The malate-aspartate shuttle is important for de novo serine biosynthesis

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Carbocyclic substrates for de novo purine biosynthesis

The carbocyclic analogues of phosphoribosylamine, glycinamide ribonucleotide, and formylglycinamide ribonucleotide have been prepared as the racemates. Carbocyclic phosphoribosylamine was utilized as a substrate by the monofunctional glycinamide ribonucleotide synthetase from Escherichia coli as well as the glycinamide ribonucleotide synthetase activity of the eucaryotic trifunctional enzyme of de novo purine biosynthesis. Furthermore, carbocyclic glycinamide ribonucleotide was processed in the reverse reaction catalyzed by these enzymes. In addition, carbocyclic formylglycinamide ribonucleotide was converted, by E. coli formylglycinamide ribonucleotide synthetase, to carbocyclic formylglycinamidine ribonucleotide, which was accepted as a substrate by the aminoimidazole ribonucleotide synthetase activity of the trifunctional enzyme. This study has afforded carbocyclic substrate analogues, in particular for the chemically labile phosphoribosyl amine, for the initial steps of de novo purine biosynthesis.     

Cloning, expression, purification, and characterization of zebrafish cytosolic serine hydroxymethyltransferase

A cDNA which encodes for zebrafish serine hydroxymethyltransferase (SHMT) has been cloned into a pET43.1a vector as a NdeI-EcoRI insert and transformed into HMS174(DE3) cells. After induction with isopropyl thiogalactoside, the enzyme was purified with a three-step purification protocol and about 15 mg of pure enzyme was obtained per liter of culture. Spectral and structural characteristics of the recombinant zebrafish SHMT are similar to the rabbit and human cytosolic SHMT. Kinetic constants for the natural substrates l-serine and tetrahydrofolate are also comparable to the values obtained previously for the rabbit and human cytosolic enzyme.     

Pyrimidine biosynthesis de novo in M. leprae

Mycobacterium leprae can synthesise pyrimidines de novo. Although pyrimidine synthesis could not be detected in intact bacteria, extracts contained all four enzymes unique to the de novo pathway which are detectable in mycobacteria by the methods used. Inhibition of aspartate transcarbamylase by UTP and ATP suggested that lack of pyrimidine synthetic activity in whole M. leprae could be a result of strong feedback inhibition.     

Serine hydroxymethyltransferase anchors de novo thymidylate synthesis pathway to nuclear lamina for DNA synthesis

The de novo thymidylate biosynthetic pathway in mammalian cells translocates to the nucleus for DNA replication and repair and consists of the enzymes serine hydroxymethyltransferase 1 and 2α (SHMT1 and SHMT2α), thymidylate synthase, and dihydrofolate reductase. In this study, we demonstrate that this pathway forms a multienzyme complex that is associated with the nuclear lamina. SHMT1 or SHMT2α is required for co-localization of dihydrofolate reductase, SHMT, and thymidylate synthase to the nuclear lamina, indicating that SHMT serves as scaffold protein that is essential for complex formation. The metabolic complex is enriched at sites of DNA replication initiation and associated with proliferating cell nuclear antigen and other components of the DNA replication machinery. These data provide a mechanism for previous studies demonstrating that SHMT expression is rate-limiting for de novo thymidylate synthesis and indicate that de novo thymidylate biosynthesis occurs at replication forks.     

Cloning and characterization of Plasmodium vivax serine hydroxymethyltransferase

Serine hydroxymethyltransferase (SHMT), which catalyzes the reversible reaction of serine and tetrahydrofolate to glycine and methylenetetrahydrofolate, is one of the three enzymes in dTMP synthesis pathway that is highly active during cell division and has been proposed as a potential chemotherapeutic target in infectious diseases and cancer. This is the first study to describe nucleotide and amino acid sequences of SHMT from the malaria parasite Plasmodium vivax. Sequencing of 12 P. vivax isolates revealed limited polymorphisms in 3 noncoding regions. Its biological function is also reported.     

Purine biosynthesis de novo in rat skeletal muscle.

Purine biosynthesis by the 'de novo' pathway was demonstrated in isolated rat extensor digitorum longus muscle with [1-14C]glycine, [3-14C]serine and sodium [14C]formate as nucleotide precursors. Evidence is presented which suggests that the source of glycine and serine for purine biosynthesis is extracellular rather than intracellular. The relative incorporation rates of the three precursors were formate greater than glycine greater than serine. Over 85% of the label from formate and glycine was recovered in the adenine nucleotides, principally ATP. Azaserine markedly inhibited purine biosynthesis from both formate and glycine. Cycloserine inhibited synthesis from serine, but not from formate. Adenine, hypoxanthine and adenosine markedly inhibited purine synthesis from sodium [14C]formate.     

Enzymes of de novo pyrimidine biosynthesis in Babesia rodhaini

The pathway of de novo pyrimidine biosynthesis in the rodent parasitic protozoa Babesia rodhaini has been investigated. Specific activities of five of the six enzymes of the pathway were determined: aspartate transcarbamylase (ATCase: E.C. 2.1.3.2); dihydroorotase (DHOase: E.C. 3.5.2.3); dihydroorotate dehydrogenase (DHO-DHase: E.C. 1.3.3.1); orotate phosphoribosyltransferase (OPRTase: E.C. 2.4.2.10); and orotidine-5'-phosphate decarboxylase (ODCase: E.C. 4.1.1.23). Michaelis constants for ATCase, DHO-DHase, OPRTase, and ODCase were determined in whole homogenates. Several substrate analogs were also investigated as inhibitors and inhibitor constants determined. N-(phosphonacetyl)-L-aspartate was shown to be an inhibitor of the ATCase with an apparent Ki of 7 microM. Dihydro-5-azaorotate inhibited the DHO-DHase (Ki, 16 microM) and 5-azaorotate (Ki, 21 microM) was an inhibitor of the OPRTase. The UMP analog, 6-aza-UMP (Ki, 0.3 microM) was a potent inhibitor of ODCase, while lower levels of inhibition were found with the product, UMP (Ki, 120 microM) and the purine nucleotide, XMP (Ki, 95 microM). Additionally, menoctone, a ubiquinone analog, was shown to inhibit DHO-DHase.     

Evidence for de novo sphingolipid biosynthesis in Toxoplasma gondii

Glycolipids are important components of cellular membranes involved in various biological functions. In this report, we describe the identification of the de novo synthesis of glycosphingolipids by Toxoplasma gondii tachyzoites. Parasite-specific glycolipids were identified by metabolic labelling of parasites with tritiated serine and galactose. These glycolipids were characterised as sphingolipids based on the labelling protocol and their insensitivity towards alkaline treatment. Synthesis of parasite glycosphingolipids were inhibited by threo-phenyl-2-palmitoylamino-3-morpholino-1-propanol and L-cycloserine, two well-established inhibitors of de novo sphingolipid biosynthesis. The identified glycolipids were insensitive towards treatment with endoglycoceramidase II indicating that they might belong to globo-type glycosphingolipids. Taken together, we provide evidence for the first time that T. gondii is capable of synthesising glycosphingolipids de novo.     

Cloning and structural analysis of Mycobacterium leprae serine hydroxymethyltransferase

Serine hydroxymethyltransferase (SHMT) plays a key role in cell physiology as it participates in the different interconversion pathway of folate coenzymes, provides almost exclusively folate one carbon fragments for the biosynthesis of a variety of end products. For the first time, Mycobacterium leprae glyA gene, encodes the enzyme serine hydroxymethyltransferase, has been cloned in Escherichia coli, over-expressed and purified the protein product (mlSHMT) for folding and stability studies under various denaturating conditions. The recombinant mlSHMT exists as homo-dimer of molecular mass about 90 kDa under physiological conditions . The studies on catalytic properties of mlSHMT show that the enzyme catalyzes the H(4)-folate dependent retro-aldol cleavage of L-serine, however, D-alanine dependent transaminase activity was absent in the enzyme. Further analysis of the enzyme kinetics for hydroxymethyltransferase reaction for mlSHMT demonstrates a comparable K(m) value for L-serine to SHMTs from other sources but significantly lower catalytic efficiency (k(cat)/K(m)). The mlSHMT is resistant to alkaline denaturation and exist as apo-dimer up to pH 10.5. Urea and guanidinium chloride induces dissociation of mlSHMT dimer into monomer at low denaturant concentrations, and leads to loss of enzymatic activity.     

Mutations in tetrapyrrole biosynthesis pathway uncouple nuclear WUSCHEL expression from de novo shoot development in Arabidopsis

Plant Cell, Tissue and Organ Culture (PCTOC) - The effect of several parameters on trans-resveratrol extracellular production in Vitis vinifera cv Monastrell suspension cultured cells elicited with...     

Evidence in support of biosynthesis de novo of free cytokinins

The four cytokinins in the tRNA from Lupinus luteus L. seeds have been purified and identified as ribosyl-cis-zeatin, 2-methylthio-ribosylzeatin, ( ²-isopentenyl)adenosine and 2-methylthio-N⁶-( ²-isopentenyl)adenosine. These structures have been assigned on the basis of their chromatographic mobilities and the spectroscopic data of the parent materials and their silylated derivatives. The tRNA isolated from Populus x robusta Schneid. leaves contained four cytokinins with identical chromatographic properties to those identified in Lupinus luteus seed tRNA. No evidence was obtained for the presence, in tRNA, of the naturally occurring free cytokinins identified in these plant species, dihydrozeatin (Lupinus luteus) and N⁶-(2-hydroxybenzyl)adenosine (Populus x robusta). This is evidence in support of the possibility that free cytokinins can arise by biosynthesis de novo and are not exclusively by-products released intact during tRNA turnover.     

The de novo biosynthesis of platelet-activating factor in rat brain

Platelet-Activating Factor (PAF, 1-alkyl-2-acetyl-sn-glycero-3-phosphocholine) is present in nervous tissue and its function is still unknown. We have demonstrated that rat brain is able to synthesize PAF from 1-alkyl-2-acetyl-sn-glycerol and CDP-choline by a "DTT-insensitive" phosphocholine transferase. This represents the last step of the de novo pathway which apparently is the only one existing in the brain for PAF biosynthesis. The enzyme has a microsomal localization, requires Mg++ and is inhibited by Ca++ as reported for phosphocholine transferase utilizing long-chain diradylglycerols as substrates. However, other properties of PAF-synthesizing enzyme (sensitivity to DTT and dependence on pH) are different from those of phosphocholine transferase responsible for the synthesis of diacyl and long-chain alkylacyl glycerophosphocholines. These observations indicate that a specific enzyme for PAF biosynthesis might exist in rat brain.     

Regulation of de novo purine biosynthesis in Chinese hamster cells

Regulation of de novo purine biosynthesis was examined in two Chinese hamster cell lines, CHO and V79. De novo purine biosynthesis is inhibited at low concentrations of adenine. The mechanism of inhibition was studied using the RNA and protein synthesis inhibitors actinomycin D, cycloheximide, and azacytidine. Although all three inhibitors rapidly inhibited de novo purine biosynthesis in vivo, neither adenine nor the RNA and protein synthesis inhibitors could be found to have an effect in vitro on either phosphoribosylpyrophosphate (PRPP) synthetase or amido phosphoribosyltransferase, the first enzymes of the de novo pathway. However, in the presence of actinomycin D, cycloheximide, and azacytidine, there was a 50% or greater reduction in PRPP concentrations. This reduction in PRPP levels is correlated with a 2-fold increase in purine nucleotides in the acid-soluble pool. It is proposed that in the presence of the metabolic inhibitors there is an increase in nucleotide pools due to degradation of RNA, with a resulting feedback inhibition on de novo purine biosynthesis. In contrast to a previous report (Martin, D. W., Jr., and Owen, N. T. (1972) J. Biol. Chem. 247, 5477-5485), we could find no evidence for a repressor type mechanism in these cells.     

Metabolic control analysis of de novo sunflower fatty acid biosynthesis

We have obtained a simulation of the final steps of de novo fatty acid biosynthesis in sunflower control line RHA-274. For this simulation, we have used data from the evolution of fatty acids during seed formation and from the biochemical characterization of beta-keto-acyl-ACP synthetase II (FASII), stearoyl-ACP desaturase (SAD) and acyl-ACP thioesterase activities and the program GEPASI (based on the metabolic control-analysis theory). When physiological data from high- and medium-stearic acid mutants seed development were used with this model the predicted changes in SAD and TE were very similar to those actually found in the biochemical characterization of these mutants. However, the model had to be modified when results from high-palmitic mutants, accumulating unusual fatty acids like palmitoleic, asclepic and palmitolinoleic acids, were used. The emerging model, that fits all of our results, predicts the existence of a dynamic channelling between the FASII complex and SAD, that channelling being responsible for the alternative pathway starting with the desaturation of palmitic acid by the stearoyl-ACP desaturase. This channelling is consistent with our previous results. For instance, the determination of SAD activity on sunflower seed crude extracts only rendered oleic acid when the stearic acid used as a substrate was obtained from a KASII assay, but not when the stearic acid came from in vitro synthesis using acyl-ACP synthetase from Escherichia coli. This theoretical approximation will be very useful in predicting the evolution of the system when introducing new or modified activities; similar approximations in other oil-seed crops could be of great interest.