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Biological processes are so complicated that to understand the mechanisms underlying the functioning of biomolecules it is inevitable to study them from various perspectives and with a wide range of tools. Understanding the function at the molecular level obviously requires the knowledge of the three dimensional structure of the biomolecules. Experimentally this can be obtained by techniques such as X‐ray crystallography and NMR studies. Computational biology has also played an important role in elucidating the structure function relationship in biomolecules. Computationally one can obtain the temporal as well as ensemble behavior of biomolecules at atomic level under conditions that are experimentally not accessible. Molecular dynamics(MD) study is a technique that can be used to obtain information of the dynamic behavior of the biomolecules. Dynamics of large systems like proteins can be investigated by classical force fields. However, the changes at the level of covalent bond involve the reorganization of electron density distribution which can be addressed only at Quantum mechanical level. In the present thesis, some of the biological systems have been characterized both at the classical and quantum mechanical level. The systems investigated by MD simulations and the insights brought from these studies are presented in Chapters 3 and 4. The unusual bonds such as pyrophosphate linkage in ATP and short strong hydrogen bonds in proteins, investigated through high level quantum chemical methods, are presented in Chapters 5, 6 and 7. Part of this thesis is aimed to address some important issues related to the dynamics of Tryptophanyl tRNA synthetase (TrpRS) which belongs to classic of aminoacyl‐tRNA synthetases (aaRS). aaRSs are extremely important class of enzymes involved in the translation of genetic code. These enzymes catalyze the aminoacylation of tRNAs to relate the cognate amino acids to the anticodon trinucleotide sequences. aaRSs are modular enzymes with distinct domains on which extensive kinetic and mutational experiments as well as structural analyses have been carried out, highlighting the role of inter‐domain communication (Alexander and Schimmel, 2001). The overall architecture of tRNA synthetases consists of primarily two domains. The active site domain is responsible for the activation of an amino acid with ATP in synthesizing an enzyme‐bound aminoacyl‐adenylate, and transfer of the aminoacyl‐adenylate intermediate to the 3’end of tRNA. The second domain is responsible for selection and binding of the cognate tRNA. aaRSs are allosteric proteins in which the binding of tRNA at the anticodon domain influences the activity at the catalytic region. These two binding sites are separated by a large distance. One of the aims of this thesis is to characterize such long distance communication (allosteric communication) at atomic level in Tryptophanyl tRNA synthetase. This is achieved by generating ensembles of conformations by MD simulations and analyzing the trajectories by novel graph theoretic approach. Graph and network based approaches are well established in the field of protein structure analysis for analyzing protein structure, stability and function (Kannan and Vishveshwara, 1999; Brinda and Vishveshwara, 2005). The parameters such as clusters, hubs and shortest paths provide valuable information on the structure and dynamics of the proteins. In this thesis, network parameters are used for the analysis of molecular dynamics MD) simulation data, to represent the global dynamic behavior of protein in a more elegant way. MD simulations are performed on some available (and modeled) structures of TrpRS bound to a variety of ligands, and the protein structure networks( PSN) of non‐covalent interactions are characterized in dynamical equilibrium. The ligand induced conformational changes are investigated through structure networks. These networks are used to understand the mode of communication between the anticodon domain and the active site. The interface dynamics is crucial for the function of TrpRS (since it is a functional dimer) and it is investigated through interface clusters. The matter embodied in the thesis is presented as 9 chapters. Chapter 1 lays the suitable background and foundation for the study, surveying relevant literature from different fields .Chapter 2 describes in detail the various materials, methods and techniques employed in the different analyses and studies presented in this thesis. A brief description of well‐known methods of molecular dynamics simulations, essential dynamics calculations, cross correlation maps, conformational clustering etc.is presented. The methods for constructing protein structure graphs and networks, developed in our lab, are described in detail. The use of network parameters for the analysis of MD simulation data to address the problem of communication between the two distal sites is also presented. Some descriptions of the ab initio quantum mechanical methods, which are used to investigate the unusual bonds in biomolecules, are also presented in this chapter. Chapter 3 is devoted in discussing the results from several normal as well as high temperature MD simulations of ligand‐free and ligand bound Bacillus stearothermophilus Tryptophanyl‐tRNA synthetase (bsTrpRS). The essential modes of the protein in the presence of different ligands are captured by essential dynamics calculations. Different conformations of the protein associated with the catalysis process of TrpRS, as captured through experiments, are discussed in the context of conformational sampling. High temperature simulations are carried out to explore the larger conformational space. Chapter 4 is focused on the results obtained from the MD simulation of human Tryptophanyl‐tRNA synthetase (hTrpRS). The structure of human TrpRS bound to the activated ligand (TrpAMP) and the cognate tRNA(tRNATRP) is modeled since no structure in the presence of both TrpAMP and tRNATRP is available. MD simulations on these modeled as well as other complexes of hTrpRS are performed to capture the dynamical process of ligand induced conformational changes (Hansiaetal., communicated). Both the local and the global changes in the protein conformation from the protein structure network (PSN) of MD snapshots are analyzed. Several important information such as the ligand induced correlation between different residues of the protein, asymmetric binding of the ligands to the two subunits of the protein, and the path of communication between the anticodon region and the aminoacylation site are obtained. Also, the role of the dimmer interface, from a dynamic perspective, is obtained for the first time. The interface dynamics which stabilize different quaternary structures of lectins (with high sequence and structure similarity) were investigated in a collaborative work (Hansiaetal.,2007). The lectin peanut agglutinin (PNA) is a tetramer with three different types of interfaces. The interface dynamics of this protein in the presence and in the absence of metal ions was investigated and the paper reporting the results from this study is included as appendix in this thesis. Chapter 5 deals with high level ab initio quantum chemical calculations on tri‐ and diphosphate fragments of adenosine triphosphate (ATP). Pyrophosphate prototypes such as methyl triphosphate and methyl diphosphate molecules in their different protonation states have been investigated at high levels of calculations (Hansiaetal., 2006a). The optimized geometries, the thermochemistry of the hydrolysis and the molecular orbitals contributing to the high energy of these compounds have been analyzed. These investigations provide insights into the‘‘highenergy’’character of ATP molecule. Further, the dependence of vibrational frequencies on the number of phosphate groups and the charged states has also been presented. These results aid in the interpretation of spectra obtained by experiments on complexes containing pyrophosphate prototypes. Hydrogen bonding is fundamental in understanding the structure and properties of molecules of biological interest including proteins. A recent analysis carried out in our lab showed that a significant number of short hydrogen bonds (SHB) are present in proteins (Rajagopal and Vishveshwara, 2005). Chapters 6 and 7 elucidate the results obtained from ab initio quantum chemical calculations on some of these SHBs to get aquantitative estimation of their geometry and strength. In chapter 6, asystematic analysis of the geometries and the energetics of possible SHB systems, which are frequently encountered in proteins, are presented at different levels of theory (HF,DFTandMP2). It is found that the SHBs involving both charged residues in the proteins are intrinsic in nature. However, two neutral residues form a SHB in the protein crystal structures either due to geometric constraints or due to the environment of these residues. This analysis enables one to distinguish SHBs which are formed because of geometric constraints from those which are formed because of the inherent property of the chemical groups involved in the hydrogen bonding. These results are useful in refining protein structures determined by crystallographic or NMR methods. In addition, sulfur atom of methionine and cysteinein proteins also participate in SHBs, which are not so well characterized. Chapter 7 presents the similar analysis carried out on short hydrogen bonds in proteins involving sulfur atom. A detailed analysis of SHBs of sulfur containing groups in a data set of proteins has been carried out. Some of the residue pairs from this analysis were considered for ab initio calculations. However, the optimization of these examples resulted in breaking of the hydrogen bonds involving sulfur atoms and formation of new hydrogen bonds with oxygen and/or nitrogen atoms. Hence model systems, which mimic the real examples, were designed to carry out ab initio studies and to investigate the short hydrogen bonds involving sulfur atoms. Another study on the protein‐water interaction, which does not fall under the realm of the main objective of the thesis, is discussed in Chapter 8. Protein–water interaction is crucial for accomplishing many biological functions of proteins. In the recent past, natural probe tryptophan, located at the protein surfaces, has been extensively investigated using femtosecond spectroscopy experiments to understand salvation dynamics (Peonetal.,2002). In this chapter a method is described to follow up the molecular events of the protein–water interactions in detail. Tryptophan–water interaction in the protein Monellin is investigated in order to get the atomic level insights into the hydration dynamics, by carrying out MD simulations on Monellin (Hansiaetal.,2006b). The results are compared with those obtained from femtosecond resolved fluorescence spectroscopy. The time constants of the survival correlation function match well with the reported experimental values.This validates the procedure, adapted here for Monellin, to investigate the hydration dynamics in general. The last chapter (Chapter9) summarizes the results obtained from various studies and discusses the future directions. First part of this thesis aims to present the analysis by carrying out MD simulations on monomeric and dimeric TrpRS protein in order to understand the two steps of the aminoacylation reaction: activation of the aminoacid Trp in the first step and the transfer of the activated amino acid in the next step. In the second part, quantitative estimation of the geometry and the strength of pyrophosphate bond and short hydrogen bonds in proteins are reported in detail by subjecting the systems to high levels of quantum mechanical calculations(QM). The use of ab initio QM/MM calculations by combining the quantum mechanics(QM) with the molecular mechanics(MM) in order to study the enzymatic reactions is discussed as the future

Le virus de l’immunodéficience humaine de type 1 (VIH-1), est un rétrovirus dont le génome est composé de deux molécules d’ARN simple brin. La transcriptase inverse codée par le VIH-1 utilise l’ARNt3Lys de la cellule hôte pour amorcer la réplication de son génome ARN en ADN proviral. L’ARNt3Lys est encapsidé dans les virions lors de l’assemblage; la lysyl-ARNt synthétase (LysRS) cellulaire est impliquée dans ce mécanisme et sert de co-transporteur à l’ARNt3Lys.Chez l’homme, il existe deux formes de LysRS, une forme cytoplasmique (cLysRS) et une forme mitochondriale (pmLysRS) qui donnera la forme mature (mLysRS) après translocation dans la mitochondrie. Les deux LysRS sont issues d’un même gène par épissage alternatif. Il a été démontré que seule la forme mitochondriale est présente dans les particules virales.Nous avons établi un modèle des interactions protéine-protéine impliquées dans la formation du complexe d’encapsidation de l’ARNt3Lys. En recherchant les interactions des précurseurs Gag et GagPol avec les LysRS et leurs domaines, nous avons démontré que seul le domaine Pol du précurseur GagPol a la capacité de s’associer à la LysRS. Ce sont les sous-domaines transframe TF et intégrase IN du domaine Pol qui permettent l’association entre LysRS et GagPol. Cette association se fait via le domaine catalytique de l’enzyme. La sélectivité de l'encapsidation de la forme mitochondriale de LysRS aux dépens de sa forme cytoplasmique pourrait résider dans la stricte compartimentation cellulaire de ces deux formes enzymatiques. Nous avons voulu établir à quel stade l’encapsidation de la LysRS mitochondriale a lieu, soit avant sa translocation mitochondriale sous forme de précurseur pmLysRS, soit après sous forme mLysRS maturée. Nous avons déterminé le site de maturation du précurseur pmLysRS puis caractérisé les deux formes mitochondriales de la LysRS, en déterminant leurs paramètres cinétiques et leur affinité pour l’ARNt3Lys. Alors que la forme pmLysRS ne forme pas de complexe stable avec l’ARNt, la forme maturée mLysRS est la plus apte à interagir avec l’ARNt3Lys. Ce serait donc la mLysRS qui serait impliquée dans le transport de l’ARNt3Lys dans les particules virales lors du bourgeonnement.Comme l'interaction GagPol:LysRS n'est pas spécifique in vitro de la forme mLysRS qui est la seule espèce de LysRS encapsidée, nous avons recherché si d’autres protéines virales pouvaient intervenir dans la formation du complexe d’encapsidation et conférer la spécificité pour la seule mLysRS. Nous avons montré que les protéines auxiliaires Rev et Vpr ont la capacité à s’associer à la LysRS sans distinction d'origine, mais ne peuvent interagir dans le contexte du complexe d'encapsidation GagPol:mLysRS:ARNt3Lys. Les différentes formes de LysRS pourraient ainsi réguler l'activité de Vpr et Rev à d'autres étapes du cycle viral. / The Human immunodeficiency virus type 1 (HIV-1) is a retrovirus with a genome composed of two molecules of single stranded RNA. The reverse transcriptase encoded by HIV-1 uses the cellular tRNA3Lys to prime the replication of its RNA genome into a proviral DNA. The tRNA3Lys is packaged into the viral particles during their assembly; the cellular lysyl-tRNA synthetase (LysRS) is involved in this mechanism as a co-carrier of tRNA3Lys.In human, there are two forms of LysRS, a cytoplasmic form (cLysRS) and a mitochondrial form (pmLysRS) that will be maturated into mLysRS after translocation into the mitochondrion. Both LysRS arise from the same gene by alternative splicing. It was demonstrated that only the mitochondrial species is present in the viral particles.We established a model of the protein-protein interactions which are implied in the formation of the packaging complex of tRNA3Lys. By searching for interactions of the viral precursors Gag and GagPol with the LysRS species and their domains, we demonstrated that only the Pol domain of the GagPol precursor has the capacity to interact with LysRS. The transframe (TF) and integrase (IN) domains of the Pol region of the polyprotein GagPol are required for association of LysRS with GagPol. This association is mediated by the catalytic domain of the enzyme. The selectivity of the packaging of the mitochondrial species of LysRS but not of its cytoplasmic species would rest on the cellular compartmentalization of these two enzyme forms. To establish at which step the mitochondrial LysRS is packaged, either as the pmLysRS precursor before its mitochondrial translocation, or after as the mature mLysRS, we determined the site of maturation of the pmLysRS precursor, then we characterized both mitochondrial forms of LysRS, by determining their kinetic parameters and their affinity for tRNA3Lys. Whereas the pmLysRS species did not form a stable complex with tRNA, the mature pmLysRS species did. Thus, mLysRS is the only LysRS species which could be implied in the transport of tRNA3Lys into the viral particles during the budding step. In vitro, the interaction GagPol:LysRS is not specific for the mLysRS species, but only the mitochondrial LysRS is packaged into the viral particles. We determined if another viral protein could impact the specificity of mLysRS packaging. We showed that the auxiliary proteins Rev and Vpr have the capacity to interact with LysRS but this intercation is not recovered in the context of the GagPol:mLysRS:tRNA3Lys packaging complex. These data suggest that the different forms of LysRS might regulate the activity of Vpr and Rev at other steps of the viral cycle.

Tuberculosis is a severe disease that causes about 2 million deaths every year. It is a worldwide threat and it is estimated that one-third of the world’s population carries the infection. The severe side effects of the present drugs, and the more than 6 months long treatment, in addition to the development of resistant bacterial strains, are the incentives for the intensified search for new drugs. In this work two potential mycobacterial drug targets have been studied: Caseinolytic protease 1 (ClpP1) from Mycobacterium tuberculosis (Mt) and Methionyl-tRNA synthetase (MetRS) from Mycobacterium smegmatis (Ms). The X-ray stucture of ClpP1 was determined to 3.0 Å resolution. The study gives details on the tetradecameric arrangement of the enzyme. Two hepameric discs assemble to form a chamber containing the catalytic activity mediated by each of the monomers. The chamber can be reached by two pores. Comparison with the human homologue reveals important structural differences. The X-ray studies on Ms MetRS were done to 2.3 Å and 2.8 Å resolution. The study gives details on the flexibility of the enzyme and how this is related to activity. Important findings are identification of an intermediate structure in which the methionine to be adenylated is bound in the catalytic site in a tight complex. The catalytic site and the anticodon recognizing domains are separated and the structural results indicate communication between the domains. The possibility to allosterically inhibit the enzyme is discussed.

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As aminoacil tRNA sintetases (AaRSs) são enzimas essenciais na síntese de proteínas assegurando a correta relação entre os aminoácidos e seus tRNA cognatos. O genoma mitocondrial dos tripanossomatídeos perdeu os genes codificantes dos tRNAs, assim os tRNA mitocondriais são codificados no núcleo e importados do citoplasma. O código genético do kinetoplasto desvia do código genético pela utilização do códon de terminação UGA para a decodificação do códon do triptofano. Um único gene codificando o tRNATrp(CCA) observado no genoma de Leismania é responsável pela incorporação do aminoácido triptofano durante a síntese proteíca na mitocôndria. Para decodificar os dois códons do Trp (UGA e UGG) a base na posição 34 do tRNATrp(CCA) passa por um evento de editoração, convertendo o ribunuclotídeo C34 em U34, produzindo o tRNATrp(UCA) capaz de decodificar o códon UGA. Nesse trabalho foram caracterizadas duas triptofanil tRNA sintetases de Leishmania major. De acordo com experimentos de ?western blotting? e análises ?in silico? das seqüências de aminoácidos, uma enzima tem localização citoplasmática (LmTrpRS1) enquanto a outra mitocondrial (LmTrpRS2). Os mRNAs dos dois genes foram definidos por experimentos de 5? e 3? RT-PCR. As duas enzimas foram clonadas em diversos vetores de expressão procariotos e eucariotos. A LmTrpRS1 foi obtida somente na fração insolúvel, já a LmTrpRS2 foi obtida na fração solúvel quando clonada no vetor de expressão pET28a. Esta porém mostrou-se instável precipitando rapidamente após sua purificação. Os ensaios enzimáticos realizados com a mesma mostraram que ela é capaz de reconhecer os tRNAsTrp editado e não editado. Modelagem molecular por homologia com as duas proteínas foi realizada usando a proteína citoplasmática humana como molde, para estudar a interação entre a proteína e o tRNATrp. Xylella fastidiosa é um bactéria gram negativa limitada ao xilema, responsável por um grande número de doenças economicamente importantes, como a doença de Pierces em videiras, Clorose variegata do Citrus (CVC) e a doença da requeima das folhas em outras plantas incluindo, amendoeira, ameixeira, louro, amoreira e café. Em todos os casos a X. fastidiosa afeta o xylema da planta causando redução na produção de frutos. Nesse trabalho nós mostramos a estrutura da Xylellaína, uma cisteíno protease desse patógeno. A estrutura foi resolvida por dispersão anômala a um único comprimento de onda, utilizando cristais de xylellaína selenometionina substituídos. A estrutura da Xylellaína foi refinada até 1,65 Å de resolução, mostrando enovelamento similar às proteínas da família da papaína, porém algumas características interessantes como uma região N-terminal composta por 38 aminoácidos cobrindo o sulco ativo da enzima, um intrigante ribonucleotídeo encontrado fora do sítio ativo da enzima e um ?loop? semelhante ao ?loop? de oclusão presente na catepsina B. / The aminoacyl tRNA synthetases (aaRSs) are essential enzymes in protein synthesis that ensure the correct match between amino acids and their cognate tRNAs. The mitochondrial (kinetoplast) genome of trypanossomatids lacks tRNA genes, and therefore nucleus-encoded tRNAs are imported from the cytoplasm, the kinetoplast genetic code deviates from the universal code in that UGA instead of UGG encodes for tryptophan. A single nucleus-encoded tRNATrp(CCA) is responsible for Trp insertion during organellar protein synthesis. To decode both Trp codons (UGA and UGG), tRNATrp(CCA) undergoes a single C to U editing event at position 34 of the anticodon yielding to versions of the tRNA in the mitochondria with anticodon CCA and UCA, permitting UGA decoding. This work have characterized two Leishmania major tryptophanyl-tRNA synthetase, acording western blotting experiments and ?in silico? sequence analisis one of cytoplasmatic localization (LmTrpRS1) and another from mitochondria localization (LmTrpRS2). The mature mRNA transcripts for both genes were defined by 5? and 3? RT-PCR. Both enzymes were cloned into several expressions vectors. LmTrpRs1 was obtained as an insoluble protein and LmTrpRs2 expressed into the soluble fraction in pET28a expression system. LmTrpRS2 protein, however, is unstable precipitating shortly after purification. The enzymatic assay showed that this enzyme is able to recognize both tRNATrp. Molecular modeling for LmTrpRS1 and LmTrpRS2 were constructed using the cytoplasmatic human tryptophanyl tRNA synthetase as a model, to study the interaction between proteins and tRNATrp. Xylella fastidiosa is a xylem-limited, gram-negative bacteria responsible for a large number of economically important plant diseases, such as Pierces disease in grapevines, citrus variegated chlorosis (CVC) in sweet oranges and leaf scorch diseases in other plants, including almond, plum, oleander, mulberry and coffee. In all cases, X. fastidiosa infects the plant xylem and impairs fruit production. Here, we report the crystal structure of xylellain, a cystein protease from X. fastidiosa. The structure was solved by single-wavelength anomalous dispersion (SAD) using seleno-methionine containing xylellain crystals. The final structure of Xylellaína was refined against the best native data set (1.65 Å) showing R/Rfree= 17/21. Xylellain shares fold similar to Papain like Family, but contains some interesting features, like a 38 N-terminal tail covering the active site cleft; one intriguing ribonucleotide found outside the active site and one loop that resemble the ocluding loop from cathepsin B.