Use of analogues of methionine and methionyl adenylate to sample conformational changes during catalysis in Escherichia coli methionyl-tRNA synthetase

MACSPI enables tissue-selective proteomic and interactomic analyses in multicellular organisms

Multicellular organisms are composed of many tissue types that have distinct morphologies and functions, which are largely driven by specialized proteomes and interactomes. To define the proteome and interactome of a specific type of tissue in an intact animal, we developed a localized proteomics approach called Methionine Analog-based Cell-Specific Proteomics and Interactomics (MACSPI). This method uses the tissue-specific expression of an engineered methionyl-tRNA synthetase to label proteins with a bifunctional amino acid 2-amino-5-diazirinylnonynoic acid in selected cells. We applied MACSPI in Caenorhabditis elegans, a model multicellular organism, to selectively label, capture, and profile the proteomes of the body wall muscle and the nervous system, which led to the identification of tissue-specific proteins. Using the photo-cross-linker, we successfully profiled HSP90 interactors in muscles and neurons and identified tissue-specific interactors and stress-related interactors. Our study demonstrates that MACSPI can be used to profile tissue-specific proteomes and interactomes in intact multicellular organisms.

Phylogeny-guided Characterization of Bacterial Hydrazine Biosynthesis Mediated by Cupin/methionyl tRNA Synthetase-like Enzymes

Cupin/methionyl-tRNA synthetase (MetRS)-like didomain enzymes catalyze nitrogen-nitrogen (N-N) bond formation between Nω-hydroxylamines and amino acids to generate hydrazines, key biosynthetic intermediates of various natural products containing N-N bonds. While the combination of these two building blocks leads to the creation of diverse hydrazine products, the full extent of their structural diversity remains largely unknown. To explore this, we herein conducted phylogeny-guided genome-mining of related hydrazine biosynthetic pathways consisting of two enzymes: flavin-dependent Nω-hydroxylating monooxygenases (NMOs) that produce Nω-hydroxylamine precursors and cupin/MetRS-like enzymes that couple the Nω-hydroxylamines with amino acids via N-N bonds. A phylogenetic analysis identified the largely unexplored sequence spaces of these enzyme families. The biochemical characterization of NMOs demonstrated their capabilities to produce various Nω-hydroxylamines, including those previously not known as precursors of N-N bonds. Furthermore, the characterization of cupin/MetRS-like enzymes identified five new hydrazine products with novel combinations of building blocks, including one containing non-amino acid building blocks: 1,3-diaminopropane and putrescine. This study substantially expanded the variety of N-N bond forming pathways mediated by cupin/MetRS-like enzymes.

Biochemical and structural characterization of chlorhexidine as an ATP-assisted inhibitor against type 1 methionyl-tRNA synthetase from Gram-positive bacteria

Methionyl-tRNA synthetase (MetRS) catalyzes the attachment of l-methionine (l-Met) to tRNAMet to generate methionyl-tRNAMet, an essential substrate for protein translation within ribosome. Owing to its indispensable biological function and the structural discrepancies with human counterpart, bacterial MetRS is considered an ideal target for developing antibacterials. Herein, chlorhexidine (CHX) was identified as a potent binder of Staphylococcus aureus MetRS (SaMetRS) through an ATP-aided affinity screening. The co-crystal structure showed that CHX simultaneously occupies the enlarged l-Met pocket (EMP) and the auxiliary pocket (AP) of SaMetRS with its two chlorophenyl groups, while its central hexyl linker swings upwards to interact with some conserved hydrophobic residues. ATP adopts alternative conformations in the active site cavity, and forms ionic bonds and water-mediated hydrogen bonds with CHX. Consistent with this synergistic binding mode, ATP concentration-dependently enhanced the binding affinity of CHX to SaMetRS from 10.2 μM (no ATP) to 0.45 μM (1 mM ATP). While it selectively inhibited two representative type 1 MetRSs from S. aureus and Enterococcus faecalis, CHX did not show significant interactions with three tested type 2 MetRSs, including human cytoplasmic MetRS, in the enzyme inhibition and biophysical binding assays, probably due to the conformational differences between two types of MetRSs at their EMP and AP. Our findings on CHX may inspire the design of MetRS-directed antimicrobials in future.

Exploring Proteus mirabilis Methionine tRNA Synthetase Active Site: Homology Model Construction, Molecular Dynamics, Pharmacophore and Docking Validation

Currently, the treatment of Proteus mirabilis infections is considered to be complicated as the organism has become resistant to numerous antibiotic classes. Therefore, new inhibitors should be developed, targeting bacterial molecular functions. Methionine tRNA synthetase (MetRS), a member of the aminoacyl-tRNA synthetase family, is essential for protein biosynthesis offering a promising target for novel antibiotics discovery. In the context of computer-aided drug design (CADD), the current research presents the construction and analysis of a comparative homology model for P. mirabilis MetRS, enabling development of novel inhibitors with greater selectivity. Molecular Operating Environment (MOE) software was used to build a homology model for P. mirabilis MetRS using Escherichia coli MetRS as a template. The model was evaluated, and the active site of the target protein predicted from its sequence using conservation analysis. Molecular dynamic simulations were performed to evaluate the stability of the modeled protein structure. In order to evaluate the predicted active site interactions, methionine (the natural substrate of MetRS) and several inhibitors of bacterial MetRS were docked into the constructed model using MOE. After validation of the model, pharmacophore-based virtual screening for a systemically prepared dataset of compounds was performed to prove the feasibility of the proposed model, identifying possible parent compounds for further development of MetRS inhibitors against P. mirabilis.

Redesigning methionyl-tRNA synthetase for β-methionine activity with adaptive landscape flattening and experiments

Amino acids (AAs) with a noncanonical backbone would be a valuable tool for protein engineering, enabling new structural motifs and building blocks. To incorporate them into an expanded genetic code, the first, key step is to obtain an appropriate aminoacyl-tRNA synthetase. Currently, directed evolution is not available to optimize AAs with noncanonical backbones, since an appropriate selective pressure has not been discovered. Computational protein design (CPD) is an alternative. We used a new CPD method to redesign MetRS and increase its activity towards β-Met, which has an extra backbone methylene. The new method considered a few active site positions for design and used a Monte Carlo exploration of the corresponding sequence space. During the exploration, a bias energy was adaptively learned, such that the free energy landscape of the apo enzyme was flattened. Enzyme variants could then be sampled, in the presence of the ligand and the bias energy, according to their β-Met binding affinities. Eighteen predicted variants were chosen for experimental testing; 10 exhibited detectable activity for β-Met adenylation. Top predicted hits were characterized experimentally in detail. Dissociation constants, catalytic rates, and Michaelis constants for both α-Met and β-Met were measured. The best mutant retained a preference for α-Met over β-Met; however, the preference was reduced, compared to the wildtype, by a factor of 29. For this mutant, high resolution crystal structures were obtained in complex with both α-Met and β-Met, indicating that the predicted, active conformation of β-Met in the active site was retained.

MARS2 drives metabolic switch of non-small-cell lung cancer cells via interaction with MCU

Mitochondrial methionyl-tRNA synthetase (MARS2) canonically mediates the formation of fMet-tRNA_i^fMet for mitochondrial translation initiation. Mitochondrial calcium uniporter (MCU) is a major gate of Ca²⁺ flux from cytosol into the mitochondrial matrix. We found that MARS2 interacts with MCU and stimulates mitochondrial Ca²⁺ influx. Methionine binding to MARS2 would act as a molecular switch that regulates MARS2-MCU interaction. Endogenous knockdown of MARS2 attenuates mitochondrial Ca²⁺ influx and induces p53 upregulation through the Ca²⁺-dependent CaMKII/CREB signaling. Subsequently, metabolic rewiring from glycolysis into pentose phosphate pathway is triggered and cellular reactive oxygen species level decreases. This metabolic switch induces inhibition of epithelial-mesenchymal transition (EMT) via cellular redox regulation. Expression of MARS2 is regulated by ZEB1 transcription factor in response to Wnt signaling. Our results suggest the mechanisms of mitochondrial Ca²⁺ uptake and metabolic control of cancer that are exerted by the key factors of the mitochondrial translational machinery and Ca²⁺ homeostasis.

Partitioning of the initial catalytic steps of leucyl-tRNA synthetase is driven by an active site peptide-plane flip

To correctly aminoacylate tRNA^Leu, leucyl-tRNA synthetase (LeuRS) catalyzes three reactions: activation of leucine by ATP to form leucyl-adenylate (Leu-AMP), transfer of this amino acid to tRNA^Leu and post-transfer editing of any mischarged product. Although LeuRS has been well characterized biochemically, detailed structural information is currently only available for the latter two stages of catalysis. We have solved crystal structures for all enzymatic states of Neisseria gonorrhoeae LeuRS during Leu-AMP formation. These show a cycle of dramatic conformational changes, involving multiple domains, and correlate with an energetically unfavorable peptide-plane flip observed in the active site of the pre-transition state structure. Biochemical analyses, combined with mutant structural studies, reveal that this backbone distortion acts as a trigger, temporally compartmentalizing the first two catalytic steps. These results unveil the remarkable effect of this small structural alteration on the global dynamics and activity of the enzyme.

Non-canonical Amino Acid Substrates of E. coli Aminoacyl-tRNA Synthetases

In this comprehensive review, I focus on the twenty E. coli aminoacyl-tRNA synthetases and their ability to charge non-canonical amino acids (ncAAs) onto tRNAs. The promiscuity of these enzymes has been harnessed for diverse applications including understanding and engineering of protein function, creation of organisms with an expanded genetic code, and the synthesis of diverse peptide libraries for drug discovery. The review catalogues the structures of all known ncAA substrates for each of the 20 E. coli aminoacyl-tRNA synthetases, including ncAA substrates for engineered versions of these enzymes. Drawing from the structures in the list, I highlight trends and novel opportunities for further exploitation of these ncAAs in the engineering of protein function, synthetic biology, and in drug discovery.

Molecular basis for diaryldiamine selectivity and competition with tRNA in a type 2 methionyl-tRNA synthetase from a Gram-negative bacterium

Gram-negative bacteria are responsible for a variety of human, animal, and plant diseases. The spread of multidrug-resistant Gram-negative bacteria poses a challenge to disease control and highlights the need for novel antimicrobials. Owing to their critical role in protein synthesis, aminoacyl-tRNA synthetases, including the methionyl-tRNA synthetases MetRS1 and MetRS2, are attractive drug targets. MetRS1 has long been exploited as a drug target in Gram-positive bacteria and protozoan parasites. However, MetRS1 inhibitors have limited action upon Gram-negative pathogens or on Gram-positive bacteria that produce MetRS2 enzymes. The underlying mechanism by which MetRS2 enzymes are insensitive to MetRS1 inhibitors is presently unknown. Herein, we report the first structures of MetRS2 from a multidrug-resistant Gram-negative bacterium in its ligand-free state and bound to its substrate or MetRS1 inhibitors. The structures reveal the binding mode of two diaryldiamine MetRS1 inhibitors that occupy the amino acid-binding site and a surrounding auxiliary pocket implicated in tRNA acceptor arm binding. The structural features associated with amino acid polymorphisms found in the methionine and auxiliary pockets reveal the molecular basis for diaryldiamine binding and selectivity between MetRS1 and MetRS2 enzymes. Moreover, we show that mutations in key polymorphic residues in the methionine and auxiliary pockets not only altered inhibitor binding affinity but also significantly reduced enzyme function. Our findings thus reinforce the tRNA acceptor arm binding site as a druggable pocket in class I aminoacyl-tRNA synthetases and provide a structural basis for optimization of MetRS2 inhibitors for the development of new antimicrobials against Gram-negative pathogens.

How evolution shapes enzyme selectivity - lessons from aminoacyl-tRNA synthetases and other amino acid utilizing enzymes

Aminoacyl-tRNA synthetases (AARSs) charge tRNA with their cognate amino acids. Many other enzymes use amino acids as substrates, yet discrimination against noncognate amino acids that threaten the accuracy of protein translation is a hallmark of AARSs. Comparing AARSs to these other enzymes allowed us to recognize patterns in molecular recognition and strategies used by evolution for exercising selectivity. Overall, AARSs are 2-3 orders of magnitude more selective than most other amino acid utilizing enzymes. AARSs also reveal the physicochemical limits of molecular discrimination. For example, amino acids smaller by a single methyl moiety present a discrimination ceiling of ~200, while larger ones can be discriminated by up to 10⁵ -fold. In contrast, substrates larger by a hydroxyl group challenge AARS selectivity, due to promiscuous H-bonding with polar active site groups. This 'hydroxyl paradox' is resolved by editing. Indeed, when the physicochemical discrimination limits are reached, post-transfer editing - hydrolysis of tRNAs charged with noncognate amino acids, evolved. The editing site often selectively recognizes the edited noncognate substrate using the very same feature that the synthetic site could not efficiently discriminate against. Finally, the comparison to other enzymes also reveals that the selectivity of AARSs is an explicitly evolved trait, showing some clear examples of how selection acted not only to optimize catalytic efficiency with the target substrate, but also to abolish activity with noncognate threat substrates ('negative selection').

Adaptive landscape flattening allows the design of both enzyme: Substrate binding and catalytic power

Designed enzymes are of fundamental and technological interest. Experimental directed evolution still has significant limitations, and computational approaches are a complementary route. A designed enzyme should satisfy multiple criteria: stability, substrate binding, transition state binding. Such multi-objective design is computationally challenging. Two recent studies used adaptive importance sampling Monte Carlo to redesign proteins for ligand binding. By first flattening the energy landscape of the apo protein, they obtained positive design for the bound state and negative design for the unbound. We have now extended the method to design an enzyme for specific transition state binding, i.e., for its catalytic power. We considered methionyl-tRNA synthetase (MetRS), which attaches methionine (Met) to its cognate tRNA, establishing codon identity. Previously, MetRS and other synthetases have been redesigned by experimental directed evolution to accept noncanonical amino acids as substrates, leading to genetic code expansion. Here, we have redesigned MetRS computationally to bind several ligands: the Met analog azidonorleucine, methionyl-adenylate (MetAMP), and the activated ligands that form the transition state for MetAMP production. Enzyme mutants known to have azidonorleucine activity were recovered by the design calculations, and 17 mutants predicted to bind MetAMP were characterized experimentally and all found to be active. Mutants predicted to have low activation free energies for MetAMP production were found to be active and the predicted reaction rates agreed well with the experimental values. We suggest the present method should become the paradigm for computational enzyme design.

Designed enzymes are of major interest. Experimental directed evolution still has significant limitations, and computational approaches are another route. Enzymes must be stable, bind substrates, and be powerful catalysts. It is challenging to design for all these properties. A method to design substrate binding was proposed recently. It used an adaptive Monte Carlo method to explore mutations of a few amino acids near the substrate. A bias energy was gradually “learned” such that, in the absence of the ligand, the simulation visited most of the possible protein mutations with comparable probabilities. Remarkably, a simulation of the protein:ligand complex, including the bias, will then preferentially sample tight-binding sequences. We generalized the method to design binding specificity. We tested it for the methionyl-tRNA synthetase enzyme, which has been engineered in order to expand the genetic code. We redesigned the enzyme to obtain variants with low activation free energies for the catalytic step. The variants proposed by the simulations were shown experimentally to be active, and the predicted activation free energies were in reasonable agreement with the experimental values. We expect the new method will become the paradigm for computational enzyme design.

Use of β3-methionine as an amino acid substrate of Escherichia coli methionyl-tRNA synthetase

Polypeptides containing β-amino acids are attractive tools for the design of novel proteins having unique properties of medical or industrial interest. Incorporation of β-amino acids in vivo requires the development of efficient aminoacyl-tRNA synthetases specific of these non-canonical amino acids. Here, we have performed a detailed structural and biochemical study of the recognition and use of β³-Met by Escherichia coli methionyl-tRNA synthetase (MetRS). We show that MetRS binds β³-Met with a 24-fold lower affinity but catalyzes the esterification of the non-canonical amino acid onto tRNA with a rate lowered by three orders of magnitude. Accurate measurements of the catalytic parameters required careful consideration of the presence of contaminating α-Met in β³-Met commercial samples. The 1.45 Å crystal structure of the MetRS: β³-Met complex shows that β³-Met binds the enzyme essentially like α-Met, but the carboxylate moiety is mobile and not adequately positioned to react with ATP for aminoacyl adenylate formation. This study provides structural and biochemical bases for engineering MetRS with improved β³-Met aminoacylation capabilities.

Targeting adenylate-forming enzymes with designed sulfonyladenosine inhibitors

Adenylate-forming enzymes are a mechanistic superfamily that are involved in diverse biochemical pathways. They catalyze ATP-dependent activation of carboxylic acid substrates as reactive acyl adenylate (acyl-AMP) intermediates and subsequent coupling to various nucleophiles to generate ester, thioester, and amide products. Inspired by natural products, acyl sulfonyladenosines (acyl-AMS) that mimic the tightly bound acyl-AMP reaction intermediates have been developed as potent inhibitors of adenylate-forming enzymes. This simple yet powerful inhibitor design platform has provided a wide range of biological probes as well as several therapeutic lead compounds. Herein, we provide an overview of the nine structural classes of adenylate-forming enzymes and examples of acyl-AMS inhibitors that have been developed for each.

Mutations in MetG (methionyl-tRNA synthetase) and TrmD [tRNA (guanine-N1)-methyltransferase] conferring meropenem tolerance in Burkholderia thailandensis

Objectives

Although meropenem is widely used to treat Burkholderia infections, the response of Burkholderia pathogens to this antibiotic is largely unexplored.

Methods

Burkholderia thailandensis, a model for Burkholderia spp., particularly Burkholderia mallei and Burkholderia pseudomallei, was challenged with a lethal level of meropenem and survivors were isolated. The genomes of two of the isolates were analysed to identify mutated genes and these genes were then specifically examined in more isolates to profile mutation diversity. Mutants were characterized to investigate the biological basis underlying survival against meropenem.

Results

One of two genes associated with tRNA metabolism [metG or trmD, encoding methionyl-tRNA synthetase or tRNA (guanine-N1)-methyltransferase, respectively] was found to be mutated in the two survivors. A single nucleotide substitution and a frameshift mutation were found in metG and trmD, respectively. Five different substitution mutations affecting methionine- or tRNA-binding sites were found in metG during further screening. The mutants exhibited slowed growth and increased tolerance not only to meropenem but also various other antibiotics. This tolerance required intact RelA, a key stringent response.

Conclusions

Specific mutations affecting the tRNA pool, particularly those in metG, play a pivotal role in the B. thailandensis response to meropenem challenge. This mechanism of antibiotic tolerance is important because it can reduce the effectiveness of meropenem and thereby facilitate chronic infection by Burkholderia pathogens. In addition, specific mutations found in MetG will prove useful in the effort to develop new drugs to completely inhibit this essential enzyme, while preventing stringent-response-mediated antibiotic tolerance in pathogens.

Identification and Characterization of Chemical Compounds that Inhibit Leucyl-tRNA Synthetase from Pseudomonas aeruginosa

BACKGROUND

Pseudomonas aeruginosa is an opportunistic multi-drug resistance pathogen implicated as the causative agent in a high-percentage of nosocomial and community acquired bacterial infections. The gene encoding leucyl-tRNA synthetase (LeuRS) from P. aeruginosa was overexpressed in Escherichia coli and the resulting protein was characterized.

METHODS

LeuRS was kinetically evaluated and the KM values for interactions with leucine, ATP and tRNA were 6.5, 330, and 3.0 μM, respectively. LeuRS was developed into a screening platform using scintillation proximity assay (SPA) technology and used to screen over 2000 synthetic and natural chemical compounds.

RESULTS

The initial screen resulted in the identification of two inhibitory compounds, BT03C09 and BT03E07. IC50s against LeuRS observed for BT03C09 and BT03E07 were 23 and 15 μM, respectively. The minimum inhibitory concentrations (MIC) were determined against nine clinically relevant bacterial strains. In time-kill kinetic analysis, BT03C09 was observed to inhibit bacterial growth in a bacteriostatic manner, while BT03E07 acted as a bactericidal agent. Neither compound competed with leucine or ATP for binding LeuRS. Limited inhibition was observed in aminoacylation assays with the human mitochondrial form of LeuRS, however when tested in cultures of human cell line, BT03C09 was toxic at all concentration whereas BT03E07 only showed toxic effects at elevated concentrations.

CONCLUSION

Two compounds were identified as inhibitors of LeuRS in a screen of over 2000 natural and synthetic compounds. After characterization one compound (BT03E07) exhibited broad spectrum antibacterial activity while maintaining low toxicity against human mitochondrial LeuRS as well as against human cell cultures.

Structural characterization of free-state and product-state Mycobacterium tuberculosis methionyl-tRNA synthetase reveals an induced-fit ligand-recognition mechanism

Mycobacterium tuberculosis (MTB) caused 10.4 million cases of tuberculosis and 1.7 million deaths in 2016. The incidence of multidrug-resistant and extensively drug-resistant MTB is becoming an increasing threat to public health and the development of novel anti-MTB drugs is urgently needed. Methionyl-tRNA synthetase (MetRS) is considered to be a valuable drug target. However, structural characterization of M. tuberculosis MetRS (MtMetRS) was lacking for decades, thus hampering drug design. Here, two high-resolution crystal structures of MtMetRS are reported: the free-state structure (apo form; 1.9 Å resolution) and a structure with the intermediate product methionyl-adenylate (Met-AMP) bound (2.4 Å resolution). It was found that free-state MtMetRS adopts a previously unseen conformation that has never been observed in other MetRS homologues. The pockets for methionine and AMP are not formed in free-state MtMetRS, suggesting that it is in a nonproductive conformation. Combining these findings suggests that MtMetRS employs an induced-fit mechanism in ligand binding. By comparison with the structure of human cytosolic MetRS, additional pockets specific to MtMetRS that could be used for anti-MTB drug design were located.

Identification and Characterization of a Chemical Compound that Inhibits Methionyl-tRNA Synthetase from Pseudomonas aeruginosa

BACKGROUND

Pseudomonas aeruginosa is an opportunistic pathogen problematic in causing nosocomial infections and is highly susceptible to development of resistance to multiple antibiotics. The gene encoding methionyl-tRNA synthetase (MetRS) from P. aeruginosa was cloned and the resulting protein characterized.

METHODS

MetRS was kinetically evaluated and the KM for its three substrates, methionine, ATP and tRNAMet were determined to be 35, 515, and 29 μM, respectively. P. aeruginosaMetRS was used to screen two chemical compound libraries containing 1690 individual compounds.

RESULTS

A natural product compound (BM01C11) was identified that inhibited the aminoacylation function. The compound inhibited P. aeruginosa MetRS with an IC50 of 70 μM. The minimum inhibitory concentration (MIC) of BM01C11 was determined against nine clinically relevant bacterial strains, including efflux pump mutants and hypersensitive strains of P. aeruginosa and E. coli. The MIC against the hypersensitive strain of P. aeruginosa was 16 μg/ml. However, the compound was not effective against the wild-type and efflux pump mutant strains, indicating that efflux may not be responsible for the lack of activity against the wild-type strains. When tested in human cell cultures, the cytotoxicity concentration (CC50) was observed to be 30 μg/ml. The compound did not compete with methionine or ATP for binding MetRS, indicating that the mechanism of action of the compound likely occurs outside the active site of aminoacylation.

CONCLUSION

An inhibitor of P. aeruginosa MetRS, BM01C11, was identified as a flavonoid compound named isopomiferin. Isopomiferin inhibited the enzymatic activity of MetRS and displayed broad spectrum antibacterial activity. These studies indicate that isopomiferin may be amenable to development as a therapeutic for bacterial infections.

Structure of the G119S Mutant Acetylcholinesterase of the Malaria Vector Anopheles gambiae Reveals Basis of Insecticide Resistance

Malaria is a devastating disease in sub-Saharan Africa and is transmitted by the mosquito Anopheles gambiae. While indoor residual spraying of anticholinesterase insecticides has been useful in controlling the spread of malaria, widespread application of these compounds has led to the rise of an insecticide-resistant mosquito strain that harbors a G119S mutation in the nervous system target enzyme acetylcholinesterase. We demonstrate the atomic basis of insecticide resistance through structure determination of the G119S mutant acetylcholinesterase of An. gambiae in the ligand-free state and bound to a potent difluoromethyl ketone inhibitor. These structures reveal specific features within the active-site gorge distinct from human acetylcholinesterase, including an open channel at the base of the gorge, and provide a means for improving species selectivity in the rational design of improved insecticides for malaria vector control.

Monitoring the Disassembly of Virus-like Particles by 19F-NMR

Virus-like particles (VLPs) are stable protein cages derived from virus coats. They have been used extensively as biomolecular platforms, e.g., nanocarriers or vaccines, but a convenient in situ technique is lacking for tracking functional status. Here, we present a simple way to monitor disassembly of 19F-labeled VLPs derived from bacteriophage Qβ by 19F NMR. Analysis of resonances, under a range of conditions, allowed determination not only of the particle as fully assembled but also as disassembled, as well as detection of a degraded state upon digestion by cells. This in turn allowed mutational redesign of disassembly and testing in both bacterial and mammalian systems as a strategy for the creation of putative, targeted-VLP delivery systems.

Prospects of In vivo Incorporation of Non-canonical Amino Acids for the Chemical Diversification of Antimicrobial Peptides

The incorporation of non-canonical amino acids (ncAA) is an elegant way for the chemical diversification of recombinantly produced antimicrobial peptides (AMPs). Residue- and site-specific installation methods in several bacterial production hosts hold great promise for the generation of new-to-nature AMPs, and can contribute to tackle the ongoing emergence of antibiotic resistance in pathogens. Especially from a pharmacological point of view, desirable improvements span pH and protease resistance, solubility, oral availability and circulation half-life. Although the primary focus of this report is on ribosomally synthesized and post-translationally modified peptides (RiPPs), we have included selected cases of peptides produced by solid phase peptide synthesis to comparatively show the potential and impact of ncAA introduction. Generally speaking, the introduction of ncAAs in recombinant AMPs delivers novel levels of chemical diversification. Cotranslationally incorporated, they can take part in AMP biogenesis either through direction interaction with elements of the post-translational modification (PTM) machinery or as untargeted sites with unique physicochemical properties and chemical handles for further modification. Together with genetic libraries, genome mining and processing by PTM machineries, ncAAs present not a mere addition to this process, but a highly diverse pool of building blocks to significantly broaden the chemical space of this valuable class of molecules. This perspective summarizes new developments of ncAA containing peptides. Challenges to be resolved in order to reach large-scale pharmaceutical production of these promising compounds and prospects for future developments are discussed.

Aminoacyl-tRNA Synthetases in the Bacterial World

Aminoacyl-tRNA synthetases (aaRSs) are modular enzymes globally conserved in the three kingdoms of life. All catalyze the same two-step reaction, i.e., the attachment of a proteinogenic amino acid on their cognate tRNAs, thereby mediating the correct expression of the genetic code. In addition, some aaRSs acquired other functions beyond this key role in translation. Genomics and X-ray crystallography have revealed great structural diversity in aaRSs (e.g., in oligomery and modularity, in ranking into two distinct groups each subdivided in 3 subgroups, by additional domains appended on the catalytic modules). AaRSs show huge structural plasticity related to function and limited idiosyncrasies that are kingdom or even species specific (e.g., the presence in many Bacteria of non discriminating aaRSs compensating for the absence of one or two specific aaRSs, notably AsnRS and/or GlnRS). Diversity, as well, occurs in the mechanisms of aaRS gene regulation that are not conserved in evolution, notably between distant groups such as Gram-positive and Gram-negative Bacteria. The review focuses on bacterial aaRSs (and their paralogs) and covers their structure, function, regulation, and evolution. Structure/function relationships are emphasized, notably the enzymology of tRNA aminoacylation and the editing mechanisms for correction of activation and charging errors. The huge amount of genomic and structural data that accumulated in last two decades is reviewed, showing how the field moved from essentially reductionist biology towards more global and integrated approaches. Likewise, the alternative functions of aaRSs and those of aaRS paralogs (e.g., during cell wall biogenesis and other metabolic processes in or outside protein synthesis) are reviewed. Since aaRS phylogenies present promiscuous bacterial, archaeal, and eukaryal features, similarities and differences in the properties of aaRSs from the three kingdoms of life are pinpointed throughout the review and distinctive characteristics of bacterium-like synthetases from organelles are outlined.

Biological chemistry of the carbon–sulfur bond

Carbon–sulfur biological chemistry encompasses a fascinating area of biochemistry and medicinal chemistry and includes the roles that methionine and S-adenosyl-l-methionine play in cells as well as the chemistry of intracellular thiols such as glutathione. This article, based on the 2014 Bernard Belleau Award lecture, provides an overview of some of the key investigations that were undertaken in this area from a bioorganic perspective. The research has ameliorated our fundamental knowledge of several of the enzymes utilizing these sulfur-containing molecules, has led to the development of several novel 19F biophysical probes, and has explored some of the medicinal chemistry associated with these processes.

Substrate-Assisted and Enzymatic Pretransfer Editing of Nonstandard Amino Acids by Methionyl-tRNA Synthetase

Aminoacyl-tRNA synthetases (aaRSs) are central to a number of physiological processes, including protein biosynthesis. In particular, they activate and then transfer their corresponding amino acid to the cognate tRNA. This is achieved with a generally remarkably high fidelity by editing against incorrect standard and nonstandard amino acids. Using docking, molecular dynamics (MD), and hybrid quantum mechanical/molecular mechanics methods, we have investigated mechanisms by which methionyl-tRNA synthetase (MetRS) may edit against the highly toxic, noncognate, amino acids homocysteine (Hcy) and its oxygen analogue, homoserine (Hse). Substrate-assisted editing of Hcy-AMP in which its own phosphate acts as the mechanistic base occurs with a rate-limiting barrier of 98.2 kJ mol(-1). This step corresponds to nucleophilic attack of the Hcy side-chain sulfur at its own carbonyl carbon (CCarb). In contrast, a new possible editing mechanism is identified in which an active site aspartate (Asp259) acts as the base. The rate-limiting step is now rotation about the substrate's aminoacyl Cβ-Cγ bond with a barrier of 27.5 kJ mol(-1), while for Hse-AMP, the rate-limiting step is cleavage of the CCarb-OP bond with a barrier of 30.9 kJ mol(-1). A similarly positioned aspartate or glutamate also occurs in the homologous enzymes LeuRS, IleRS, and ValRS, which also discriminate against Hcy. Docking and MD studies suggest that at least in the case of LeuRS and ValRS, a similar editing mechanism may be possible.

Screening of quinoline, 1,3-benzoxazine, and 1,3-oxazine-based small molecules against isolated methionyl-tRNA synthetase and A549 and HCT116 cancer cells including an in silico binding mode analysis

Elevated activity of methionyl-tRNA synthetase (MRS) in many cancers renders it a possible drug target in this disease area, as well as in a series of parasitic diseases. In the present work, we report the synthesis and in vitro screening of a library of 1,3-oxazines, benzoxazines and quinoline scaffolds against human MRS. Among the compounds tested, 2-(2-butyl-4-chloro-1-(4-phenoxybenzyl)-1H-imidazol-5-yl)-5-(4-methoxyphenyl)-1-oxa-3-azaspiro[5.5]undecane (compound 21) and 2-(2-butyl-4-chloro-1-(4-nitrobenzyl)-1H-imidazol-5-yl)-2,4-dihydro-1H-benzo[d][1,3]oxazine (compound 8) were found to be potent inhibitors of MRS. Additionally, these compounds significantly suppressed the proliferation of A549 and HCT116 cells with IC50 values of 28.4, 17.7, 41.9, and 19.8 μM respectively. Molecular docking studies suggested that the ligand binding orientation overlaps with the original positions of both methionine and adenosine of MRS. This suggests the binding of compound 21 against MRS, which might lead the inhibitory activity towards cancer cells.

Toward an era of utilizing methionine overproducing hosts for recombinant protein production in Escherichia coli

Amino acid sequence variants, especially variants containing non-canonical amino acids such as norleucine and norvaline, are a concern during therapeutic protein production in microbial systems. Substitution of methionine residues with norleucine in recombinant proteins produced in Escherichia coli is well known. Continuous feeding of amino acids such as methionine is commonly used in E. coli fermentation processes to control incorporation of norleucine in the recombinant protein. There are several disadvantages associated with continuous feeding during a fermentation process. For example, a continuous feed increases the operational complexity and cost of a manufacturing process and results in dilution of culture medium which could result in lower cell densities and product yields. To overcome the limitations of existing approaches to prevent norleucine incorporation during E. coli fermentations, a new approach using an engineered host was developed that overproduces methionine in the cell to prevent norleucine incorporation without negatively impacting fermentation process performance and product yields. In this commentary, the results on using methionine overproducing hosts for recombinant protein production in E. coli and some "watch outs" when using these hosts for recombinant protein production are discussed.

Determinants for tRNA-dependent pretransfer editing in the synthetic site of isoleucyl-tRNA synthetase

The accurate expression of genetic information relies on the fidelity of amino acid–tRNA coupling by aminoacyl-tRNA synthetases (aaRS). When the specificity against structurally similar noncognate amino acids in the synthetic reaction does not support a threshold fidelity level for translation, the aaRS employ intrinsic hydrolytic editing to correct errors in aminoacylation. Escherichia coli isoleucyl-tRNA synthetase (EcIleRS) is a class I aaRS that is notable for its use of tRNA-dependent pretransfer editing to hydrolyze noncognate valyl-adenylate prior to aminoacyl-tRNA formation. On the basis of the finding that IleRS possessing an inactivated post-transfer editing domain is still capable of robust tRNA-dependent editing, we have recently proposed that the pretransfer editing activity resides within the synthetic site. Here we apply an improved methodology that allows quantitation of the AMP fraction that arises particularly from tRNA-dependent aa-AMP hydrolysis. By this approach, we demonstrate that tRNA-dependent pretransfer editing accounts for nearly one-third of the total proofreading by EcIleRS and that a highly conserved tyrosine within the synthetic site modulates both editing and aminoacylation. Therefore, synthesis of aminoacyl-tRNA and hydrolysis of aminoacyl-adenylates employ overlapping amino acid determinants. We suggest that this overlap hindered the evolution of synthetic site-based pretransfer editing as the predominant proofreading pathway, because that activity is difficult to accommodate in the context of efficient aminoacyl-tRNA synthesis. Instead, the acquisition of a spatially separate domain dedicated to post-transfer editing alone allowed for the development of a powerful deacylation machinery that effectively competes with dissociation of misacylated tRNAs.

Novel hybrid virtual screening protocol based on molecular docking and structure-based pharmacophore for discovery of methionyl-tRNA synthetase inhibitors as antibacterial agents

Methione tRNA synthetase (MetRS) is an essential enzyme involved in protein biosynthesis in all living organisms and is a potential antibacterial target. In the current study, the structure-based pharmacophore (SBP)-guided method has been suggested to generate a comprehensive pharmacophore of MetRS based on fourteen crystal structures of MetRS-inhibitor complexes. In this investigation, a hybrid protocol of a virtual screening method, comprised of pharmacophore model-based virtual screening (PBVS), rigid and flexible docking-based virtual screenings (DBVS), is used for retrieving new MetRS inhibitors from commercially available chemical databases. This hybrid virtual screening approach was then applied to screen the Specs (202,408 compounds) database, a structurally diverse chemical database. Fifteen hit compounds were selected from the final hits and shifted to experimental studies. These results may provide important information for further research of novel MetRS inhibitors as antibacterial agents.

Emergence and evolution

The aminoacyl-tRNA synthetases (aaRSs) are essential components of the protein synthesis machinery responsible for defining the genetic code by pairing the correct amino acids to their cognate tRNAs. The aaRSs are an ancient enzyme family believed to have origins that may predate the last common ancestor and as such they provide insights into the evolution and development of the extant genetic code. Although the aaRSs have long been viewed as a highly conserved group of enzymes, findings within the last couple of decades have started to demonstrate how diverse and versatile these enzymes really are. Beyond their central role in translation, aaRSs and their numerous homologs have evolved a wide array of alternative functions both inside and outside translation. Current understanding of the emergence of the aaRSs, and their subsequent evolution into a functionally diverse enzyme family, are discussed in this chapter.

A novel crystal form of pyrrolysyl-tRNA synthetase reveals the pre- and post-aminoacyl-tRNA synthesis conformational states of the adenylate and aminoacyl moieties and an asparagine residue in the catalytic site

Structures of Methanosarcina mazei pyrrolysyl-tRNA synthetase (PylRS) have been determined in a novel crystal form. The triclinic form crystals contained two PylRS dimers (four monomer molecules) in the asymmetric unit, in which the two subunits in one dimer each bind N(ℇ)-(tert-butyloxycarbonyl)-L-lysyladenylate (BocLys-AMP) and the two subunits in the other dimer each bind AMP. The BocLys-AMP molecules adopt a curved conformation and the C(α) position of BocLys-AMP protrudes from the active site. The β7-β8 hairpin structures in the four PylRS molecules represent distinct conformations of different states of the aminoacyl-tRNA synthesis reaction. Tyr384, at the tip of the β7-β8 hairpin, moves from the edge to the inside of the active-site pocket and adopts multiple conformations in each state. Furthermore, a new crystal structure of the BocLys-AMPPNP-bound form is also reported. The bound BocLys adopts an unusually bent conformation, which differs from the previously reported structure. It is suggested that the present BocLys-AMPPNP-bound, BocLys-AMP-bound and AMP-bound complexes represent the initial binding of an amino acid (or pre-aminoacyl-AMP synthesis), pre-aminoacyl-tRNA synthesis and post-aminoacyl-tRNA synthesis states, respectively. The conformational changes of Asn346 that accompany the aminoacyl-tRNA synthesis reaction have been captured by X-ray crystallographic analyses. The orientation of the Asn346 side chain, which hydrogen-bonds to the carbonyl group of the amino-acid substrate, shifts by a maximum of 85-90° around the C(β) atom.

An in silico approach to evaluate the polyspecificity of methionyl-tRNA synthetases

Residue-specific incorporation is a technique used to replace natural amino acids with their close structural analogs, unnatural amino acids (UAAs), during protein synthesis. This is achieved by exploiting the substrate promiscuity of the wild type amino acyl tRNA synthetase (AARS) towards the close structural analogs of their cognate amino acids. In the past few decades, seleno-methionine was incorporated into proteins, using the substrate promiscuity of wild type AARSs, to resolve their crystal structures. Later, the incorporation of many UAAs showed that the AARSs are polyspecific to the close structural analogs of their cognate amino acids and that they maintain fidelity for the 19 natural amino acids. This polyspecificity helps to expand the use of this powerful tool to incorporate various UAA residues specifically through in vivo and in vitro approaches. Incorporation of UAAs is expensive, tedious and time-consuming. For the efficient incorporation of UAAs, it is important to screen substrate selectivity prior to their incorporation. As an initial study, using a docking tool, we analyzed the polyspecificity of the methionyl-tRNA synthetases (MetRSs) towards multiple reported and virtually generated methionine analogs. Based on the interaction result of these docking simulations, we predicted the substrate selectivity of the MetRS and the key residues responsible for the recognition of methionine analogs. Similarly, we compared the active site residues of the MetRSs of different species and identified the conserved amino acids in their active sites. Given the close similarity in the active site residues of these systems, we evaluated the polyspecificity of MetRSs.

Aminoacyl-tRNA Synthetases in the Bacterial World

Aminoacyl-tRNAsynthetases (aaRSs) are modular enzymesglobally conserved in the three kingdoms of life. All catalyze the same two-step reaction, i.e., the attachment of a proteinogenic amino acid on their cognate tRNAs, thereby mediating the correct expression of the genetic code. In addition, some aaRSs acquired other functions beyond this key role in translation.Genomics and X-ray crystallography have revealed great structural diversity in aaRSs (e.g.,in oligomery and modularity, in ranking into two distinct groups each subdivided in 3 subgroups, by additional domains appended on the catalytic modules). AaRSs show hugestructural plasticity related to function andlimited idiosyncrasies that are kingdom or even speciesspecific (e.g.,the presence in many Bacteria of non discriminating aaRSs compensating for the absence of one or two specific aaRSs, notably AsnRS and/or GlnRS).Diversity, as well, occurs in the mechanisms of aaRS gene regulation that are not conserved in evolution, notably betweendistant groups such as Gram-positive and Gram-negative Bacteria.Thereview focuses on bacterial aaRSs (and their paralogs) and covers their structure, function, regulation,and evolution. Structure/function relationships are emphasized, notably the enzymology of tRNA aminoacylation and the editing mechanisms for correction of activation and charging errors. The huge amount of genomic and structural data that accumulatedin last two decades is reviewed,showing how thefield moved from essentially reductionist biologytowards more global and integrated approaches. Likewise, the alternative functions of aaRSs and those of aaRSparalogs (e.g., during cellwall biogenesis and other metabolic processes in or outside protein synthesis) are reviewed. Since aaRS phylogenies present promiscuous bacterial, archaeal, and eukaryal features, similarities and differences in the properties of aaRSs from the three kingdoms of life are pinpointedthroughout the reviewand distinctive characteristics of bacterium-like synthetases from organelles are outlined.

Structural diversity and protein engineering of the aminoacyl-tRNA synthetases

Aminoacyl-tRNA synthetases (aaRS) are the enzymes that ensure faithful transmission of genetic information in all living cells, and are central to the developing technologies for expanding the capacity of the translation apparatus to incorporate nonstandard amino acids into proteins in vivo. The 24 known aaRS families are divided into two classes that exhibit functional evolutionary convergence. Each class features an active site domain with a common fold that binds ATP, the amino acid, and the 3'-terminus of tRNA, embellished by idiosyncratic further domains that bind distal portions of the tRNA and enhance specificity. Fidelity in the expression of the genetic code requires that the aaRS be selective for both amino acids and tRNAs, a substantial challenge given the presence of structurally very similar noncognate substrates of both types. Here we comprehensively review central themes concerning the architectures of the protein structures and the remarkable dual-substrate selectivities, with a view toward discerning the most important issues that still substantially limit our capacity for rational protein engineering. A suggested general approach to rational design is presented, which should yield insight into the identities of the protein-RNA motifs at the heart of the genetic code, while also offering a basis for improving the catalytic properties of engineered tRNA synthetases emerging from genetic selections.

Structural dynamics of the aminoacylation and proofreading functional cycle of bacterial leucyl-tRNA synthetase

Leucyl-tRNA synthetase (LeuRS) produces error-free leucyl-tRNA(Leu) by coordinating translocation of the 3' end of (mis-)charged tRNAs from its synthetic site to a separate proofreading site for editing. Here we report cocrystal structures of the Escherichia coli LeuRS-tRNA(Leu) complex in the aminoacylation or editing conformations, showing that translocation involves correlated rotations of four flexibly linked LeuRS domains. This pivots the tRNA to guide its charged 3' end from the closed aminoacylation state to the editing site. The editing domain unexpectedly stabilizes the tRNA during aminoacylation, and a large rotation of the leucine-specific domain positions the conserved KMSKS loop to bind the 3' end of the tRNA, promoting catalysis. Our results give new insight into the structural dynamics of a molecular machine that is essential for accurate protein synthesis.

Allosteric communication in cysteinyl tRNA synthetase: a network of direct and indirect readout

Protein structure networks are constructed for the identification of long-range signaling pathways in cysteinyl tRNA synthetase (CysRS). Molecular dynamics simulation trajectory of CysRS-ligand complexes were used to determine conformational ensembles in order to gain insight into the allosteric signaling paths. Communication paths between the anticodon binding region and the aminoacylation region have been identified. Extensive interaction between the helix bundle domain and the anticodon binding domain, resulting in structural rigidity in the presence of tRNA, has been detected. Based on the predicted model, six residues along the communication paths have been examined by mutations (single and double) and shown to mediate a coordinated coupling between anticodon recognition and activation of amino acid at the active site. This study on CysRS clearly shows that specific key residues, which are involved in communication between distal sites in allosteric proteins but may be elusive in direct structure analysis, can be identified from dynamics of protein structure networks.

Fluorine-protein interactions and ¹⁹F NMR isotropic chemical shifts: An empirical correlation with implications for drug design

An empirical correlation between the fluorine isotropic chemical shifts, measured by ¹⁹F NMR spectroscopy, and the type of fluorine-protein interactions observed in crystal structures is presented. The CF, CF₂, and CF₃ groups present in fluorinated ligands found in the Protein Data Bank were classified according to their ¹⁹F NMR chemical shifts and their close intermolecular contacts with the protein atoms. Shielded fluorine atoms, i.e., those with increased electron density, are observed primarily in close contact to hydrogen bond donors within the protein structure, suggesting the possibility of intermolecular hydrogen bond formation. Deshielded fluorines are predominantly found in close contact with hydrophobic side chains and with the carbon of carbonyl groups of the protein backbone. Correlation between the ¹⁹F NMR chemical shift and hydrogen bond distance, both derived experimentally and computed through quantum chemical methods, is also presented. The proposed "rule of shielding" provides some insight into and guidelines for the judicious selection of appropriate fluorinated moieties to be inserted into a molecule for making the most favorable interactions with the receptor.

Structure of Leishmania major methionyl-tRNA synthetase in complex with intermediate products methionyladenylate and pyrophosphate

Leishmania parasites cause two million new cases of leishmaniasis each year with several hundreds of millions of people at risk. Due to the paucity and shortcomings of available drugs, we have undertaken the crystal structure determination of a key enzyme from Leishmania major in hopes of creating a platform for the rational design of new therapeutics. Crystals of the catalytic core of methionyl-tRNA synthetase from L. major (LmMetRS) were obtained with the substrates MgATP and methionine present in the crystallization medium. These crystals yielded the 2.0 Å resolution structure of LmMetRS in complex with two products, methionyladenylate and pyrophosphate, along with a Mg(2+) ion that bridges them. This is the first class I aminoacyl-tRNA synthetase (aaRS) structure with pyrophosphate bound. The residues of the class I aaRS signature sequence motifs, KISKS and HIGH, make numerous contacts with the pyrophosphate. Substantial differences between the LmMetRS structure and previously reported complexes of Escherichia coli MetRS (EcMetRS) with analogs of the methionyladenylate intermediate product are observed, even though one of these analogs only differs by one atom from the intermediate. The source of these structural differences is attributed to the presence of the product pyrophosphate in LmMetRS. Analysis of the LmMetRS structure in light of the Aquifex aeolicus MetRS-tRNA(Met) complex shows that major rearrangements of multiple structural elements of enzyme and/or tRNA are required to allow the CCA acceptor triplet to reach the methionyladenylate intermediate in the active site. Comparison with sequences of human cytosolic and mitochondrial MetRS reveals interesting differences near the ATP- and methionine-binding regions of LmMetRS, suggesting that it should be possible to obtain compounds that selectively inhibit the parasite enzyme.

Role for a conserved structural motif in assembly of a class I aminoacyl-tRNA synthetase active site

The catalytic domains of class I aminoacyl-tRNA synthetases are built around a conserved Rossmann nucleotide binding fold, with additional polypeptide domains responsible for tRNA binding or hydrolytic editing of misacylated substrates. Structural comparisons identified a conserved motif bridging the catalytic and anticodon binding domains of class Ia and Ib enzymes. This stem contact fold (SCF) has been proposed to globally orient each enzyme's cognate tRNA by interacting with the inner corner of the L-shaped tRNA. Despite the structural similarity of the SCF among class Ia/Ib enzymes, the sequence conservation is low. We replaced amino acids of the MetRS SCF with portions of the structurally similar glutaminyl-tRNA synthetase (GlnRS) motif or with alanine residues. Chimeric variants retained significant tRNA methionylation activity, indicating that structural integrity of the helix-turn-strand-helix motif contributes more to tRNA aminoacylation than does amino acid identity. In contrast, chimeras were significantly reduced in methionyl adenylate synthesis, suggesting a role for the SCF in formation of a structured active site domain. A highly conserved aspartic acid within the MetRS SCF is proposed to make an electrostatic interaction with an active site lysine; these residues were replaced with alanines or conservative substitutions. Both methionyl adenylate formation and methionine transfer were impaired, and activity was not significantly recovered by making the compensatory double substitution.

A homology model for Clostridium difficile methionyl tRNA synthetase: active site analysis and docking interactions

Treatment of C. difficile infection is one of the most difficult biomedical challenges. To develop novel antibacterials, researchers have been targeting bacterial molecular functions that are essential for its growth. The methionyl tRNA synthetase (MetRS) is strictly required for protein biosynthesis and success was reported in developing antibacterials to inhibit this enzyme. The present study was aimed at building and analyzing a homology model for C. difficile MetRS in the context of drug design. A homology model of C. difficile MetRS was constructed using Molecular Operating Environment (MOE) software. A. aeolicus MetRS was the main template while the query zinc binding domain was modeled using T. thermophilus MetRS. The model has been assessed and compared to its main template (Ramachandran, ERRAT and ProSA). The active site of the query protein has been predicted from its sequence using a detailed conservation analysis (ClustalW2). Using MOE software, suitable ligands were docked in the constructed model, including a C. difficile MetRS inhibitor REP3123 and the enzyme natural substrate, and the key active site residues and interactions were identified. These docking studies have validated the active site conformation in the constructed model and identified binding interactions.

Flexibility and communication within the structure of the Mycobacterium smegmatis methionyl-tRNA synthetase

Two structures of monomeric methionyl-tRNA synthetase, from Mycobacterium smegmatis, in complex with the ligands methionine/adenosine and methionine, were analyzed by X-ray crystallography at 2.3 Å and at 2.8 Å, respectively. The structures demonstrated the flexibility of the multidomain enzyme. A new conformation of the structure was identified in which the connective peptide domain bound more closely to the catalytic domain than described previously. The KMSKS(301-305) loop in our structures was in an open and inactive conformation that differed from previous structures by a rotation of the loop of about 90° around hinges located at Asn297 and Val310. The binding of adenosine to the methionyl-tRNA synthetase methionine complex caused a shift in the KMSKS domain that brought it closer to the catalytic domain. The potential use of the adenosine-binding site for inhibitor binding was evaluated and a potential binding site for a specific allosteric inhibitor was identified.

Effect of a domain-spanning disulfide on aminoacyl-tRNA synthetase activity

Enzymes regulated by allostery undergo conformational rearrangement upon binding effector molecules. For modular proteins, a flexible interface may mediate reorientation of the protein domains and transmit binding events to activate catalysis at a distance. Aminoacyl-tRNA synthetases (aaRSs) that use tRNA anticodons as identity elements can be considered allosteric enzymes in which aminoacylation of the tRNA acceptor stem is enhanced upon anticodon binding. We reasoned that anticodon-triggered conformational change might be restricted upon introduction of a disulfide linkage near the core of an aaRS. Here we show that a double cysteine mutation engineered at the Escherichia coli MetRS domain interface spontaneously generates a disulfide linkage. This disulfide clamp has no effect on methionyl adenylate formation but reduces the level of tRNA(Met) aminoacylation approximately 2-fold. Activity is restored upon chemical reduction of the disulfide, demonstrating that E. coli MetRS requires a flexible interface domain for full catalytic efficiency.

Toward the catalytic mechanism of a cysteine ligase (MshC) from Mycobacterium smegmatis: an enzyme involved in the biosynthetic pathway of mycothiol

Mycobacterium tuberculosis and other members of the actinomycete family produce mycothiol (MSH or acetylcysteine-glucosamine-inositol, AcCys-GlcN-Ins) to protect the organism against oxidative and antibiotic stress. The biosynthesis of MSH proceeds via a five-step process that involves four unique enzymes, MshA-D, which represent specific targets for inhibitor design. Recombinant Mycobacterium smegmatis MshC catalyzes the ATP-dependent condensation of glucosamine-inositol (GlcN-Ins) and cysteine to form Cys-GlcN-Ins. The 1.6 A three-dimensional structure of MshC in complex with a tight binding bisubstrate analogue, 5'-O-[N-(L-cysteinyl)sulfamonyl]adenosine (CSA), has suggested specific roles for T46, H55, T83, W227, and D251. In addition, a catalytic role for H55 has been proposed on the basis of studies of related aminoacyl-tRNA synthetases. Site-directed mutagenesis was conducted to evaluate the functional roles of these highly conserved residues. All mutants exhibited significantly decreased k(cat) values, with the exception of T83V for which a <7-fold decrease was observed compared to that of the wild type (WT). For the T46V, H55A, W227F, and D251N mutants, the rate of cysteine activation decreased 100-1400-fold compared to that of WT, consistent with the important roles of these residues in the first half-reaction. The approximately 2000-fold decrease in k(cat)/K(m) as well as the approximately 20-fold decrease in K(m) for cysteine suggested a significant role for T46 in cysteine binding. Kinetic studies also indicate a function for W227 in cysteine binding but not in substrate discrimination against serine. H55 was also observed to play a significant role in ATP binding as well as cysteine adenylation. The activity of H55A was partially rescued with exogenous imidazole at acidic pH values, suggesting that the protonated form of histidine is exerting a catalytic role. The pH dependence of the kinetic parameters with the WT enzyme suggests an additional requirement for a catalytic base in cysteinyl ligation.

Switching from an induced-fit to a lock-and-key mechanism in an aminoacyl-tRNA synthetase with modified specificity

Methionyl-tRNA synthetase (MetRS) specifically binds its methionine substrate in an induced-fit mechanism, with methionine binding causing large rearrangements. Mutated MetRS able to efficiently aminoacylate the methionine (Met) analog azidonorleucine (Anl) have been identified by saturation mutagenesis combined with in vivo screening procedures. Here, the crystal structure of such a mutated MetRS was determined in the apo form as well as complexed with Met or Anl (1.4 to 1.7 A resolution) to reveal the structural basis for the altered specificity. The mutations result in both the loss of important contacts with Met and the creation of new contacts with Anl, thereby explaining the specificity shift. Surprisingly, the conformation induced by Met binding in wild-type MetRS already occurs in the apo form of the mutant enzyme. Therefore, the mutations cause the enzyme to switch from an induced-fit mechanism to a lock-and-key one, thereby enhancing its catalytic efficiency.

Discovery of Escherichia coli methionyl-tRNA synthetase mutants for efficient labeling of proteins with azidonorleucine in vivo

Incorporation of noncanonical amino acids into cellular proteins often requires engineering new aminoacyl-tRNA synthetase activity into the cell. A screening strategy that relies on cell-surface display of reactive amino acid side-chains was used to identify a diverse set of methionyl-tRNA synthetase (MetRS) mutants that allow efficient incorporation of the methionine (Met) analog azidonorleucine (Anl). We demonstrate that the extent of cell-surface labeling in vivo is a good indicator of the rate of Anl activation by the MetRS variant harbored by the cell. By screening at low Anl concentrations in Met-supplemented media, MetRS variants with improved activities toward Anl and better discrimination against Met were identified.

Modeling of tRNA-assisted mechanism of Arg activation based on a structure of Arg-tRNA synthetase, tRNA, and an ATP analog (ANP)

The ATP-pyrophosphate exchange reaction catalyzed by Arg-tRNA, Gln-tRNA and Glu-tRNA synthetases requires the assistance of the cognate tRNA. tRNA also assists Arg-tRNA synthetase in catalyzing the pyrophosphorolysis of synthetic Arg-AMP at low pH. The mechanism by which the 3'-end A76, and in particular its hydroxyl group, of the cognate tRNA is involved with the exchange reaction catalyzed by those enzymes has yet to be established. We determined a crystal structure of a complex of Arg-tRNA synthetase from Pyrococcus horikoshii, tRNA(Arg)(CCU) and an ATP analog with Rfactor = 0.213 (Rfree = 0.253) at 2.0 A resolution. On the basis of newly obtained structural information about the position of ATP bound on the enzyme, we constructed a structural model for a mechanism in which the formation of a hydrogen bond between the 2'-OH group of A76 of tRNA and the carboxyl group of Arg induces both formation of Arg-AMP (Arg + ATP --> Arg-AMP + pyrophosphate) and pyrophosphorolysis of Arg-AMP (Arg-AMP + pyrophosphate --> Arg + ATP) at low pH. Furthermore, we obtained a structural model of the molecular mechanism for the Arg-tRNA synthetase-catalyzed deacylation of Arg-tRNA (Arg-tRNA + AMP --> Arg-AMP + tRNA at high pH), in which the deacylation of aminoacyl-tRNA bound on Arg-tRNA synthetase and Glu-tRNA synthetase is catalyzed by a quite similar mechanism, whereby the proton-donating group (-NH-C+(NH2)2 or -COOH) of Arg and Glu assists the aminoacyl transfer from the 2'-OH group of tRNA to the phosphate group of AMP at high pH.

The 1.6 A crystal structure of Mycobacterium smegmatis MshC: the penultimate enzyme in the mycothiol biosynthetic pathway

Mycobacterium smegmatis MshC catalyzes the ATP-dependent condensation of GlcN-Ins and l-cysteine to form l-Cys-GlcN-Ins, the penultimate step in mycothiol biosynthesis. Attempts to crystallize the native, full-length MshC have been unsuccessful. However, incubation of the enzyme with the cysteinyl adenylate analogue, 5'-O-[N-(l-cysteinyl)-sulfamonyl]adenosine (CSA), followed by a 24-h limited trypsin proteolysis yielded an enzyme preparation that readily crystallized. The three-dimensional structure of MshC with CSA bound in the active site was solved and refined to 1.6 A. The refined structure exhibited electron density corresponding to the entire 47 kDa MshC molecule, with the exception of the KMSKS loop (residues 285-297), a loop previously implicated in the formation of the adenylate in related tRNA synthases. The overall tertiary fold of MshC is similar to that of cysteinyl-tRNA synthetase, with a Rossmann fold catalytic domain. The interaction of the thiolate of CSA with a zinc ion at the base of the active site suggests that the metal ion participates in amino acid binding and discrimination. A number of active site residues were observed to interact with the ligand, suggesting a role in substrate binding and catalysis. Analysis utilizing modeling of the proteolyzed loop and GlcN-Ins docking, as well as the examination of sequence conservation in the active site suggests similarities and differences between cysteinyl-tRNA synthetases and MshC in recognition of the substrates for their respective reactions.

Inhibition of methionyl-tRNA synthetase by REP8839 and effects of resistance mutations on enzyme activity

REP8839 is a selective inhibitor of methionyl-tRNA synthetase (MetRS) with antibacterial activity against a variety of gram-positive organisms. We determined REP8839 potency against Staphylococcus aureus MetRS and assessed its selectivity for bacterial versus human orthologs of MetRS. The inhibition constant (K(i)) of REP8839 was 10 pM for Staphylococcus aureus MetRS. Inhibition of MetRS by REP8839 was competitive with methionine and uncompetitive with ATP. Thus, high physiological ATP levels would actually facilitate optimal binding of the inhibitor. While many gram-positive bacteria, such as Staphylococcus aureus, express exclusively the MetRS1 subtype, many gram-negative bacteria express an alternative homolog called MetRS2. Some gram-positive bacteria, such as Streptococcus pneumoniae and Bacillus anthracis, express both MetRS1 and MetRS2. MetRS2 orthologs were considerably less susceptible to REP8839 inhibition. REP8839 inhibition of human mitochondrial MetRS was 1,000-fold weaker than inhibition of Staphylococcus aureus MetRS; inhibition of human cytoplasmic MetRS was not detectable, corresponding to >1,000,000-fold selectivity for the bacterial target relative to its cytoplasmic counterpart. Mutations in MetRS that confer reduced susceptibility to REP8839 were examined. The mutant MetRS enzymes generally exhibited substantially impaired catalytic activity, particularly in aminoacylation turnover rates. REP8839 K(i) values ranged from 4- to 190,000-fold higher for the mutant enzymes than for wild-type MetRS. These observations provide a potential mechanistic explanation for the reduced growth fitness observed with MetRS mutant strains relative to that with wild-type Staphylococcus aureus.

Antibacterial 5'-O-(N-dipeptidyl)-sulfamoyladenosines

The aminoacyl-tRNA synthetase (aaRS) class of enzymes is a validated target for antimicrobial development. Aminoacyl analogues of 5'-O-(N-L-aminoacyl)-sulfamoyladenosines are known to be potent inhibitors of aaRS, but whole cell antibacterial activity of these compounds is very limited, and poor penetration into bacteria has been proposed as the main reason for this. Aiming to find derivatives that better penetrate bacteria, we developed a simple and short method to prepare dipeptidyl-derivatives of 5'-O-(N-L-aminoacyl)-sulfamoyladenosines, and used this method to prepare 18 5'-O-(N-dipeptidyl)-sulfamoyladenosines. The antibacterial activity of these derivatives and a number of reference compounds against S. aureus, E. faecalis and E. coli was determined. Several of the new derivatives showed improved antibacterial activity and an altered spectrum of antibacterial activity.