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

The carbonyl-sulfur chalcogen bonding interaction: Rotational spectroscopic study of the 2,2,4,4-tetrafluoro-1,3-dithietane···formaldehyde complex

The rotational spectrum of a binary molecular cluster consisting of 2,2,4,4-tetrafluoro-1,3-dithietane (C2S2F4) and formaldehyde (H2CO) was studied by means of high-resolution Fourier transform microwave spectroscopy in conjunction with quantum chemical calculations. One of the three isomers predicted at the B3LYP-D3(BJ)/def2-TZVP level of theory was successfully detected in the supersonic expansion. Theoretical analyses using the non-covalent interactions and natural bond orbital methods reveal that the observed isomer is primarily stabilized by one C=O⋯S chalcogen bond and two C-H⋯F hydrogen bonds. The distance between the oxygen atom of H2CO and the nearest sulfur atom of C2S2F4 within the observed isomer is 2.9260(1) Å and the angle ∠O⋯S-C is 161.83(1)°. The analysis utilizing the symmetry-adapted perturbation theory approach demonstrates that electrostatic interactions play a significant role in stabilizing the studied complex, with the contribution of dispersion interactions being comparable to that of electrostatic ones.

Exploring the influence of (n - 1)d subvalence correlation and of spin-orbit coupling on chalcogen bonding

This article presents a comprehensive computational investigation into chalcogen bonding interactions, focusing specifically on elucidating the role of subvalence (n - 1)d and (n - 1)sp correlation. The incorporation of inner-shell (n - 1)d correlation leads to a decrease in interaction energies for chalcogen-bonded systems (at least those studied herein), contradicting the observations regarding halogen bonding documented by Kesharwani et al. in J. Phys. Chem. A, 2018, 122 (8), 2184-2197. The significance of (n - 1)sp subvalence correlation appears to be lower by an order of magnitude. Notably, among the various components of interaction energies computed at the PNO-LCCSD(T) or DF-CCSD levels, we identify the PNO-LMP2 or DF-MP2 component of the (n - 1)d correlation as predominant. Furthermore, we delve into the impact of second-order spin-orbit coupling (SOC2) on these interactions. The SOC2 effects appear to be less significant than the (n - 1)d correlation; however, they remain non-trivial, particularly for Te complexes. For the Se complexes, SOC2 is much less important. Generally, SOC2 stabilizes monomers more than dimers, resulting in reduced binding of the latter. Notably, at equilibrium and stretched geometries, SOC2 and (n - 1)d destabilize the complex; however, at compressed geometries, they exhibit opposing effects, with (n - 1)d becoming stabilizing.

Parameterization of a single H-bond in Orange Carotenoid Protein by atomic mutation reveals principles of evolutionary design of complex chemical photosystems

Introduction: Dissecting the intricate networks of covalent and non-covalent interactions that stabilize complex protein structures is notoriously difficult and requires subtle atomic-level exchanges to precisely affect local chemical functionality. The function of the Orange Carotenoid Protein (OCP), a light-driven photoswitch involved in cyanobacterial photoprotection, depends strongly on two H-bonds between the 4-ketolated xanthophyll cofactor and two highly conserved residues in the C-terminal domain (Trp288 and Tyr201). Method: By orthogonal translation, we replaced Trp288 in Synechocystis OCP with 3-benzothienyl-L-alanine (BTA), thereby exchanging the imino nitrogen for a sulphur atom. Results: Although the high-resolution (1.8 Å) crystal structure of the fully photoactive OCP-W288_BTA protein showed perfect isomorphism to the native structure, the spectroscopic and kinetic properties changed distinctly. We accurately parameterized the effects of the absence of a single H-bond on the spectroscopic and thermodynamic properties of OCP photoconversion and reveal general principles underlying the design of photoreceptors by natural evolution. Discussion: Such "molecular surgery" is superior over trial-and-error methods in hypothesis-driven research of complex chemical systems.

Chalcogen bond-guided conformational isomerization enables catalytic dynamic kinetic resolution of sulfoxides

Conformational isomerization can be guided by weak interactions such as chalcogen bonding (ChB) interactions. Here we report a catalytic strategy for asymmetric access to chiral sulfoxides by employing conformational isomerization and chalcogen bonding interactions. The reaction involves a sulfoxide bearing two aldehyde moieties as the substrate that, according to structural analysis and DFT calculations, exists as a racemic mixture due to the presence of an intramolecular chalcogen bond. This chalcogen bond formed between aldehyde (oxygen atom) and sulfoxide (sulfur atom), induces a conformational locking effect, thus making the symmetric sulfoxide as a racemate. In the presence of N-heterocyclic carbene (NHC) as catalyst, the aldehyde moiety activated by the chalcogen bond selectively reacts with an alcohol to afford the corresponding chiral sulfoxide products with excellent optical purities. This reaction involves a dynamic kinetic resolution (DKR) process enabled by conformational locking and facile isomerization by chalcogen bonding interactions.

CHAL336 Benchmark Set: How Well Do Quantum-Chemical Methods Describe Chalcogen-Bonding Interactions?

We present the CHAL336 benchmark set-the most comprehensive database for the assessment of chalcogen-bonding (CB) interactions. After careful selection of suitable systems and identification of three high-level reference methods, the set comprises 336 dimers each consisting of up to 49 atoms and covers both σ- and π-hole interactions across four categories: chalcogen-chalcogen, chalcogen-π, chalcogen-halogen, and chalcogen-nitrogen interactions. In a subsequent study of DFT methods, we re-emphasize the need for using proper London dispersion corrections when treating noncovalent interactions. We also point out that the deterioration of results and systematic overestimation of interaction energies for some dispersion-corrected DFT methods does not hint at problems with the chosen dispersion correction but is a consequence of large density-driven errors. We conclude this work by performing the most detailed DFT benchmark study for CB interactions to date. We assess 109 variations of dispersion-corrected and dispersion-uncorrected DFT methods and carry out a detailed analysis of 80 of them. Double-hybrid functionals are the most reliable approaches for CB interactions, and they should be used whenever computationally feasible. The best three double hybrids are SOS0-PBE0-2-D3(BJ), revDSD-PBEP86-D3(BJ), and B2NCPLYP-D3(BJ). The best hybrids in this study are ωB97M-V, PW6B95-D3(0), and PW6B95-D3(BJ). We do not recommend using the popular B3LYP functional nor the MP2 approach, which have both been frequently used to describe CB interactions in the past. We hope to inspire a change in computational protocols surrounding CB interactions that leads away from the commonly used, popular methods to the more robust and accurate ones recommended herein. We would also like to encourage method developers to use our set for the investigation and reduction of density-driven errors in new density functional approximations.

Conserved metabolite regulation of stress granule assembly via AdoMet

Stress granules (SGs) are evolutionarily conserved condensates of ribonucleoproteins that assemble in response to metabolic stresses. Because aberrant SG formation is associated with amyotrophic lateral sclerosis (ALS), understanding the connection between metabolic activity and SG composition can provide therapeutic insights into neurodegeneration. Here, we identify 17 metabolic enzymes recruited to yeast SGs in response to physiological growth stress. Furthermore, the product of one of these enzymes, AdoMet, is a regulator of SG assembly and composition. Decreases in AdoMet levels increase SG formation, while chronic elevation of AdoMet produces SG remnants lacking proteins associated with the 5′ end of transcripts. Interestingly, acute elevation of AdoMet blocks SG formation in yeast and motor neurons. Treatment of ALS-derived motor neurons with AdoMet also suppresses the formation of TDP-43–positive SGs, a hallmark of ALS. Together, these results argue that AdoMet is an evolutionarily conserved regulator of SG composition and assembly with therapeutic potential in neurodegeneration.

Nature of the E⋯E′ interactions (E, E′ = O, S, Se, and Te) at naphthalene 1,8-positions with fine details of the structures: experimental and theoretical investigations

The nature of E⋯E′ in 1-RE–8-R′E′C₁₀H₆ (E/E′ = O, S, Se and Te) is clarified with the QTAIM approach and NBO analysis, after structural determinations.

Rotational characterization of S⋯F chalcogen bonds in the complex of 2,2,4,4-tetrafluoro-1,3-dithietane and difluoromethane

The nature of S⋯F chalcogen bonds and C–H⋯F and C–F⋯F–C contacts was characterized by rotational spectroscopy for the first time.

Chalcogen Bonding in Protein-Ligand Complexes: PDB Survey and Quantum Mechanical Calculations

A chalcogen bond is a nonclassical noncovalent interaction which can stabilise small-molecule crystals as well as protein structures. Here, we systematically explore the stabilising potential of chalcogen bonding in protein-ligand complexes in the Protein Data Bank (PDB). We have found that a large fraction (23 %) of complexes with a S/Se-containing ligand feature close S/Se⋅⋅⋅O/N/S contacts. Eleven non-redundant representative potential S/Se⋅⋅⋅O chalcogen-bond motifs were selected and truncated to model systems and seven more model systems were prepared by S-to-Se substitution. These systems were then subjected to analysis by quantum chemical (QM) methods-electrostatic potential, geometry optimisation or interaction energy calculations, including solvent effects. The QM calculations indicate that chalcogen bonding does indeed play a dominant role in stabilising some of the interaction motifs studied. We thus advocate further exploration of chalcogen bonding with the aim of potential future use in structure-based drug design.

S⋯S and S⋯P chalcogen bonding in solution: a cryospectroscopic study of the complexes of 2,2,4,4-tetrafluoro-1,3-dithietane with dimethyl sulfide and trimethylphosphine

Experimental evidence on the formation of S⋯S and P⋯S chalcogen bonded complexes between 2,2,4,4-tetrafluoro-1,3-dithiethane and the Lewis bases dimethyl sulfide and trimethylphosphine is obtained using infrared spectroscopy of solutions in liquid krypton.

Chalcogen- and halogen-bonds involving SX2 (X = F, Cl, and Br) with formaldehyde

The capacity of SX2 (X = F, Cl, and Br) to engage in different kinds of noncovalent bonds was investigated by ab initio calculations. SCl2 (SBr2) has two σ-holes upon extension of Cl (Br)-S bonds, and two σ-holes upon extension of S-Cl (Br) bonds. SF2 contains only two σ-holes upon extension of the F-S bond. Consequently, SCl2 and SBr2 form chalcogen and halogen bonds with the electron donor H2CO while SF2 forms only a chalcogen bond, i.e., no F···O halogen bond was found in the SF2:H2CO complex. The S···O chalcogen bond between SF2 and H2CO is the strongest, while the strongest halogen bond is Br···O between SBr2 and H2CO. The nature of these two types of noncovalent interaction was probed by a variety of methods, including molecular electrostatic potentials, QTAIM, energy decomposition, and electron density shift maps. Termolecular complexes X2S···H2CO···SX'2 (X = F, Cl, Br, and X' = Cl, Br) were constructed to study the interplay between chalcogen bonds and halogen bonds. All these complexes contained S···O and Cl (Br)···O bonds, with longer intermolecular distances, smaller values of electron density, and more positive three-body interaction energies, indicating negative cooperativity between the chalcogen bond and the halogen bond. In addition, for all complexes studied, interactions involving chalcogen bonds were more favorable than those involving halogen bonds. Graphical Abstract Molecular electrostatic potential and contour map of the Laplacian of the electron density in Cl2S···H2CO···SCl2 complex.

Designing metal hydride complexes for water splitting reactions: a molecular electrostatic potential approach

The hydridic character of octahedral metal hydride complexes of groups VI, VII and VIII has been systematically studied using molecular electrostatic potential (MESP) topography. The absolute minimum of MESP at the hydride ligand (Vmin) and the MESP value at the hydride nucleus (VH) are found to be very good measures of the hydridic character of the hydride ligand. The increasing/decreasing electron donating feature of the ligand environment is clearly reflected in the increasing/decreasing negative character of Vmin and VH. The formation of an outer sphere metal hydride-water complex showing the HH dihydrogen interaction is supported by the location and the value of Vmin near the hydride ligand. A higher negative MESP suggested lower activation energy for H2 elimination. Thus, MESP features provided a way to fine-tune the ligand environment of a metal-hydride complex to achieve high hydridicity for the hydride ligand. The applicability of an MESP based hydridic descriptor in designing water splitting reactions is tested for group VI metal hydride model complexes of tungsten.

Chalcogen bonding in solution: interactions of benzotelluradiazoles with anionic and uncharged Lewis bases

Chalcogen bonding is the noncovalent interaction between an electron-deficient, covalently bonded chalcogen (Te, Se, S) and a Lewis base. Although substantial evidence supports the existence of chalcogen bonding in the solid state, quantitative data regarding the strengths of the interactions in the solution phase are lacking. Herein, determinations of the association constants of benzotelluradiazoles with a variety of Lewis bases (Cl(-), Br(-), I(-), NO3(-) and quinuclidine, in organic solvent) are described. The participation of the benzotelluradiazoles in chalcogen bonding interactions was probed by UV-vis, (1)H and (19)F NMR spectroscopy as well as nano-ESI mass spectrometry. Trends in the free energy of chalcogen bonds upon variation of the donor, acceptor and solvent are evident from these data, including a linear free energy relationship between chalcogen bond donor ability and calculated electrostatic potential at the tellurium center. Calculations using the dispersion-corrected B97-D3 functional were found to give good agreement with the experimental free energies of chalcogen bonding.

Hypervalent nonbonded interactions of a divalent sulfur atom. Implications in protein architecture and the functions

In organic molecules a divalent sulfur atom sometimes adopts weak coordination to a proximate heteroatom (X). Such hypervalent nonbonded S···X interactions can control the molecular structure and chemical reactivity of organic molecules, as well as their assembly and packing in the solid state. In the last decade, similar hypervalent interactions have been demonstrated by statistical database analysis to be present in protein structures. In this review, weak interactions between a divalent sulfur atom and an oxygen or nitrogen atom in proteins are highlighted with several examples. S···O interactions in proteins showed obviously different structural features from those in organic molecules (i.e., π(o) → σ(s)* versus n(o) → σ(s)* directionality). The difference was ascribed to the HOMO of the amide group, which expands in the vertical direction (π(o)) rather than in the plane (n(o)). S···X interactions in four model proteins, phospholipase A₂ (PLA₂), ribonuclease A (RNase A), insulin, and lysozyme, have also been analyzed. The results suggested that S···X interactions would be important factors that control not only the three-dimensional structure of proteins but also their functions to some extent. Thus, S···X interactions will be useful tools for protein engineering and the ligand design.

Polar coordinate representation of Hb(rc) versus (h2/8m)nabla2rhob(rc) at BCP in AIM analysis: classification and evaluation of weak to strong interactions

Polar coordinate (R, theta) representation is proposed for the plot of Hb(rc) versus (h2/8m)nabla2rhob(rc) in AIM analysis to classify, evaluate, and understand weak to strong interactions in a unified way and in more detail; Hb(rc) and nabla2rhob(rc) are total electron energy densities and the Laplacian of rhob(rc) at bond critical points (BCPs: rc), respectively, where rhob(rc) are electron densities at rc. Both the x- and y-axes of the plot are expressed in the common unit of energy since Hb(rc) = Gb(rc) + Vb(rc) and (h2/8m)nabla2rhob(rc) = Hb(rc) - Vb(rc)/2 (= Gb(rc) + Vb(rc)/2), where Gb(rc) and Vb(rc) are kinetic energy densities and potential energy densities, respectively. Data employed for the plot are calculated at BCPs for full-optimized structures and optimized structures with the fixed distances (r) of r = r(o) + wa(o), where r(o) are the full-optimized distances, a(o) is the Bohr radius, and w = +/-0.1 and +/-0.2. The plot draws a helical stream starting from near origin (Hb(rc) = (h2/8m)nabla2rhob(rc) = 0) for very weak interactions and turns to the right as interactions become stronger. The helical stream is well described by the polar coordinate representation with (R, theta); R is given in the energy unit, and theta in degrees is measured from the y-axis. The ratio of Vb(rc)/Gb(rc) (= k) controls theta, of which an acceptable range in the plot is 45.0 < theta < 206.6 degrees. Each plot for an interaction gives a curve, which supplies important information. It is expressed by theta(p) and kappa(p); theta(p) corresponds to the tangent line measured from the y-direction, and kappa(p) is the curvature of the plot at w = 0. The polar coordinate (R, theta) representation with (theta(p), kappa(p)) helps us to classify, evaluate, and understand the nature of weak to strong interactions in a unified way.

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.

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.

Proteomic survey of copper-binding proteins inArabidopsis roots by immobilized metal affinity chromatography and mass spectrometry

To plants, copper is vitally essential at low concentrations but extremely toxic at elevated concentrations. Plants have evolved a suite of mechanisms that modulate the uptake, distribution, and utilization of copper ions. These mechanisms require copper-interacting proteins for transporting, chelating, and sequestrating copper ions. In this study, we have systematically screened for copper-interacting proteins in Arabidopsis roots via copper-immobilized metal affinity chromatography (Cu-IMAC). We also compared Arabidopsis root metalloproteomes with affinity to Cu-IMAC and Zn-IMAC. From the identities of 38 protein spots with affinity to Cu-IMAC, 35 unique proteins were identified. Functional classification of these proteins includes redox/hydrolytic reactions, amino acid metabolism, glutathione metabolism, phosphorylation, translation machinery, membrane-associated proteins, and vegetative storage proteins. Potential copper-interacting motifs were predicted and scored. Six candidate motifs, H-(X)5 -H, H-(X)7 -H, H-(X)12 -H, H-(X)6 -M, M-(X)7 -H, and H-(X)3 -C, are present in Cu-IMAC-isolated proteins with higher frequency than in the whole Arabidopsis proteome.

Possible roles of S···O and S···N interactions in the functions and evolution of phospholipase A2

To investigate possible roles of S···X (X= O, N, S) interactions in the functions and evolution of a protein, two types of database analyses were carried out for a vertebrate phospholipase A₂ (PLA₂) family. A comprehensive search for close S···X contacts in the structures retrieved from protein data bank (PDB) revealed that there are four common S···O interactions and one common S···N interaction for the PLA₂ domain group (PLA₂-DG), while an additional three S···O interactions were found for the snake PLA₂ domain group (sPLA₂-DG). On the other hand, a phylogenetic analysis on the conservation of the observed S···O and S···N interactions over various amino acid sequences of sPLA₂-DG demonstrated probable clustering of the interactions on the dendrogram. Most of the interactions characterized for PLA₂ were found to reside in the vicinity of the active site and to be able to tolerate the conformational changes due to the substrate binding. These observations suggested that the S···X interactions play some role in the functions and evolution of the PLA₂ family.

Mutational analysis of methionine adenosyltransferase from Leishmania donovani

The 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. This report describes the mutational analysis of the amino acids involved in both the ATP and L-methionine binding sites of Leishmania donovani MAT (GenBank accession number AF179714) the aetiological agent of visceral leishmaniasis. Site-directed mutagenesis was used to substitute neutral residues for the basic amino acid (Lys168, Lys256, Lys276, Lys280 and His17), acidic residues (Asp19, Asp121, Asp166, Asp249, Asp277 and Asp288) and Phe241 involved in AdoMet synthesis and PPPi hydrolysis. With the exception of D116N, none of these mutants was able to synthesize AdoMet at a significant rate, although H17A, H17N, K256A, K280A, D19N, D121N, D166N, D249N and D282N showed measurable tripolyphosphatase activity. Finally, the C-terminus domain of L. donovani MAT was truncated at three points (F382Stop, D375Stop, F368Stop), deleting a 3(10) one-turn helix motif in all three cases. Whilst none of the truncated proteins conserved MAT activity, they were able to hydrolyse PPPi, albeit at a lower rate than the wild-type enzyme. A fourth protein with an internal deletion (E376DeltaF382) in the C-terminal domain conserved high tripolyphosphatase activity, which was not, however, induced by 50 microM AdoMet.

Methionine adenosyltransferase as a useful molecular systematics tool revealed by phylogenetic and structural analyses

Structural and phylogenetic relationships among Bacteria and Eukaryota were analyzed by examining 292 methionine adenosyltransferase (MAT) amino acid sequences with respect to the crystal structure of this enzyme established for Escherichia coli and rat liver. Approximately 30% of MAT residues were found to be identical in all species. Five highly conserved amino acid sequence blocks did not vary in the MAT family. We detected specific structural features that correlated with sequence signatures for several clades, allowing taxonomical identification by sequence analysis. In addition, the number of amino acid residues in the loop connecting beta-strands A2 and A3 served to clearly distinguish sequences between eukaryotes and eubacteria. The molecular phylogeny of MAT genes in eukaryotes can be explained in terms of functional diversification coupled to gene duplication or alternative splicing and adaptation through strong structural constraints. Sequence analyses and intron/exon junction positions among nematodes, arthropods and vertebrates support the traditional Coelomata hypothesis. In vertebrates, the liver MAT I isoenzyme has gradually adapted its sequence towards one providing a more specific liver function. MAT phylogeny also served to cluster the major bacterial groups, demonstrating the superior phylogenetic performance of this ubiquitous, housekeeping gene in reconstructing the evolutionary history of distant relatives.

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.

The crystal structure of tetrameric methionine adenosyltransferase from rat liver reveals the methionine-binding site

Most of the transmethylation reactions use the same methyl donor, S-adenosylmethionine (SAM), that is synthesised from methionine and ATP by methionine adenosyltransferase (MAT). In mammals, two MAT enzymes have been detected, one ubiquitous and another liver specific. The liver enzyme exists in two oligomeric forms, a tetramer (MAT I) and a dimer (MAT III), MAT I being the one that shows a higher level of affinity for methionine but a lower SAM synthesis capacity. We have solved the crystal structure of rat liver MAT I at 2.7 A resolution, complexed with a methionine analogue: l-2-amino-4-methoxy-cis-but-3-enoic acid (l-cisAMB). The enzyme consists of four identical subunits arranged in two tight dimers that are related by crystallographic 2-fold symmetry. The crystal structure shows the positions of the relevant cysteine residues in the chain, and that Cys35 and Cys61 are perfectly oriented for forming a disulphide link. This result leads us to propose a hypothesis to explain the control of MAT I/III exchange and hence, the effects observed on activity. We have identified the methionine-binding site into the active-site cavity, for the first time. The l-cisAMB inhibitor is stacked against Phe251 aromatic ring in a rather planar conformation, and its carboxylate group coordinates a Mg(2+), which, in turn, is linked to Asp180. The essential role of the involved residues in MAT activity has been confirmed by site-directed mutagenesis. Phe251 is exposed to solvent and is located in the beginning of the flexible loop Phe251-Ala260 that is connecting the N-terminal domain to the central domain. We postulate that a conformational change may take place during the enzymatic reaction and this is possibly the reason of the unusual two-step mechanism involving tripolyphosphate hydrolysis. Other important mechanistic implications are discussed on the light of the results. Moreover, the critical role that certain residues identified in this study may have in methionine recognition opens further possibilities for rational drug design.

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.