Ardala Breda

Purine Salvage Pathway in Mycobacterium tuberculosis

The causative agent of tuberculosis (TB), Mycobacterium tuberculosis, infects one-third of the world population. TB remains the leading cause of mortality due to a single bacterial pathogen. The worldwide increase in incidence of M. tuberculosis has been attributed to the high proliferation rates of multi and extensively drug-resistant strains, and to co-infection with the human immunodeficiency virus. There is thus a continuous requirement for studies on mycobacterial metabolism to identify promising targets for the development of new agents to combat TB. Singular characteristics of this pathogen, such as functional and structural features of enzymes involved in fundamental metabolic pathways, can be evaluated to identify possible targets for drug development. Enzymes involved in the pyrimidine salvage pathway might be attractive targets for rational drug design against TB, since this pathway is vital for all bacterial cells, and is composed of enzymes considerably different from those present in humans. Moreover, the enzymes of the pyrimidine salvage pathway might have an important role in the mycobacterial latent state, since M. tuberculosis has to recycle bases and/or nucleosides to survive in the hostile environment imposed by the host. The present review describes the enzymes of M. tuberculosis pyrimidine salvage pathway as attractive targets for the development of new antimycobacterial agents. Enzyme functional and structural data have been included to provide a broader knowledge on which to base the search for compounds with selective biological activity.

Virtual Screening of Drugs: Score Functions, Docking, and Drug Design

The computational approach for new drug design and/or identification, was initially proposed in mid 70s. The virtual screening of chemical libraries against a biological target has proven its reliability on structure-based drug design, for instance, for many HIV virus protein inhibitors and for the development of Cyclin-Dependent Kinase inhibitors. Target- based virtual screening, allied to docking studies, enables searches on larger data set of probable ligands, with less costs than the traditional experimental screening. The increasing availability of small molecules databases and its free online distribution is now allowing not only pharmaceutical industries, but independent research labs as well, to apply this methodology on early stages of drug discovery. When the protein target structure is available, and a chemical virtual library is accessible, following questions need to be answered: how the target and the ligand interact and how these interactions may be evaluated? Several docking algorithms for the identification of the molecular features responsible for binding specificity are available. While such algorithms are very robust and accurate, the scoring functions remain more questionable in the sense of what parameters should be considered when defining protein-ligand binding affinity when ranking candidates pointed-out by the virtual screening to the next step on drug testing. Aside conformational and chemical information, pharmacokinetics properties should be considered as well when selecting potential new drugs. Along with structural well-match, appropriate molecular features that define desired kinetics characteristics should be consistently addressed for usefulness of virtual screening results. The present review is focused on these questions and their implication for virtual screening.

Recombinant Escherichia coli GMP reductase: kinetic, catalytic and chemical mechanisms, and thermodynamics of enzyme–ligand binary complex formation

Guanosine monophosphate (GMP) reductase catalyzes the reductive deamination of GMP to inosine monophosphate (IMP). GMP reductase plays an important role in the conversion of nucleoside and nucleotide derivatives of guanine to adenine nucleotides. In addition, as a member of the purine salvage pathway, it also participates in the reutilization of free intracellular bases. Here we present cloning, expression and purification of Escherichia coli guaC-encoded GMP reductase to determine its kinetic mechanism, as well as chemical and thermodynamic features of this reaction. Initial velocity studies and isothermal titration calorimetry demonstrated that GMP reductase follows an ordered bi-bi kinetic mechanism, in which GMP binds first to the enzyme followed by NADPH binding, and NADP(+) dissociates first followed by IMP release. The isothermal titration calorimetry also showed that GMP and IMP binding are thermodynamically favorable processes. The pH-rate profiles showed groups with apparent pK values of 6.6 and 9.6 involved in catalysis, and pK values of 7.1 and 8.6 important to GMP binding, and a pK value of 6.2 important for NADPH binding. Primary deuterium kinetic isotope effects demonstrated that hydride transfer contributes to the rate-limiting step, whereas solvent kinetic isotope effects arise from a single protonic site that plays a modest role in catalysis. Multiple isotope effects suggest that protonation and hydride transfer steps take place in the same transition state, lending support to a concerted mechanism. Pre-steady-state kinetic data suggest that product release does not contribute to the rate-limiting step of the reaction catalyzed by E. coli GMP reductase.

A Hybrid Method for the Protein Structure Prediction Problem

This article provides the initial results of our effort to develop a hybrid prediction method, combining the principles of de novoand homology modeling, to help solve the protein three-dimensional (3-D) structure prediction problem. A target protein amino acid sequence is fragmented into many short contiguous fragments. Clustered short templates fragments, obtained from experimental protein structures in the Protein Data Bank (PDB), using the NCBI BLASTp program, were used for building an initial conformation, which was further refined by molecular dynamics simulations. We tested our method with the artificially designed alpha helical hairpin (PDB ID: 1ZDD) starting with its amino acids sequence only. The structure obtained with the proposed method is topologically a helical hairpin, with a C( RMSD of ~ 5.0 A with respect to the experimental PDB structure for all 34 amino acids residues, and only ~ 2.0 A when considering amino acids 1 to 22. We discuss further improvements to the method.

Wild-Type Phosphoribosylpyrophosphate Synthase (PRS) from Mycobacterium tuberculosis: A Bacterial Class II PRS?

The 5-phospho-α-D-ribose 1-diphosphate (PRPP) metabolite plays essential roles in several biosynthetic pathways, including histidine, tryptophan, nucleotides, and, in mycobacteria, cell wall precursors. PRPP is synthesized from α-D-ribose 5-phosphate (R5P) and ATP by the Mycobacterium tuberculosis prsA gene product, phosphoribosylpyrophosphate synthase (MtPRS). Here, we report amplification, cloning, expression and purification of wild-type MtPRS. Glutaraldehyde cross-linking results suggest that MtPRS predominates as a hexamer, presenting varied oligomeric states due to distinct ligand binding. MtPRS activity measurements were carried out by a novel coupled continuous spectrophotometric assay. MtPRS enzyme activity could be detected in the absence of Pi. ADP, GDP and UMP inhibit MtPRS activity. Steady-state kinetics results indicate that MtPRS has broad substrate specificity, being able to accept ATP, GTP, CTP, and UTP as diphosphoryl group donors. Fluorescence spectroscopy data suggest that the enzyme mechanism for purine diphosphoryl donors follows a random order of substrate addition, and for pyrimidine diphosphoryl donors follows an ordered mechanism of substrate addition in which R5P binds first to free enzyme. An ordered mechanism for product dissociation is followed by MtPRS, in which PRPP is the first product to be released followed by the nucleoside monophosphate products to yield free enzyme for the next round of catalysis. The broad specificity for diphosphoryl group donors and detection of enzyme activity in the absence of Pi would suggest that MtPRS belongs to Class II PRS proteins. On the other hand, the hexameric quaternary structure and allosteric ADP inhibition would place MtPRS in Class I PRSs. Further data are needed to classify MtPRS as belonging to a particular family of PRS proteins. The data here presented should help augment our understanding of MtPRS mode of action. Current efforts are toward experimental structure determination of MtPRS to provide a solid foundation for the rational design of specific inhibitors of this enzyme.

The conserved Lysine69 residue plays a catalytic role in Mycobacterium tuberculosis shikimate dehydrogenase

BackgroundThe shikimate pathway is an attractive target for the development of antitubercular agents because it is essential in Mycobacterium tuberculosis, the causative agent of tuberculosis, but absent in humans. M. tuberculosis aroE-encoded shikimate dehydrogenase catalyzes the forth reaction in the shikimate pathway. Structural and functional studies indicate that Lysine69 may be involved in catalysis and/or substrate binding in M. tuberculosis shikimate dehydrogenase. Investigation of the kinetic properties of mutant enzymes can bring important insights about the role of amino acid residues for M. tuberculosis shikimate dehydrogenase.FindingsWe have performed site-directed mutagenesis, steady-state kinetics, equilibrium binding measurements and molecular modeling for both the wild-type M. tuberculosis shikimate dehydrogenase and the K69A mutant enzymes. The apparent steady-state kinetic parameters for the M. tuberculosis shikimate dehydrogenase were determined; the catalytic constant value for the wild-type enzyme (50 s-1) is 68-fold larger than that for the mutant K69A (0.73 s-1). There was a modest increase in the Michaelis-Menten constant for DHS (K69A = 76 μM; wild-type = 29 μM) and NADPH (K69A = 30 μM; wild-type = 11 μM). The equilibrium dissociation constants for wild-type and K69A mutant enzymes are 32 (± 4) μM and 134 (± 21), respectively.ConclusionOur results show that the residue Lysine69 plays a catalytic role and is not involved in substrate binding for the M. tuberculosis shikimate dehydrogenase. These efforts on M. tuberculosis shikimate dehydrogenase catalytic mechanism determination should help the rational design of specific inhibitors, aiming at the development of antitubercular drugs.

Bioinformatics tools for HIV-1 identification in southern brazilian states

HIV/AIDS pandemic affected 39.4 million people at the end of 2004, spreading at the rate of 15.000 new infections per day [1]. Although Brazil ranks in fourth in number of reported AIDS cases, limited information concerning the molecular diversity of HIV-1 circulating subtypes is known [3]. Southern Brazil has a particular HIV-1 epidemic, whereas subtype B dominates other regions of the country and subtype C reported cases are rare, in southern states the subtypes C and B have equivalent proportions, and the subtype C seems to be growing up since it was first described in Porto Alegre city, capital of Rio Grande do Sul (RS), at 90s.