Structural and functional roles of cysteine 90 and cysteine 240 in S-adenosylmethionine synthetase

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.

Identification of S-Nitrosylated (SNO) Proteins in Entamoeba histolytica Adapted to Nitrosative Stress: Insights into the Role of SNO Actin and In vitro Virulence

We have recently reported that Entamoeba histolytica trophozoites can adapt to toxic levels of the nitric oxide (NO) donor, S-nitrosoglutathione (GSNO). Even if the consequences of this adaptation on the modulation of gene expression in NO-adapted trophozoites (NAT) have been previously explored, insight on S-nitrosylated (SNO) proteins in NAT is missing. Our study aims to fill this knowledge gap by performing a screening of SNO proteins in NAT. Employing SNO resin-assisted capture (RAC), we identified 242 putative SNO proteins with key functions in calcium binding, enzyme modulation, redox homeostasis, and actin cytoskeleton. Of the SNO proteins in NAT, proteins that are associated with actin family cytoskeleton protein are significantly enriched. Here we report that the formation of actin filaments (F-actin) is impaired in NAT. Consequently, the ability of NAT to ingest erythrocytes and their motility and their cytopathic activity are impaired. These phenotypes can be imitated by treating control parasite with cytochalasin D (CytD), a drug that binds to F-actin polymer and prevent polymerization of actin monomers. Removal of GSNO from the culture medium of NAT restored the sensitivity of the parasite to nitrosative stress (NS) and its ability to form F-actin formation and its virulence. These results establish the central role of NO in shaping the virulence of the parasite through its effect on F-actin formation and highlight the impressive ability of this parasite to adapt to NS.

Characterization and redox regulation of Plasmodium falciparum methionine adenosyltransferase

As a methyl group donor for biochemical reactions, S-adenosylmethionine plays a central metabolic role in most organisms. Depletion of S-adenosylmethionine has downstream effects on polyamine metabolism and methylation reactions, and is an effective way to combat pathogenic microorganisms such as malaria parasites. Inhibition of both the methylation cycle and polyamine synthesis strongly affects Plasmodium falciparum growth. Despite its central position in the methylation cycle, not much is currently known about P. falciparum methionine adenosyltransferase (PfalMAT). Notably, however, PfalMAT has been discussed as a target of different redox regulatory modifications. Modulating the redox state of critical cysteine residues is a way to regulate enzyme activity in different pathways in response to changes in the cellular redox state. In the present study, we optimized an assay for detailed characterization of enzymatic activity and redox regulation of PfalMAT. While the presence of reduced thioredoxin increases the activity of the enzyme, it was found to be inhibited upon S-glutathionylation and S-nitrosylation. A homology model and site-directed mutagenesis studies revealed a contribution of the residues Cys52, Cys113 and Cys187 to redox regulation of PfalMAT by influencing its structure and activity. This phenomenon connects cellular S-adenosylmethionine synthesis to the redox state of PfalMAT and therefore to the cellular redox homeostasis.

Multiple inputs control sulfur-containing amino acid synthesis in Saccharomyces cerevisiae

Genes in Saccharomyces cerevisiae involved in sulfur-containing amino acid synthesis are transcriptionally induced by either cysteine or S-adenosyl-methionine deficiency, as well as defects in phosphatidylcholine synthesis. Met30p, a regulator of these genes, changes physically in inducing conditions, which may mediate its regulatory activity.

In Saccharomyces cerevisiae, transcription of the MET regulon, which encodes the proteins involved in the synthesis of the sulfur-containing amino acids methionine and cysteine, is repressed by the presence of either methionine or cysteine in the environment. This repression is accomplished by ubiquitination of the transcription factor Met4, which is carried out by the SCF(Met30) E3 ubiquitin ligase. Mutants defective in MET regulon repression reveal that loss of Cho2, which is required for the methylation of phosphatidylethanolamine to produce phosphatidylcholine, leads to induction of the MET regulon. This induction is due to reduced cysteine synthesis caused by the Cho2 defects, uncovering an important link between phospholipid synthesis and cysteine synthesis. Antimorphic mutants in S-adenosyl-methionine (SAM) synthetase genes also induce the MET regulon. This effect is due, at least in part, to SAM deficiency controlling the MET regulon independently of SAM's contribution to cysteine synthesis. Finally, the Met30 protein is found in two distinct forms whose relative abundance is controlled by the availability of sulfur-containing amino acids. This modification could be involved in the nutritional control of SCF(Met30) activity toward Met4.

An investigation of the catalytic mechanism of S-adenosylmethionine synthetase by QM/MM calculations

Catalysis by S-adenosylmethionine synthetase has been investigated by quantum mechanical/molecular mechanical calculations, exploiting structures of the active crystalline enzyme. The transition state energy of +19.1 kcal/mol computed for a nucleophilic attack of the methionyl sulfur on carbon-5' of the nucleotide was indistinguishable from the experimental (solution) value when the QM residues were an uncharged histidine that hydrogen bonds to the leaving oxygen-5' and an aspartate that chelates a Mg2+ ion, and was similar (+18.8 kcal/mol) when the QM region also included the active site arginine and lysines. The computed energy difference between reactant and product was also consistent with their equimolar abundance in co-crystals. The calculated geometrical changes support catalysis of a S(N)2 reaction through hydrogen bonding of the liberated oxygen-5' to the histidine, charge neutralization by the two Mg2+ ions, and stabilization of the product sulfonium cation through a close, non-bonded, contact between the sulfur and the ribose oxygen-4'.

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.

Identification of a mutation in the Bacillus subtilis S-adenosylmethionine synthetase gene that results in derepression of S-box gene expression

Genes in the S-box family are regulated by binding of S-adenosylmethionine (SAM) to the 5' region of the mRNA of the regulated gene. SAM binding was previously shown to promote a rearrangement of the RNA structure that results in premature termination of transcription in vitro and repression of expression of the downstream coding sequence. The S-box RNA element therefore acts as a SAM-binding riboswitch in vitro. In an effort to identify factors other than SAM that could be involved in the S-box regulatory mechanism in vivo, we searched for trans-acting mutations in Bacillus subtilis that act to disrupt repression of S-box gene expression during growth under conditions where SAM pools are elevated. We identified a single mutant that proved to have one nucleotide substitution in the metK gene, encoding SAM synthetase. This mutation, designated metK10, resulted in a 15-fold decrease in SAM synthetase activity and a 4-fold decrease in SAM concentration in vivo. The metK10 mutation specifically affected S-box gene expression, and the increase in expression under repressing conditions was dependent on the presence of a functional transcriptional antiterminator element. The observation that the mutation identified in this search affects SAM production supports the model that the S-box RNAs directly monitor SAM in vivo, without a requirement for additional factors.

Conformational dynamics of the active site loop of S-adenosylmethionine synthetase illuminated by site-directed spin labeling

S-adenosylmethionine synthetase (ATP: L-methionine S-adenosyltransferase, methionine adenosyltransferase, a.k.a. MAT) is one of numerous enzymes that have a flexible polypeptide loop that moves to gate access to the active site in a motion that is closely coupled to catalysis. Crystallographic studies of this tetrameric enzyme have shown that the loop is closed in the absence of bound substrates. However, the loop must open to allow substrate binding and a variety of data indicate that the loop is closed during the catalytic steps. Previous kinetic studies indicate that during turnover loop motion occurs on a time scale of 10(-2)s, ca. 10-fold faster than chemical transformations and turnover. Site-directed spin labeling has been used to introduce nitroxide groups at two positions in the loop to illuminate how the motion of the loop is affected by substrate binding. The two loop mutants constructed, G105C and D107C, retain wild type levels of MAT activity; attachment of a methanethiosulfonate spin label to convert the cysteine to the "R1" residue reduced the k(cat) only for the labeled D107R1 form (7-fold). The K(m) value for methionine increased 2- to 4-fold for the cysteine mutants and 2- to 7-fold for the labeled proteins, whereas the K(m) for ATP was changed by at most 2-fold. EPR spectra for both labeled proteins are nearly identical and show the presence of two major spin label environments with rotational diffusion rates differing by approximately 10-fold; the slower rate is ca. 4-fold faster than the estimated protein rotational rate. The spectra are not altered by addition of substrates or products. At both positions the less mobile conformation constitutes ca. 65% of the total species, indicating an equilibrium that only slightly favors one form, that in which the label is more immobilized. The equilibrium constant that relates the two forms is comparable to the equilibrium constant of 1.5 for a conformational change that was previously deduced from the viscosity dependence of the rate of AdoMet formation. The results suggest that the motion of the loop may be an intrinsic property of the protein and not be strictly ligand modulated.

Characterisation of methionine adenosyltransferase from Mycobacterium smegmatis and M. tuberculosis

BACKGROUND

Tuberculosis remains a serious world-wide health threat which requires the characterisation of novel drug targets for the development of future antimycobacterials. One of the key obstacles in the definition of new targets is the large variety of metabolic alterations that occur between cells in the active growth and chronic/dormant phases of tuberculosis. The ideal biochemical target should be active in both growth phases. Methionine adenosyltransferase, which catalyses the formation of S-adenosylmethionine from methionine and ATP, is involved in polyamine biosynthesis during active growth and is also required for the methylation and cyclopropylation of mycolipids necessary for survival in the chronic phase.

RESULTS

The gene encoding methionine adenosyltransferase has been cloned from Mycobacterium tuberculosis and the model organism M. smegmatis. Both enzymes retained all amino acids known to be involved in catalysing the reaction. While the M. smegmatis enzyme could be functionally expressed, the M. tuberculosis homologue was insoluble and inactive under a large variety of expression conditions. For the M. smegmatis enzyme, the Vmax for S-adenosylmethionine formation was 1.30 micromol/min/mg protein and the Km for methionine and ATP was 288 microM and 76 microM respectively. In addition, the enzyme was competitively inhibited by 8-azaguanine and azathioprine with a Ki of 4.7 mM and 3.7 mM respectively. Azathioprine inhibited the in vitro growth of M. smegmatis with a minimal inhibitory concentration (MIC) of 500 microM, while the MIC for 8-azaguanine was >1.0 mM.

CONCLUSION

The methionine adenosyltransferase from both organisms had a primary structure very similar those previously characterised in other prokaryotic and eukaryotic organisms. The kinetic properties of the M. smegmatis enzyme were also similar to known prokaryotic methionine adenosyltransferases. Inhibition of the enzyme by 8-azaguanine and azathioprine provides a starting point for the synthesis of higher affinity purine-based inhibitors.

S-adenosylmethionine transport in Rickettsia prowazekii

Rickettsia prowazekii, the causative agent of epidemic typhus, is an obligate, intracellular, parasitic bacterium that grows within the cytoplasm of eucaryotic host cells. Rickettsiae exploit this intracellular environment by using transport systems for the compounds available in the host cell's cytoplasm. Analysis of the R. prowazekii Madrid E genome sequence revealed the presence of a mutation in the rickettsial metK gene, the gene encoding the enzyme responsible for the synthesis of S-adenosylmethionine (AdoMet). Since AdoMet is required for rickettsial processes, the apparent inability of this strain to synthesize AdoMet suggested the presence of a rickettsial AdoMet transporter. We have confirmed the presence of an AdoMet transporter in the rickettsiae which, to our knowledge, is the first bacterial AdoMet transporter identified. The influx of AdoMet into rickettsiae was a saturable process with a K(T) of 2.3 micro M. Transport was inhibited by S-adenosylethionine and S-adenosylhomocysteine but not by sinfungin or methionine. Transport was also inhibited by 2,4-dinitrophenol, suggesting an energy-linked transport mechanism, and by N-ethylmaleimide. AdoMet transporters with similar properties were also identified in the Breinl strain of R. prowazekii and in Rickettsia typhi. By screening Escherichia coli clone banks for AdoMet transport, the R. prowazekii gene coding for a transporter, RP076 (sam), was identified. AdoMet transport in E. coli containing the R. prowazekii sam gene exhibited kinetics similar to that seen in rickettsiae. The existence of a rickettsial transporter for AdoMet raises intriguing questions concerning the evolutionary relationship between the synthesis and transport of this essential metabolite.

Leishmania donovani methionine adenosyltransferase. Role of cysteine residues in the recombinant enzyme

Methionine adenosyltransferase (MAT, EC 2.5.1.6)-mediated synthesis of S-adenosylmethionine (AdoMet) is a two-step process consisting of the formation of AdoMet and the subsequent cleavage of the tripolyphosphate (PPPi) molecule, a reaction induced, in turn, by AdoMet. The fact that the two activities, AdoMet synthesis and tripolyphosphate hydrolysis, can be measured separately is particularly useful when the site-directed mutagenesis approach is used to determine the functional role of the amino acid residues involved in each. The present report describes the cloning and subsequent functional refolding, using a bacterial expression system, of the MAT gene (GenBank accession number AF179714) from Leishmania donovani, the etiological agent of visceral leishmaniasis. The absolute need to include a sulfhydryl-protection reagent in the refolding buffer for this protein, in conjunction with the rapid inactivation of the functionally refolded protein by N-ethylmaleimide, suggests the presence of crucial cysteine residues in the primary structure of the MAT protein. The seven cysteines in L. donovani MAT were mutated to their isosterical amino acid, serine. The C22S, C44S, C92S and C305S mutants showed a drastic loss of AdoMet synthesis activity compared to the wild type, and the C33S and C47S mutants retained a mere 12% of wild-type MAT activity. C106S mutant activity and kinetics remained unchanged with respect to the wild-type. Cysteine substitutions also modified PPPi cleavage and AdoMet induction. The C22S, C44S and C305S mutants lacked in tripolyphosphatase activity altogether, whereas C33S, C47S and C92S retained low but detectable activity. The behavior of the C92S mutant was notable: its inability to synthesize AdoMet combined with its retention of tripolyphosphatase activity appear to be indicative of the specific involvement of the respective residue in the first step of the MAT reaction.

A stereospecific colorimetric assay for (S,S)-adenosylmethionine quantification based on thiopurine methyltransferase-catalyzed thiol methylation

S-Adenosyl-L-methionine (AdoMet) which is biologically synthesized by AdoMet synthetase bears an S configuration at the sulfur atom. The chiral sulfonium spontaneously racemizes to form a mixture of S and R isomers of AdoMet under physiological conditions or normal storage conditions. The chirality of AdoMet greatly affects its activity; the R isomer is not accepted as a substrate for AdoMet-dependent methyltransferases. We report a stereospecific colorimetric assay for (S,S)-adenosylmethionine quantification based on an enzyme-coupled reaction in which (S,S)-AdoMet reacts with 2-nitro-5-thiobenzoic acid to form AdoHcy and 2-nitro-5-methylthiobenzoic acid. The transformation is catalyzed by recombinant human thiopurine S-methyltransferase (TPMT, EC 2.1.1.67) and is associated with a large spectral change at 410 nm. Accumulation of the S-adenosylhomocysteine (AdoHcy) product, a feedback inhibitor of TPMT, slows the assay. AdoHcy nucleosidase (EC 3.2.2.9) irreversibly cleaves AdoHcy to adenine and S-ribosylhomocysteine, significantly shortening the assay time to less than 10 min. The assay is linear from 5 to at least 60 microM (S,S)-AdoMet.

The active site loop of S-adenosylmethionine synthetase modulates catalytic efficiency

Crystallographic studies of Escherichia coli S-adenosylmethionine synthetase (ATP:L-methionine S-adenosyltransferase, MAT) have defined a flexible polypeptide loop that can gate access to the active site without contacting the substrates. The influence of the length and sequence of this active site loop on catalytic efficiency has been characterized in a mutant in which the E. coli MAT sequence (DRADPLEQ) has been replaced with the distinct sequence of the corresponding region of the otherwise highly homologous rat liver enzyme (HDLRNEEDV). Four additional mutants in which the entire DRADPLEQ sequence was replaced by five, six, seven, or eight glycines have been studied to unveil the effects of loop length and the influence of side chains. In all of the mutants, the maximal rate of S-adenosylmethionine formation (k(cat)) is diminished by more than 200-fold whereas the rate of hydrolysis of the tripolyphosphate intermediate is decreased by less than 3-fold. Thus, the function of the loop is localized to the first step in the overall reaction. The K(m) for methionine increases in all of the oligoglycine mutants, whereas the K(m) values for ATP are not substantially different. The k(cat) for the wild-type enzyme is decreased by increases in solution microviscosity with 55% of the maximal dependence. Thus, a diffusional event is coupled to the chemical step of AdoMet formation, which is known to be rate-limiting. The results indicate that a conformational change, possibly loop closure, is associated with AdoMet synthesis. The data integrate a previously discovered conformational change associated with PPP(i) binding to the E x AdoMet complex into the reaction sequence, reflecting a difference in protein conformation in the E x AdoMet x PPP(i) complex whether it is formed from the E x ATP x methionine complex or from binding of exogenous PPP(i). The temperature dependence of the k(cat) for S-adenosylmethionine formation shows that the removal of the side chains in the glycine mutants causes the activation enthalpy of the reaction to approximately double in each case, while the activation entropy changes from negative in the wild-type enzyme to positive in the mutants. The favorable activation entropy in the mutant-catalyzed reactions may reflect release of water during catalysis, while the negative activation entropy in the reaction catalyzed by the wild-type enzyme apparently reflects reorganization of the loop. The observations point to how nature can fine-tune the activity of an enzyme by modifying substrate and product access to the active site rather than by altering the enzyme x substrate contacts or the catalytic machinery itself.

The bifunctional active site of S-adenosylmethionine synthetase. Roles of the basic residues

S-adenosylmethionine (AdoMet) synthetase catalyzes a unique two-step enzymatic reaction leading to formation of the primary biological alkylating agent. The crystal structure of Escherichia coli AdoMet synthetase shows that the active site, which lies between two subunits, contains four lysines and one histidine as basic residues. In order to test the proposed charge and hydrogen bonding roles in catalytic function, each lysine has been changed to an uncharged methionine or alanine, and the histidine has been altered to asparagine. The resultant enzyme variants are all tetramers like the wild type enzyme; however, circular dichroism spectra show reductions in helix content for the K245*M and K269M mutants. (The asterisk denotes that the residue is in the second subunit.) Four mutants have k(cat) reductions of approximately 10(3)-10(4)-fold in AdoMet synthesis; however, the k(cat) of K165*M variant is only reduced 2-fold. In each mutant, there is a smaller catalytic impairment in the partial reaction of tripolyphosphate hydrolysis. The K165*A enzyme has a 100-fold greater k(cat) for tripolyphosphate hydrolysis than the wild type enzyme, but this mutant is not activated by AdoMet in contrast to the wild type enzyme. The properties of these mutants require reassessment of the catalytic roles of these residues.

Identification of a highly diverged class of S-adenosylmethionine synthetases in the archaea

S-adenosylmethionine is the primary alkylating agent in all known organisms. ATP:L-methionine S-adenosyltransferase (MAT) catalyzes the only known biosynthetic route to this central metabolite. Although the amino acid sequence of MAT is strongly conserved among bacteria and eukarya, no homologs have been recognized in the completed genome sequences of any archaea. In this study, MAT has been purified to homogeneity from the archaeon Methanococcus jannaschii, and the gene encoding it has been identified by mass spectrometry. The peptide mass map identifies the gene encoding MAT as MJ1208, a hypothetical open reading frame. The gene was cloned in Escherichia coli, and expressed enzyme has been purified and characterized. This protein has only 22 and 23% sequence identity to the E. coli and human enzymes, respectively, whereas those are 59% identical to each other. The few identical residues include the majority of those constituting the polar active site residues. Each complete archaeal genome sequence contains a homolog of this archaeal-type MAT. Surprisingly, three bacterial genomes encode both the archaeal and eukaryal/bacterial types of MAT. This identification of a second major class of MAT emphasizes the long evolutionary history of the archaeal lineage and the structural diversity found even in crucial metabolic enzymes.

The bifunctional active site of s-adenosylmethionine synthetase. Roles of the active site aspartates

S-Adenosylmethionine (AdoMet) synthetase catalyzes the biosynthesis of AdoMet in a unique enzymatic reaction. Initially the sulfur of methionine displaces the intact tripolyphosphate chain (PPP(i)) from ATP, and subsequently PPP(i) is hydrolyzed to PP(i) and P(i) before product release. The crystal structure of Escherichia coli AdoMet synthetase shows that the active site contains four aspartate residues. Aspartate residues Asp-16* and Asp-271 individually provide the sole protein ligand to one of the two required Mg(2+) ions (* denotes a residue from a second subunit); aspartates Asp-118 and Asp-238* are proposed to interact with methionine. Each aspartate has been changed to an uncharged asparagine, and the metal binding residues were also changed to alanine, to assess the roles of charge and ligation ability on catalytic efficiency. The resultant enzyme variants all structurally resemble the wild type enzyme as indicated by circular dichroism spectra and are tetramers. However, all have k(cat) reductions of approximately 10(3)-fold in AdoMet synthesis, whereas the MgATP and methionine K(m) values change by less than 3- and 8-fold, respectively. In the partial reaction of PPP(i) hydrolysis, mutants of the Mg(2+) binding residues have >700-fold reduced catalytic efficiency (k(cat)/K(m)), whereas the D118N and D238*N mutants are impaired less than 35-fold. The catalytic efficiency for PPP(i) hydrolysis by Mg(2+) site mutants is improved by AdoMet, like the wild type enzyme. In contrast AdoMet reduces the catalytic efficiency for PPP(i) hydrolysis by the D118N and D238*N mutants, indicating that the events involved in AdoMet activation are hindered in these methionyl binding site mutants. Ca(2+) uniquely activates the D271A mutant enzyme to 15% of the level of Mg(2+), in contrast to the approximately 1% Ca(2+) activation of the wild type enzyme. This indicates that the Asp-271 side chain size is a discriminator between the activating ability of Ca(2+) and the smaller Mg(2+).

The active-site arginine of S-adenosylmethionine synthetase orients the reaction intermediate

S-Adenosylmethionine (AdoMet) synthetase catalyzes the formation of AdoMet and tripolyphosphate (PPPi) from ATP and L-methionine and the subsequent hydrolysis of the PPPi to PPi and Pi before product release. Little is known about the roles of active-site residues involved in catalysis of the two sequential reactions that occur at opposite ends of the polyphosphate chain. Crystallographic studies of Escherichia coli AdoMet synthetase showed that arginine-244 is the only arginine near the polyphosphate-binding site. Arginine-244 is embedded as the seventh residue in the conserved sequence DxGxTxxKxI which is also found at the active site of inorganic pyrophosphatases, suggesting a potential pyrophosphate-binding motif. Chemical modification of AdoMet synthetase by the arginine-specific reagents phenylglyoxal or p-hydroxyphenylglyoxal inactivates the enzyme. ATP and PPPi protect the enzyme from inactivation, consistent with the presence of an important arginine residue in the vicinity of the polyphosphate-binding site. Site-specific mutagenesis has been used to change the conserved arginine-244 to either leucine (R244L) or histidine (R244H). In the overall reaction, the R244L mutant has the kcat reduced approximately 10(3)-fold, with a 7 to 10-fold increase in substrate Km values; the R244H mutant has an approximately 10(5)-fold decrease in kcat. In contrast, the kcat values for hydrolysis of added PPPi by the R244L and R244H mutants have been reduced by less than 2 orders of magnitude. In contrast to the wild-type enzyme in which 98% of the Pi formed originates as the gamma-phosphoryl group of ATP, in the R244L mutant the orientation of the PPPi intermediate equilibrates at the active site yielding equal amounts of Pi from the alpha- and gamma-phosphoryl groups of ATP. Thus, the active-site arginine has a profound role in the cleavage of PPPi from ATP during AdoMet formation and in maintaining the orientation of PPPi in the active site, while playing a lesser role in the subsequent PPPi hydrolytic reaction.

Nucleotide sequence and developmental expression of Acanthamoeba S-adenosylmethionine synthetase gene

We have isolated and characterized a cDNA (cDNA1) from an Acanthamoeba cDNA library encoding the enzyme S-adenosylmethionine (SAM) synthetase (ATP: L-methionine S-adenosyltransferase; EC 2.5.1.6). The nucleotide sequence exhibits about 61-73% overall similarity to the corresponding gene of other organisms. The cDNA displays extreme codon bias with a preference for C or G in the third position. A putative initiation site and an ATP-binding site are identified. An amino acid content of 388 and a molecular mass of about 44,000 Daltons are deduced for the enzyme. Putative phosphorylation sites which might be involved in regulation of the enzyme are revealed. The cDNA was expressed in Escherichia coli BL21(DE3), and the identity of the protein product confirmed by Western blotting analysis. Northern analyses of the expression of the Acanthamoeba SAM synthetase gene during development revealed a pronounced reduction in the level of transcripts as amoebae converted to cysts.

Recombinant rat liver S-adenosyl-L-methionine synthetase tetramers and dimers are in equilibrium

Rat liver S-adenosyl-L-methionine synthetase is present in two oligomeric forms, tetramers and dimers, with different substrate kinetics and regulation. In vivo the relative amounts of both forms may change in some instances. The basis of this regulatory mechanism is not known. When rat liver cDNA was used to express the protein in Escherichia coli the two oligomeric forms were found. Gel filtration chromatography of the purified recombinant enzyme suggested that these two isoforms might be in equilibrium. This was confirmed by kinetic experiments which showed that the specific activity of the enzyme was dependent on the protein concentration. From these experiments, apparent equilibrium constants of (5.6 +/- 0.4) x 10(5) M-1 and (3.5 +/- 0.9) x 10(5) M-1 were obtained at 2mM and 60 microM methionine concentrations, respectively. Using hydrophobic chromatography on phenyl-Sepharose to separate the tetrameric and dimeric forms, an equilibrium constant of (4.9 +/- 0.7) x 10(5) M-1 was calculated. A rate constant for the dissociation of the tetramer of k-1 = (8.1 +/- 0.4) x 10(-4) s-1 at 4 degrees C was also calculated using the same approach. In summary, we have shown that the rat liver S-adenosyl-L-methionine synthetase produced in bacterial cells is present in two oligomeric forms, tetramers and dimers, which are in equilibrium. This system might be useful for studying the dynamics and the regulation of the distribution of oligomeric forms in the mammalian liver.

Site-directed mutagenesis of rat liver S-adenosylmethionine synthetase. Identification of a cysteine residue critical for the oligomeric state

We have examined the functional importance of the cysteine residues of rat liver S-adenosylmethionine synthetase. For this purpose the ten cysteine residues of the molecule were changed to serines by site-directed mutagenesis. Ten recombinant enzyme mutants were obtained by using a bacterial expression system. The same level of expression was obtained for the wild type and mutants, but the ratio of S-adenosylmethionine synthetase between soluble and insoluble fractions differed for some of the mutant forms. The immunoreactivity against an anti-(rat liver S-adenosylmethionine synthetase) antibody was equivalent in all the cases. Effects on S-adenosylmethionine synthetase activities were also measured. Mutants C57S, C69S, C105S and C121S showed decreased relative specific activity of 68, 85, 63 and 29%, respectively, compared with wild-type, whereas C312S resulted in an increase of 1.6-fold. Separation of tetramer and dimer forms for wild type and mutants was carried out by using phenyl-Sepharose columns. The dimer/tetramer ratio was calculated based on the activity and on the protein level estimated by immunoblotting. No monomeric forms of the enzyme were detected in any case. Comparison of dimer/tetramer ratios indicates the importance of cysteine-69 (dimer/tetramer protein ratio of 88 versus 10.2 in the wild type) in maintaining the oligomeric state of rat liver S-adenosylmethionine synthetase. Moreover, all the mutations carried out of cysteine residues between cysteine-35 and cysteine-105 altered the ratio between oligomeric forms.