The three-dimensional structures of biological macromolecules and molecular assemblies are becoming increasingly important with the changing methodologies of drug discovery. The structures aid in understanding of protein function at the molecular level: be it a macromolecular assembly, a cytosolic enzyme or an intermembrane receptor molecule. X-ray crystallography is the most powerful technique to obtain the three-dimensional structures of such molecules at or near atomic resolution. With such a wide-spread importance, crystallography is an integral part of structural biology and also of the current drug discovery programs. The present thesis mainly deals with application of the crystallographic techniques for understanding the structure and function of adenylosuccinate lyase (ASL) from bacterial pathogens Salmonella typhimurium and Mycobacterium tuberculosis as well as its non-pathogenic counterpart Mycobacterium smegmatis. Studies were also carried out to understand the structure-function relationship of the protease in the plant virus Sesbania Mosaic Virus (SeMV). The thesis has been divided into six chapters. The first chapter contains an introduction to nucleotide synthesis and ASL superfamily of enzymes known as the aspartase/fumarase superfamily based on the published literature. Chapter 2 provides the details of the techniques used for the investigations presented in this thesis. Chapters 3-5 deal with the structural and functional studies carried out on ASL from the three bacterial organisms. Chapter 6 deals with the simulation studies carried out on SeMV protease. Mechanism and importance of nucleotide synthesis is introduced in Chapter 1, with special emphasis on purine de novo and salvage pathways. ASL is introduced as an important enzyme for purine synthesis. Its superfamily, the aspartase/fumarase superfamily of enzymes is described in detail with respect to its structure, function and pathophysiology. Objectives of the present study are outlined towards the end of the chapter. The experimental and computational techniques utilized during the course of my research are described in Chapter 2. These techniques include gene cloning, protein expression and purification, kinetic and biophysical characterization of proteins, crystallization, X-ray diffraction, data collection and processing, structure solution, refinement, model building, validation and structural analysis, phylogenetic studies, molecular docking and molecular dynamic simulation studies. Adenylosuccinate lyase is an important enzyme participating in purine biosynthesis. With the emergence of drug resistant variants of various pathogens, ASL has been recognized as a drug target against microbial infections. Chapter 3 deals with the structural and functional characterization of ASL from Salmonella typhimurium. Two constructs of the StASL gene were cloned and expressed leading to the purification of truncated (residues 1-366) and full-length (residues 1-456) polypeptides. Crystallization of the two polypeptides resulted in three independent structures. The full-length structure was very similar to the E. coli ASL structure consistent with 95% amino acid sequence identity between the two polypeptides. However, the truncated structures showed large distortions, especially of the active site residues, accounting for the catalytic inactivity of the truncated polypeptide in spite of retaining all residues considered important for function. The full-length ASL was catalytically active. A unique feature observed in StASL, not reported in other ASLs, was its allosteric regulation by the substrate. Kinetic studies also revealed hysteretic behavior of the enzyme. The electron density map of the full-length structure showed two novel densities on the molecular 2-fold axis into each of which a molecule of cadavarine could be fitted. Docking studies revealed a ligand-binding site at the inter-subunit interface between the two observed densities which might represent a potential allosteric site. Combining the structural and kinetic results, a possible morpheein model of allosteric regulation of StASL was hypothesized. Chapter 4 deals with the crystallographic and kinetic investigations on ASL from Mycobacterium smegmatis and Mycobacterium tuberculosis. MsASL and MtbASL were cloned, purified and crystallized. The X-ray crystal structure of MsASL was determined at 2.16 Å resolution. It is the first report of an apo-ASL structure with a partially ordered active site C3 loop. Diffracting crystals of MtbASL could not be obtained and a model for its structure was derived using MsASL as a template. Most of the active site residues were found to be conserved with the exception of Ser 148 and Gly 319 of MsASL. Ser 148 is structurally equivalent to a threonine in most other ASLs. Gly 319 is replaced by an arginine residue in most ASLs. The two enzymes were catalytically much less active when compared to ASLs from other organisms. Arg319Gly substitution and reduced flexibility of the C3 loop might account for the low catalytic activity of mycobacterial ASLs. The low activity is consistent with the slow growth rate of Mycobacteria, their high GC containing genomes as well as with their dependence on other salvage pathways for the supply of purine nucleotides. Chapter 5 deals with the identification of the catalytic residues important for ASL catalysis in view of the earlier conflicting reports on the identity of these residues. pH-dependent kinetic studies were performed on full-length StASL. The theory behind these studies is also described in this chapter. Two residues with pKa values of 6.6 and 7.7 were identified as essential for the enzymatic activity. These results were interpreted along with structural comparison of MsASL and other superfamily enzymes with ordered C3 loops. They suggest that His 149 and either Lys 285 or Ser 279 of MsASL are the residues most likely to function as the catalytic acid and base, respectively. The final Chapter 6 of the thesis deals with the structural and dynamic studies carried out on Sesbania mosaic virus (SeMV) protease. The chapter begins with a general introduction to viruses, followed by a brief summary of SeMV. The goal of this study is to understand the interactions between the protease and VPg at a structural level using the information available from biochemical studies. Crystallographic studies initiated for the mutant H275APro and Y315APro were unsuccessful due to the insolubility of the proteins. Co-crystallization or soaking experiments of wild type protease with cognate peptides were unsuccessful due to the inability of the enzyme to bind to its substrates in the absence of VPg. Higher resolution structure of wild type protease did not yield any new insights when compared to the earlier reported structure determined at a lower resolution. In the absence of structural insights, molecular dynamic simulations were carried out on wild type protease structure and in silico generated mutants using GROMACS package. The studies showed the importance of flipping of residue Phe 301 and opening-closing of the loop region corresponding to residues 301-308 for the catalytic mechanism. The thesis concludes with Future perspectives of the various studies carried out on ASL and SeMV protease. The atomic coordinates determined from the work presented in this thesis have been deposited in the PDB and the assigned PDB codes are reported in the respective chapters. Publications cited in the thesis are listed in the Bibliography section.