Structural investigations on two mycobacterial enzymes were carried out. Tuberculosis still remains a major threat to mankind even though drugs against it have been in use for many decades. The emergence of drug resistant strains of the bacteria calls for the identification of new targets based on which new drugs can be developed to combat the disease. A thorough understanding of the functioning of the target molecules is essential for this approach. We have taken up the structural studies on two such molecules, aspartic proteinases and adenylyl cyclases, of Mycobacterium tuberculosis with a view to obtain insights into their mechanisms of action at the atomic level. The work presented in the thesis includes (i) the identification, cloning, expression, purification and structure determination of a putative aspartic proteinase domain of M. tuberculosis and (ii) the crystal structure of an adenylyl cyclase of M. tuberculosis and its mutant; and also of an adenylyl cyclase from M. avium. Chapter 1 presents an overview of aspartic proteinases and nucleotide cyclases with an emphasis on their structural features. The methods employed during the course of the present work are described in Chapter 2. Work on the putative aspartic proteinase domain identified in M. tuberculosis is presented in Chapter 3. The structure of the aspartic proteinase domain is the first structural report of such domain from any bacteria. A search in the genome of M. tuberculosis showed a weak similarity to the HIV aspartic proteinase sequence. This region corresponds to the C-terminal domain of a PE family protein in M. tuberculosis. The presence of two signature motifs, DTG and DSG, of aspartic proteinases in the full sequence of this domain encouraged us to take up further studies on this domain. Previous reports identifying the protein as a surface antigen and our findings on the occurrence of similar domains in two other PE proteins of M. tuberculosis and also in other pathological strains of Mycobacteria indicated that these domains probably play an important role in infecting the host. The crystal structure of one of the domains showed that it has a pepsin-like fold and the catalytic site architecture of known aspartic proteinases. However, no proteolytic activity was detected. The size of the molecule is intermediate to eukaryotic pepsins and the homodimeric retroviral pepsins. A close examination of the binding site revealed subtle differences when compared to the active enzyme structures. Appropriate mutations of some of the residues in this region to convert it to an active enzyme did not make it active. Once the in vivo function of these putative aspartic proteinase domains is established, their potential to act as drug targets can be probed as the PE proteins are present exclusively in pathogenic Mycobacteria. As part of an ongoing project on adenylyl cyclases of Mycobacteria, we have taken up the structure analysis of the catalytic domains of two adenylyl cyclases; Rv1625c from M. tuberculosis and Ma1120 from M. avium. This work is described in Chapter 4. The wild-type of Rv1625c crystallized as a domain swapped head to head inactive dimer even though it is an active dimer in solution and expected to have a head to tail arrangement as in the previously reported structures of the active forms of the enzyme. Mutation of a phenylalanine residue presumed to occur at the subunit interface of the active dimeric structure of the enzyme to an arginine residue, a conserved residue of guanylyl cyclases, resulted in reduced adenylyl cyclase activity. This mutant crystallized as a monomer though it was expected to be an active dimer. Similarly, Ma1120 also has a monomeric structure in the crystal in spite of showing activity in solution. Though our aim was to capture the active dimers in the crystalline state we did not succeed in this effort in any of the three cases. The catalytic reaction probably takes place very rapidly with the formation of a transient active form of the dimer which cannot be easily crystallized. However, the analysis revealed new structures which are likely to represent the stable states of the enzyme when it is required to stay inactive in certain conditions. We have also established that the N-terminal segments of the enzyme, a loop at the dimeric interface and external factors like pH are involved in determining the oligomeric status of the enzyme thereby regulating its function. Publications 1 Crystal structure of a putative aspartic proteinase domain of the Mycobacterium tuberculosis cell surface antigen PE_PGRS16; Deivanayaga V. Barathy and K. Suguna; FEBS Open Bio (In Press) 2 New structural forms of mycobacterial adenylyl cyclases (in preparation)