Glycine N-methyltransferase expression in the hippocampus and its role in neurogenesis and cognitive performance.

The hippocampus is a brain area characterized by its high plasticity, observed at all levels of organization: molecular, synaptic, and cellular, the latter referring to the capacity of neural precursors within the hippocampus to give rise to new neurons throughout life. Recent findings suggest that promoter methylation is a plastic process subjected to regulation, and this plasticity seems to be particularly important for hippocampal neurogenesis. We have detected the enzyme GNMT (a liver metabolic enzyme) in the hippocampus. GNMT regulates intracellular levels of SAMe, which is a universal methyl donor implied in almost all methylation reactions and, thus, of prime importance for DNA methylation. In addition, we show that deficiency of this enzyme in mice (Gnmt-/-) results in high SAMe levels within the hippocampus, reduced neurogenic capacity, and spatial learning and memory impairment. In vitro, SAMe inhibited neural precursor cell division in a concentration-dependent manner, but only when proliferation signals were triggered by bFGF. Indeed, SAMe inhibited the bFGF-stimulated MAP kinase signaling cascade, resulting in decreased cyclin E expression. These results suggest that alterations in the concentration of SAMe impair neurogenesis and contribute to cognitive decline.

Human cystathionine ß-synthase (CBS) contains two classes of binding sites for S-adenosylmethionine (SAM): complex regulation of CBS activity and stability by SAM.

CBS (cystathionine ß-synthase) is a multidomain tetrameric enzyme essential in the regulation of homocysteine metabolism, whose activity is enhanced by the allosteric regulator SAM (S-adenosylmethionine). Missense mutations in CBS are the major cause of inherited HCU (homocystinuria). In the present study we apply a novel approach based on a combination of calorimetric methods, functional assays and kinetic modelling to provide structural and energetic insight into the effects of SAM on the stability and activity of WT (wild-type) CBS and seven HCU-causing mutants. We found two sets of SAM-binding sites in the C-terminal regulatory domain with different structural and energetic features: a high affinity set of two sites, probably involved in kinetic stabilization of the regulatory domain, and a low affinity set of four sites, which are involved in the enzyme activation. We show that the regulatory domain displays a low kinetic stability in WT CBS, which is further decreased in many HCU-causing mutants. We propose that the SAM-induced stabilization may play a key role in modulating steady-state levels of WT and mutant CBS in vivo. Our strategy may be valuable for understanding ligand effects on proteins with a complex architecture and their role in human genetic diseases and for the development of novel pharmacological strategies.

Folates and S-adenosylmethionine for major depressive disorder.

Interest in nonpharmaceutical supplements for treating major depressive disorder (MDD) has increased significantly, both among patients and among clinicians during the past decades. Despite the large array of antidepressants (ADs) available, many patients continue to experience relatively modest response and remission rates, in addition to a burden of side effects that can hinder treatment compliance and acceptability. In this article, we review the literature on folates and S-adenosylmethionine (SAMe), 2 natural compounds linked in the 1-carbon cycle metabolic pathway, for which substantial evidence supports their involvement in mood disorders. Background information, efficacy data, proposed mechanisms of action, and side effects are reviewed. Based on existing data, supplementation with SAMe, as well as with various formulations of folates, appears to be efficacious and well tolerated in reducing depressive symptoms. Compared with other forms of folates, 5-methyltetrahydrofolate (L-methylfolate or 5-MTHF) may represent a preferable treatment option for MDD given its greater bioavailability in patients with a genetic polymorphism, and the lower risk of specific side effects associated with folic acid. Although further randomized controlled trials in this area appear warranted, SAMe and L-methylfolate may represent a useful addition to the AD armamentarium.

Nutritional interventions to prevent and treat osteoarthritis. Part II: focus on micronutrients and supportive nutraceuticals.

Osteoarthritis (OA) is the most common cause of musculoskeletal disability in the elderly, and it places an enormous economic burden on society, which will remain a major health care challenge with an aging population. Management of OA is primarily focused on palliative relief using agents such as nonsteroidal anti-inflammatory drugs (NSAID) and analgesics. However, such an approach is limited by a narrow therapeutic focus that fails to address the progressive and multimodal nature of OA. Given the favorable safety profile of most nutritional interventions, identifying disease-modifying pharmaconutrients capable of improving symptoms and also preventing, slowing, or even reversing the degenerative process in OA should remain an important paradigm in translational and clinical research. The goals of pharmaconutrition for metabolic optimization are to drive biochemical reactions in a desired direction and to meet health condition-specific metabolic demands. Applying advances in nutritional science to musculoskeletal medicine remains challenging, given the fluid and dynamic nature of the field, along with a rapidly developing regulatory climate over manufacturing and commerce requirements. The purpose of this article is to review the available literature on effectiveness and potential mechanism for OA of micronutrient vitamins; minerals; glycosaminoglycans; avocado-soybean unsaponifiable fractions; methylsulfonylmethane; s-adenosylmethionine; undenatured and hydrolyzed collagen preparations; phytoflavonoid compounds found in fruits, vegetables, spices, teas, and nuts; and other nutrients on the horizon. There also is a discussion on the concept of rational polysupplementation via the strategic integration of multiple nutraceuticals with potential complementary mechanisms for improving outcomes in OA. As applied nutritional science evolves, it will be important to stay on the forefront of proteomics, metabolomics, epigenetics, and nutrigenomics, because they hold enormous potential for developing novel therapeutic and prognostic breakthroughs in many areas of medicine, including OA.

S-adenosyl methionine regulates ubiquitin-conjugating enzyme 9 protein expression and sumoylation in murine liver and human cancers.

UNLABELLED: Ubiquitin-conjugating enzyme 9 (Ubc9) is required for sumoylation and is overexpressed in several malignancies, but its expression in hepatocellular carcinoma (HCC) is unknown. Hepatic S-adenosyl methionine (SAMe) levels decrease in methionine adenosyltransferase 1A (Mat1a) knockout (KO) mice, which develop HCC, and in ethanol-fed mice. We examined the regulation of Ubc9 by SAMe in murine liver and human HCC, breast, and colon carcinoma cell lines and specimens. Real-time polymerase chain reaction and western blotting measured gene and protein expression, respectively. Immunoprecipitation followed by western blotting examined protein-protein interactions. Ubc9 expression increased in HCC and when hepatic SAMe levels decreased. SAMe treatment in Mat1a KO mice reduced Ubc9 protein, but not messenger RNA (mRNA) levels, and lowered sumoylation. Similarly, treatment of liver cancer cell lines HepG2 and Huh7, colon cancer cell line RKO, and breast cancer cell line MCF-7 with SAMe or its metabolite 5'-methylthioadenosine (MTA) reduced only Ubc9 protein level. Ubc9 posttranslational regulation is unknown. Ubc9 sequence predicted a possible phosphorylation site by cell division cycle 2 (Cdc2), which directly phosphorylated recombinant Ubc9. Mat1a KO mice had higher phosphorylated (phospho)-Ubc9 levels, which normalized after SAMe treatment. SAMe and MTA treatment lowered Cdc2 mRNA and protein levels, as well as phospho-Ubc9 and protein sumoylation in liver, colon, and breast cancer cells. Serine 71 of Ubc9 was required for phosphorylation, interaction with Cdc2, and protein stability. Cdc2, Ubc9, and phospho-Ubc9 levels increased in human liver, breast, and colon cancers. CONCLUSION: Cdc2 expression is increased and Ubc9 is hyperphosphorylated in several cancers, and this represents a novel mechanism to maintain high Ubc9 protein expression that can be inhibited by SAMe and MTA.

Folding of the SAM-I riboswitch: a tale with a twist.

Riboswitches are ligand-dependent RNA genetic regulators that control gene expression by altering their structures. The elucidation of riboswitch conformational changes before and after ligand recognition is crucial to understand how riboswitches can achieve high ligand binding affinity and discrimination against cellular analogs. The detailed characterization of riboswitch folding pathways suggest that they may use their intrinsic conformational dynamics to sample a large array of structures, some of which being nearly identical to ligand-bound molecules. Some of these structural conformers can be "captured" upon ligand binding, which is crucial for the outcome of gene regulation. Recent studies about the SAM-I riboswitch have revealed unexpected and previously unknown RNA folding mechanisms. For instance, the observed helical twist of the P1 stem upon ligand binding to the SAM-I aptamer adds a new element in the repertoire of RNA strategies for recognition of small metabolites. From an RNA folding perspective, these findings also strongly indicate that the SAM-I riboswitch could achieve ligand recognition by using an optimized combination of conformational capture and induced-fit approaches, a feature that may be shared by other RNA regulatory sequences.

S-adenosyl-L-methionine induces compaction of nascent peptide chain inside the ribosomal exit tunnel upon translation arrest in the Arabidopsis CGS1 gene.

Expression of the Arabidopsis CGS1 gene, encoding the first committed enzyme of methionine biosynthesis, is feedback-regulated in response to S-adenosyl-L-methionine (AdoMet) at the mRNA level. This regulation is first preceded by temporal arrest of CGS1 translation elongation at the Ser-94 codon. AdoMet is specifically required for this translation arrest, although the mechanism by which AdoMet acts with the CGS1 nascent peptide remained elusive. We report here that the nascent peptide of CGS1 is induced to form a compact conformation within the exit tunnel of the arrested ribosome in an AdoMet-dependent manner. Cysteine residues introduced into CGS1 nascent peptide showed reduced ability to react with polyethyleneglycol maleimide in the presence of AdoMet, consistent with a shift into the ribosomal exit tunnel. Methylation protection and UV cross-link assays of 28 S rRNA revealed that induced compaction of nascent peptide is associated with specific changes in methylation protection and UV cross-link patterns in the exit tunnel wall. A 14-residue stretch of amino acid sequence, termed the MTO1 region, has been shown to act in cis for CGS1 translation arrest and mRNA degradation. This regulation is lost in the presence of mto1 mutations, which cause single amino acid alterations within MTO1. In this study, both the induced peptide compaction and exit tunnel change were found to be disrupted by mto1 mutations. These results suggest that the MTO1 region participates in the AdoMet-induced arrest of CGS1 translation by mediating changes of the nascent peptide and the exit tunnel wall.

Radical SAM activation of the B12-independent glycerol dehydratase results in formation of 5'-deoxy-5'-(methylthio)adenosine and not 5'-deoxyadenosine.

Activation of glycyl radical enzymes (GREs) by S-adenosylmethonine (AdoMet or SAM)-dependent enzymes has long been shown to proceed via the reductive cleavage of SAM. The AdoMet-dependent (or radical SAM) enzymes catalyze this reaction by using a [4Fe-4S] cluster to reductively cleave AdoMet to form a transient 5'-deoxyadenosyl radical and methionine. This radical is then transferred to the GRE, and methionine and 5'-deoxyadenosine are also formed. In contrast to this paradigm, we demonstrate that generation of a glycyl radical on the B(12)-independent glycerol dehydratase by the glycerol dehydratase activating enzyme results in formation of 5'-deoxy-5'-(methylthio)adenosine and not 5'-deoxyadenosine. This demonstrates for the first time that radical SAM activases are also capable of an alternative cleavage pathway for SAM.

Oxidative stress and the pathogenesis of cholestasis.

Cholestasis is a reduction in bile flow that occurs from a variety of causes in humans. This produces hepatocellular injury and fibrosis. Considering that there are limited therapies for this disease, there has been interest in understanding the mechanism by which cholestasis produces injury. Studies have demonstrated that oxidative stress occurs in livers of humans with cholestasis. In vitro studies have demonstrated that bile acids kill hepatocytes by a mechanism that depends upon reactive oxygen species (ROS). Further studies, however, have demonstrated that this mechanism is of limited importance in vivo. Cholestasis also initiates an inflammatory response resulting in accumulation of neutrophils in the liver. Inhibition of neutrophil function reduces oxidative stress and liver injury suggesting that neutrophils are an important source of damaging ROS in vivo. Furthermore, inhibition of ROS during cholestasis reduces fibrosis. Collectively, these studies suggest that ROS are important for pathologic changes that occur during cholestasis.

Changes in the expression of methionine adenosyltransferase genes and S-adenosylmethionine homeostasis during hepatic stellate cell activation.

UNLABELLED: Hepatic stellate cell (HSC) activation is an essential event during liver fibrogenesis. Methionine adenosyltransferase (MAT) catalyzes biosynthesis of S-adenosylmethionine (SAMe), the principle methyl donor. SAMe metabolism generates two methylation inhibitors, methylthioadenosine (MTA) and S-adenosylhomocysteine (SAH). Liver cell proliferation is associated with induction of two nonliver-specific MATs: MAT2A, which encodes the catalytic subunit alpha2, and MAT2beta, which encodes a regulatory subunit beta that modulates the activity of the MAT2A-encoded isoenzyme MATII. We reported that MAT2A and MAT2beta genes are required for liver cancer cell growth that is induced by the profibrogenic factor leptin. Also, MAT2beta regulates leptin signaling. The strong association of MAT genes with proliferation and leptin signaling in liver cells led us to examine the role of these genes during HSC activation. MAT2A and MAT2beta are induced in culture-activated primary rat HSCs and HSCs from 10-day bile duct ligated (BDL) rat livers. HSC activation led to a decline in intracellular SAMe and MTA levels, a drop in the SAMe/SAH ratio, and global DNA hypomethylation. The decrease in SAMe levels was associated with lower MATII activity during activation. MAT2A silencing in primary HSCs and MAT2A or MAT2beta silencing in the human stellate cell line LX-2 resulted in decreased collagen and alpha-smooth muscle actin (alpha-SMA) expression and cell growth and increased apoptosis. MAT2A knockdown decreased intracellular SAMe levels in LX-2 cells. Activation of extracellular signal-regulated kinase and phosphatidylinositol-3-kinase signaling in LX-2 cells required the expression of MAT2beta but not that of MAT2A. CONCLUSION: MAT2A and MAT2beta genes are induced during HSC activation and are essential for this process. The SAMe level falls, resulting in global DNA hypomethylation.

Excessive S-adenosyl-L-methionine-dependent methylation increases levels of methanol, formaldehyde and formic acid in rat brain striatal homogenates: possible role in S-adenosyl-L-methionine-induced Parkinson's disease-like disorders.

AIMS: Excessive methylation may be a precipitating factor for Parkinson's disease (PD) since S-adenosylmethionine (SAM), the endogenous methyl donor, induces PD-like changes when injected into the rat brain. The hydrolysis of the methyl ester bond of the methylated proteins produces methanol. Since methanol is oxidized into formaldehyde, and formaldehyde into formic acid in the body, we investigated the effects of SAM on the production of methanol, formaldehyde and formic acid in rat brain striatal homogenates and the toxicity of these products in PC12 cells. MAIN METHODS: Radio-enzymatic and colorimetric assays, cell viability, Western blot. KEY FINDINGS: SAM increased the formation of methanol, formaldehyde and formic acid in a concentration and time-dependent manner. Concentrations of [3H-methyl]-SAM at 0.17, 0.33, 0.67 and 1.34 nM produced 3.8, 8.0, 18.3 and 34.4 fmol/mg protein/h of [3H] methanol in rat striatal homogenates, respectively. SAM also significantly generated formaldehyde and formic acid in striatal homogenates. Formaldehyde was the most toxic metabolite to differentiated PC12 pheochromocytoma cells in cell culture studies, indicating that formaldehyde formed endogenously may contribute to neuronal damage in excessive methylation conditions. Subtoxic concentration of formaldehyde decreased the expression of tyrosine hydroxylase, the limiting factor in dopamine synthesis. Formaldehyde was more toxic to catecholaminergic PC12 cells than C6 glioma cells, indicating that neurons are more vulnerable to formaldehyde than glia cells. SIGNIFICANCE: We suggest that excessive carboxylmethylation of proteins might be involved in the SAM-induced PD-like changes and in the aging process via the toxic effects of formaldehyde.

Over-expression of an S-adenosylmethionine-dependent methyltransferase leads to an extended lifespan of Podospora anserina without impairments in vital functions.

PaMTH1, a putative methyltransferase previously described to increase in abundance in total protein extracts during aging of Podospora anserina is demonstrated to accumulate in the mitochondrial cell fraction of senescent cultures. The protein is localized in the mitochondrial matrix and displays a methyltransferase activity utilizing flavonoids as substrates. Constitutive over-expression of PaMth1 in P. anserina results in a reduced carbonylation of proteins and an extended lifespan without impairing vital functions suggesting a protecting role of PaMTH1 against oxidative stress.

S-adenosylmethionine mediates glutathione efficacy by increasing glutathione S-transferase activity: implications for S-adenosyl methionine as a neuroprotective dietary supplement.

When maintained on a folate-deficient, iron-rich diet, transgenic mice lacking in apolipoprotein E (ApoE-/- mice) demonstrate impaired activity of glutathione S-transferase (GST), resulting in increased oxidative species within brain tissue despite abnormally high levels of glutathione. These mice also exhibit reduced levels of S-adenosyl methionine (SAM) and increased levels of its hydrolysis product S-adenosyl homocysteine, which inhibits SAM usage. Supplementation of the above diet with SAM restored GST activity and eliminated reactive oxygen species at the expense of stockpiled glutathione, suggesting that one or more SAM-dependent reactions were required to maintain GST activity. We examined herein the impact of SAM on GST activity using a cell-free assay. SAM stimulated GST activity in a dose-response manner when added to homogenates derived from the above ApoE-/- mice. SAM also increased activity of purified rat liver GST and recombinant GST. Filtering of SAM through a 4 kDa cutoff and systematic withholding of reaction components eliminated the possibility of any additional contaminating enzyme. These findings confirm that SAM can exert a direct effect on GST activity. Since Alzheimer's disease is accompanied by reduced GST activity, diminished SAM and increased SAH, these findings underscore the critical role of SAM in maintenance of neuronal health.

S-Adenosylmethionine in cell growth, apoptosis and liver cancer.

S-Adenosylmethionine (SAMe), the principal biological methyl donor, is synthesized from methionine and ATP in a reaction catalyzed by methionine adenosyltransferase (MAT). In mammals, two genes (MAT1A and MAT2A), encode for two homologous MAT catalytic subunits, while a third gene MAT2beta, encodes for the beta-subunit that regulates MAT2A-encoded isoenzyme. Normal liver expresses MAT1A, whereas extrahepatic tissues express MAT2A. MAT2A and MAT2 beta are induced in human hepatocellular carcinoma (HCC), which facilitate cancer cell growth. Patients with cirrhosis of various etiologies, including alcohol, have decreased hepatic MAT activity and SAMe biosynthesis. Consequences of hepatic SAMe deficiency as illustrated by the Mat1a knock-out mouse model include increased susceptibility to steatosis and oxidative liver injury, spontaneous development of steatohepatitis and HCC. Predisposition to HCC can be partly explained by the effect of SAMe on growth. Thus, SAMe inhibits the mitogenic effect of growth factors such as hepatocyte growth factor and, following partial hepatectomy, a fall in SAMe level is required for the liver to regenerate. During liver regeneration, the fall in hepatic SAMe is transient. If the fall were to persist, it would favor a proliferative phenotype and, ultimately, development of HCC. Not only does SAMe control liver growth, it also regulates apoptosis. Interestingly, SAMe is anti-apoptotic in normal hepatocytes but pro-apoptotic in liver cancer cells. In liver cancer cells but not in normal human hepatocytes, SAMe can selectively induce Bcl-x(S), an alternatively spliced isoform of Bcl-x(L) that promotes apoptosis. This should make SAMe an attractive agent for both chemoprevention and treatment of HCC.

A decrease in S-adenosyl-L-methionine potentiates arachidonic acid cytotoxicity in primary rat hepatocytes enriched in CYP2E1.

Previous studies show that treatment with a polyunsaturated fatty acid, arachidonic acid (AA), or high concentrations of cycloleucine, an inhibitor of methionine adenosyltransferase (MAT), which lowers levels of S-adenosyl-L-methionine (SAM), increased toxicity in hepatocytes from pyrazole-treated rats which expressed high levels of cytochrome P450 2E1 (CYP2E1). In this study, I used concentrations of cycloleucine or AA, which by themselves do not produce any toxicity, to evaluate whether a decrease in SAM sensitizes hepatocytes to AA toxicity, especially in hepatocytes enriched in CYP2E1. Levels of SAM were lower by 50% in hepatocytes from pyrazole- compared to saline-treated rats. Cycloleucine treatment caused a 50% decline in SAM levels with both hepatocyte preparations and SAM levels were lowest in the pyrazole-treated hepatocytes. The combination of cycloleucine plus AA produced some toxicity and apoptosis in hepatocytes from saline-treated rats but increased toxicity and apoptosis was found in the hepatocytes from pyrazole-treated rats. Cytotoxicity could be prevented by incubation with SAM, the antioxidant trolox, and the mitochondrial permeability transition inhibitor trifluoperazine. The enhanced cytotoxicity could also be protected by treating rats with chlormethiazole, a specific inhibitor of CYP2E1, thus validating the role of CYP2E1. Cycloleucine plus AA treatment elevated production of reactive oxygen species (ROS) and lipid peroxidation to greater extents with the hepatocytes from pyrazole-treated rats than that from the saline-treated rats. I hypothesize that increased production of ROS by hepatocytes enriched in CYP2E1 potentiates AA-induced lipid peroxidation and toxicity when hepatoprotective levels of SAM are lowered. Such interactions, e.g. induction of CYP2E1, decline in SAM and polyunsaturated fatty acid-induced lipid peroxidation, may contribute to alcohol-induced liver injury.

Role of S-adenosyl-L-methionine in liver health and injury.

S-adenosylmethionine (SAMe) has rapidly moved from being a methyl donor to a key metabolite that regulates hepatocyte growth, death, and differentiation. Biosynthesis of SAMe occurs in all mammalian cells as the first step in methionine catabolism in a reaction catalyzed by methionine adenosyltransferase (MAT). Decreased hepatic SAMe biosynthesis is a consequence of all forms of chronic liver injury. In an animal model of chronic liver SAMe deficiency, the liver is predisposed to further injury and develops spontaneous steatohepatitis and hepatocellular carcinoma. However, impaired SAMe metabolism, which occurs in patients with mutations of glycine N-methyltransferase (GNMT), can also lead to liver injury. This suggest that hepatic SAMe level needs to be maintained within a certain range, and deficiency or excess can both lead to abnormality. SAMe treatment in experimental animal models of liver injury shows hepatoprotective properties. Meta-analyses also show it is effective in patients with cholestatic liver diseases. Recent data show that exogenous SAMe can regulate hepatocyte growth and death, independent of its role as a methyl donor. This raises the question of its mechanism of action when used pharmacologically. Indeed, many of its actions can be recapitulated by methylthioadenosine (MTA), a by-product of SAMe that is not a methyl donor. A better understanding of why liver injury occurs when SAMe homeostasis is perturbed and mechanisms of action of pharmacologic doses of SAMe are essential in defining which patients will benefit from its use.

A pathogenic linked mutation in the catalytic core of human cystathionine beta-synthase disrupts allosteric regulation and allows kinetic characterization of a full-length dimer.

Cystathionine beta-synthase catalyzes the condensation of serine and homocysteine to yield cystathionine and is the single most common locus of mutations associated with homocystinuria. In this study, we have examined the kinetic consequences of a pair of linked patient mutations, P78R/K102N, that are housed in the catalytic core of the protein and compared it to the effects of the corresponding single mutations. The P78R mutation affords purification of a mixture of higher order oligomers, P78R-I, which resembles the mixed quaternary state associated with wild-type enzyme. However, unlike wild-type enzyme, P78R-I converts over time to P78R-II, which exists predominantly as a full-length dimer. The specific activities of the K102N, P78R-I, and P78R-II mutants in the absence of AdoMet are approximately 3-, 9-, and 3-fold lower than of wild-type enzyme and are stimulated 2.9-, 2.5-, and 1.4-fold respectively by AdoMet. However, when linked, the specific activity of the resulting double mutant is comparable to that of wild-type enzyme but it is unresponsive to AdoMet, revealing that interactions between the two sites modulate the phenotype of the enzyme. Steady-state kinetic analysis for the double mutant reveals a sigmoidal dependence on homocysteine that is not observed with wild-type enzyme, which is ascribed to the mutation at the K102 locus and indicates changes in subunit interactions. Hydrogen-deuterium mass spectrometric analysis reveals that, even in the absence of AdoMet, the double mutant is locked in an activated conformation that is observed for wild-type enzyme in the presence of AdoMet, providing a structural rationale for loss of this allosteric regulation. To our knowledge, this is the first example of mutations in the catalytic core of cystathionine beta-synthase that result in failure of AdoMet-dependent regulation. Furthermore, analysis of individual single mutations has permitted, for the first time, partial kinetic characterization of a full-length dimeric form of human cystathionine beta-synthase.

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.

S-adenosylmethionine and radical-based catalysis.

S-adenosylmethionine is the major methyl donor in all living organisms, but it is also involved in many other reactions occurring through radical-based catalysis. The structure and function of some of these enzymes, including those involved in the synthesis of the molybdenum cofactors, biotin, lipoate, will be discussed.

Flipping off the riboswitch: RNA structures that control gene expression.

Riboswitches are metabolite-sensing RNA structures that have been discovered in regulatory regions of messenger RNA (mRNA). They have the remarkable ability to shut off the transcription or translation of their own mRNAs in response to binding a specific metabolite. In other words, riboswitches regulate their own genes using RNA instead of protein. Three new crystal structures reveal how S-adenosylmethionine and thiamine pyrophosphate riboswitches accomplish this task.