The K+-Dependent and -Independent Pyruvate Kinases Acquire the Active Conformation by Different Mechanisms.

Eukarya pyruvate kinases possess glutamate at position 117 (numbering of rabbit muscle enzyme), whereas bacteria have either glutamate or lysine. Those with E117 are K+-dependent, whereas those with K117 are K+-independent. In a phylogenetic tree, 80% of the sequences with E117 are occupied by T113/K114/T120 and 77% of those with K117 possess L113/Q114/(L,I,V)120. This work aims to understand these residues' contribution to the K+-independent pyruvate kinases using the K+-dependent rabbit muscle enzyme. Residues 117 and 120 are crucial in the differences between the K+-dependent and -independent mutants. K+-independent activity increased with L113 and Q114 to K117, but L120 induced structural differences that inactivated the enzyme. T120 appears to be key in folding the protein and closure of the lid of the active site to acquire its active conformation in the K+-dependent enzymes. E117K mutant was K+-independent and the enzyme acquired the active conformation by a different mechanism. In the K+-independent apoenzyme of Mycobacterium tuberculosis, K72 (K117) flips out of the active site; in the holoenzyme, K72 faces toward the active site bridging the substrates through water molecules. The results provide evidence that two different mechanisms have evolved for the catalysis of this reaction.

Energetic and structural effects of the Tanford transition on ligand recognition of bovine ß-lactoglobulin.

Bovine ß-lactoglobulin, an abundant protein in whey, is a promising nanocarrier for peroral administration of drug-like hydrophobic molecules, a process that involves transit through the different acidic conditions of the human digestive tract. Among the several pH-induced conformational rearrangements that this lipocalin undergoes, the Tanford transition is particularly relevant. This transition, which occurs with a midpoint around neutral pH, involves a conformational change of the E-F loop that regulates accessibility to the primary binding site. The effect of this transition on the ligand binding properties of this protein has scarcely been explored. In this study, we carried out an energetic and structural characterization of ß-lactoglobulin molecular recognition at pH values above and below the zone in which the Tanford transition occurs. The combined analysis of crystallographic, calorimetric, and molecular dynamics data sheds new light on the interplay between self-association, ligand binding, and the Tanford pre- and post-transition conformational states, revealing novel aspects underlying the molecular recognition mechanism of this enigmatic lipocalin.

Structural Insights into the Polyphyletic Origins of Glycyl tRNA Synthetases.

Glycyl tRNA synthetase (GlyRS) provides a unique case among class II aminoacyl tRNA synthetases, with two clearly widespread types of enzymes: a dimeric (α2) species present in some bacteria, archaea, and eukaryotes; and a heterotetrameric form (α2ß2) present in most bacteria. Although the differences between both types of GlyRS at the anticodon binding domain level are evident, the extent and implications of the variations in the catalytic domain have not been described, and it is unclear whether the mechanism of amino acid recognition is also dissimilar. Here, we show that the α-subunit of the α2ß2 GlyRS from the bacterium Aquifex aeolicus is able to perform the first step of the aminoacylation reaction, which involves the activation of the amino acid with ATP. The crystal structure of the α-subunit in the complex with an analog of glycyl adenylate at 2.8 Å resolution presents a conformational arrangement that properly positions the cognate amino acid. This work shows that glycine is recognized by a subset of different residues in the two types of GlyRS. A structural and sequence analysis of class II catalytic domains shows that bacterial GlyRS is closely related to alanyl tRNA synthetase, which led us to define a new subclassification of these ancient enzymes and to propose an evolutionary path of α2ß2 GlyRS, convergent with α2 GlyRS and divergent from AlaRS, thus providing a possible explanation for the puzzling existence of two proteins sharing the same fold and function but not a common ancestor.

Kinetics of tRNA folding monitored by aminoacylation.

We describe a strategy for tracking Mg²âº-initiated folding of ³²P-labeled tRNA molecules to their native structures based on the capacity for aminoacylation by the cognate aminoacyl-tRNA synthetase enzyme. The approach directly links folding to function, paralleling a common strategy used to study the folding of catalytic RNAs. Incubation of unfolded tRNA with magnesium ions, followed by the addition of aminoacyl-tRNA synthetase and further incubation, yields a rapid burst of aminoacyl-tRNA formation corresponding to the prefolded tRNA fraction. A subsequent slower increase in product formation monitors continued folding in the presence of the enzyme. Further analysis reveals the presence of a parallel fraction of tRNA that folds more rapidly than the majority of the population. The application of the approach to study the influence of post-transcriptional modifications in folding of Escherichia coli tRNA1(Gln) reveals that the modified bases increase the folding rate but do not affect either the equilibrium between properly folded and misfolded states or the folding pathway. This assay allows the use of ³²P-labeled tRNA in integrated studies combining folding, post-transcriptional processing, and aminoacylation reactions.

Substrate binding in the allosteric site mimics homotropic cooperativity in the SIS-fold glucosamine-6-phosphate deaminases.

Glucosamine-6-phosphate (GlcN6P) deaminases from Escherichia coli (EcNagBI) and Shewanella denitrificans (SdNagBII) are special examples of what constitute nonhomologous isofunctional enzymes due to their convergence, not only in catalysis, but also in cooperativity and allosteric properties. Additionally, we found that the sigmoidal kinetics of SdNagBII cannot be explained by the existing models of homotropic activation. This study describes the regulatory mechanism of SdNagBII using enzyme kinetics, isothermal titration calorimetry (ITC), and X-ray crystallography. ITC experiments revealed two different binding sites with distinctive thermodynamic signatures: a single binding site per monomer for the allosteric activator N-acetylglucosamine 6-phosphate (GlcNAc6P) and two binding sites per monomer for the transition-state analog 2-amino-2-deoxy-D-glucitol 6-phosphate (GlcNol6P). Crystallographic data demonstrated the existence of an unusual allosteric site that can bind both GlcNAc6P and GlcNol6P, implying that the homotropic activation of this enzyme arises from the occupation of the allosteric site by the substrate. In this work we describe the presence of this novel allosteric site in the SIS-fold deaminases, which is responsible for the homotropic and heterotropic activation of SdNagBII by GlcN6P and GlcNAc6P, respectively. This study unveils an original mechanism to generate a high degree of homotropic activation in SdNagBII, mimicking the allosteric and cooperative properties of hexameric EcNagBI but with a reduced number of subunits.

Angucyclinones rescue PhLOPSA antibiotic activity by inhibiting Cfr-dependent antibiotic resistance.

IMPORTANCE: Cfr is an antibiotic resistance enzyme that inhibits five clinically important antibiotic classes, is genetically mobile, and has a minimal fitness cost, making Cfr a serious threat to antibiotic efficacy. The significance of our work is in discovering molecules that inhibit Cfr-dependent methylation of the ribosome, thus protecting the efficacy of the PhLOPSA antibiotics. These molecules are the first reported inhibitors of Cfr-mediated ribosome methylation and, as such, will guide the further discovery of chemical scaffolds against Cfr-mediated antibiotic resistance. Our work acts as a foundation for further development of molecules that safeguard the PhLOPSA antibiotics from Cfr.

Structure-Based Discovery of Inhibitors of the SARS-CoV-2 Nsp14 N7-Methyltransferase.

An under-explored target for SARS-CoV-2 is the S-adenosyl methionine (SAM)-dependent methyltransferase Nsp14, which methylates the N7-guanosine of viral RNA at the 5'-end, allowing the virus to evade host immune response. We sought new Nsp14 inhibitors with three large library docking strategies. First, up to 1.1 billion lead-like molecules were docked against the enzyme's SAM site, leading to three inhibitors with IC50 values from 6 to 50 µM. Second, docking a library of 16 million fragments revealed 9 new inhibitors with IC50 values from 12 to 341 µM. Third, docking a library of 25 million electrophiles to covalently modify Cys387 revealed 7 inhibitors with IC50 values from 3.5 to 39 µM. Overall, 32 inhibitors encompassing 11 chemotypes had IC50 values < 50 µM and 5 inhibitors in 4 chemotypes had IC50 values < 10 µM. These molecules are among the first non-SAM-like inhibitors of Nsp14, providing starting points for future optimization.

A native IgE in complex with profilin provides insights into allergen recognition and cross-reactivity.

Allergies have become a rising health problem, where plentiful substances can trigger IgE-mediated allergies in humans. While profilins are considered minor allergens, these ubiquitous proteins are primary molecules involved in cross-reactivity and pollen-food allergy syndrome. Here we report the first crystal structures of murine Fab/IgE, with its chains naturally paired, in complex with the allergen profilin from Hevea brasiliensis (Hev b 8). The crystallographic models revealed that the IgE's six complementarity-determining regions (CDRs) interact with the allergen, comprising a rigid paratope-epitope surface of 926 Å2, which includes an extensive network of interactions. Interestingly, we also observed previously unreported flexibility at Fab/IgE's elbow angle, which did not influence the shape of the paratope. The Fab/IgE exhibits a high affinity for Hev b 8, even when using 1 M NaCl in BLI experiments. Finally, based on the encouraging cross-reactivity assays using two mutants of the maize profilin (Zea m 12), this antibody could be a promising tool in IgE engineering for diagnosis and research applications.

A rationally engineered misacylating aminoacyl-tRNA synthetase.

Information transfer from nucleic acid to protein is mediated by aminoacyl-tRNA synthetases, which catalyze the specific pairings of amino acids with transfer RNAs. Despite copious sequence and structural information on the 22 tRNA synthetase families, little is known of the enzyme signatures that specify amino acid selectivities. Here, we show that transplanting a conserved arginine residue from glutamyl-tRNA synthetase (GluRS) to glutaminyl-tRNA synthetase (GlnRS) improves the K(M) of GlnRS for noncognate glutamate. Two crystal structures of this C229R GlnRS mutant reveal that a conserved twin-arginine GluRS amino acid identity signature cannot be incorporated into GlnRS without disrupting surrounding protein structural elements that interact with the tRNA. Consistent with these findings, we show that cumulative replacement of other primary binding site residues in GlnRS, with those of GluRS, only slightly improves the ability of the GlnRS active site to accommodate glutamate. However, introduction of 22 amino acid replacements and one deletion, including substitution of the entire primary binding site and two surface loops adjacent to the region disrupted in C229R, improves the capacity of Escherichia coli GlnRS to synthesize misacylated Glu-tRNA(Gln) by 16,000-fold. This hybrid enzyme recapitulates the function of misacylating GluRS enzymes found in organisms that synthesize Gln-tRNA(Gln) by an alternative pathway. These findings implicate the RNA component of the contemporary GlnRS-tRNA(Gln) complex in mediating amino acid specificity. This role for tRNA may persist as a relic of primordial cells in which the evolution of the genetic code was driven by RNA-catalyzed amino acid-RNA pairing.

The structure of a novel membrane-associated 6-phosphogluconate dehydrogenase from Gluconacetobacter diazotrophicus (Gd6PGD) reveals a subfamily of short-chain 6PGDs.

The enzyme 6-phosphogluconate dehydrogenase catalyzes the conversion of 6-phosphogluconate to ribulose-5-phosphate. It represents an important reaction in the oxidative pentose phosphate pathway, producing a ribose precursor essential for nucleotide and nucleic acid synthesis. We succeeded, for the first time, to determine the three-dimensional structure of this enzyme from an acetic acid bacterium, Gluconacetobacter diazotrophicus (Gd6PGD). Active Gd6PGD, a homodimer (70 kDa), was present in both the soluble and the membrane fractions of the nitrogen-fixing microorganism. The Gd6PGD belongs to the newly described subfamily of short-chain (333 AA) 6PGDs, compared to the long-chain subfamily (480 AA; e.g., Ovis aries, Homo sapiens). The shorter amino acid sequence in Gd6PGD induces the exposition of hydrophobic residues in the C-terminal domain. This distinct structural feature is key for the protein to associate with the membrane. Furthermore, in terms of function, the short-chain 6PGD seems to prefer NAD+ over NADP+ , delivering NADH to the membrane-bound NADH dehydrogenase of the microorganisms required by the terminal oxidases to reduce dioxygen to water for energy conservation. ENZYME: ECnonbreakingspace1.1.1.343. DATABASE: Structural data are available in PDB database under the accession number 6VPB.

Synthesis of Glu-tRNA(Gln) by engineered and natural aminoacyl-tRNA synthetases.

A protein engineering approach to delineating which distinct elements of homologous tRNA synthetase architectures are responsible for divergent RNA-amino acid pairing specificities is described. Previously, we constructed a hybrid enzyme in which 23 amino acids from the catalytic domain of Escherichia coli glutaminyl-tRNA synthetase (GlnRS) were replaced with the corresponding residues of human glutamyl-tRNA synthetase (GluRS). The engineered hybrid (GlnRS S1/L1/L2) synthesizes Glu-tRNA(Gln) more than 10(4)-fold more efficiently than GlnRS. Detailed comparison of kinetic parameters between GlnRS S1/L1/L2 and the naturally occurring Methanothermobacter thermautotrophicus GluRS(ND), which is also capable of Glu-tRNA(Gln) synthesis, now shows that both k(cat) and K(m) for glutamate are recapitulated in the engineered enzyme, but that K(m) for tRNA is 200-fold higher. Thus, the simultaneous optimization of paired amino acid and tRNA binding sites found in a naturally occurring enzyme is not recapitulated in a hybrid that is successfully engineered for amino acid complementarity. We infer that the GlnRS architecture has differentiated to match only cognate amino acid-RNA pairs, and that the substrate selection functions do not operate independently of each other. Design and characterization of four additional hybrids identify further residues involved in improving complementarity for glutamate and in communicating between amino acid and tRNA binding sites. The robust catalytic function demonstrated in this engineered system offers a novel platform for exploring the stereochemical origins of coding as a property of the ancient Rossmann fold.

The substrate of the glucose-6-phosphate dehydrogenase of Pseudomonas aeruginosa provides structural stability.

In general, eukaryotic glucose-6-phosphate dehydrogenases (G6PDHs) are structurally stabilized by NADP+. Here we show by spectrofluorometric analysis, thermal and urea denaturation, and trypsin proteolysis, that a different mechanism stabilizes the enzyme from Pseudomonas aeruginosa (PaG6PDH) (EC 1.1.1.363). The spectrofluorometric analysis of the emission of 8-anilino-1-naphthalenesulfonic acid (ANS) indicates that this stabilization is the result of a structural change in the enzyme caused by G6P. The similarity between the Kd values determined for the PaG6PDH-G6P complex (78.0 ±â€¯7.9 µM) and the K0.5 values determined for G6P (57.9 ±â€¯2.5 and 104.5 ±â€¯9.3 µM in the NADP+- and NAD+-dependent reactions, respectively) suggests that the structural changes are the result of G6P binding to the active site of PaG6PDH. Modeling of PaG6PDH indicated the residues that potentially bind the ligand. These results and a phylogenetic analysis of the amino acid sequences of forty-four G6PDHs, suggest that the stabilization observed for PaG6PDH could be a characteristic that distinguishes this and other G6PDHs that use NAD+ and NADP+ from those that use NADP+ only or preferentially, such as those found in eukaryotes. This characteristic could be related to the metabolic roles these enzymes play in the organisms to which they belong.

Novel murine mAbs define specific and cross-reactive epitopes on the latex profilin panallergen Hev b 8.

The production of specific antibodies able to recognize allergens from different sources or block interactions between allergens and antibodies mediating allergic reactions is crucial for developing successful tools for diagnostics and therapeutics. Panallergens are highly conserved proteins present in widely different species, implicated in relevant cross-reactions. The panallergen latex profilin (Hev b 8) has been associated with the latex-food-pollen syndrome. We generated five monoclonal IgGs and one IgE from murine hybridomas against recombinant Hev b 8 and evaluated their interaction with this allergen using ELISA and biolayer interferometry (BLI). Affinity purified mAbs exhibited high binding affinities towards rHev b 8, with KD1 values ranging from 10-10 M to 10-11 M. Some of these antibodies also recognized the recombinant profilins from maize and tomato (Zea m 12 and Sola l 1), and the ash tree pollen (Fra e 2). Competition ELISA demonstrated that some mAb pairs could bind simultaneously to rHev b 8. Using BLI, we detected competitive, non-competitive, and partial-competition interactions between pairs of mAbs with rHev b 8, suggesting the existence of at least two non-overlapping epitopes on the surface of this allergen. Three-dimensional models of the Fv of 1B4 and 2D10 IgGs and docking simulations of these Fvs with rHev b 8 revealed these epitopes. Furthermore, these two mAbs inhibited the interaction of polyclonal IgE and IgG4 antibodies from profilin-allergic patients with rHev b 8, indicating that the mAbs and the antibodies present in sera from allergic patients bind to overlapping epitopes on the allergen. These mAbs can be useful tools for immune-localization studies, immunoassay development, or standardization of allergenic products.

The interplay of protein-ligand and water-mediated interactions shape affinity and selectivity in the LAO binding protein.

The study of binding thermodynamics is essential to understand how affinity and selectivity are acquired in molecular complexes. Periplasmic binding proteins (PBPs) are macromolecules of biotechnological interest that bind a broad number of ligands and have been used to design biosensors. The lysine-arginine-ornithine binding protein (LAO) is a PBP of 238 residues that binds the basic amino acids l-arginine and l-histidine with nm and µm affinity, respectively. It has been shown that the affinity difference for arginine and histidine binding is caused by enthalpy, this correlates with the higher number of protein-ligand contacts formed with arginine. In order to elucidate the structural bases that determine binding affinity and selectivity in LAO, the contribution of protein-ligand contacts to binding energetics was assessed. To this end, an alanine scanning of the LAO-binding site residues was performed and arginine and histidine binding were characterized by isothermal titration calorimetry and X-ray crystallography. Although unexpected enthalpy and entropy changes were observed in some mutants, thermodynamic data correlated with structural information, especially, the binding heat capacity change. We found that selectivity is conferred by several residues rather than exclusive arginine-protein interactions. Furthermore, crystallographic structures revealed that protein-ligand contributions to binding thermodynamics are highly influenced by the solvent. Finally, we found a similar backbone conformation in all the closed structures obtained, but different structures in the open state, suggesting that the binding site residues of LAO play an important role in stabilizing not only the holo conformation, but also the apo state. DATABASE: Structural data are available in the Protein Data Bank database under the accession numbers 6MLE, 6MLN, 6MLG, 6MKX, 6MLI, 6MLA, 6MKU, 6MKW, 6ML0, 6MLD, 6MLV, 6MLO, 6MLP, 6ML9, 6MLJ.

A biophysical and structural study of two chitinases from Agave tequilana and their potential role as defense proteins.

Plant chitinases are enzymes that have several functions, including providing protection against pathogens. Agave tequilana is an economically important plant that is poorly studied. Here, we identified a chitinase from short reads of the A. tequilana transcriptome (AtChi1). A second chitinase, differing by only six residues from the first, was isolated from total RNA of plants infected with Fusarium oxysporum (AtChi2). Both enzymes were overexpressed in Escherichia coli and analysis of their sequences indicated that they belong to the class I glycoside hydrolase family19, whose members exhibit two domains: a carbohydrate-binding module and a catalytic domain, connected by a flexible linker. Activity assays and thermal shift experiments demonstrated that the recombinant Agave enzymes are highly thermostable acidic endochitinases with Tm values of 75 °C and 71 °C. Both exhibit a molecular mass close to 32 kDa, as determined by MALDI-TOF, and experimental pIs of 3.7 and 3.9. Coupling small-angle x-ray scattering information with homology modeling and docking simulations allowed us to structurally characterize both chitinases, which notably show different interactions in the binding groove. Even when the six different amino acids are all exposed to solvent in the loops located near the linker and opposite to the binding site, they confer distinct kinetic parameters against colloidal chitin and similar affinity for (GlnNAc)6, as shown by isothermal titration calorimetry. Interestingly, binding is more enthalpy-driven for AtChi2. Whereas the physiological role of these chitinases remains unknown, we demonstrate that they exhibit important antifungal activity against chitin-rich fungi such as Aspergillus sp. DATABASE: SAXS structural data are available in the SASBDB database with accession numbers SASDDE7 and SASDDA6. ENZYMES: Chitinases (EC3.2.1.14).

Amino and carboxy-terminal extensions of yeast mitochondrial DNA polymerase assemble both the polymerization and exonuclease active sites.

Human and yeast mitochondrial DNA polymerases (DNAPs), POLG and Mip1, are related by evolution to bacteriophage DNAPs. However, mitochondrial DNAPs contain unique amino and carboxyl-terminal extensions that physically interact. Here we describe that N-terminal deletions in Mip1 polymerases abolish polymerization and decrease exonucleolytic degradation, whereas moderate C-terminal deletions reduce polymerization. Similarly, to the N-terminal deletions, an extended C-terminal deletion of 298 amino acids is deficient in nucleotide addition and exonucleolytic degradation of double and single-stranded DNA. The latter observation suggests that the physical interaction between the amino and carboxyl-terminal regions of Mip1 may be related to the spread of pathogenic POLG mutant along its primary sequence.

Structural and mechanistic basis for enhanced translational efficiency by 2-thiouridine at the tRNA anticodon wobble position.

The 2-thiouridine (s(2)U) at the wobble position of certain bacterial and eukaryotic tRNAs enhances aminoacylation kinetics, assists proper codon-anticodon base pairing at the ribosome A-site, and prevents frameshifting during translation. By mass spectrometry of affinity-purified native Escherichia coli tRNA1(Gln)UUG, we show that the complete modification at the wobble position 34 is 5-carboxyaminomethyl-2-thiouridine (cmnm(5)s(2)U). The crystal structure of E. coli glutaminyl-tRNA synthetase (GlnRS) bound to native tRNA1(Gln) and ATP demonstrates that cmnm(5)s(2)U34 improves the order of a previously unobserved 11-amino-acid surface loop in the distal ß-barrel domain of the enzyme and imparts other local rearrangements of nearby amino acids that create a binding pocket for the 2-thio moiety. Together with previously solved structures, these observations explain the degenerate recognition of C34 and modified U34 by GlnRS. Comparative pre-steady-state aminoacylation kinetics of native tRNA1(Gln), synthetic tRNA1(Gln) containing s(2)U34 as sole modification, and unmodified wild-type and mutant tRNA1(Gln) and tRNA2(Gln) transcripts demonstrates that the exocyclic sulfur moiety improves tRNA binding affinity to GlnRS 10-fold compared with the unmodified transcript and that an additional fourfold improvement arises from the presence of the cmnm(5) moiety. Measurements of Gln-tRNA(Gln) interactions at the ribosome A-site show that the s(2)U modification enhances binding affinity to the glutamine codons CAA and CAG and increases the rate of GTP hydrolysis by E. coli EF-Tu by fivefold.

Heat maps for intramolecular communication in an RNP enzyme encoding glutamine.

Allosteric signaling within large ribonucleoproteins modulates both catalytic function and biological specificity, but the spatial extent and quantitative magnitudes of long-distance free-energy couplings have yet to be well characterized. Here, we employ pre-steady-state kinetics to generate a comprehensive mapping of intramolecular communication in the glutaminyl-tRNA synthetase:tRNA(Gln) complex. Alanine substitution at 29 positions across the protein-RNA interface reveals distinct coupling amplitudes for glutamine binding and aminoacyl-tRNA formation on the enzyme, respectively, implying the existence of multiple signaling pathways. Structural models suggest that long-range signal propagation from the tRNA anticodon is dynamically driven, whereas shorter pathways are mediated by induced-fit rearrangements. Seven protein contacts with the distal tRNA vertical arm each weaken glutamine binding affinity across distances up to 40 Å, demonstrating that negative allosteric coupling plays a key role in enforcing the selective RNA-amino acid pairing at the heart of the genetic code.