Structural and kinetic properties of Bacillus subtilis S-adenosylmethionine synthetase expressed in Escherichia coli

Characterization of an iron-induced enzyme, nicotianamine synthase, from giant leucaena

Metal homeostasis in giant leucaena (Leucaena leucocephala subsp. glabrata) is of interest due to the plant's production of mimosine, an iron-chelating secondary metabolite. Real-time PCR performed on root and foliar tissue showed the upregulation of 19 genes following exogenous application of iron. Notable genes affected include glutathione synthase (20-fold increase in leaf), ferric chelate reductase (15-fold increase in root), mimosinase (20-fold increase in leaf) and nicotianamine synthase (30-fold increase in root). Transcriptome sequence data and 5'-RLM-RACE methods identified the complete nicotianamine synthase coding sequence, which was cloned for heterologous expression and in vitro assays. To properly assay nicotianamine synthase activity, due to strong feedback inhibition by 5'-methylthoadenosine, the giant leucaena 5'-methylthoadenosine nucleosidase was cloned and purified as well. Additional inhibition produced by the substrate compound, S-adenosylmethionine (SAM), was discovered in this study by utilizing a recombinant SAM-synthetase. Nicotianamine synthase is sensitive to racemic mixtures of SAM, which is inevitably produced in commercial SAM solutions. When substrate was produced in situ, using SAM-synthetase, nicotianamine synthase activity was 5-fold faster. Thus, in vitro nicotianamine synthase activity depends highly on two additional enzymes, the inclusion of MTA-nucleosidase being vital. Although promising, cell-free nicotianamine production methods are not yet efficient enough for industry-scale efforts. Sequence and structural analyses suggest residues involved in azetidine ring formation and other aspects of the mechanism are explored.

Efficient whole-cell biosynthesis of S-adenosyl-L-methionine by the engineered Escherichia coli with high ATP regenerating system

S-adenosyl-L-methionine (SAM) is an important intermediate metabolite and widely used in the treatment of liver disease, arthritis, and depression. In this work, a whole-cell catalysis strategy was employed to enhance SAM production by combining the SAM biosynthetic pathway with an adenosine triphosphate (ATP) regeneration system in Escherichia coli. Specifically, the ado1, ack, and adk genes were previously introduced into the genome of the host strain. We then confirmed the availability of the ATP regeneration system under the condition of adding adenosine monophosphate (AMP) and acetyl phosphate (ACP) as supplements. To improve the SAM production, the sam2 gene derived from Saccharomyces cerevisiae was overexpressed using the plasmid pGEX-2TK in the strain and the conditions of biocatalytic process were optimized. Under the optimal biocatalytic conditions, the recombinant strain RS01 (pGEX-2TK-SAM2) achieved a SAM titer of 11.4 g/L after 10 h cultivation. This work not only provides a new platform for the efficient production of SAM but also offers insights into the biosynthesis of other ATP-dependent products.

Efficient spermidine production using a multi-enzyme cascade system utilizing methionine adenosyltransferase from Lactobacillus fermentum with Reduced Product Inhibition and Acidic pH Preference

Methionine adenosyltransferases (MATs; EC 2.5.1.6) are key enzymes that catalyze a crucial step in the spermidine biosynthesis pathway. Due to MAT's significant product inhibition, S-adenosylmethionine (SAM) and spermidine production faces challenges. We evaluated MATs from 20 lactic acid bacteria (LAB) to identify enzymes with acidic preference and lower susceptibility to product inhibition. Lactobacillus fermentum's MAT (LfMAT) emerged as a candidate with desirable characteristics. LfMAT exhibited strong activity in acidic environments, maintaining over 85 % activity between pH 6.0-8.5 for 60 min, with peak efficacy at pH 7.0. LfMAT produced 4.2 mM SAM from 5 mM substrate, indicating reduced product inhibition. Ultimately, using an in vitro multi-enzyme cascade system containing LfMAT, S-adenosylmethionine decarboxylase, and spermidine synthase, we successfully produced 12.9 g·L-1 of spermidine. This study establishes a cascade reaction platform, offering a novel approach for the efficient synthesis of spermidine and other polyamines.

Characterization of a plant S-adenosylmethionine synthetase from Acacia koa

The Acacia koa S-adenosylmethionine (SAM) synthetase was identified from transcriptome data and cloned into the T7-expression vector pEt14b. Assays indicate a thermoalkaliphic enzyme which tolerates conditions up to pH 10.5, 55 °C and 3 M KCl. In vitro examples of plant SAM-synthetase activity are scarce, however this study provides supporting evidence that these extremophilic properties may actually be typical for this plant enzyme. Enzyme kinetic constants (Km = 1.44 mM, Kcat = 1.29 s-1, Vmax 170 μM. min-1) are comparable to nonplant SAM-synthetases except that substrate inhibition was not apparent at 10 mM ATP/L-methionine. Methods were explored in this study to reduce feedback inhibition, which is known to limit SAM-synthetase activity in vitro. Four single-point mutation variants of the Acacia koa SAM-synthetase were produced, each with varying degrees of reduced reaction rate, greater sensitivity to product inhibition and loss of thermophilic properties. Although an enhanced mutant was not produced, this study describes the first mutagenesis of a plant SAM-synthetase. Overcoming feedback inhibition was accomplished by the addition of organic solvent to enzyme assays. Acetonitrile, methanol or dimethylformamide, when included as 25% of the assay volume, improved total SAM production by 30-65%.

Enhanced selenocysteine biosynthesis for seleno-methylselenocysteine production in Bacillus subtilis

Seleno-methylselenocysteine (SeMCys) is an effective component for selenium supplementation with anti-carcinogenic potential and can ameliorate neuropathology and cognitive deficits. In this study, we aimed to engineer Bacillus subtilis 168 for the microbial production of SeMCys. First, the accumulation of intracellular selenocysteine (SeCys) as the precursor of SeMCys was enhanced through overexpression of serine O-acetyltransferase, which was desensitized against feedback inhibition by cysteine. Next, the S-adenosylmethionine (SAM) synthetic pathway was optimized to improve methyl donor availability through expression of S-adenosylmethionine synthetase. Further, SeMCys was successfully produced through expression of the selenocysteine methyltransferase in SeCys and SAM-producing strain. The increased expression level of selenocysteine methyltransferase benefited the SeMCys production. Finally, all the heterologous genes were integrated into the genome of B. subtilis, and the strain produced SeMCys at a titer of 18.4 μg/L in fed-batch culture. This is the first report on the metabolic engineering of B. subtilis for microbial production of SeMCys and provides a good starting point for future pathway engineering to achieve the industrial-grade production of SeMCys. KEY POINTS: • Expression of the feedback-insensitive serine O-acetyltransferase provided B. subtilis the ability of accumulating SeCys. • SAM production was enhanced through expressing S-adenosylmethionine synthetase in B. subtilis. • Expression of selenocysteine methyltransferase in SeCys and SAM-accumulating strain facilitated SeMCys production.

High-Throughput Screening and Directed Evolution of Methionine Adenosyltransferase from Escherichia coli

S-adenosyl-L-methionine (SAM) is the active form of methionine, which participates in various metabolic reactions and plays a vital role. It is mainly used as a precursor by three key metabolic pathways: trans-methylation, trans-sulfuration, and trans-aminopropylation. Methionine adenosyltransferase (MAT) is the only enzyme to produce SAM from methionine and ATP. However, there is no efficient and accurate method for high-throughput detection of SAM, which is the major obstacles of directed evolution campaigns for MAT. Herein, we established a colorimetric method for directed evolution of MAT based on detecting SAM by using glycine oxidase and glycine/sarcosine N-methyltransferase enzyme. Screening of MAT libraries revealed variant I303V/Q22R with 2.13-fold improved activity towards SAM in comparison to the wild type. Molecular dynamic simulation indicates that the loops more flexible and more conducive to SAM release.

Heterologous expression and characterization of a thermoalkaliphilic SAM-synthetase from giant leucaena (Leucaena leucocephala subsp glabrata)

The cDNA encoding S-adenosylmethionine (SAM) synthetase was isolated from giant leucaena (Leucaena leucocephala subsp. glabrata) root tissue mRNA. Transcriptome data and 5'-RLM-RACE were used to obtain the transcript sequence and clone into the T₇-expression vector pEt14b. N-terminal Histidine-tagged recombinant protein was expressed highly in Escherichia coli, purified and characterized by activity assays. A straightforward method using isocratic reverse-phase HPLC analysis (mobile phase: 0.02M o-phosphoric acid) of enzyme assays determined optimal enzyme activity at pH 10.0, 55 °C and 200 mM KCl. In addition to thermophilic activity, giant leucaena SAM-synthetase remains highly active in solutions containing up to 4 M KCl and accepts Na⁺ to some extent as a substitute for K⁺, a known required cofactor for SAM-synthetases. The enzyme followed Michaelis-Menten kinetics (K_m = 1.82 mM, K_cat = 1.17 s^-1, V_max 243.9 μM. min^-1) and was not inhibited by spermidine, spermine or nicotianamine. Giant leucaena SAM-synthetase is a highly tolerant enzyme to extreme conditions, suggesting further studies on plant SAM-synthetases.

Purification and structural elucidation of a cobalamin-dependent radical SAM enzyme

The cobalamin (Cbl)-dependent radical S-adenosylmethionine (SAM) enzymes use a [4Fe-4S] cluster, SAM, and Cbl to carry out remarkable catalytic feats in a large number of biosynthetic pathways. However, despite the abundance of annotated Cbl-dependent radical SAM enzymes, relatively few molecular details exist regarding how these enzymes function. Traditionally, challenges associated with purifying and reconstituting Cbl-dependent radical SAM enzymes have hindered biochemical studies aimed at elucidating the structures and mechanisms of these enzymes. Herein, we describe a bottom-up approach that was used to crystallize OxsB, learn about the overall architecture of a Cbl-dependent radical SAM enzyme, and facilitate mechanistic studies. We report lessons learned from the crystallization of different states of OxsB, including the apo-, selenomethionine (SeMet)-labeled, and fully reconstituted form of OxsB that has a [4Fe-4S] cluster, SAM, and Cbl bound. Further, we suggest that, when appropriate, this bottom-up method can be used to facilitate studies on enzymes in this class for which there are challenges associated with purifying and reconstituting the active enzyme.

Deciphering the Role of S-adenosyl Homocysteine Nucleosidase in Quorum Sensing Mediated Biofilm Formation

Abstract:

S-adenosylhomocysteine nucleosidase (MTAN) is a protein that plays a crucial role in several pathways of bacteria that are essential for its survival and pathogenesis. In addition to the role of MTAN in methyl-transfer reactions, methionine biosynthesis, and polyamine synthesis, MTAN is also involved in bacterial quorum sensing (QS). In QS, chemical signaling autoinducer (AI) secreted by bacteria assists cell to cell communication and is regulated in a cell density-dependent manner. They play a significant role in the formation of bacterial biofilm. MTAN plays a major role in the synthesis of these autoinducers. Signaling molecules secreted by bacteria, i.e., AI-1 are recognized as acylated homoserine lactones (AHL) that function as signaling molecules within bacteria. QS enables bacteria to establish physical interactions leading to biofilm formation. The formation of biofilm is a primary reason for the development of multidrug-resistant properties in pathogenic bacteria like Enterococcus faecalis (E. faecalis). In this regard, inhibition of E. faecalis MTAN (EfMTAN) will block the QS and alter the bacterial biofilm formation. In addition to this, it will also block methionine biosynthesis and many other critical metabolic processes. It should also be noted that inhibition of EfMTAN will not have any effect on human beings as this enzyme is not present in humans. This review provides a comprehensive overview of the structural-functional relationship of MTAN. We have also highlighted the current status, enigmas that warrant further studies, and the prospects for identifying potential inhibitors of EfMTAN for the treatment of E. faecalis infections. In addition to this, we have also reported structural studies of EfMTAN using homology modeling and highlighted the putative binding sites of the protein.

Identification of methionine adenosyltransferase with high diastereoselectivity for biocatalytic synthesis of (S)-S-adenosyl-l-methionine and exploring its relationship with fluorinated biosynthetic pathway

Natural fluorinated products are rare and attract great attention. The de novo fluorometabolites biosynthetic pathway in microbes has been studied. It is revealed that the carbon-fluorine (C-F) bond is formed by an exotic enzyme called fluorinase (FLA) when using fluorine ions and S-adenosyl-l-methionine (SAM) as substrates. However, the resource of the precursor SAM is still elusive. To solve this, a novel methionine adenosyltransferase from Streptomyces xinghaiensis (SxMAT) was identified and characterized. We proved that SAM was enzymatically synthesized by SxMAT, an enzyme that mediated the reaction between adenosine triphosphate (ATP) and l-methionine (l-Met) with 99% diastereoisomeric excess (d.e.) and 80% yield. Such high diastereoselectivity had never been reported before. SxMAT was a Co²⁺-dependent metalloenzyme. The results showed that the metal cobalt ion contributes to the activity and selectivity of SxMAT. Molecular docking was performed to reveal its catalytic mechanism. The optimal temperature and pH were 55 °C and 8.5, respectively. Lastly, a two-step tandem enzymatic reaction using SxMAT and FLA both from S. xinghaiensis to generate 5'-fluoro-deoxyadenosine (5'-FDA) was performed. This implied that SxMAT may be present in this fluorometabolites biosynthetic route. These results suggested that SxMAT could be a useful biocatalyst for the synthesis of optically pure (S)-S-adenosyl-l-methionine, an important nutraceutical. In addition, SxMAT will probably play an important role in the biosynthetic pathway of fluorinated natural products in bacteria.

Semi-rationally engineered variants of S-adenosylmethionine synthetase from Escherichia coli with reduced product inhibition and improved catalytic activity

S-adenosylmethionine synthetase (MAT) catalyzes the synthesis of S-adenosylmethionine (SAM) from ATP and L-methionine. SAM is the major methyl donor for more than 100 transmethylation reactions. It is also a common cosubstrate involved in transsulfuration and aminopropylation. However, product inhibition largely restrains the activity of MAT and limits the enzymatic synthesis of SAM. In this research, the product inhibition of MAT from Escherichia coli was reduced via semi-rational modification. A triple variant (Variant III, I303 V/I65 V/L186 V) showed a 42-fold increase in K_i,ATP and a 2.08-fold increase in specific activity when compared to wild-type MAT. Its K_i,ATP was 0.42 mM and specific acitivity was 3.78 ±0.19 U/mg. Increased K_i,ATP means reduced product inhibition which enhances SAM accumulation. The SAM produced by Variant III could reach to 3.27 mM while SAM produced by wild-type MAT was 1.62 mM in the presence of 10 mM substrates. When the residue in 104^th of Variant III was further optimized by site-saturated mutagenesis, the specific activity of Variant IV (I303 V/I65 V/L186 V/N104 K) reached to 6.02 ±0.22 U/mg at 37 °C, though the SAM concentration decreased to 2.68 mM with 10 mM substrates. Analysis of protein 3D structure suggests that changes in hydrogen bonds or other ligand interactions around active site may account for the variety of product inhibition and enzyme activity. The Variant III and Variant IV with reduced inhibition and improved enzyme activity in the study would be more suitable candidates for SAM production in the future.

Metal-nucleobase hybrid nanoparticles for enhancing the activity and stability of metal-activated enzymes

A novel strategy for enhancing the activity and stability of metal-activated enzyme methionine adenosyltransferase (MAT) by allosteric control and confinement of metal-nulceobase hybrid coordination.

Identification of a natural inhibitor of methionine adenosyltransferase 2A regulating one-carbon metabolism in keratinocytes

Background

Psoriasis is a common chronic inflammatory skin disease which lacks effective strategies for the treatment. Natural compounds with biological activities are good tools to identify new targets with therapeutic potentials. Acetyl-11-keto-β-boswellic acid (AKBA) is the most bioactive ingredient of boswellic acids, a group of compounds with anti-inflammatory and anti-cancer properties. Target identification of AKBA and metabolomics analysis of psoriasis helped to elucidate the molecular mechanism underlying its effect, and provide new target(s) to treat the disease.

Methods

To explore the targets and molecular mechanism of AKBA, we performed affinity purification, metabolomics analysis of HaCaT cells treated with AKBA, and epidermis of imiquimod (IMQ) induced mouse model of psoriasis and psoriasis patients.

Findings

AKBA directly interacts with methionine adenosyltransferase 2A (MAT2A), inhibited its enzyme activity, decreased level of S-adenosylmethionine (SAM) and SAM/SAH ratio, and reprogrammed one‑carbon metabolism in HaCaT cells. Untargeted metabolomics of epidermis showed one‑carbon metabolism was activated in psoriasis patients. Topical use of AKBA improved inflammatory phenotype of IMQ induced psoriasis-like mouse model. Molecular docking and site-directed mutagenesis revealed AKBA bound to an allosteric site at the interface of MAT2A dimer.

Interpretation

Our study extends the molecular mechanism of AKBA by revealing a new interacting protein MAT2A. And this leads us to find out the dysregulated one‑carbon metabolism in psoriasis, which indicates the therapeutic potential of AKBA in psoriasis.

Fund

The National Natural Science Foundation, the National Program on Key Basic Research Project, the Shanghai Municipal Commission, the Leading Academic Discipline Project of the Shanghai Municipal Education Commission.

Biosynthesis of S-Adenosylmethionine by Magnetically Immobilized Escherichia coli Cells Highly Expressing a Methionine Adenosyltransferase Variant

S-Adenosylmethionine (SAM) is a natural metabolite having important uses in the treatment of various diseases. To develop a simple and effective way to produce SAM, immobilized Escherichia coli cells highly expressing an engineered variant of methionine adenosyltransferase (MAT) were employed to synthesize SAM. The recombinant I303V MAT variant was successfully produced at approximately 900 mg/L in a 10-L bioreactor and exhibited significantly less product inhibition and had a four-fold higher specific activity (14.2 U/mg) than the wild-type MAT (3.6 U/mg). To reduce the mass transfer resistance, the free whole-cells were permeabilized and immobilized using gellan gum gel as support in the presence of 100 mg/L Fe₃O₄ nanoparticles, and the highest activity (4152.4 U/L support) was obtained, with 78.2% of the activity recovery. The immobilized cells were more stable than the free cells under non-reactive conditions, with a half-life of 9.1 h at 50 °C. Furthermore, the magnetically immobilized cells were employed to produce SAM at a 40-mM scale. The residual activity of the immobilized cells was 67% of its initial activity after 10 reuses, and the conversion rate of ATP was ≥95% in all 10 batches. These results indicated that magnetically immobilized cells should be a promising biocatalyst for the biosynthesis of SAM.

Enzymatic Synthesis of S-Adenosylmethionine Using Immobilized Methionine Adenosyltransferase Variants on the 50-mM Scale

S-adenosylmethionine (SAM), an important metabolite in all living organisms, has been widely used to treat various diseases. To develop a simple and efficient method to produce SAM, an engineered variant of the methionine adenosyltransferase (MAT) from Escherichia coli was investigated for its potential use in the enzymatic synthesis of SAM due to its significantly decreased product inhibition. The recombinant I303V MAT variant was successfully produced at a high level (~800 mg/L) with approximately four-fold higher specific activity than the wild-type MAT. The recombinant I303V MAT was covalently immobilized onto the amino resin and epoxy resin in order to obtain a robust biocatalyst to be used in industrial bioreactors. The immobilized preparation using amino resin exhibited the highest activity coupling yield (~84%), compared with approximately 3% for epoxy resin. The immobilized enzyme was more stable than the soluble enzyme under the reactive conditions, with a half-life of 229.5 h at 37 °C. The KmATP value (0.18 mM) of the immobilized enzyme was ca. two-fold lower than that of the soluble enzyme. Furthermore, the immobilized enzyme showed high operational stability during 10 consecutive 8 h batches, with the substrate adenosine triphosphate (ATP) conversion rate above 95% on the 50-mM scale.

A thermostable archaeal S-adenosylmethionine synthetase: a promising tool to improve the synthesis of adenosylmethionine analogs of biotechnological interest

The naturally and widely occurring sulfonium compound, S-adenosylmethionine (AdoMet), one of nature's most versatile molecules, is biosynthesized from methionine and ATP by AdoMet synthetase or methionine adenosyltransferase (MAT) in a 2-step reaction in which the energy-rich sulfonium compound is formed by dephosphorylation of ATP. All living cells, with the only exception of some parasites and infectious agents, express MAT.

Rationally engineered variants of S-adenosylmethionine (SAM) synthase: reduced product inhibition and synthesis of artificial cofactor homologues

S-Adenosylmethionine (SAM) synthase was engineered for biocatalytic production of SAM and long-chain analogues by rational re-design. Substitution of two conserved isoleucine residues extended the substrate spectrum of the enzyme to artificial S-alkylhomocysteines. The variants proved to be beneficial in preparative synthesis of SAM (and analogues) due to a much reduced product inhibition.

Biochemical characterization of a thermostable adenosylmethionine synthetase from the archaeon Pyrococcus furiosus with high catalytic power

Adenosylmethionine synthetase plays a key role in the biogenesis of the sulfonium compound S-adenosylmethionine, the principal widely used methyl donor in the biological methylations. We report here, for the first time, the characterization of adenosylmethionine synthetase from the hyperthermophilic archaeon Pyrococcus furiosus (PfMAT). The gene PF1866 encoding PfMAT was cloned and expressed, and the recombinant protein was purified to homogeneity. PfMAT shares 51, 63, and 82% sequence identity with the homologous enzymes from Sulfolobus solfataricus, Methanococcus jannaschii, and Thermococcus kodakarensis, respectively. PfMAT is a homodimer of 90 kDa highly thermophilic with an optimum temperature of 90 °C and is characterized by remarkable thermodynamic stability (Tm, 99 °C), kinetic stability, and resistance to guanidine hydrochloride-induced unfolding. The latter process is reversible as demonstrated by the analysis of the refolding process by activity assays and fluorescence measurements. Limited proteolysis experiments indicated that the proteolytic cleavage site is localized at Lys148 and that the C-terminal peptide is necessary for the integrity of the active site. PfMAT shows kinetic features that make it the most efficient catalyst for S-adenosylmethionine synthesis among the characterized MAT from Bacteria and Archaea. Molecular and structural characterization of PfMAT could be useful to improve MAT enzyme engineering for biotechnological applications.

A sensitive mass spectrum assay to characterize engineered methionine adenosyltransferases with S-alkyl methionine analogues as substrates

Methionine adenosyltransferases (MATs) catalyze the formation of S-adenosyl-l-methionine (SAM) inside living cells. Recently, S-alkyl analogues of SAM have been documented as cofactor surrogates to label novel targets of methyltransferases. However, these chemically synthesized SAM analogues are not suitable for cell-based studies because of their poor membrane permeability. This issue was recently addressed under a cellular setting through a chemoenzymatic strategy to process membrane-permeable S-alkyl analogues of methionine (SAAMs) into the SAM analogues with engineered MATs. Here we describe a general sensitive activity assay for engineered MATs by converting the reaction products into S-alkylthioadenosines, followed by high-performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS) quantification. With this assay, 40 human MAT mutants were evaluated against 7 SAAMs as potential substrates. The structure-activity relationship revealed that, besides better engaged SAAM binding by the MAT mutants (lower Km value in contrast to native MATs), the gained activity toward the bulky SAAMs stems from their ability to maintain the desired linear SN2 transition state (reflected by higher kcat value). Here the I117A mutant of human MATI was identified as the most active variant for biochemical production of SAM analogues from diverse SAAMs.

Expression, purification, and characterization of a recombinant methionine adenosyltransferase pDS16 in Pichia pastoris

Methionine adenosyltransferase (MAT, EC2.5.1.6) catalyzes the synthesis of S-adenosylmethionine (SAM) using L-methionine and adenosine triphosphate (ATP) as substrates. The mutant MAT pDS16 was obtained through DNA shuffling previously in our lab. Overexpression of pDS16 in Pichia pastoris led to about 65 % increase of MAT activity and SAM accumulation, compared with the strain overexpressing Saccharomyces cerevisiae MAT gene SAM2. Different strategies were tested to facilitate the expression and purification of pDS16. However, addition of the hexahistidine tag to pDS16 was shown to decrease the enzyme activity, and the yeast α-factor signal sequence could not effectivley direct the secretion of pDS16. The intracellular pDS16 was purified by a simple two-step procedure combining an ion exchange and hydrophobic interaction chromatography. Protein purity was verified by sodium dodecyl sulfate polyacrylamide gel electrophoresis to be 93%, with the specific activity of 1.828 U/mg. Two-dimensional electrophoresis revealed pI of ∼5.5. The purified enzyme followed Michaelis kinetics with a Km of 1.72 and 0.85 mM, and Vmax of 1.54 and 1.15 μmol/min/mg for ATP and L-methionine, respectively. pDS16 exhibited optimal activity at pH 8.5 and 45 °C with the requirement of divalent cation Mg(2+) and was slightly stimulated by the monovalent cation K(+). It showed an improved thermostability, about 50% of the enzyme activity was retained even after preincubation at 50 °C for 2 h.

Structural and functional characterisation of the methionine adenosyltransferase from Thermococcus kodakarensis

BACKGROUND

Methionine adenosyltransferases catalyse the synthesis of S-adenosylmethionine, a cofactor abundant in all domains of life. In contrast to the enzymes from bacteria and eukarya that show high sequence similarity, methionine adenosyltransferases from archaea diverge on the amino acid sequence level and only few conserved residues are retained.

RESULTS

We describe the initial characterisation and the crystal structure of the methionine adenosyltransferase from the hyperthermophilic archaeon Thermococcus kodakarensis. As described for other archaeal methionine adenosyltransferases the enzyme is a dimer in solution and shows high temperature stability. The overall structure is very similar to that of the bacterial and eukaryotic enzymes described, with some additional features that might add to the stability of the enzyme. Compared to bacterial and eukaryotic structures, the active site architecture is largely conserved, with some variation in the substrate/product-binding residues. A flexible loop that was not fully ordered in previous structures without ligands in the active side is clearly visible and forms a helix that leaves an entrance to the active site open.

CONCLUSIONS

The similar three-dimensional structures of archaeal and bacterial or eukaryotic methionine adenosyltransferases support that these enzymes share an early common ancestor from which they evolved independently, explaining the low similarity in their amino acid sequences. Furthermore, methionine adenosyltransferase from T. kodakarensis is the first structure without any ligands bound in the active site where the flexible loop covering the entrance to the active site is fully ordered, supporting a mechanism postulated earlier for the methionine adenosyltransferase from E. coli. The structure will serve as a starting point for further mechanistic studies and permit the generation of enzyme variants with different characteristics by rational design.

Engineered Pichia pastoris for enhanced production of S-adenosylmethionine

A genetically engineered strain of Pichia pastoris expressing S-adenosylmethionine synthetase gene from Saccharomyces cerevisiae under the control of AOX 1 promoter was developed. Induction of recombinant strain with 1% methanol resulted in the expression of SAM2 protein of ~ 42 kDa, whereas control GS115 showed no such band. Further, the recombinant strain showed 17-fold higher enzyme activity over control. Shake flask cultivation of engineered P. pastoris in BMGY medium supplemented with 1% L-methionine yielded 28 g/L wet cell weight and 0.6 g/L S-adenosylmethionine, whereas control (transformants with vector alone) with similar wet cell weight under identical conditions accumulated 0.018 g/L. The clone cultured in the bioreactor containing enriched methionine medium showed increased WCW (117 g/L) as compared to shake flask cultures and yielded 2.4 g/L S-adenosylmethionine. In spite of expression of SAM 2 gene up to 90 h, S-adenosylmethionine accumulation tended to plateau after 72 h, presumably because of the limited ATP available in the cells at stationery phase. The recombinant P pastoris seems promising as potential source for industrial production of S-adenosylmethionine.

Structural basis for the stability of a thermophilic methionine adenosyltransferase against guanidinium chloride

The methionine adenosyltransferase from the thermophile Methanococcus jannaschii is fully and irreversibly unfolded in the presence of guanidinium chloride. Unfolding of this dimeric protein is a three-state process in which a dimeric intermediate could be identified. The less stable secondary structural elements of the protein are the C-terminal ends of β-strands E2 and E6, as deduced from the behavior of tyrosine to tryptophan mutants at residues 72 and 170, which are located in the subunit interface. Unraveling of these elements at the monomer interface may soften intersubunit interactions, leading to the observed 85% activity loss. Accumulation of the intermediate was associated with maintenance of residual activity, an increase in the elution volume of the protein upon gel filtration and a decrease in the sedimentation coefficient. Elimination of the remaining enzymatic activity occurred in conjunction with a 50% reduction in helicity and fluorescence alterations illustrating a transient burial of tryptophans at β-strands E2, E3 and E9. The available 3D-model predicted that these β-strands are involved in the central and N-terminal domains of the monomer structure. Severe perturbation of this area of the monomer-monomer interface may destroy the remaining intermolecular interactions, thus leading to dissociation and aggregation. Finally, transition to the denatured state includes completion of the changes detected in the microenvironments around tryptophans included at α-helixes H5 and H6, the loops connecting H5-E8 and E9, β-strands E3 and E12.

Structural and functional characterization of S-adenosylmethionine (SAM) synthetase from Pichia ciferrii

S-adenosylmethionine synthetase (SAM-s) catalyzes the synthesis of S-adenosylmethionine (SAM), which is essential for methylation, transcription, proliferation, and production of secondary metabolites. Here SAM-s from Pichia ciferrii were selectively cloned using RNA CapFishing and rapid amplification of cDNA ends (RACE). The putative full-length cDNA of SAM-s encoded a 383 amino acid protein (42.6 kDa), which has highly conserved metal binding sites, a phosphate-binding site, and functionally important motifs. The corresponding enzyme was over-expressed in a heterologous host of Pichia pastoris, and then purified to a homogenous form. Enzyme kinetics, immunoblotting, circular dichroism (CD), high performance liquid chromatography (HPLC), and molecular modeling were conducted to characterize the SAM-s from P. ciferrii. Structural and functional studies of SAM-s will provide important insights for industrial applications.

Refolding and characterization of methionine adenosyltransferase from Euglena gracilis

Methionine adenosyltransferase from Euglena gracilis (MATX) is a recently discovered member of the MAT family of proteins that synthesize S-adenosylmethionine. Heterologous overexpression of MATX in Escherichia coli rendered the protein mostly in inclusion bodies under all conditions tested. Therefore, a refolding and purification procedure from these aggregates was developed to characterize the enzyme. Maximal recovery was obtained using inclusion bodies devoid of extraneous proteins by washing under mild urea (2M) and detergent (5%) concentrations. Refolding was achieved in two steps following solubilization in the presence of Mg(2+); chaotrope dilution to <1M and dialysis under reducing conditions. Purified MATX is a homodimer that exhibits Michaelis kinetics with a V(max) of 1.46 μmol/min/mg and K(m) values of approximately 85 and 260 μM for methionine and ATP, respectively. The activity is dependent on Mg(2+) and K(+) ions, but is not stimulated by dimethylsulfoxide. MATX exhibits tripolyphosphatase activity that is stimulated in the presence of S-adenosylmethionine. Far-UV circular dichroism revealed β-sheet and random coil as the main secondary structure elements of the protein. The high level of sequence conservation allowed construction of a structural model that preserved the main features of the MAT family, the major changes involving the N-terminal domain.

Methylthioadenosine/S-adenosylhomocysteine nucleosidase, a critical enzyme for bacterial metabolism

The importance of methylthioadenosine/S-adenosylhomocysteine (MTA/SAH) nucleosidase in bacteria has started to be appreciated only in the past decade. A comprehensive analysis of its various roles here demonstrates that it is an integral component of the activated methyl cycle, which recycles adenine and methionine through S-adenosylmethionine (SAM)-mediated methylation reactions, and also produces the universal quorum-sensing signal, autoinducer-2 (AI-2). SAM is also essential for synthesis of polyamines, N-acylhomoserine lactone (autoinducer-1), and production of vitamins and other biomolecules formed by SAM radical reactions. MTA, SAH and 5'-deoxyadenosine (5'dADO) are product inhibitors of these reactions, and are substrates of MTA/SAH nucleosidase, underscoring its importance in a wide array of metabolic reactions. Inhibition of this enzyme by certain substrate analogues also limits synthesis of autoinducers and hence causes reduction in biofilm formation and may attenuate virulence. Interestingly, the inhibitors of MTA/SAH nucleosidase are very effective against the Lyme disease causing spirochaete, Borrelia burgdorferi, which uniquely expresses three homologous functional enzymes. These results indicate that inhibition of this enzyme can affect growth of different bacteria by affecting different mechanisms. Therefore, new inhibitors are currently being explored for development of potential novel broad-spectrum antimicrobials.

Subunit association as the stabilizing determinant for archaeal methionine adenosyltransferases

Archaea contain a class of methionine adenosyltransferases (MATs) that exhibit substantially higher stability than their mesophilic counterparts. Their sequences are highly divergent, but preserve the essential active site motifs of the family. We have investigated the origin of this increased stability using chemical denaturation experiments on Methanococcus jannaschii MAT (Mj-MAT) and mutants containing single tryptophans in place of tyrosine residues. The results from fluorescence, circular dichroism, hydrodynamic, and enzyme activity measurements showed that the higher stability of Mj-MAT derives largely from a tighter association of its subunits in the dimer. Local fluorescence changes, interpreted using secondary structure predictions, further identify the least stable structural elements as the C-terminal ends of beta-strands E2 and E6, and the N-terminus of E3. Dimer dissociation however requires a wider perturbation of the molecule. Additional analysis was initially hindered by the lack of crystal structures for archaeal MATs, a limitation that we overcame by construction of a 3D-homology model of Mj-MAT. This model predicts preservation of the chain topology and three-domain organization typical of this family, locates the least stable structural elements at the flat contact surface between monomers, and shows that alterations in all three domains are required for dimer dissociation.