not similar in the MetRSs of both these organisms. Furthermore the formation of cliques/communities and hubs in the communication paths are more in A. aeolicus compared to E.coli. The participation of structurally homologous linker peptide, essential for orienting the two domains for efficient communication is same in both the organisms although, the residues composition near domain interface regions including the linker peptide is different. Thus, the diversity in the functioning of two different MetRS has been brought out, by comparing the E.coli and Aquifex aeolicus systems. Protein Structure network analysis of MD simulated trajectories of various ligand bound complexes of Escherichia coli Cysteinyl-tRNA synthetase (CysRS) have been discussed in Chapter 5. The modeling of the complex is done by docking the ligand CysAMP into the tRNA bound structure of E.coli Cysteinyl tRNA synthetase. Molecular dynamics simulations have been performed on the modeled structure and the paths of communications were evaluated using a similar method as used in finding communication paths for MetRS enzymes. Compared to MetRS the evaluation of communication paths in CysRS is complicated due to presence of both direct and indirect readouts. The direct and indirect readouts (DR/IR) involve interaction of protein residues with base-specific functional group and sugar-phosphate backbone of nucleic acids respectively. Two paths of communication between the anticodon region and the activation site has been identified by combining the cross correlation information with the protein structure network constructed on the basis of non-covalent interaction. The complete paths include DR/IR interactions with tRNA. Cliques/communities of non-covalently interacting residues imparting structural rigidity are present along the paths. The reduction of cooperative fluctuation due to the presence of community is compensated by IR/DR interaction and thus plays a crucial role in communication of CysRS. Chapter 6 focuses on free energy calculations of aminoacyl tRNA synthetases with various ligands. The free energy contributions to the binding of the substrates are calculated using a method called MM-PBSA (Massova and Kollman, 2000). The binding free energies were calculated as the difference between the free energy of the enzyme-ligand complex, and the free ligand and protein. The ligand unbinding energy values obtained from the umbrella sampling MD correlates well with the ligand binding energies obtained from MM-PBSA method. Furthermore the essential dynamics was captured from MD simulations trajectories performed on E.coli MetRS, A. aeolius MetRS and E.coli CysRS in terms of the eigenvalues. The top two modes account for more than 50% of the motion in essential space for systems E.coli MetRS, A. aeolius MetRS and E.coli CysRS. Population distribution of protein conformation states are looked at the essential plane defined by the two principal components with highest eigenvalues. This shows how aaRSs existed as a population of conformational states and the variation with the addition of ligands. The population of conformational states is converted into Free energy contour surface. From free energy surfaces, it is evident that the E.coli tRNAMet bound MetRS conformational fluctuations are more, which attributes to less rigidity in the complex. Whereas E.coli tRNACys bound CysRS conformational fluctuations are less and this is reflected in the increase in rigidity of the complex as confirmed by its entropic contribution. Future directions have been discussed in the final chapter (Chapter 7). Specifically, it deals with the ab-initio QM/MM study of the enzymatic reaction involved in the active site of E.coli Methionyl tRNA synthetase. To achieve this, two softwares are integrated: the Quantum Mechanics (QM) part includes small ligands and the Molecular Mechanics (MM) part as protein MetRS are handled using CPMD and Gromacs respectively. The inputs for two reactions pathways are prepared. First reaction involves cyclization reaction of homocysteine in the active site of MetRS and the second reaction deals with charging of methionine in the presence of ATP and magnesium ion. These simulations require very high power computing systems and also time of computation is also very large. With the available computational power we could simulate up to 10ps and it is insufficient for analysis. The future direction will involve the simulations of these systems for longer time, followed by the analysis for reaction pathways.

Malgré l’intérêt et la diversité des propriétés pharmacologiques des 2,5-dicétopipérazines (DKP), les voies de biosynthèse de ces molécules d’origine microbienne sont très peu connues. L’objectif de mes travaux de thèse a été i) de documenter de nouvelles voies de biosynthèse de DKP qui se caractérisent par la présence d’une synthase de cyclodipeptides (CDPS) travaillant souvent de concert avec une ou plusieurs enzymes de modification des cyclodipeptides et ii) d’explorer la diversité chimique codée par ces voies. Dans un premier temps, je me suis intéressée aux CDPS. Après la sélection par bioinformatique de candidats dans les bases de données génomiques, j’ai pu identifier 51 nouvelles CDPS actives et montrer que ces enzymes peuvent incorporer 17 des 20 acides aminés naturels. Par ailleurs, ce travail a permis de mieux caractériser la famille des CDPS, de définir l’existence de plusieurs sous-familles aux signatures fonctionnelles spécifiques et d’établir les premiers éléments d’un code de spécificité pour la synthèse de cyclodipeptides. Dans un second temps, je me suis attachée à caractériser les enzymes de modification associées aux nouvelles CDPS et, en particulier, les dioxygénases dépendant du Fe(II) et du 2-oxoglutarate (OG) qui sont très représentées dans ces voies. J’ai ainsi pu détecter une activité in vivo pour 11 OG et poursuivre la caractérisation in vitro pour l’une de ces OG, ce qui a permis de caractériser les DKP qu’elle synthétise et d’ainsi montrer la complexité des modifications chimiques introduites. L’ensemble de ces travaux a donc permis d’identifier et de caractériser de nouvelles voies de biosynthèse qui donnent accès à une diversité accrue de DKP. / Despite the interest and diversity of the pharmacological properties of 2,5-diketopiperazines (DKPs), the biosynthetic pathways of these microbial molecules are poorly documented. The aim of my doctoral work was i) to identify new DKP biosynthetic pathways that are characterized by the presence of a cyclodipeptide synthase (CDPS) often associated with one or more cyclodipeptide-tailoring enzymes and ii) to explore the chemical diversity encoded by these pathways. First of all, my study focused on CDPSs. After the bioinformatics-based selection of candidates, 51 novel CDPS were characterized, revealing the incorporation of 17 of the 20 proteinogenic amino acids. Moreover, this work has allowed a better characterization of the CDPS family, by showing the existence of several subfamilies with specific functional signatures and laying the foundations of a specificity conferring code for the synthesis of cyclodipeptides. Second, I characterized the tailoring enzymes associated with the newly identified CDPSs and, in particular, the Fe(II) and oxoglutarate dependent dioxygenases (OGs) that are highly represented in these pathways. I detected the in vivo activity for 11 OGs and characterized the in vitro activity for one of them, showing the complexity of the chemical modifications introduced into the cyclodipeptide. This work has led to identify and characterize novel biosynthetic pathways that provide access to a greater diversity of DKPs.