Caracterização cinética e bioquímica da enzima inosina monofosfato desidrogenase (EC 1.1.1.205) de Mycobacterium tuberculosis como alvo para o desenvolvimento de inibidores

• Autor(a) principal: Rostirolla, Diana Carolina
• Data de Publicação: 2013
• Tipo de documento: Tese
• Idioma: por
• Título da fonte: Biblioteca Digital de Teses e Dissertações da PUC_RS
• Texto Completo: http://tede2.pucrs.br/tede2/handle/tede/1749
• topic: BIOLOGIA MOLECULAR; FARMACOLOGIA MOLECULAR; TUBERCULOSE; ENZIMAS - FARMACOLOGIA; CNPQ::CIENCIAS DA SAUDE::MEDICINA

Tuberculosis (TB) remains a leading infectious killer and its causative agent, Mycobacterium tuberculosis, infects one third of the world population. Despite 50 years of available drug treatments, TB continues to cause considerable mortality worldwide. The TBHIV co-infection and the emergence of multidrug and extensively-resistant TB have provided a very alarming challenge to global health and led us to focus on the research for new and more effective therapeutics against the disease. Advances in the identification of new TB drug targets have been driven by the availability of the genome sequence of M. tuberculosis H37Rv, and include elucidation of the role played by proteins of essential biochemical pathways for mycobacterial growth. Inosine Monophosphate Dehydrogenase (IMPDH, EC 1.1.1.205) catalyzes the penultimate and rate-limiting step in guanine nucleotide biosynthesis, the oxidation of inosine 5 -monophosphate (IMP) to xanthosine 5 - monophosphate (XMP), with concomitant reduction of nicotinamide adenine dinucleotide (NAD+) to NADH. This reaction controls the guanine nucleotide pool and IMPDH inhibitors are used as immunosuppresive, anticancer, and antiviral agents. Recently, IMPDH from M. tuberculosis (MtIMPDH) has gained attention as a promising anti-mycobacterial target, associated with a number of distinct inhibitor scaffolds. In the present work, the guaB2- encoded MtIMPDH has been cloned, expressed and purified to homogeneity. The recombinant MtIMPDH has a subunit molecular weight of 54,775 Da and Inductively-Coupled Plasma Optical Emission Spectroscopy (ICP-OES) and Flame Atomic Absorption Spectroscopy (FAAS) identified a K+ ion per subunit. Glutaraldehyde cross-linking analysis suggests that MtIMPDH predominates as a tetramer. Steady-state kinetics showed that MtIMPDH optimal activity is dependent on the presence of a monovalent cation, mainly K+. Initial velocity and product inhibition patterns suggested a steady-state ordered Bi Bi kinetic mechanism in which IMP binds first followed by NAD+, and hydride transfer is not the ratelimiting step in the MtIMPDH catalyzed reaction. In addition, NAD+ substrate inhibition implies that product release is ordered, with NADH released first. The pH-rate profile indicated one deprotonated group essential for catalysis and that groups with pK values of 7.5 and 9.0 are important for NAD+ binding. The data presented here are discussed in light of the kinetic and structural information available for IMPDHs investigated to date and should inform us on how to better design inhibitors targeting this enzyme. Additionaly, a vital part of drug development is the identification of gene products that are critical for bacterial growth and survival. In this regard, we have performed site-directed mutagenesis by gene replacement in order to evaluate the importance of the guaB2 gene for mycobacteria growth. Our results suggest that this gene is essential for M. tuberculosis H37Rv in vitro growth, and the confirmation of gene essentiality will point out the guaB2-encoded IMPDH as a potential drug target. Data resulting from enzyme s characterization and gene replacement were the starting point for MtIMPDH inhibitors planning, selection and testing. A compound was identified as promising lead molecule, with Ki values in nanomolar range and it was characterized as noncompetitive and uncompetitive inhibitor towards MtIMPDH substrates NAD+ and IMP, respectively. MtIMPDH characterization and results on primary inhibitors identification are crucial step towards a potential novel drug development for TB treatment. Altogether, these data may be useful for a better understanding about the biology of M. tuberculosis and as a framework on which to base efforts towards the development of efficient therapeutic strategies, aiming TB control.