Amino acids 39-456 of the large subunit and 210-262 of the small subunit constitute the minimal functionally interacting fragments of the unusual heterodimeric topoisomerase IB of Leishmania

DNA Topoisomerases in the Unicellular Protozoan Parasites: Unwinding the Mystery

DNA topoisomerases are a group of enzymes present ubiquitously in all organisms from unicellular protozoan parasites to humans. These enzymes control the topological problems caused by DNA double helix in the cell during nucleic acid metabolism. Certain types of topoisomerases present in unicellular parasites are quite different from human topoisomerases (hTop) concerning structure, expression, and function. Many protozoan parasites causing fatal diseases have DNA topoisomerases, which play vital roles in their survival. Given the fact that the structures of the protozoan parasite topoisomerases are different from humans, DNA topoisomerase acts as an essential target for potent drug development for parasitic diseases. Moreover, various studies revealed the therapeutic potential of these drugs targeting the parasitic topoisomerases. Therefore, the characterization of parasitic topoisomerases is pivotal for the development of future potential drug targets. Considering the importance of this ubiquitous enzyme as a potential drug target, we describe in detail all the reported protozoan topoisomerases in an organized manner including Leishmania, Trypanosoma, Plasmodium, Giardia, Entamoeba, Babesia, Theileria, Crithidia, Cryptosporidium, Toxoplasma, etc. This review highlights the unique attributes associated with the structure and function of different types of DNA topoisomerases from the unicellular protozoan parasites. So, it would be beneficial for researchers to obtain awareness about the currently characterized topoisomerases and the ones that need better characterization, understand the structure-function relationship of parasitic topoisomerases, to develop the potent anti-parasitic drugs, and also provides a future platform for therapeutic development.

ATP independent type IB topoisomerase of Leishmania donovani is stimulated by ATP: an insight into the functional mechanism

Most type IB topoisomerases do not require ATP and Mg(2+) for activity. However, as shown previously for vaccinia topoisomerase I, we demonstrate that ATP stimulates the relaxation activity of the unusual heterodimeric type IB topoisomerase from Leishmania donovani (LdTOP1L/S) in the absence of Mg(2+). The stimulation is independent of ATP hydrolysis but requires salt as a co-activator. ATP binds to LdTOP1L/S and increases its rate of strand rotation. Docking studies indicate that the amino acid residues His93, Tyr95, Arg188 and Arg190 of the large subunit may be involved in ATP binding. Site directed mutagenesis of these four residues individually to alanine and subsequent relaxation assays reveal that the R190A mutant topoisomerase is unable to exhibit ATP-mediated stimulation in the absence of Mg(2+). However, the ATP-independent relaxation activities of all the four mutant enzymes remain unaffected. Additionally, we provide evidence that ATP binds LdTOP1L/S and modulates the activity of the otherwise ATP-independent enzyme. This study establishes ATP as an activator of LdTOP1L/S in the absence of Mg(2+).

Role of a differentially expressed cAMP phosphodiesterase in regulating the induction of resistance against oxidative damage in Leishmania donovani

Differentiation-coupled induction of resistance of Leishmania parasites to macrophage oxidative damage was shown to be associated with an increased cAMP response. This study explores the significance of the cAMP response in the parasite by identifying a differentially expressed cAMP phosphodiesterase (LdPDEA) and deciphering its role in regulating antioxidant machineries in the parasite. LdPDEA, a high K(M) class I cytosolic cAMP phosphodiesterase, was expressed maximally in log-phase promastigotes, but was significantly reduced in stationary-phase promastigotes and amastigotes. Chemical inhibition or silencing of PDEA conferred enhanced resistance to pro-oxidants in these cells and this led to studies on trypanothione biosynthesis and utilization, as trypanothione is one of the major modulators of antioxidant defense in kinetoplastidae. Despite enhanced arginase and ornithine decarboxylase activity, trypanothione biosynthesis seemed to be unaffected by PDEA blockage, whereas significant elevations in the expression of tryparedoxin peroxidase, ascorbate peroxidase, and tryparedoxin were detected, suggesting a definite shift of trypanothione-pool utilization bias toward antioxidant defense. Moreover, parasites that overexpressed PDEA showed reduced resistance to oxidative damage and reduced infectivity toward activated macrophages. This study reveals the significance of a cAMP phosphodiesterase in the infectivity of Leishmania parasites.

Deletion study of DNA topoisomerase IB from Leishmania donovani: searching for a minimal functional heterodimer

The substantial differences between trypanosomal and leishmanial DNA topoisomerase IB concerning to their homologues in mammals have provided a new lead in the study of the structural determinants that can be effectively targeted. Leishmania donovani, the causative agent of visceral leishmaniasis, contains an unusual heterodimeric DNA topoisomerase IB. The catalytically active enzyme consists of a large subunit (LdTopIL), which contains the non-conserved N-terminal end and the phylogenetically conserved “core” domain, and of a small subunit (LdTopIS) which harbors the C-terminal region with the characteristic tyrosine residue in the active site. Heterologous co-expression of LdTopIL and LdTopIS genes in a topoisomerase I deficient yeast strain, reconstitutes a fully functional enzyme LdTopIL/S which can be used for structural studies. An approach by combinatorial cloning of deleted genes encoding for truncated versions of both subunits was used in order to find out structural insights involved in enzyme activity or protein-protein interaction. The role played by the non-conserved N-terminal extension of LdTopIL in both relaxation activity and CPT sensitivity has been examined co-expressing the full-length LdTopIS and a fully active LdTopIΔS deletion with several deletions of LdTopIL lacking growing sequences of the N-terminal end. The sequential deletion study shows that the first 26 amino acids placed at the N-terminal end and a variable region comprised between Ala548 to end of the C-terminal extension of LdTopIL were enzymatically dispensable. Altogether this combinatorial approach provides important structural insights of the regions involved in relaxation activity and for understanding the atypical structure of this heterodimeric enzyme.