The serine hydroxymethyltransferase of Plasmodium lophurae

Characterization of Plasmodium falciparum serine hydroxymethyltransferase-A potential antimalarial target

Serine hydroxymethyltransferase (SHMT) is a ubiquitous enzyme required for folate recycling and dTMP synthesis. A cDNA encoding Plasmodium falciparum (Pf) SHMT was expressed as a hexa-histidine tagged protein in Escherichia coli BL21-CodonPlus (DE3)-RIL. The protein was purified and the process yielded 3.6 mg protein/l cell culture. Recombinant His(6)-tagged PfSHMT exhibits a visible spectrum characteristic of pyridoxal-5'-phosphate enzyme and catalyzes the reversible conversion of l-serine and tetrahydrofolate (H(4)folate) to glycine and 5,10-methylenetetrahydrofolate (CH(2)-H(4)folate). Steady-state kinetics study indicates that His(6)-tagged PfSHMT catalyzes the reaction by a ternary-complex mechanism. The sequence of substrate binding to the enzyme was also examined by glycine product inhibition. A striking property that is unique for His(6)-tagged PfSHMT is the ability to use D-serine as a substrate in the folate-dependent serine-glycine conversion. Kinetic data in combination with expression result support the proposal of SHMT reaction being a regulatory step for dTMP cycle. This finding suggests that PfSHMT can be a potential target for antimalarial chemotherapy.

Catalytic and ligand-binding characteristics of Plasmodium falciparum serine hydroxymethyltransferase

The plant-like, bifunctional dihydrofolate reductase-thymidylate synthase (DHFR-TS) from malaria parasites has been a good target for drug development. Dihydrofolate reductase (DHFR) is inhibited by clinically established antimalarials, pyrimethamine and cycloguanil. Thymidylate synthase (TS) is the target of potent experimental antimalarials such as 5-fluoroorotate and 1843U89. Another enzyme in folate recycling, serine hydroxymethyltransferase (SHMT), produces 5,10-methylenetetrahydrofolate which, in many cells, is required for the de novo, biosynthesis of thymidine and methionine. Thus, the biochemical characterization of malarial SHMT was of interest. The principle, active Plasmodium falciparum SHMT (PfSHMT) was expressed in E. coli and purified using an N-terminal histidine tag. Unlike the plant enzyme, but like the host enzyme, PfSHMT requires the cofactor pyridoxal 5'-phosphate for enzymatic activity. The substrate specificities for serine, tetrahydrofolate, and pyridoxal 5'-phosphate were comparable to those for SHMT from other organisms. Antifolates developed for DHFR and TS inhibited SHMT in the mid-micromolar range, offering insights into the binding preferences of SHMT but clearly leaving room for improved new inhibitors. As previously seen with P. falciparum DHFR-TS, PfSHMT bound its cognate mRNA but not control RNA for actin. RNA binding was not reversed with enzyme substrates. Unlike DHFR-TS, the SHMT RNA-protein interaction was not tight enough to inhibit translation. Another gene PF14_0534, previously proposed to code for an alternate mitochondrial SHMT, was also expressed in E. coli but found to be inactive. This protein, nor DHFR-TS, enhanced the catalytic activity of PfSHMT. The present results set the stage for developing specific, potent inhibitors of SHMT from P. falciparum.

Isolation of second-generation schizonts of avian coccidia and their use in biochemical investigations

A technique is described for the isolation of a pure suspension of second-generation schizonts ofEimeria necatrixandE. tenella. Most of the schizonts appeared viable as judged by their ability to exclude Trypan blue and the use of this criterion is examined. They were able to metabolize radio-isotope labelled glucose and accumulate thiamine against a concentration gradient. A formula allowing detailed consideration of the latter process is introduced. Crude extracts of these schizonts exhibited serine hydroxymethyl transferase activity which is significantly different from that of the host tissue in terms of optimal pH (8·3 for the parasite and 7·8 for the avian intestinal cells) and Michaelis constant (approximately 2·6 × 10−3mand 0·6 × 10−3mrespectively). Differences in response to potential inhibitors are also shown.

A three-dimensional structure of Plasmodium falciparum serine hydroxymethyltransferase in complex with glycine and 5-formyl-tetrahydrofolate. Homology modeling and molecular dynamics

Cytosolic Plasmodium falciparum serine hydroxymethyltransferase (pfSHMT) is a potential target for antimalarial chemotherapy. Contrasting with the other enzymes involved in the parasite folate cycle, little information is available about this enzyme, and its crystallographic structure is unknown yet. In this paper, we propose a theoretical low-resolution 3D model for pfSHMT in complex with glycine and 5-formyl tetrahydrofolate (5-FTHF) based on homology modeling by multiple alignment followed by intensive optimization, validation and dynamics simulations in water. Comparison between the active sites of our model and that of crystallographic Human SHMT (hSHMT) revealed key differences that could be useful for the design of new selective inhibitors of pfSHMT.

Gene organization of a Plasmodium falciparum serine hydroxymethyltransferase and its functional expression in Escherichia coli

The global emergence of drug-resistant malarial parasites necessitates identification and characterization of novel drug targets. Three reactions are involved in methylenetetrahydrofolate recycling: Thymidylate synthase (TS), dihydrofolate reductase (DHFR), and serine hydroxymethyltransferase (SHMT). Malarial bifunctional DHFR-TS is a well-studied, important target of established drugs such as pyrimethamine and cycloguanil. In sharp contrast, malarial SHMT remains largely uncharacterized. In the present study, a Plasmodium falciparum SHMT coding region was characterized. It had 1603 bp including two introns near the 5'-end of the gene: one 118 bp intron immediately after the start methionine and a 159 bp intron after an additional 34 amino acids. The three exons together coded for a 442 amino acid protein with 38-47% identity to SHMT sequences from other species. Expression of malarial SHMT coding sequence (minus the introns) into glyA mutants of Escherichia coli relieved glycine auxotrophy and permitted direct assay of SHMT catalytic activity in bacterial cell lysates. This is the first SHMT cloned and expressed from a protozoan parasite. The molecular tools developed in this study will be useful for developing potential antimalarials directed at SHMT.

Purification and some properties of serine hydroxymethyltransferase from Trypanosoma cruzi

A single form of serine hydroxymethyltransferase (SHMT) was detected in epimastigotes of Trypanosoma cruzi, in contrast to the three isoforms of the enzyme characterized from another trypanosomatid, Crithidia fasciculata [Capelluto D.G.S., Hellman U., Cazzulo J.J. & Cannata J.J.B. (1999) Mol. Biochem. Parasitol. 98, 187-201]. The T. cruzi SHMT was found to be highly unstable in crude extracts. In the presence of the cysteine proteinase inhibitors N-alpha-p-tosyl-L-lysine chloromethyl ketone and Ltrans-3-carboxyoxiran-2-carbonyl-L-leucylagmatine, however, the enzyme could be purified to homogeneity. Digitonin treatment of intact cells suggested that the enzyme is cytosolic. T. cruzi SHMT presents a monomeric structure shown by the apparent molecular masses of 69 kDa (native) and 55 kDa (subunit) determined by Sephadex G-200 gel filtration and SDS/PAGE, respectively. This is in contrast to the tetrameric SHMTs described in C. fasciculata and other eukaryotes. The enzyme was pyridoxal phosphate-dependent after L-cysteine and hydroxylamine treatments and it was strongly inhibited by the substrate analog folate, which was competitive towards tetrahydrofolate and noncompetitive towards L-serine. Partial sequencing of tryptic internal peptides of the enzyme indicate considerable similarity with other SHMTs, particularly from those of plant origin.

Serine hydroxymethyltransferase from pyrimethamine-sensitive and -resistant strains of Plasmodium chabaudi

Serine hydroxymethyltransferase (EC 2.1.2.1) was partially purified from a pyrimethamine sensitive strain of Plasmodium chabaudi. Km values of 2.91 and 1.08 mM were determined for tetrahydrofolate and serine, respectively. The effects of pH, of temperature and of some potential inhibitors were determined. The enzyme was also partially purified from a pyrimethamine-resistant strain of P. chabaudi and subjected to the same regime. No differences between the enzymes from the two sources could be detected. It would appear that the changes in properties in the enzymes dihydrofolate reductase and thymidylate synthetase associated with the development of drug resistance in P. chabaudi were not reflected in any obvious alterations in serine hydroxymethyltransferase.

Pyridoxine kinase in Plasmodium lophurae and duckling erythrocytes

Pyridoxine kinase enzyme activity was greatly increased in duckling erythrocytes infected with Plasmodium lophurae. Pyridoxine kinase activity in parasites freed from erythrocytes was much greater than that of uninfected erythrocytes. The apparent Km for pyridoxine of the parasite enzyme was 6.6 times 10(-5) M whereas the host red cell enzyme Km was 1.9 times 10(-6) M. Deoxypyridoxine inhibited host and parasite pyridoxine kinase activity with an apparent Ki of 1.5 times 10(-6) and 8.6 times 10(-6) M, respectively. These results suggest that the vitamin B6 metabolism of the malaria parasites is distinct and separate from that of the host erythrocytes.