Methionine Derivatives Research Articles

Unravelling the Structure of One-Electron Oxidation Products of Model Peptide Backbones Containing Methionine by IRMPD Spectroscopy: the Effect of the Neighbouring Groups.

A series of Methionine (Met) derivatives, where either the amino group and/or the carboxylic acid group is blocked by acetyl and/or methyl ester functionalities, has been investigated by Collision Induced Dissociation-tandem mass spectrometry (CID-MS2) and Infrared Multiple Photon Dissociation (IRMPD) spectroscopy. The CID-MS2 experiments were performed using a Fourier-transform ion-cyclotron-resonance (FT-ICR) mass spectrometer equipped with an electrospray ionization (ESI) source. The IRMPD spectra were recorded employing a Paul type ion-trap coupled with the free-electron laser (FEL) FELIX in the fingerprint region from 600 to 2000 cm-1. We show that the oxidation of the methionine residue with protected terminal groups induces the formation of a sulfoxide function. However, compared to the IRMPD spectrum of protonated methionine and methionine sulfoxide, significant spectral differences are observed in the spectra of model protected peptide backbones containing methionine. DFT calculations show that protonation occurs on the sulfoxide group in the gas phase for these model peptide backbones containing methionine, shifting the diagnostic signature of the sulfoxide group from 1000 to 862 cm-1.

Antioxidant cysteine and methionine derivatives show trachea disruption in insects.

To prevent the deterioration of the global environment, the reduction of chemical pesticide use and the development of eco-friendly pest control technologies are urgent issues. Our recent study revealed that the production of reactive oxygen species (ROS) by dual oxidase (Duox) plays a pivotal role in stabilizing the tracheal network by intermediating the tyrosine cross-linking of proteins that constitute trachea. Notably, the formation of dityrosine bonds by ROS can be inhibited by the intake of an antioxidant cysteine derivative N-acetyl-L-cysteine (NAC), which can suppress insect respiration. In this study, we screened for the derivatives showing insecticidal activity and tracheal formation inhibition. As a result of investigating the soybean pest bug Riptortus pedestris, cysteine and methionine derivatives showed respiratory formation inhibition and high insecticidal activity. In particular, NAC had a slow-acting insecticidal effect, while L-cysteine methyl ester (L-CME) showed relatively fast-acting insecticidal activity. Furthermore, the insecticidal activity of these derivatives was also detected in Drosophila, mealworms, cockroaches, termites, and plant bugs. Our results suggest that some antioxidant compounds have specific tracheal inhibitory activity in different insect species and they may be used as novel pest control agents upon further characterization.

Metabolization of Free and Peptide-Bound Oxidized Methionine Derivatives by Saccharomyces cerevisiae in a Model System.

In the brewing process, methionine is a decisive amino acid for (off-)flavor formation. A significant part of methionine is oxidized to methionine sulfoxide (MetSO) in malt. We hypothesized that MetSO and MetSO2 are metabolized to volatile compounds during yeast fermentation and examined whether the yeast Saccharomyces cerevisiae is able to catabolize l-MetSO and l-MetSO2 in free and dipeptide-bound forms. We also investigated the stability of l-methionine sulfoximine and S-methylmethionine. Cell viability in the presence of the test compounds was at least 90%. Both free and peptide-bound test substances were metabolized by Saccharomyces cerevisiae. l-MetSO was degraded most rapidly as the free amino acid, while l-MetSO2 was degraded most rapidly bound in dipeptides. We observed a different degradation behavior of the (R) and (S) diastereoisomers for l-MetSO and l-methionine sulfoximine. Furthermore, we detected methionol as the only metabolite of MetSO. Methionol sulfoxide was not formed. MetSO2 was not converted to methionol or methionol sulfone but to the respective α-hydroxy acid. We conclude that the reduction of MetSO to methionine proceeds faster than transamination. The occurrence of MetSO or MetSO2 in brewing malt will not lead to the formation of hitherto unknown volatile metabolites of the Ehrlich pathway.

Identifying distinct markers in two Sorghum varieties for baijiu fermentation using untargeted metabolomics and molecular network approaches

The quality of strong-flavor Baijiu, a prominent Chinese liquor, is intricately tied to the choice of sorghum variety used in fermentation. However, a significant gap remains in our understanding of how glutinous and non-glutinous sorghum varieties comprehensively impact Baijiu flavor formation through fermentation metabolites. This study employed untargeted metabolomics combined with feature-based molecular networking (FBMN) to explore the unique metabolic characteristics of these two sorghum varieties during fermentation. FBMN analysis revealed 267 metabolites within both types of fermented sorghum (Zaopei) in the cellar. Further multidimensional statistical analyses highlighted sphingolipids, 2,5-diketopiperazines, and methionine derivatives as critical markers for quality control. These findings represent a significant advancement in our understanding and provide valuable insights for regulating the quality of Baijiu flavors.

Synthesis of novel zwitterionic nido-carborane-containing derivatives of cysteine and methionine

Synthesis of novel zwitterionic nido-carborane-containing derivatives of cysteine and methionine

Surface Interaction Effect and Mechanism of Methionine Derivatives as Novel Inhibitors for Alkaline Copper CMP: Insights from Molecular Simulation and Experimental Analysis

To prevent excessive corrosion caused by the slurry in the copper (Cu) chemical mechanical polishing (CMP) process, a corrosion inhibitor is normally required. In this study, the methionine (Met) derivative FMOC-L-Methionine (Fmoc-Met-OH) was explored as a corrosion inhibitor for Cu film CMP in weak alkaline conditions (pH = 8.5). A comprehensive evaluation was conducted to confirm the efficiency of Fmoc-Met-OH as a corrosion inhibitor, combining experiments and theoretical calculations. The results showed that Fmoc-Met-OH could effectively inhibit the corrosion of Cu, with a high inhibition efficiency (IE) of 78.26% while maintaining a high removal rate (RR) of 5703 Å min−1, a low static etch rate (SER) of 676 Å min−1, and a low surface root mean square deviation (Sq) of 1.41 nm. Simultaneously, the results of X-ray photoelectron spectroscopy (XPS) tests and electrochemical analysis confirm that Fmoc-Met-OH molecules can form a dense and ordered adsorption film on the Cu surface. According to the density functional theory (DFT) calculations and molecular dynamics (MD) simulation, it was verified that Fmoc-Met-OH exhibited strong chemical adsorption on Cu substrates, as evidenced by the high binding energy (E Binding) value, low energy gap (ΔE), and radial distribution function (RDF) analysis. The findings provided theoretical evidence of the better inhibition effectiveness of Fmoc-Met-OH at a molecular or atomic level.

Methionine in Poultry Nutrition: A Review

Methionine is an essential amino acid which is commonly used as a supplement in poultry diets. Multiple systems are involved in the absorption and transportation of liquid and powder Methionine in the segment of the jejunum. Methionine supplementation in a low-protein diet alleviates the negative effects of heat stress and improves the performance of chickens. The supplementation of the synthetic Methionine improves the amino acid balance and consequently promotes growth performance by enhancing quantity and quality of egg production, feed efficiency, and protein synthesis, as well as decreasing fat synthesis in poultry breeds (Broiler Chicken, Laying hen, Turkey, Duck, Guinea fowl, Quail breeder, and Gees). Methionine supplementation also improves the immune response through the direct effects on protein synthesis and breakdown and indirect effects on the derivatives of Methionine. The variables, such as growing period, type of production, sex, and breed, influence the Methionine requirement. Moreover, the Methionine requirement expressed as a percentage of diet declines during the starter and grower phases.

Homocysteine Metabolism in Pregnancy and Developmental Impacts.

Homocysteine is a metabolite generated by methionine cycle metabolism, comprising the demethylated derivative of methionine. Homocysteine can be metabolised by the transsulphuration pathway to cystathionine, which requires vitamin B6, or can undergo remethylation to methionine. Homocysteine remethylation to methionine is catalysed by methionine synthase activity which requires vitamin B12, regenerating methionine to allow synthesis of the universal methyl donor S-adenosylmethionine required for methylation and gene transcription regulation. The methyl-group donated for homocysteine remethylation comes from 5-methyltetrahydrofolate generated by the folate cycle, which allows tetrahydrofolate to be returned to the active folate pool for nucleotide biosynthesis. Therefore the integrated actions of the methionine and folate cycles, required to metabolise homocysteine, also perpetuate methylation and nucleotide synthesis, vitally important to support embryonic growth, proliferation and development. Dysregulated activities of these two interdependent metabolic cycles, arising from maternal suboptimal intake of nutrient co-factors such as folate and vitamin B12 or gene polymorphisms resulting in reduced enzymatic activity, leads to inefficient homocysteine metabolic conversion causing elevated concentrations, known as hyperhomocysteinemia. This condition is associated with multiple adverse pregnancy outcomes including neural tube defects (NTDs). Raised homocysteine is damaging to cellular function, binding to proteins thereby impairing their function, with perturbed homocysteine metabolism impacting negatively on embryonic development. This review discusses the “cross-talk” of maternal-fetal homocysteine interrelationships, describes the placental transport of homocysteine, homocysteine impacts on pregnancy outcomes, homocysteine and methylation effects linking to NTD risk and proposes a putative pathway for embryonic provision of folate and vitamin B12, homocysteine-modulating nutrients that ameliorate NTD risk.

Mechanisms of Substrate Selectivity and Transport by a Bacterial Methionine ABC Importer

ABC transporters are central in many cellular functions including nutrient uptake, signal transduction membrane assembly, and cellular detoxification. Both structural and functional studies have revealed insights into the high‐affinity uptake mechanism of the MetNI methionine ABC transporter. Using the energy from ATP binding and hydrolysis, the MetNI‐Q system can import L‐Met, D‐Met, and other methionine derivatives against concentration gradients. Many mechanistic studies of ABC transporter propose a model in which cognate binding proteins sequester substrates in the periplasm and deliver them to the transporter. In contrast, recent in vivo and crystallographic studies of MetNI‐Q suggest that some substrates may be able to access the transporter through a solvent accessible tunnel in the cognate binding protein‐transporter complex. These studies suggest that the substrate delivery to MetNI transporter may be more flexible than other ABC transport systems.To test if the MetNI‐Q system transport system uses two mechanisms of substrate delivery, we designed a series of experiments to dissect the individual steps in the transport cycle. As the starting point, we have developed a fluorescent anisotropy assay to measure the binding affinity between the MetQ substrate‐binding protein and the MetNI transporter in the presence of different methionine derivatives. Our preliminary data show that MetNI binds to apo‐MetQ ~10 fold tighter than L‐Met liganded MetQ, and additionally may differ in the presence of D‐Met. This suggests that different methionine derivatives may utilize discrete transport models.

Quinine as a highly responsive chiral sensor for the 1H and 19F NMR enantiodiscrimination of N-trifluoroacetyl amino acids with free carboxyl functions.

Hydrogen-bond accepting and enantiodiscriminating abilities of quinine (Qui) have been exploited in the enantiodiscrimination of N-trifluoroacetyl (TFA) derivatives of amino acids by nuclear magnetic resonance (NMR) spectroscopy. 1H and 19F NMR resonances of derivatives of alanine, valine, leucine, norvaline, phenylalanine, phenylglycine, methionine, glutamic acid, proline, and tryptophan were well differentiated employing CDCl3 and/or C6D6 as solvent, with Qui acting in some cases not only as enantiodiscriminating agent, but also as solubility promoter. For derivatives soluble in both solvents, the best results were obtained in benzene-d6, with very high nonequivalence values, which were detectable not only starting from very low equimolar concentrations of 0.1 mM, but also in the presence of sub-stoichiometric amounts of Qui. The quality of enantiodifferentiation has been also evaluated by means of the enantioresolution quotient E. The method has been applied to the detection and quantification of mixtures of amino acid derivatives by single point measurements.

A Study of the Antibacterial Activities and the Mode of Action of L-Methionine and L-Cystine Based Surfactants and their Interaction with Bovine Serum Albumin Using Fluorescence Spectroscopy and in Silico Modelling

Surfactants are versatile excipients that are commonly employed in diverse pharmaceutical formulations given their broad range of antimicrobial activities. Sulfur-based amino acid surfactants are promising candidates as substitutes for conventional antibacterial agents in light of the current antibiotic resistance crisis. Dodecyl esters of the free amine (SURF1), cationic ester hydrochloride (SURF2), and quaternary ammonium (QUAT3) derivatives of methionine, and the Gemini diester of cystine (GEM4) were synthesized and evaluated for antibacterial activity against Gram-positive and Gram-negative pathogens. All surfactants displayed moderate to high antibacterial activities, particularly on Gram-positive bacteria, with QUAT3 showing the highest activity. Among Gram-negative bacteria, QUAT3, GEM4, and SURF2 were mostly active towards K. pneumoniae with minimum inhibitory concentrations (MIC) ranging from 0.004 to 0.441 mM. QUAT3 and GEM4 displayed a bacteriostatic behavior similar to that of tetracycline, as assessed by broth macrodilution assays. Fluorescence binding studies with 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) revealed that the antibacterial activities could be attributed to a combination of electrostatic and hydrophobic interactions. Bovine serum albumin (BSA) fluorescence and molecular docking studies indicated that SURF1 and QUAT3 interact mainly via van der Waals' forces and hydrogen bonding while SURF2 binds through hydrophobic interactions. QUAT3, which displays broad-spectrum activity, has the potential to combat drug-resistant bacteria.

Pharmacokinetic properties of a novel formulation of S-adenosyl-l-methionine phytate

S-adenosyl-l-methionine (SAM), the main endogenous methyl donor, is the adenosyl derivative of the amino acid methionine, which displays many important roles in cellular metabolism. It is widely used as a food supplement and in some countries is also marketed as a drug. Its interesting nutraceutical and pharmacological properties prompted us to evaluate the pharmacokinetics of a new form of SAM, the phytate salt. The product was administered orally to rats and pharmacokinetic parameters were evaluated by comparing the results with that obtained by administering the SAM tosylated form (SAM PTS). It was found that phytate anion protects SAM from degradation, probably because of steric hindrance exerted by the counterion, and that the SAM phytate displayed significant better pharmacokinetic parameters compared to SAM PTS. These results open to the perspective of the use of new salts of SAM endowed with better pharmacokinetic properties.

Antitumor and Antioxidant Activity of S-Methyl Methionine Sulfonium Chloride against Liver Cancer Induced in Wistar Albino Rats by Diethyl Nitrosamine and Carbon Tertrachloride

Liver disease, especially liver cancer, has become a threat facing the world. Now, antioxidant products are garnering great attention for the treatment and prevention of many diseases. S-Methyl methionine sulfonium chloride (MMSC) is a methionine derivative and is present in many vegetables and has anti-inflammatory effects and antioxidants. This is the first study aiming to investigate the antitumor activity of the MMSC. This study was carried out on 60 male Wistar albino rats (4–6 weeks old age) and divided into four groups, with the first group as normal control, second group as hepatocarcinoma induced by diethyl nitrosamine and carbon tetrachloride (DEN/CCL4) group, third group as normal rats treated with MMSC, and fourth group as hepatocellular carcinoma (HCC) induced rats treated with MMSC. Our findings revealed that MMSC administration after HCC induction significantly improved (p < 0.05) the liver function biomarkers, including AST, GGT, albumin, globulin, and albumin/globulin ratio (A/G), in comparison with those in the HCC group. Moreover, the histopathological changes of the liver tissue in the HCC group were improved by MMSC treatment. Likewise, the expression levels of tumor necrosis factor-alpha (TNF-α), induced nitric oxide synthase (iNOS), transforming growth factor (TGF-1β), and glypican 3 (GP3) were downregulated by MMSC treatment after HCC induction in comparison with those in the HCC-induced group. In conclusion, MMSC showed antitumor activity against HCC induction by DEN/CCl4 through decreasing lipid peroxide formation, the expression level of an inflammatory cytokines such as (TNF-α), immunoregulatory cytokines such as (TGF-1β), induced nitric oxide synthase, and glypican 3.

Amino acid derivatives of natural chlorins as a platform for the creation of targeted photosensitizers in oncology

Objectives. This study aims to obtain the amino acid derivatives of chlorophyll a and bacteriochlorophyll a for the targeted delivery of pigments to tumor foci. This will increase biocompatibility and, as a result, reduce toxic side effects. In addition to photodynamic efficiency, an additional cytotoxic effect is expected for the obtained conjugates of photosensitizers (PSs) with amino acids. This is owing to the participation of the latter in intracellular biochemical processes, including interaction with the components of the glutathione antioxidant system, leading to the vulnerability of tumor cells to oxidative stress.Methods. In this work, we have implemented the optimization of the structure of a highly efficient infrared PS based on O-propyloxim-N-propoxybacteriopurpurinimide (DPBP), absorbing at 800 nm and showing photodynamic efficacy for the treatment of deep-seated and pigmented tumors, by introducing L-lysine, L-arginine, methionine sulfoximine (MSO), and buthionine sulfoximine (BSO) methyl esters. The structure of the obtained compounds was proved by mass spectrometry and nuclear magnetic resonance spectroscopy, and the photoinduced cytotoxicity was studied in vitro on the HeLa cell line.Results. Conjugates of DPBP with amino acids and their derivatives, such as lysine, arginine, MSO, and BSO have been prepared. The chelating ability of DPBP conjugate with lysine was shown, and its Sn(IV) complex was obtained.Conclusions. Biological testing of DPBP with MSO and BSO showed a 5–6-fold increase in photoinduced cytotoxicity compared to the parent DPBP PS. Additionally, a high internalization of pigments by tumor cells was found, and the dark cytotoxicity (in the absence of irradiation) of DPBP-MSO and DPBP-BSO increased fourfold compared to the initial DPBP compound. This can be explained by the participation of methionine derivatives in the biochemical processes of the tumor cell.

An Engineered Escherichia coli Strain with Synthetic Metabolism for in-Cell Production of Translationally Active Methionine Derivatives.

In the last decades, it has become clear that the canonical amino acid repertoire codified by the universal genetic code is not up to the needs of emerging biotechnologies. For this reason, extensive genetic code re‐engineering is essential to expand the scope of ribosomal protein translation, leading to reprogrammed microbial cells equipped with an alternative biochemical alphabet to be exploited as potential factories for biotechnological purposes. The prerequisite for this to happen is a continuous intracellular supply of noncanonical amino acids through synthetic metabolism from simple and cheap precursors. We have engineered an Escherichia coli bacterial system that fulfills these requirements through reconfiguration of the methionine biosynthetic pathway and the introduction of an exogenous direct trans‐sulfuration pathway. Our metabolic scheme operates in vivo, rescuing intermediates from core cell metabolism and combining them with small bio‐orthogonal compounds. Our reprogrammed E. coli strain is capable of the in‐cell production of l‐azidohomoalanine, which is directly incorporated into proteins in response to methionine codons. We thereby constructed a prototype suitable for economic, versatile, green sustainable chemistry, pushing towards enzyme chemistry and biotechnology‐based production.

Novel (p)ppGpp0 suppressor mutations reveal an unexpected link between methionine catabolism and GTP synthesis in Bacillus subtilis

In bacteria, guanosine (penta)tetra-phosphate ([p]ppGpp) is essential for controlling intracellular metabolism that is needed to adapt to environmental changes, such as amino acid starvation. The (p)ppGpp0 strain of Bacillus subtilis, which lacks (p)ppGpp synthetase, is unable to form colonies on minimal medium. Here, we found suppressor mutations in the (p)ppGpp0 strain, in the purine nucleotide biosynthesis genes, prs, purF and rpoB/C, which encode RNA polymerase core enzymes. In comparing our work with prior studies of ppGpp0 suppressors, we discovered that methionine addition masks the suppression on minimal medium, especially of rpoB/C mutations. Furthermore, methionine addition increases intracellular GTP in rpoB suppressor and this effect is decreased by inhibiting GTP biosynthesis, indicating that methionine addition activated GTP biosynthesis and inhibited growth under amino acid starvation conditions in (p)ppGpp0 backgrounds. Furthermore, we propose that the increase in intracellular GTP levels induced by methionine is due to methionine derivatives that increase the activity of the de novo GTP biosynthesis enzyme, GuaB. Our study sheds light on the potential relationship between GTP homeostasis and methionine metabolism, which may be the key to adapting to environmental changes.

Intra‐ and Intermolecular Fluorescence Quenching of Alkylthio‐Substituted Phthalimides by Photoinduced Electron Transfer: Distance, Position and Conformational Dependence

AbstractThe photophysical properties of fluorescent phthalimides with thioether groups directly connected to the chromophore or separated by alkyl spacers, respectively, were studied. Intermolecular fluorescence quenching by electron transfer from dimethylsulfide to the 4,5‐dimethoxy phthalimide (DMPht) model compound 6 is dynamic and fast. The fluorescence properties of 6 and the remote C5‐spaced thioether derivative 5 are nearly identical. In compounds 1–4 with shorter spacer lengths, fluorescence quenching is strong for C2 and C3‐spaced 2 and 3 and less pronounced for C1‐ and C4‐spaced compounds 1 and 4, mapping the conformational landscape of these molecules. The o‐,m‐,p‐substituted N‐(thiomethyl)benzyl DMPht 7 are almost non‐fluorescent which correlates very well with the intermolecular thioanisole fluorescence quenching of 6. In contrast, the 3‐ and 4‐thiomethyl phthalimides 8 and 9 show divergent fluorescence that is also rationalized by DFT calculation results. The fluorescence properties can be switched by oxidation of the thioethers to sulfoxides with H2O2 or 1O2 (off→on for 1,4,7 and on→off for 9). Further functionalized molecules that were based on the model compounds are the sulfur‐containing amino acid methionine derivative 10, dipeptides 11 a,b, and S‐alkylated cysteine derivatives 12 to 14 a–d with strong side‐chain‐dependent fluorescence.

Structures of the Neisseria meningitides methionine-binding protein MetQ in substrate-free form and bound to l- and d-methionine isomers.

The bacterial periplasmic methionine‐binding protein MetQ is involved in the import of methionine by the cognate MetNI methionine ATP binding cassette (ABC) transporter. The MetNIQ system is one of the few members of the ABC importer family that has been structurally characterized in multiple conformational states. Critical missing elements in the structural analysis of MetNIQ are the structure of the substrate‐free form of MetQ, and detailing how MetQ binds multiple methionine derivatives, including both l‐ and d‐methionine isomers. In this study, we report the structures of the Neisseria meningitides MetQ in substrate‐free form and in complexes with l‐methionine and with d‐methionine, along with the associated binding constants determined by isothermal titration calorimetry. Structures of the substrate‐free (N238A) and substrate‐bound N. meningitides MetQ are related by a “Venus‐fly trap” hinge‐type movement of the two domains accompanying methionine binding and dissociation. l‐ and d‐methionine bind to the same site on MetQ, and this study emphasizes the important role of asparagine 238 in ligand binding and affinity. A thermodynamic analysis demonstrates that ligand‐free MetQ associates with the ATP‐bound form of MetNI ∼40 times more tightly than does liganded MetQ, consistent with the necessity of dissociating methionine from MetQ for transport to occur.

Surfaces based on amino acid functionalized polyelectrolyte films towards active surfaces for enzyme immobilization.

Surface based on polyelectrolytes functionalized with amino acids onto amino-terminated solid surfaces of silicon wafers was prepared, with the purpose of evaluate the chemical functionality of the polyelectrolyte films in adsorption and catalytic activity of an enzyme. In this work, the adsorption of the enzyme glucose 6-phosphate dehydrogenase from Leuconostoc mesenteroides (LmG6PD) was studied as model. The polyelectrolytes were obtained from poly (maleic anhydride-alt-vinylpyrrolidone) [poly(MA-alt-VP)] and functionalized with amino acids of different hydropathy index: glutamine (Gln), tyrosine (Tyr) and methionine (Met). The polyelectrolytes were adsorbed onto the amino-terminated silicon wafer at pH 3.5 and 4.5 and at low and high ionic strength. At low ionic strength and pH 3.5, the largest quantity of adsorbed polyelectrolyte was on the films containing glutamine moiety as the most hydrophilic amino acid in the side chain of polymer chain (5.88 mg/m2), whereas at high ionic strength and pH 4.5, the lowest quantity was in films containing tyrosine moiety in the side chain (1.88 mg/m2). The films were characterized by ellipsometry, contact angle measurements and atomic force microscopy (AFM). The polyelectrolyte films showed a moderate degree of hydrophobicity, the methionine derivative being the most hydrophobic film. With the aim of evaluate the effect of the amino acid moieties on the ability of the surface to adsorb enzymes, we study the activity of the enzyme on these surfaces. We observed that the polarity of the side chain of the amino acid in the polyelectrolyte affected the quantity of LmG6PD adsorbed, as well as its specific activity, showing that films prepared from poly(MA-alt-VP) functionalized with Met provide the best enzymatic performance. The results obtained demonstrated that the surfaces prepared from polyelectrolytes functionalized with amino acids could be an attractive and simple platform for the immobilization of enzymes, which could be of interest for biocatalysis applications.

Noncanonical role for the binding protein in substrate uptake by the MetNI methionine ATP Binding Cassette (ABC) transporter

The Escherichia coli methionine ABC transporter MetNI exhibits both high-affinity transport toward l-methionine and broad specificity toward methionine derivatives, including d-methionine. In this work, we characterize the transport of d-methionine derivatives by the MetNI transporter. Unexpectedly, the N229A substrate-binding deficient variant of the cognate binding protein MetQ was found to support high MetNI transport activity toward d-selenomethionine. We determined the crystal structure at 2.95 Å resolution of the ATPγS-bound MetNIQ complex in the outward-facing conformation with the N229A apo MetQ variant. This structure revealed conformational changes in MetQ providing substrate access through the binding protein to the transmembrane translocation pathway. MetQ likely mediates uptake of methionine derivatives through two mechanisms: in the methionine-bound form delivering substrate from the periplasm to the transporter (the canonical mechanism) and in the apo form by facilitating ligand binding when complexed to the transporter (the noncanonical mechanism). This dual role for substrate-binding proteins is proposed to provide a kinetic strategy for ABC transporters to transport both high- and low-affinity substrates present in a physiological concentration range.