Cloning and biochemical characterization of genes encoding two isozymes of cysteine synthase from Entamoeba dispar

Current and future perspectives on the chemotherapy of the parasitic protozoa Trichomonas vaginalis and Entamoeba histolytica

Trichomonas vaginalis and Entamoeba histolytica are clinically important protozoa that affect humans. T. vaginalis produces sexually transmitted infections and E. histolytica is the causative agent of amebic dysentery. Metronidazole, a compound first used to treat T. vaginalis in 1959, is still the main drug used worldwide to treat these pathogens. It is essential to find new biochemical differences in these organisms that could be exploited to develop new antiprotozoal chemotherapeutics. Recent findings associated with T. vaginalis and E. histolytica biochemistry and host–pathogen interactions are surveyed. Knowledge concerning the biochemistry of these parasites is serving to form the foundation for the development of new approaches to control these important human pathogens.

Isoform-dependent feedback regulation of serine O-acetyltransferase isoenzymes involved in L-cysteine biosynthesis of Entamoeba histolytica

Serine acetyltransferase (SAT; EC 2.3.1.30) catalyzes the CoA-dependent acetylation of the side chain hydroxyl group of l-serine to form O-acetyl serine, in the first step of the L-cysteine biosynthetic pathway. Since this pathway is selectively present in a few parasitic protists and absent in mammals, it represents a reasonable target to develop new chemotherapeutics. Entamoeba histolytica apparently possesses three SAT isotypes (EhSAT1-3) showing 48-73% mutual identity, a calculated molecular mass of 34.4-37.7 kDa, and an isoelectric point of 5.70-6.63. To better understand the role of individual SAT isotypes, we determined kinetic and inhibitory parameters of recombinant SAT isotypes. While the three SAT isotypes showed comparable Km and k(cat) for L-serine and acetyl-CoA, they showed remarkable differences in their sensitivity to inhibition by L-cysteine. The Ki values for L-cysteine varied by 100-fold (4.7-460 microM) among SAT isotypes (EhSAT1<EhSAT2<EhSAT3). Consequently, these EhSAT isotypes revealed remarkable differences in activity in the presence of physiological L-serine and L-cysteine concentrations. We propose that multiple SAT isotypes with different properties may play complementary roles in the regulation of the cysteine biosynthetic pathway in E. histolytica under different conditions, e.g. during colonization of the intestine and tissue invasion.

Current therapeutics, their problems, and sulfur-containing-amino-acid metabolism as a novel target against infections by "amitochondriate" protozoan parasites

The "amitochondriate" protozoan parasites of humans Entamoeba histolytica, Giardia intestinalis, and Trichomonas vaginalis share many biochemical features, e.g., energy and amino acid metabolism, a spectrum of drugs for their treatment, and the occurrence of drug resistance. These parasites possess metabolic pathways that are divergent from those of their mammalian hosts and are often considered to be good targets for drug development. Sulfur-containing-amino-acid metabolism represents one such divergent metabolic pathway, namely, the cysteine biosynthetic pathway and methionine gamma-lyase-mediated catabolism of sulfur-containing amino acids, which are present in T. vaginalis and E. histolytica but absent in G. intestinalis. These pathways are potentially exploitable for development of drugs against amoebiasis and trichomoniasis. For instance, L-trifluoromethionine, which is catalyzed by methionine gamma-lyase and produces a toxic product, is effective against T. vaginalis and E. histolytica parasites in vitro and in vivo and may represent a good lead compound. In this review, we summarize the biology of these microaerophilic parasites, their clinical manifestation and epidemiology of disease, chemotherapeutics, the modes of action of representative drugs, and problems related to these drugs, including drug resistance. We further discuss our approach to exploit unique sulfur-containing-amino-acid metabolism, focusing on development of drugs against E. histolytica.

Sulfur-Containing Amino Acid Metabolism in Parasitic Protozoa

Sulfur-containing amino acids play indispensable roles in a wide variety of biological activities including protein synthesis, methylation, and biosynthesis of polyamines and glutathione. Biosynthesis and catabolism of these amino acids need to be carefully regulated to achieve the requirement of the above-mentioned activities and also to eliminate toxicity attributable to the amino acids. Genome-wide analyses of enzymes involved in the metabolic pathways of sulfur-containing amino acids, including transsulfuration, sulfur assimilatory de novo cysteine biosynthesis, methionine cycle, and degradation, using genome databases available from a variety of parasitic protozoa, reveal remarkable diversity between protozoan parasites and their mammalian hosts. Thus, the sulfur-containing amino acid metabolic pathways are a rational target for the development of novel chemotherapeutic and prophylactic agents against diseases caused by protozoan parasites. These pathways also demonstrate notable heterogeneity among parasites, suggesting that the metabolism of sulfur-containing amino acids reflects the diversity of parasitism among parasite species, and probably influences their biology and pathophysiology such as virulence competence and stress defense.

Molecular and structural characterization of NADPH-dependent d-glycerate dehydrogenase from the enteric parasitic protist Entamoeba histolytica

Putative NADPH-dependent GDH (L-glycerate dehydrogenase) of the protozoan parasite Entamoeba histolytica (EhGDH) has been characterized. The EhGDH gene encodes a protein of 318 amino acids with a calculated isoelectric point of 6.29 and a molecular mass of 35.8 kDa. EhGDH showed highest identities with GDH from epsilon-proteobacteria. This close kinship was also supported by phylogenetic analyses, suggesting possible lateral transfer of the gene from epsilon-proteobacteria to E. histolytica. In contrast with the implications from protein alignment and phylogenetic analysis, kinetic studies revealed that the amoebic GDH showed biochemical properties similar to those of mammalian GDH, i.e. a preference for NADPH as cofactor and higher affinities towards NADPH and beta-hydroxypyruvate than towards NADP+ and L-glycerate. Whereas the amino acids involved in nucleotide binding and catalysis are totally conserved in EhGDH, substitution of a negatively charged amino acid with a non-charged hydroxy-group-containing amino acid is probably responsible for the observed high affinity of EhGDH for NADP+/NADPH. In addition, the amoebic GDH, dissimilar to the bacterial and mammalian GDHs, lacks glyoxylate reductase activity. Native and recombinant EhGDH showed comparable subunit structure, kinetic parameters and elution profiles on anion-exchange chromatography. We propose that the GDH enzyme is likely to be involved in regulation of the intracellular concentration of serine, and, thus, also in controlling cysteine biosynthesis located downstream of serine metabolic pathways in this protist.

Characterization of transsulfuration and cysteine biosynthetic pathways in the protozoan hemoflagellate, Trypanosoma cruzi. Isolation and molecular characterization of cystathionine beta-synthase and serine acetyltransferase from Trypanosoma

Sulfur-containing amino acids play an important role in a variety of cellular functions such as protein synthesis, methylation, and polyamine and glutathione synthesis. We cloned and characterized cDNA encoding cystathionine beta-synthase (CBS), which is a key enzyme of transsulfuration pathway, from a hemoflagellate protozoan parasite Trypanosoma cruzi. T. cruzi CBS, unlike mammalian CBS, lacks the regulatory carboxyl terminus, does not contain heme, and is not activated by S-adenosylmethionine. T. cruzi CBS mRNA is expressed as at least six independent isotypes with sequence microheterogeneity from tandemly linked multicopy genes. The enzyme forms a homotetramer and, in addition to CBS activity, the enzyme has serine sulfhydrylase and cysteine synthase (CS) activities in vitro. Expression of the T. cruzi CBS in Saccharomyces cerevisiae and Escherichia coli demonstrates that the CBS and CS activities are functional in vivo. Enzymatic studies on T. cruzi extracts indicate that there is an additional CS enzyme and stage-specific control of CBS and CS expression. We also cloned and characterized cDNA encoding serine acetyltransferase (SAT), a key enzyme in the sulfate assimilatory cysteine biosynthetic pathway. Dissimilar to bacterial and plant SAT, a recombinant T. cruzi SAT showed allosteric inhibition by l-cysteine, l-cystine, and, to a lesser extent, glutathione. Together, these studies demonstrate the T. cruzi is a unique protist in possessing both transsulfuration and sulfur assimilatory pathways.