Mutational analysis of methionine adenosyltransferase from Leishmania donovani

Polyamine Metabolism for Drug Intervention in Trypanosomatids

Neglected tropical diseases transmitted by trypanosomatids include three major human scourges that globally affect the world's poorest people: African trypanosomiasis or sleeping sickness, American trypanosomiasis or Chagas disease and different types of leishmaniasis. Different metabolic pathways have been targeted to find antitrypanosomatid drugs, including polyamine metabolism. Since their discovery, the naturally occurring polyamines, putrescine, spermidine and spermine, have been considered important metabolites involved in cell growth. With a complex metabolism involving biosynthesis, catabolism and interconversion, the synthesis of putrescine and spermidine was targeted by thousands of compounds in an effort to produce cell growth blockade in tumor and infectious processes with limited success. However, the discovery of eflornithine (DFMO) as a curative drug against sleeping sickness encouraged researchers to develop new molecules against these diseases. Polyamine synthesis inhibitors have also provided insight into the peculiarities of this pathway between the host and the parasite, and also among different trypanosomatid species, thus allowing the search for new specific chemical entities aimed to treat these diseases and leading to the investigation of target-based scaffolds. The main molecular targets include the enzymes involved in polyamine biosynthesis (ornithine decarboxylase, S-adenosylmethionine decarboxylase and spermidine synthase), enzymes participating in their uptake from the environment, and the enzymes involved in the redox balance of the parasite. In this review, we summarize the research behind polyamine-based treatments, the current trends, and the main challenges in this field.

Polar Interactions at the Dimer-Dimer Interface of Methionine Adenosyltransferase MAT I Control Tetramerization

Catalytic MATα1 subunits associate into kinetically distinct homo-dimers (MAT III) and homo-tetramers (MAT I) that synthesize S-adenosylmethionine in the adult liver. Pathological reductions in S-adenosylmethionine levels correlate with MAT III accumulation; thus, it is important to know the determinants of dimer–dimer associations. Here, polar interactions (<3.5 Å) at the rat MAT I dimer–dimer interface were disrupted by site-directed mutagenesis. Heterologous expression rendered decreased soluble mutant MATα1 levels that appeared mostly as dimers. Substitutions at the B1–B2 or B3–C1 β-strand loops, or changes in charge on helix α2 located behind, induced either MAT III or MAT I accumulation. Notably, double mutants combining neutral changes on helix α2 with substitutions at either β-strand loop further increased MAT III content. Mutations had negligible impact on secondary or tertiary protein structure, but induced changes of 5–10 °C in thermal stability. All mutants preserved tripolyphosphatase activity, although AdoMet synthesis was only detected in single mutants. Kinetic parameters were altered in all purified proteins, their AdoMet synthesis V_max and methionine affinities correlating with the association state induced by the corresponding mutations. In conclusion, polar interactions control MATα1 tetramerization and kinetics, diverse effects being induced by changes on opposite β-sheet loops putatively leading to subtle variations in central domain β-sheet orientation.

Molecular cloning and characterization of S-adenosylmethionine synthetase gene from Lycoris radiata

S-adenosylmethionine (SAM) synthetase catalyzes the synthesis of SAM, a molecule important for all cellular organisms. It is also considered to play an important role in salt tolerance of plants. Here, we cloned a Lycoris radiata (L. radiata) SAM synthetase gene LrSAMS to determine its biological function. The gene encodes a protein of 401 amino acids with a calculated molecular weight of 43.9 kDa. Amino acid sequence analysis of the deduced protein LrSAMS reveals high sequence identity to SAM synthetases from other organisms, such as Arabidopsis thaliana and Oryza sativa. The deduced LrSAMS protein contains conserved amino acids residues and sequences motifs that closely related to the function of SAM synthetase. Otherwise, the transcript levels of LrSAMS were significantly induced by NaCl treatment in L. radiata leaves, which implied that LrSAMS might play an important role in tolerance to salt stress in L.radiata. Complete ORF of LrSAMS was inserted into expression vector pET-29a(+) and transformed into Escherichia coli BL21 (DE3). The difference between the growth curve of the transgenic strain and control strain with blank vector showed that over-expressing LrSAMS could provide growth advantage to the engineered strain in high salt concentration.

Structure-function relationships in methionine adenosyltransferases

Methionine adenosyltransferases (MATs) are the family of enzymes that synthesize the main biological methyl donor, S-adenosylmethionine. The high sequence conservation among catalytic subunits from bacteria and eukarya preserves key residues that control activity and oligomerization, which is reflected in the protein structure. However, structural differences among complexes with substrates and products have led to proposals of several reaction mechanisms. In parallel, folding studies begin to explain how the three intertwined domains of the catalytic subunit are produced, and to highlight the importance of certain intermediates in attaining the active final conformation. This review analyzes the available structural data and proposes a consensus interpretation that facilitates an understanding of the pathological problems derived from impairment of MAT function. In addition, new research opportunities directed toward clarification of aspects that remain obscure are also identified.

Pneumocystis encodes a functional S-adenosylmethionine synthetase gene

S-adenosylmethionine (AdoMet) synthetase (EC 2.5.1.6) is the enzyme that catalyzes the synthesis of AdoMet, a molecule important for all cellular organisms. We have cloned and characterized an AdoMet synthetase gene (sam1) from Pneumocystis spp. This gene was transcribed primarily as an approximately 1.3-kb mRNA which encodes a protein containing 381 amino acids in P. carinii or P. murina and 382 amino acids in P. jirovecii. sam1 was also transcribed as part of an apparent polycistronic transcript of approximately 5.6 kb, together with a putative chromatin remodeling protein homologous to Saccharomyces cerevisiae, CHD1. Recombinant Sam1, when expressed in Escherichia coli, showed functional enzyme activity. Immunoprecipitation and confocal immunofluorescence analysis using an antipeptide antibody showed that this enzyme is expressed in P. murina. Thus, Pneumocystis, like other organisms, can synthesize its own AdoMet and may not depend on its host for the supply of this important molecule.

S-Adenosylmethionine in protozoan parasites: Functions, synthesis and regulation

S-adenosylmethionine is one of the most frequently used enzymatic substrates in all living organisms. It plays a role in all biological methyl transfer reactions in as much as it is a donor of propylamine groups in the synthesis of the polyamines spermidine and spermine, it participates in the trans-sulphuration pathway to cysteine one of the three amino acids involved in glutathione and trypanothione synthesis in trypanosomatids and finally it is a source of the 5-deoxyadenosyl radicals, which are involved in many reductive metabolic processes, biodegradative pathways, tRNA modification and DNA repair. This mini-review is an update of the progress on the S-adenosylmethionine synthesis in different representative protozoan parasites responsible for many of the most devastating so-called tropical diseases that have an enormous impact on global health.