Carboxyl-carboxylate interactions in proteins

Malate initiates a proton-sensing pathway essential for pH regulation of inflammation

Metabolites can double as a signaling modality that initiates physiological adaptations. Metabolism, a chemical language encoding biological information, has been recognized as a powerful principle directing inflammatory responses. Cytosolic pH is a regulator of inflammatory response in macrophages. Here, we found that L-malate exerts anti-inflammatory effect via BiP-IRF2BP2 signaling, which is a sensor of cytosolic pH in macrophages. First, L-malate, a TCA intermediate upregulated in pro-inflammatory macrophages, was identified as a potent anti-inflammatory metabolite through initial screening. Subsequent screening with DARTS and MS led to the isolation of L-malate-BiP binding. Further screening through protein‒protein interaction microarrays identified a L-malate-restrained coupling of BiP with IRF2BP2, a known anti-inflammatory protein. Interestingly, pH reduction, which promotes carboxyl protonation of L-malate, facilitates L-malate and carboxylate analogues such as succinate to bind BiP, and disrupt BiP-IRF2BP2 interaction in a carboxyl-dependent manner. Both L-malate and acidification inhibit BiP-IRF2BP2 interaction, and protect IRF2BP2 from BiP-driven degradation in macrophages. Furthermore, both in vitro and in vivo, BiP-IRF2BP2 signal is required for effects of both L-malate and pH on inflammatory responses. These findings reveal a previously unrecognized, proton/carboxylate dual sensing pathway wherein pH and L-malate regulate inflammatory responses, indicating the role of certain carboxylate metabolites as adaptors in the proton biosensing by interactions between macromolecules.

Cryo-EM characterization of the anydromuropeptide permease AmpG central to bacterial fitness and β-lactam antibiotic resistance

Bacteria invest significant resources into the continuous creation and tailoring of their essential protective peptidoglycan (PG) cell wall. Several soluble PG biosynthesis products in the periplasm are transported to the cytosol for recycling, leading to enhanced bacterial fitness. GlcNAc-1,6-anhydroMurNAc and peptide variants are transported by the essential major facilitator superfamily importer AmpG in Gram-negative pathogens including Escherichia coli, Klebsiella pneumoniae, Acinetobacter baumannii, and Pseudomonas aeruginosa. Accumulation of GlcNAc-1,6-anhydroMurNAc-pentapeptides also results from β-lactam antibiotic induced cell wall damage. In some species, these products upregulate the β-lactamase AmpC, which hydrolyzes β-lactams to allow for bacterial survival and drug-resistant infections. Here, we have used cryo-electron microscopy and chemical synthesis of substrates in an integrated structural, biochemical, and cellular analysis of AmpG. We show how AmpG accommodates the large GlcNAc-1,6-anhydroMurNAc peptides, including a unique hydrophobic vestibule to the substrate binding cavity, and characterize residues involved in binding that inform the mechanism of proton-mediated transport.

A low-barrier proton shared between two aspartates acts as a conformational switch that changes the substrate specificity of the β-lactamase BlaC

Serine β-lactamases inactivate β-lactam antibiotics in a two-step mechanism comprising acylation and deacylation. For the deacylation step, a water molecule is activated by a conserved glutamate residue to release the adduct from the enzyme. The third-generation cephalosporin ceftazidime is a poor substrate for the class A β-lactamase BlaC from Mycobacterium tuberculosis but it can be hydrolyzed faster when the active site pocket is enlarged, as was reported for mutant BlaC P167S. The conformational change in the Ω-loop of the P167S mutant displaces the conserved glutamate (Glu166), suggesting it is not required for deacylation of the ceftazidime adduct. Here, we report the characterization of wild type BlaC and BlaC E166A at various pH values. The presence of Glu166 strongly enhances activity against nitrocefin but not ceftazidime, indicating it is indeed not required for deacylation of the adduct of the latter substrate. At high pH wild type BlaC was found to exist in two states, one of which converts ceftazidime much faster, resembling the open state previously reported for the BlaC mutant P167S. The pH-dependent switch between the closed and open states is caused by the loss at high pH of a low-barrier hydrogen bond, a proton shared between Asp172 and Asp179. These results illustrate how readily shifts in substrate specificity can occur as a consequence of subtle changes in protein structure.

Asp179 in the class A β-lactamase from Mycobacterium tuberculosis is a conserved yet not essential residue due to epistasis

Conserved residues are often considered essential for function, and substitutions in such residues are expected to have a negative influence on the properties of a protein. However, mutations in a few highly conserved residues of the β-lactamase from Mycobacterium tuberculosis, BlaC, were shown to have no or only limited negative effect on the enzyme. One such mutant, D179N, even conveyed increased ceftazidime resistance upon bacterial cells, while displaying good activity against penicillins. The crystal structures of BlaC D179N in resting state and in complex with sulbactam reveal subtle structural changes in the Ω-loop as compared to the structure of wild-type BlaC. Introducing this mutation in four other β-lactamases, CTX-M-14, KPC-2, NMC-A and TEM-1, resulted in decreased antibiotic resistance for penicillins and meropenem. The results demonstrate that the Asp in position 179 is generally essential for class A β-lactamases but not for BlaC, which can be explained by the importance of the interaction with the side chain of Arg164 that is absent in BlaC. It is concluded that Asp179 though conserved is not essential in BlaC, as a consequence of epistasis.

Two-dimensional separation of water-soluble polymers using size exclusion and reversed phase chromatography employing capillary-channeled polymer fiber columns

Polymeric materials are readily available, durable materials that have piqued the interest of many diverse fields, ranging from biomedical engineering to construction. The physiochemical properties of a polymer dictate the behavior and function, where large polydispersity among polymer properties can lead to problems; however, current polymer analysis methods often only report results for one particular property. Two-dimensional liquid chromatography (2DLC) applications have become increasingly popular due to the ability to implement two chromatographic modalities in one platform, meaning the ability to simultaneously address multiple physiochemical aspects of a polymer sample, such as functional group content and molar mass. The work presented employs size exclusion chromatography (SEC) and reversed-phase (RP) chromatography, through two coupling strategies: SEC x RP and RP x RP separations of the water-soluble polymers poly(methacrylic acid) (PMA) and polystyrene sulfonic acid (PSSA). Capillary-channeled polymer (C-CP) fiber (polyester and polypropylene) stationary phases were used for the RP separations. Particularly attractive is the fact that they are easily implemented as the second dimension in 2DLC workflows due to their low backpressure (<1000 psi at ∼70 mm sec-1) and fast separation times. In-line multi-angle light scattering (MALS) was also implemented for molecular weight determinations of the polymer samples, with the molecular weight of PMA ranging from 5 × 104 to 2 × 105 g mol-1, while PSSA ranges from 105 to 108 g mol-1. While the orthogonal pairing of SEC x RP addresses polymer sizing and chemistry, this approach is limited by long separation times (80 min), the need for high solute concentrations (PMA = 1.79 mg mL-1 and PSSA = 0.175 mg mL-1 to yield comparable absorbance responses) due to on-column dilution and subsequently limited resolution in the RP separation space. With RP x RP couplings, separation times were significantly reduced (40 min), with lower sample concentrations (0.595 mg mL-1 of PMA and 0.05 mg mL-1 of PSSA) required. The combined RP strategy provided better overall distinction in the chemical distribution of the polymers, yielding 7 distict species versus 3 for the SEC x RP coupling.

Computing the Differences between Asn-X and Gln-X Deamidation and Their Impact on Pharmaceutical and Physiological Proteins: A Theoretical Investigation Using Model Dipeptides

Protein deamidation is a degradation mechanism that significantly impacts both pharmaceutical and physiological proteins. Deamidation impacts two amino acids, Asn and Gln, where the net neutral residues are converted into their acidic forms. While there are multiple similarities between the reaction mechanisms of the two residues, the impact of Gln deamidation has been noted to be most significant on physiological proteins while Asn deamidation has been linked to both pharmaceutical and physiological proteins. For this purpose, we sought to analyze the thermochemical and kinetic properties of the different reactions of Gln deamidation relative to Asn deamidation. In this study, we mapped the deamidation of Gln-X dipeptides into Glu-X dipeptides using density functional theory (DFT). Full network mapping facilitated the prediction of reaction selectivity between the two primary pathways, as well as between the two products of Gln-X deamidation as a function of solvent dielectric. To achieve this analysis, we studied a total of 77 dipeptide reactions per solvent dielectric (308 total reactions). Modeled at a neutral pH and using quantum chemical and statistical thermodynamic methods, we computed the following values: enthalpy of reaction (ΔH_RXN), entropy (ΔS_RXN), Gibbs free energy of reaction (ΔG_RXN), activation energy (E_A), and the Arrhenius preexponential factor (log(A)) for each dipeptide. Additionally, using chemical reaction principles, we generated a database of computed rate coefficients for all possible N-terminus Gln-X deamidation reactions at a neutral pH, predicted the most likely deamidation reaction mechanism for each dipeptide reaction, analyzed our results against our prior study on Asn-X deamidation, and matched our results against qualitative trends previously noted by experimental literature.

Processive cleavage of substrate at individual proteolytic active sites of the Lon protease complex

The Lon protease is the prototype of a family of proteolytic machines with adenosine triphosphatase modules built into a substrate degradation chamber. Lon is known to degrade protein substrates in a processive fashion, cutting a protein chain processively into small peptides before commencing cleavages of another protein chain. Here, we present structural and biochemical evidence demonstrating that processive substrate degradation occurs at each of the six proteolytic active sites of Lon, which forms a deep groove that partially encloses the substrate polypeptide chain by accommodating only the unprimed residues and permits processive cleavage in the C-to-N direction. We identify a universally conserved acidic residue at the exit side of the binding groove indispensable for the proteolytic activity. This noncatalytic residue likely promotes processive proteolysis by carboxyl-carboxylate interactions with cleaved intermediates. Together, these results uncover a previously unrecognized mechanism for processive substrate degradation by the Lon protease.

Elucidation of the molecular interactions that enable stable assembly and structural diversity in multicomponent immune receptors

Multicomponent immune receptors are essential complexes in which distinct ligand-recognition and signaling subunits are held together by interactions between acidic and basic residues of their transmembrane helices. A 2:1 acidic-to-basic motif in the transmembrane domains of the subunits is necessary and sufficient to assemble these receptor complexes. Here, we study a prototype for these receptors, a DAP12-NKG2C 2:1 heterotrimeric complex, in which the two DAP12 subunits each contribute a single transmembrane Asp residue, and the NKG2C subunit contributes a Lys to form the complex. DAP12 can also associate with 20 other subunits using a similar motif. Here, we use molecular-dynamics simulations to understand the basis for the high affinity and diversity of interactions in this group of receptors. Simulations of the transmembrane helices with differing protonation states of the Asp-Asp-Lys triad identified a structurally stable interaction in which a singly-protonated Asp-Asp pair forms a hydrogen-bonded carboxyl-carboxylate clamp that clasps onto a charged Lys side chain. This polar motif was also supported by density functional theory and a Protein Data Bank-wide search. In contrast, the helices are dynamic at sites distal to the stable carboxyl-carboxylate clamp motif. Such a locally stable but globally dynamic structure is well suited to accommodate the sequence and structural variations in the transmembrane helices of multicomponent receptors, which mix and match subunits to create combinatorial functional diversity from a limited number of subunits. It also supports a signaling mechanism based on multisubunit clustering rather than propagation of rigid conformational changes through the membrane.

Sol-gel process: the inorganic approach in protein imprinting

Proteins play a central role in the signal transmission in living systems since they are able to recognize specific biomolecules acting as cellular receptors, antibodies or enzymes, being themselves recognized by other proteins in protein/protein interactions, or displaying epitopes suitable for antibody binding. In this context, the specific recognition of a given protein unlocks a range of interesting applications in diagnosis and in targeted therapies. Obviously, this role is already fulfilled by antibodies with unquestionable success. However, the design of synthetic artificial systems able to endorse this role is still challenging with a special interest to overcome limitations of antibodies, in particular their production and their stability. Molecular Imprinted Polymers (MIPs) are attractive recognition systems which could be an alternative for the specific capture of proteins in complex biological fluids. MIPs can be considered as biomimetic receptors or antibody mimics displaying artificial paratopes. However, MIPs of proteins remains a challenge due to their large size and conformational flexibility, their complex chemical nature with multiple recognition sites and their low solubility in most organic solvents. Classical MIP synthesis conditions result in large polymeric cavities and unspecific binding sites on the surface. In this review, the potential of the sol-gel process as inorganic polymerization strategy to overcome the drawbacks of protein imprinting is highlighted. Thanks to the mild and biocompatible experimental conditions required and the use of water as a solvent, the inorganic polymerization approach better suited to proteins than organic polymerization. Through numerous examples and applications of MIPs, we proposed a critical evaluation of the parameters that must be carefully controlled to achieve sol-gel protein imprinting (SGPI), including the choice of the monomers taking part in the polymerization.

Bipolymeric Pectin Millibeads Doped with Functional Polymers as Matrices for the Controlled and Targeted Release of Mesalazine

Targeted drug delivery systems are a very convenient method of treating inflammatory bowel disease. The properties of pectin make this biopolymer a suitable drug carrier. These properties allow pectin to overcome the diverse environment of the digestive tract and deliver the drug to the large intestine. This investigation proposed bipolymeric formulations consisting of the natural polymer pectin and a synthetic polymer containing the drug 5-aminosalicylic acid. Pectin beads were prepared via ionotropic gelation involving the interaction between the hydrophilic gel and calcium ions. The obtained formulations consisted of natural polymer, 5-aminosalicylic acid (5-ASA) and one of the synthetic polymers, such as polyacrylic acid, polyvinylpyrrolidone, polyethylene glycol or aristoflex. The release of the drug was carried out employing a basket apparatus (USP 1). The acceptor fluid was pH = 7.4 buffer with added enzyme pectinase to reflect the colon environment. The amount of the released drug was determined using UV-Vis spectrophotometry at a wavelength of λ = 330 nm. The kinetics of the drug dissolution revealed that none of the employed models was appropriate to describe the release process. A kinetic analysis of the release profile during two release stages was carried out. The fastest drug release occurred during the first stage from a formulation containing pectin and polyethylene glycol. However, according to the applied kinetic models, the dissolution of 5-ASA was rather high in the formulation without the synthetic polymer during the second stage. Depending on the formulation, 68-77% of 5-ASA was released in an 8-hour time period. The FTIR and DSC results showed that there was no interaction between the drug and the polymers, but interactions between pectin and synthetic polymers were found.

The structure of a calsequestrin filament reveals mechanisms of familial arrhythmia

Mutations in the calcium-binding protein calsequestrin cause the highly lethal familial arrhythmia catecholaminergic polymorphic ventricular tachycardia (CPVT). In vivo, calsequestrin multimerizes into filaments, but an atomic-resolution structure of a calsequestrin filament is lacking. We report a crystal structure of a human cardiac calsequestrin filament with supporting mutational analysis and in vitro filamentation assays. We identify and characterize a novel disease-associated calsequestrin mutation, S173I, that is located at the filament-forming interface, and further show that a previously reported dominant disease mutation, K180R, maps to the same surface. Both mutations disrupt filamentation, suggesting that disease pathology is due to defects in multimer formation. An ytterbium-derivatized structure pinpoints multiple credible calcium sites at filament-forming interfaces, explaining the atomic basis of calsequestrin filamentation in the presence of calcium. Our study thus provides a unifying molecular mechanism by which dominant-acting calsequestrin mutations provoke lethal arrhythmias.

A Slippery Scaffold: Synthesis and Recycling of the Bacterial Cell Wall Carrier Lipid

The biosynthesis of bacterial cell envelope polysaccharides such as peptidoglycan relies on the use of a dedicated carrier lipid both for the assembly of precursors at the cytoplasmic face of the plasma membrane and for the translocation of lipid linked oligosaccharides across the plasma membrane into the periplasmic space. This dedicated carrier lipid, undecaprenyl phosphate, results from the dephosphorylation of undecaprenyl pyrophosphate, which is generated de novo in the cytoplasm by undecaprenyl pyrophosphate synthase and released as a by-product when newly synthesized glycans are incorporated into the existing cell envelope. The de novo synthesis of undecaprenyl pyrophosphate has been thoroughly characterized from a structural and mechanistic standpoint; however, its dephosphorylation to the active carrier lipid form, both in the course of de novo synthesis and recycling, has only been begun to be studied in depth in recent years. This review provides an overview of bacterial carrier lipid synthesis and presents the current state of knowledge regarding bacterial carrier lipid recycling.

Impact of Pesticide Type and Emulsion Fat Content on the Bioaccessibility of Pesticides in Natural Products

There is interest in incorporating nanoemulsions into certain foods and beverages, including dips, dressings, drinks, spreads, and sauces, due to their potentially beneficial attributes. In particular, excipient nanoemulsions can enhance the bioavailability of nutraceuticals in fruit- and vegetable-containing products consumed with them. There is, however, potential for them to also raise the bioavailability of undesirable substances found in these products, such as pesticides. In this research, we studied the impact of excipient nanoemulsions on the bioaccessibility of pesticide-treated tomatoes. We hypothesized that the propensity for nanoemulsions to raise pesticide bioaccessibility would depend on the polarity of the pesticide molecules. Bendiocarb, parathion, and chlorpyrifos were therefore selected because they have Log P values of 1.7, 3.8, and 5.3, respectively. Nanoemulsions with different oil contents (0%, 4%, and 8%) were fabricated to study their impact on pesticide uptake. In the absence of oil, the bioaccessibility increased with increasing pesticide polarity (decreasing Log P): bendiocarb (92.9%) > parathion (16.4%) > chlorpyrifos (2.8%). Bendiocarb bioaccessibility did not depend on the oil content of the nanoemulsions, which was attributed to its relatively high water-solubility. Conversely, the bioaccessibility of the more hydrophobic pesticides (parathion and chlorpyrifos) increased with increasing oil content. For instance, for chlorpyrifos, the bioaccessibility was 2.8%, 47.0%, and 70.7% at 0%, 4%, and 8% oil content, respectively. Our findings have repercussions for the utilization of nanoemulsions as excipient foods in products that may have high levels of undesirable non-polar substances, such as pesticides.

Geometric H/D isotope effect in a series of organic salts involving short O–H⋯O hydrogen bonds between carboxyl and carboxylate groups

Noticeable elongations of donor–acceptor distances upon deuteration are confirmed in short O–H⋯O hydrogen bonds between carboxyl and carboxylate groups.

Crystal structure of an intramembranal phosphatase central to bacterial cell-wall peptidoglycan biosynthesis and lipid recycling

Undecaprenyl pyrophosphate phosphatase (UppP) is an integral membrane protein that recycles the lipid carrier essential to the ongoing biosynthesis of the bacterial cell wall. Individual building blocks of peptidoglycan are assembled in the cytoplasm on undecaprenyl phosphate (C55-P) before being flipped to the periplasmic face, where they are polymerized and transferred to the existing cell wall sacculus, resulting in the side product undecaprenyl pyrophosphate (C55-PP). Interruption of UppP’s regeneration of C55-P from C55-PP leads to the buildup of cell wall intermediates and cell lysis. We present the crystal structure of UppP from Escherichia coli at 2.0 Å resolution, which reveals the mechanistic basis for intramembranal phosphatase action and substrate specificity using an inverted topology repeat. In addition, the observation of key structural motifs common to a variety of cross membrane transporters hints at a potential flippase function in the specific relocalization of the C55-P product back to the cytosolic space.

Undecaprenyl pyrophosphate phosphatase (UppP) recycles the lipid carrier essential for bacterial cell wall synthesis. Here authors present the crystal structure of UppP from E. coli at 2.0 Å resolution, which sheds light on its phosphatase mechanism and indicates a potential flippase role for UppP.

Geometric isotope effect of deuteration in a hydrogen-bonded host-guest crystal

Deuteration of a hydrogen bond by replacing protium (H) with deuterium (D) can cause geometric changes in the hydrogen bond, known as the geometric H/D isotope effect (GIE). Understanding the GIEs on global structures and bulk properties is of great importance to study structure–property relationships of hydrogen-bonded systems. Here, we report a hydrogen-bonded host–guest crystal, imidazolium hydrogen terephthalate, that exemplifies striking GIEs on its hydrogen bonds, phases, and bulk dielectric transition property. Upon deuteration, the donor–acceptor distance in the O–H···O hydrogen bonds in the host structure is found to increase, which results in a change in the global hydrogen-bonded supramolecular structure and the emergence of a new phase (i.e., isotopic polymorphism). Consequently, the dynamics of the confined guest, which depend on the internal pressure exerted by the host framework, are substantially altered, showing a downward shift of the dielectric switching temperature.

Deuterating a hydrogen bond can change the bond’s geometry, a phenomenon known as the geometric isotope effect (GIE). Here, the authors find that a hydrogen-bonded host–guest crystal, imidazolium hydrogen terephthalate, exhibits significant GIE on its hydrogen bonds, changing its crystal phases and bulk dielectric properties.

Characterization of Ionizable Groups' Environments in Proteins and Protein-Ligand Complexes through a Statistical Analysis of the Protein Data Bank

We conduct a statistical analysis of the molecular environment of common ionizable functional groups in both protein-ligand complexes and inside proteins from the Protein Data Bank (PDB). In particular, we characterize the frequency, type, and density of the interacting atoms as well as the presence of a potential counterion. We found that for ligands, most guanidinium groups, half of primary and secondary amines, and one-fourth of imidazole neighbor a carboxylate group. Tertiary amines bind more rarely near carboxylate groups, which may be explained by a crowded neighborhood and hydrophobic character. In comparison to the environment seen by the ligands, inside proteins, an environment enriched in main-chain atoms is found, and the prevalence of direct charge neutralization by carboxylate groups is different. When the ionizable character of water molecules and phenolic or hydroxyl groups is accounted, considering a high-resolution dataset (less than 1.5 Å), charge neutralization could occur for well above 80% of the ligand functional groups considered, but for tertiary amines.

Impact of the environmental conditions and substrate pre-treatment on whey protein hydrolysis: A review

Proteins in solution are subject to myriad forces stemming from interactions with each other as well as with the solvent media. The role of the environmental conditions, namely pH, temperature, ionic strength remains under-estimated yet it impacts protein conformations and consequently its interaction with, and susceptibility to, the enzyme. Enzymes, being proteins are also amenable to the environmental conditions because they are either activated or denatured depending on the choice of the conditions. Furthermore, enzyme specificity is restricted to a narrow regime of optimal conditions while opportunities outside the optimum conditions remain untapped. In addition, the composition of protein substrate (whether mixed or single purified) have been underestimated in previous studies. In addition, protein pre-treatment methods like heat denaturation prior to hydrolysis is a complex phenomenon whose progression is influenced by the environmental conditions including the presence or absence of sugars like lactose, ionic strength, purity of the protein, and the molecular structure of the mixed proteins particularly presence of free thiol groups. In this review, we revisit protein hydrolysis with a focus on the impact of the hydrolysis environment and show that preference of peptide bonds and/or one protein over another during hydrolysis is driven by the environmental conditions. Likewise, heat-denaturing is a process which is dependent on not only the environment but the presence or absence of other proteins.

Higher-Resolution Structure of the Human Insulin Receptor Ectodomain: Multi-Modal Inclusion of the Insert Domain

Insulin receptor (IR) signaling is critical to controlling nutrient uptake and metabolism. However, only a low-resolution (3.8 Å) structure currently exists for the IR ectodomain, with some segments ill-defined or unmodeled due to disorder. Here, we revise this structure using new diffraction data to 3.3 Å resolution that allow improved modeling of the N-linked glycans, the first and third fibronectin type III domains, and the insert domain. A novel haptic interactive molecular dynamics strategy was used to aid fitting to low-resolution electron density maps. The resulting model provides a foundation for investigation of structural transitions in IR upon ligand binding.

Electrostatic Architecture of the Infectious Salmon Anemia Virus (ISAV) Core Fusion Protein Illustrates a Carboxyl-Carboxylate pH Sensor

Segment 5, ORF 1 of the infectious salmon anemia virus (ISAV) genome, encodes for the ISAV F protein, which is responsible for viral-host endosomal membrane fusion during a productive ISAV infection. The entry machinery of ISAV is composed of a complex of the ISAV F and ISAV hemagglutinin esterase (HE) proteins in an unknown stoichiometry prior to receptor engagement by ISAV HE. Following binding of the receptor to ISAV HE, dissociation of the ISAV F protein from HE, and subsequent endocytosis, the ISAV F protein resolves into a fusion-competent oligomeric state. Here, we present a 2.1 Å crystal structure of the fusion core of the ISAV F protein determined at low pH. This structure has allowed us to unambiguously demonstrate that the ISAV entry machinery exhibits typical class I viral fusion protein architecture. Furthermore, we have determined stabilizing factors that accommodate the pH-dependent mode of ISAV transmission, and our structure has allowed the identification of a central coil that is conserved across numerous and varied post-fusion viral glycoprotein structures. We then discuss a mechanistic model of ISAV fusion that parallels the paramyxoviral class I fusion strategy wherein attachment and fusion are relegated to separate proteins in a similar fashion to ISAV fusion.

A comprehensive classification and nomenclature of carboxyl-carboxyl(ate) supramolecular motifs and related catemers: implications for biomolecular systems

Carboxyl and carboxylate groups form important supramolecular motifs (synthons). Besides carboxyl cyclic dimers, carboxyl and carboxylate groups can associate through a single hydrogen bond. Carboxylic groups can further form polymeric-like catemer chains within crystals. To date, no exhaustive classification of these motifs has been established. In this work, 17 association types were identified (13 carboxyl-carboxyl and 4 carboxyl-carboxylate motifs) by taking into account the syn and anti carboxyl conformers, as well as the syn and anti lone pairs of the O atoms. From these data, a simple rule was derived stating that only eight distinct catemer motifs involving repetitive combinations of syn and anti carboxyl groups can be formed. Examples extracted from the Cambridge Structural Database (CSD) for all identified dimers and catemers are presented, as well as statistical data related to their occurrence and conformational preferences. The inter-carboxyl(ate) and carboxyl(ate)-water hydrogen-bond properties are described, stressing the occurrence of very short (strong) hydrogen bonds. The precise characterization and classification of these supramolecular motifs should be of interest in crystal engineering, pharmaceutical and also biomolecular sciences, where similar motifs occur in the form of pairs of Asp/Glu amino acids or motifs involving ligands bearing carboxyl(ate) groups. Hence, we present data emphasizing how the analysis of hydrogen-containing small molecules of high resolution can help understand structural aspects of larger and more complex biomolecular systems of lower resolution.

Computational design of a pH stable enzyme: understanding molecular mechanism of penicillin acylase's adaptation to alkaline conditions

Protein stability provides advantageous development of novel properties and can be crucial in affording tolerance to mutations that introduce functionally preferential phenotypes. Consequently, understanding the determining factors for protein stability is important for the study of structure-function relationship and design of novel protein functions. Thermal stability has been extensively studied in connection with practical application of biocatalysts. However, little work has been done to explore the mechanism of pH-dependent inactivation. In this study, bioinformatic analysis of the Ntn-hydrolase superfamily was performed to identify functionally important subfamily-specific positions in protein structures. Furthermore, the involvement of these positions in pH-induced inactivation was studied. The conformational mobility of penicillin acylase in Escherichia coli was analyzed through molecular modeling in neutral and alkaline conditions. Two functionally important subfamily-specific residues, Gluβ482 and Aspβ484, were found. Ionization of these residues at alkaline pH promoted the collapse of a buried network of stabilizing interactions that consequently disrupted the functional protein conformation. The subfamily-specific position Aspβ484 was selected as a hotspot for mutation to engineer enzyme variant tolerant to alkaline medium. The corresponding Dβ484N mutant was produced and showed 9-fold increase in stability at alkaline conditions. Bioinformatic analysis of subfamily-specific positions can be further explored to study mechanisms of protein inactivation and to design more stable variants for the engineering of homologous Ntn-hydrolases with improved catalytic properties.

2-Amino-4,6-dimethyl-pyrimidin-1-ium 2,3,5-triiodo-benzoate 2,3,5-triiodo-benzoic acid monosolvate

In the crystal structure of the title compound, C₆H₁₀N₃⁺·C₇H₂I₃O₂⁻·C₇H₃I₃O₂, two R₂²(8) motifs are observed. One is generated by the inter­action of the 2-amino-4,6-dimethyl­pyrimidin-1-ium cation with the carboxyl­ate group of the 2,3,5-triiodo­benzoate anion via N—H⋯O hydrogen bonds. The other R₂²(8) motif is formed by the inter­action of two centrosymmentrically related pyrimidine moieties through N—H⋯N hydrogen bonds. The two motifs combine to form a linear heterotetra­meric unit. Heterotetra­meric units are linked by a carbox­yl–carboxyl­ate O—H⋯O hydrogen bond (involving the O—H group of neutral 2,3,5-triiodo­benzoic acid and an O atom of the anion), forming a supra­molecular chain along the a axis. In addition, components are held by weak I⋯O interactions in the range 3.023 (5) to 3.382 (5) Å and I⋯I inter­actions in the range 3.6327 (7) to 4.0025 (8) Å.

A flexible cation binding site in the multidrug major facilitator superfamily transporter LmrP is associated with variable proton coupling

The multidrug major facilitator superfamily transporter LmrP from Lactococcus lactis mediates protonmotive-force dependent efflux of amphiphilic ligands from the cell. We compared the role of membrane-embedded carboxylates in transport and binding of divalent cationic propidium and monovalent cationic ethidium. D235N, E327Q, and D142N replacements each resulted in loss of electrogenicity in the propidium efflux reaction, pointing to electrogenic 3H(+)/propidium(2+) antiport. During ethidium efflux, single D142N and D235N replacements resulted in apparent loss of electrogenicity, whereas the E327Q substitution did not affect the energetics, consistent with electrogenic 2H(+)/ethidium(+) antiport. Different roles of carboxylates were also observed in fluorescence anisotropy-based ligand-binding assays. Whereas D235 and E327 were both involved in propidium binding, the loss of one of these carboxylates could be compensated for by the other in ethidium binding. The D142N replacement did not affect the binding of either ligand. These data point to the presence of a dedicated proton binding site containing D142, and a flexible proton/ligand binding site containing D235 and E327, the contributions to proton and ligand binding of which depend on the chemical structure of the bound ligand. Our findings provide the first evidence that multidrug transport by secondary-active transporters can be associated with variable ion coupling.

Short strong hydrogen bonds in proteins: a case study of rhamnogalacturonan acetylesterase

An extremely low-field signal (at approximately 18 p.p.m.) in the (1)H NMR spectrum of rhamnogalacturonan acetylesterase (RGAE) shows the presence of a short strong hydrogen bond in the structure. This signal was also present in the mutant RGAE D192N, in which Asp192, which is part of the catalytic triad, has been replaced with Asn. A careful analysis of wild-type RGAE and RGAE D192N was conducted with the purpose of identifying possible candidates for the short hydrogen bond with the 18 p.p.m. deshielded proton. Theoretical calculations of chemical shift values were used in the interpretation of the experimental (1)H NMR spectra. The crystal structure of RGAE D192N was determined to 1.33 A resolution and refined to an R value of 11.6% for all data. The structure is virtually identical to the high-resolution (1.12 A) structure of the wild-type enzyme except for the interactions involving the mutation and a disordered loop. Searches of the Cambridge Structural Database were conducted to obtain information on the donor-acceptor distances of different types of hydrogen bonds. The short hydrogen-bond interactions found in RGAE have equivalents in small-molecule structures. An examination of the short hydrogen bonds in RGAE, the calculated pK(a) values and solvent-accessibilities identified a buried carboxylic acid carboxylate hydrogen bond between Asp75 and Asp87 as the likely origin of the 18 p.p.m. signal. Similar hydrogen-bond interactions between two Asp or Glu carboxy groups were found in 16% of a homology-reduced set of high-quality structures extracted from the PDB. The shortest hydrogen bonds in RGAE are all located close to the active site and short interactions between Ser and Thr side-chain OH groups and backbone carbonyl O atoms seem to play an important role in the stability of the protein structure. These results illustrate the significance of short strong hydrogen bonds in proteins.

Calcium binding to skeletal muscle troponin C and the regulation of muscle contraction

Skeletal muscle contraction is initiated by Ca2+ ion binding to troponin C (TnC), a protein of the thin filament. Our three-dimensional structure determination of turkey skeletal TnC at 2.8 A resolution revealed an extended molecule consisting of two domains connected through a long nine-turn alpha-helix. The C-terminal domain has two Ca2+ ions bound in the expected manner of EF hands, whereas the N-terminal regulatory domain is Ca2+-free with a helix-loop-helix conformation different from that of an EF hand. The refinement of TnC at 2.2 A resolution highlights the intricate hydrogen-bonded network common to the Ca2+-bound loops and provides an explanation for the presence of a water molecule in the 5th coordination position of the Ca2+ ion. We propose that Ca2+ binding to the regulatory domain is accompanied by a conformational transition by which its structure becomes similar to that of the C-terminal domain. Dramatic movements of residues in the B and C helices and the connecting peptide of up to 14 A constitute the bulk of this change. A hydrophobic site that could be the site of interaction with troponin I is thereby exposed. We have also demonstrated that this model of the Ca2+-bound conformation can be reached from the Ca2+-free state without having to surmount large energy barriers.

Structure of acid-sensing ion channel 1 at 1.9 Å resolution and low pH

Acid-sensing ion channels (ASICs) are voltage-independent, proton-activated receptors that belong to the epithelial sodium channel/degenerin family of ion channels and are implicated in perception of pain, ischaemic stroke, mechanosensation, learning and memory. Here we report the low-pH crystal structure of a chicken ASIC1 deletion mutant at 1.9 A resolution. Each subunit of the chalice-shaped homotrimer is composed of short amino and carboxy termini, two transmembrane helices, a bound chloride ion and a disulphide-rich, multidomain extracellular region enriched in acidic residues and carboxyl-carboxylate pairs within 3 A, suggesting that at least one carboxyl group bears a proton. Electrophysiological studies on aspartate-to-asparagine mutants confirm that these carboxyl-carboxylate pairs participate in proton sensing. Between the acidic residues and the transmembrane pore lies a disulphide-rich 'thumb' domain poised to couple the binding of protons to the opening of the ion channel, thus demonstrating that proton activation involves long-range conformational changes.

Geometry of nonbonded interactions involving planar groups in proteins

Although hydrophobic interaction is the main contributing factor to the stability of the protein fold, the specificity of the folding process depends on many directional interactions. An analysis has been carried out on the geometry of interaction between planar moieties of ten side chains (Phe, Tyr, Trp, His, Arg, Pro, Asp, Glu, Asn and Gln), the aromatic residues and the sulfide planes (of Met and cystine), and the aromatic residues and the peptide planes within the protein tertiary structures available in the Protein Data Bank. The occurrence of hydrogen bonds and other nonconventional interactions such as C-H...pi, C-H...O, electrophile-nucleophile interactions involving the planar moieties has been elucidated. The specific nature of the interactions constraints many of the residue pairs to occur with a fixed sequence difference, maintaining a sequential order, when located in secondary structural elements, such as alpha-helices and beta-turns. The importance of many of these interactions (for example, aromatic residues interacting with Pro or cystine sulfur atom) is revealed by the higher degree of conservation observed for them in protein structures and binding regions. The planar residues are well represented in the active sites, and the geometry of their interactions does not deviate from the general distribution. The geometrical relationship between interacting residues provides valuable insights into the process of protein folding and would be useful for the design of protein molecules and modulation of their binding properties.

Analysis of pH-dependent elements in proteins: geometry and properties of pairs of hydrogen-bonded carboxylic acid side-chains

A rather frequent but so far little discussed observation is that pairs of carboxylic acid side-chains in proteins can share a proton in a hydrogen bond. In the present article, quantum chemical calculations of simple model systems for carboxyl-carboxylate interactions are compared with structural observations from proteins. A detailed structural analysis of the proteins deposited in the PDB revealed that, in a subset of proteins sharing less than 90% sequence identity, 19% (314) contain at least one pair of carboxylic acids with their side-chain oxygen atoms within hydrogen-bonding distance. As the distance between those interacting oxygen atoms is frequently very short ( approximately 2.55 A), many of these carboxylic acids are suggested to share a proton in a strong hydrogen bond. When situated in an appropriate structural environment (low dielectric constant), some might even form a low barrier hydrogen bond. The quantum chemical studies show that the most frequent geometric features of carboxyl-carboxylate pairs found in proteins, and no or symmetric ligation, are also the most stable arrangements at low dielectric constants, and they also suggest at medium and low pH a higher stability than for isosteric amide-carboxylate pairs. The presence of these pairs in 119 different enzymes found in the BRENDA database is set in relation to their properties and functions. This analysis shows that pH optima of enzymes with carboxyl-carboxylate pairs are shifted to lower than average values, whereas temperature optima seem to be increased. The described structural principles can be used as guidelines for rational protein design (e.g., in order to improve pH or temperature stability).

Mitochondrial carriers in the cytoplasmic state have a common substrate binding site

Mitochondrial carriers link biochemical pathways in the cytosol and mitochondrial matrix by transporting substrates across the inner mitochondrial membrane. Substrate recognition is specific for each carrier, but sequence similarities suggest the carriers have similar structures and mechanisms of substrate translocation. By considering conservation of amino acids, distance and chemical constraints, and by modeling family members on the known structure of the ADP/ATP translocase, we have identified a common substrate binding site. It explains substrate selectivity and proton coupling and provides a mechanistic link to carrier opening by substrate-induced perturbation of the salt bridges that seal the pathway to and from the mitochondrial matrix. It enables the substrate specificity of uncharacterized mitochondrial carriers to be predicted.

Crystal Structures of 2-Methylisocitrate Lyase in Complex with Product and with Isocitrate Inhibitor Provide Insight into Lyase Substrate Specificity, Catalysis and Evolution,

Two crystal structures of the C123S mutant of 2-methylisocitrate lyase have been determined, one with the bound reaction products, Mg(2+)-pyruvate and succinate, and the second with a bound Mg(2+)-(2R,3S)-isocitrate inhibitor. Comparison with the structure of the wild-type enzyme in the unbound state reveals that the enzyme undergoes a conformational transition that sequesters the ligand from solvent, as previously observed for two other enzyme superfamily members, isocitrate lyase and phosphoenolpyruvate mutase. The binding modes reveal the determinants of substrate specificity and stereoselectivity, and the stringent specificity is verified in solution using various potential substrates. A model of bound 2-methylisocitrate has been developed based on the experimentally determined structures. We propose a catalytic mechanism involving an alpha-carboxy-carbanion intermediate/transition state, which is consistent with previous stereochemical experiments showing inversion of configuration at the C(3) of 2-methylisocitrate. Structure-based sequence analysis and phylogenic tree construction reveal determinants of substrate specificity, highlight nodes of divergence of families, and predict enzyme families with new functions.

The molecular structure and catalytic mechanism of a novel carboxyl peptidase from Scytalidium lignicolum

The molecular structure of the pepstatin-insensitive carboxyl peptidase from Scytalidium lignicolum, formerly known as scytalidopepsin B, was solved by multiple isomorphous replacement phasing methods and refined to an R factor of 0.230 (R(free) = 0.246) at 2.1-A resolution. In addition to the structure of the unbound peptidase, the structure of a product complex of cleaved angiotensin II bound in the active site of the enzyme was also determined. We propose the name scytalidocarboxyl peptidase B (SCP-B) for this enzyme. On the basis of conserved, catalytic residues identified at the active site, we suggest the name Eqolisin for the enzyme family. The previously uninvestigated SCP-B fold is that of a beta-sandwich; each sheet has seven antiparallel strands. A tripeptide product, Ala-Ile-His, bound in the active site of SCP-B has allowed for identification of the catalytic residues and the residues in subsites S1, S2, and S3, which are important for substrate binding. The most likely hydrolytic mechanism involves nucleophilic attack of a general base (Glu-136)-activated water (OH(-)) on the si-face of the scissile peptide carbonylcarbon atom to form a tetrahedral intermediate. Electrophilic assistance and oxyanion stabilization is provided by the side-chain amide of Gln-53. Protonation of the leaving-group nitrogen is accomplished by the general acid function of the protonated carboxyl group of Glu-136.

Mechanism and Molecular Determinant for Regulation of Rabbit Transient Receptor Potential Type 5 (TRPV5) Channel by Extracellular pH

The transient receptor potential type 5 (TRPV5) channel is present in kidney and intestine and important for transepithelial (re)absorption of calcium in these tissues. We report that in whole-cell patch clamp recording extracellular acidification inhibited rabbit TRPV5 with apparent pKa approximately 6.55. The two extracellular loops between the fifth and sixth transmembrane segments of TRPV5 presumably form part of the outer opening of the pore and likely are important in binding and regulation by external protons. We found that mutation of glutamate 522 to glutamine (E522Q) decreased the sensitivity of the channel to extracellular acidification. Mutations of other titratable amino acids within the two extracellular loops to non-titratable amino acids had no effect on pH sensitivity. Substitutions of aspartate or other titratable amino acids for glutamate 522 conferred an increase in pH sensitivity. The pH sensitivity mediated by glutamate 522 was independent of extracellular or intracellular Mg2+. Single channel analysis revealed that extracellular acidification reduced single channel conductance as well as open probability of the wild type channel. In contrast to wild type channel, extracellular acidification did not reduce open probability for E522Q mutant. Methanethiosulfonate reagents inhibited the activity of glutamine 522 to cysteine mutant channel with a reaction rate constant approaching that with free thiols in solution, suggesting that glutamate 522 is located on the surface of the channel. These data suggest that glutamate 522 of the rabbit TRPV5 is a "pH sensor," and extracellular protons inhibit TRPV5 likely by altering conformation of the channel protein.

Probing pH-dependent functional elements in proteins: modification of carboxylic acid pairs in Trichoderma reesei cellobiohydrolase Cel6A

Two carboxylic acid side chains can, depending on their geometry and environment, share a proton in a hydrogen bond and form a carboxyl-carboxylate pair. In the Trichoderma reesei cellobiohydrolase Cel6A structure, five carboxyl-carboxylate pairs are observed. One of these pairs (D175-D221) is involved in catalysis, and three other pairs are found in, or close to the two surface loops covering the active site tunnel of the catalytic domain. To stabilize Cel6A at alkaline pH values, where deprotonation of the carboxylic acids leads to repulsion of their side chains, we designed two mutant enzymes. In the first mutant, one carboxyl-carboxylate pair (E107-E399) was replaced by a corresponding amide-carboxylate pair (Q107-E399), and in the second mutant, all three carboxyl-carboxylate pairs (E107-E399, D170-E184, and D366-D419) were mutated in a similar manner. The unfolding studies using both intrinsic tryptophan fluorescence and far-ultraviolet circular dichroism spectroscopy at different pH values demonstrate that the unfolding temperature (T(m)) of both mutants has changed, resulting in destabilization of the mutant enzymes at acidic pH and stabilization at alkaline pH. The effect of stabilization seems additive, as a Cel6A triple mutant is the most stable enzyme variant. This increased stability is also reflected in the 2- or 4-fold increased half-life of the two mutants at alkaline pH, while the catalytic rate on cellotetraose (at t = 0) has not changed. Increased operational stability at alkaline pH was also observed on insoluble cellulosic substrates. Local conformational changes are suggested to take place in the active site loops of Cel6A wild-type enzyme at elevated pHs (pH 7), affecting to the end-product spectrum on insoluble cellulose. The triple mutant does not show such pH-dependent behavior. Overall, our results demonstrate that carboxyl-carboxylate pair engineering is a useful tool to alter pH-dependent protein behavior.

Geometry of interaction of the histidine ring with other planar and basic residues

Among the aromatic residues in protein structures, histidine (His) is unique, as it can exist in the neutral or positively charged form at the physiological pH. As such, it can interact with other aromatic residues as well as form hydrogen bonds with polar and charged (both negative and positive) residues. We have analyzed the geometry of interaction of His residues with nine other planar side chains containing aromatic (residues Phe, Tyr, Trp, and His), carboxylate (Asp and Glu), carboxamide (Asn and Gln) and guanidinium (Arg) groups in 432 polypeptide chains. With the exception of the aspartic (Asp) and glutamic (Glu) acid side-chains, all other residues prefer to interact in a face-to-face or offset-face-stacked orientation with the His ring. Such a geometry is different from the edge-to-face relative orientation normally associated with the aromatic-aromatic interaction. His-His pair prefers to interact in a face-to-face orientation; however, when both the residues bind the same metal ion, the interplanar angle is close to 90 degrees. The occurrence of different interactions (including the nonconventional N-H...pi and C-H...pi hydrogen bonds) have been correlated with the relative orientations between the interacting residues. Several structural motifs, mostly involved in binding metal ions, have been identified by considering the cases where His residues are in contact with four other planar moieties. About 10% of His residues used here are also found in sequence patterns in PROSITE database. There are examples of the amino end of the Lys side chain interacting with His residues in such a way that it is located on an arc around a ring nitrogen atom.

Role of outer ring carboxylates of the rat skeletal muscle sodium channel pore in proton block

Voltage-gated Na+ current is reduced by acid solution. Protons reduce peak Na+ conductance by lowering single channel conductance and shift the voltage range of gating by neutralizing surface charges. Structure-function studies identify six carboxyls and a lysine in the channel's outer vestibule. We examined the roles of the superficial ring of carboxyls in acid block of Na(v)1.4 (the rat skeletal muscle Na+ channel isoform) by measuring the effects of their neutralization or their substitution by lysine on sensitivity to acid solutions, using the two-micropipette voltage clamp in Xenopus oocytes. Alteration of the outer ring of carboxylates had little effect on the voltage for half-activation of Na+ current, as if they are distant from the channels' voltage sensors. The mutations did not abolish proton block; rather, they all shifted the pK(a) (-log of the dissociation constant) in the acid direction. Effects of neutralization on pK(a) were not identical for different mutations, with E758Q > D1241A > D1532N > E403Q. E758K showed double the effect of E758Q, and the other lysine mutations all produced larger effects than the neutralizing mutations. Calculation of the electrostatic potential produced by these carboxylates using a pore model showed that the pK(a) values of carboxylates of Glu-403, Glu-758, and Asp-1532 are shifted to values similar to the experimentally measured pK(a). Calculations also predict the experimentally observed changes in pK(a) that result from mutational neutralization or introduction of a positive charge. We propose that proton block results from partial protonation of these outer ring carboxylates and that all of the carboxylates contribute to a composite Na+ site.

The structure of the PII-ATP complex

PII is a signal transduction protein that is part of the cellular machinery used by many bacteria to regulate the activity of glutamine synthetase and the transcription of its gene. The structure of PII was solved using a hexagonal crystal form (form I). The more physiologically relevant form of PII is a complex with small molecule effectors. We describe the structure of PII with ATP obtained by analysis of two different crystal forms (forms II and III) that were obtained by co-crystallization of PII with ATP. Both structures have a disordered recognition (T) loop and show differences at their C termini. Comparison of these structures with the form I protein reveals changes that occur on binding ATP. Surprisingly, the structure of the PII/ATP complex differs with that of GlnK, a functional homologue. The two proteins bind the base and sugar of ATP in a similar manner but show differences in the way that they interact with the phosphates. The differences in structure could account for the differences in their activities, and these have been attributed to a difference in sequence at position 82. It has been demonstrated recently that PII and GlnK form functional heterotrimers in vivo. We construct models of the heterotrimers and examine the junction between the subunits.

The core lipocalin, bovine beta-lactoglobulin

The lipocalin family became established shortly after the structural similarity was noted between plasma retinol binding protein and the bovine milk protein, beta-lactoglobulin. During the past 60 years, beta-lactoglobulin has been studied by essentially every biochemical technique available and so there is a huge literature upon its properties. Despite all of these studies, no specific biological function has been ascribed definitively to the protein, although several possibilities have been suggested. During the processing of milk on an industrial scale, the unpredictable nature of the process has been put down to the presence of beta-lactoglobulin and certainly the whey protein has been implicated in the initiation of aggregation that leads to the fouling of heat exchangers. This short review of the properties of the protein will concentrate mainly on studies carried out under essentially physiological conditions and will review briefly some of the possible functions for the protein that have been described.

Acid potentiation of the capsaicin receptor determined by a key extracellular site

The capsaicin (vanilloid) receptor, VR1, is a sensory neuron-specific ion channel that serves as a polymodal detector of pain-producing chemical and physical stimuli. The response of VR1 to capsaicin or noxious heat is dynamically potentiated by extracellular protons within a pH range encountered during tissue acidosis, such as that associated with arthritis, infarction, tumor growth, and other forms of injury. A molecular determinant for this important physiological activity was localized to an extracellular Glu residue (E600) in the region linking the fifth transmembrane domain with the putative pore-forming region of the channel. We suggest that this residue serves as a key regulatory site of the receptor by setting sensitivity to other noxious stimuli in response to changes in extracellular proton concentration. We also demonstrate that protons, vanilloids, and heat promote channel opening through distinct pathways, because mutations at a second site (E648) selectively abrogate proton-evoked channel activation without diminishing responses to other noxious stimuli. Our findings provide molecular evidence for stimulus-specific steps in VR1 activation and offer strategies for the development of novel analgesic agents.

Isolation of a single carboxyl-carboxylate proton binding site in the pore of a cyclic nucleotide-gated channel

The pore of the catfish olfactory cyclic nucleotide-gated (CNG) channel contains four conserved glutamate residues, one from each subunit, that form a high-affinity binding site for extracellular divalent cations. Previous work showed that these residues form two independent and equivalent high-pKa (approximately 7.6) proton binding sites, giving rise to three pH-dependent conductance states, and it was suggested that the sites were formed by pairing of the glutamates into two independent carboxyl-carboxylates. To test further this physical picture, wild-type CNG subunits were coexpressed in Xenopus oocytes with subunits lacking the critical glutamate residue, and single channel currents through hybrid CNG channels containing one to three wild-type (WT) subunits were recorded. One of these hybrid channels had two pH-dependent conductance states whose occupancy was controlled by a single high-pKa protonation site. Expression of dimers of concatenated CNG channel subunits confirmed that this hybrid contained two WT and two mutant subunits, supporting the idea that a single protonation site is made from two glutamates (dimer expression also implied the subunit makeup of the other hybrid channels). Thus, the proton binding sites in the WT channel occur as a result of the pairing of two glutamate residues. This conclusion places these residues in close proximity to one another in the pore and implies that at any instant in time detailed fourfold symmetry is disrupted.

Aspartate substitutions establish the concerted action of P-region glutamates in repeats I and III in forming the protonation site of L-type Ca2+ channels

Hydrogen ions reduce ion flux through voltage-gated Ca2+ channels by binding to a single protonation site with an unusually high pKa. Recent evidence localizes the protonation site to the same locus that supports high affinity Ca2+ binding and selectivity, a set of four conserved glutamate residues near the external mouth of the pore. Remaining controversy concerns the question of whether the protonation site arises from a single glutamate, Glu-1086 (EIII), or a combination of Glu-1086 and Glu-334 (EI) working in concert. We tested these hypotheses with individual Glu --> Asp substitutions. The Glu --> Asp replacements in repeats I and III stood out in two ways. First, in both EID and EIIID, protonation was destabilized relative to wild type, whereas it was unchanged in EIID and stabilized in EIVD. The changes in affinity were entirely due to alterations in H+ off-rate. Second, the ratio of protonated conductance to deprotonated conductance was significantly closer to unity for EID and EIIID than for wild-type channels or other Asp mutants. Both results support the idea that EI and EIII act together to stabilize a single titratable H+ ion and behave nearly symmetrically in influencing pore conductance. Neutralization of EIII by alanine replacement clearly failed to abolish susceptibility to protonation, indicating that no single glutamate was absolutely required. Taken together, all the evidence supports a model in which multiple carboxylates work in concert to form a single high affinity protonation site.

Refined X-ray crystallographic structure of the poliovirus 3C gene product

The X-ray crystallographic structure of the recombinant poliovirus 3C gene product (Mahoney strain) has been determined by single isomorphous replacement and non-crystallographic symmetry averaging and refined at 2.1 A resolution. Poliovirus 3C is comprised of two six-stranded antiparallel beta-barrel domains and is structurally similar to the chymotrypsin-like serine proteinases. The shallow active site cleft is located at the junction of the two beta-barrel domains and contains a His40, Glu71, Cys147 catalytic triad. The polypeptide loop preceding Cys147 is flexible and likely undergoes a conformational change upon substrate binding. The specificity pockets for poliovirus 3C are well-defined and modeling studies account for the known substrate specificity of this proteinase. Poliovirus 3C also participates in the formation of the viral replicative initiation complex where it specifically recognizes and binds the RNA stem-loop structure in the 5' non-translated region of its own genome. The RNA recognition site of 3C is located on the opposite side of the molecule in relation to its proteolytic active site and is centered about the conserved KFRDIR sequence of the domain linker. The recognition site is well-defined and also includes residues from the amino and carboxy-terminal helices. The two molecules in the asymmetric unit are related by an approximate 2-fold, non-crystallographic symmetry and form an intermolecular antiparallel beta-sheet at their interface.

Crystal structures of the wild type and the Glu376Gly/Thr255Glu mutant of human medium-chain acyl-CoA dehydrogenase: influence of the location of the catalytic base on substrate specificity

Crystal structures of the wild type human medium-chain acyl-CoA dehydrogenase (MCADH) and a double mutant in which its active center base-arrangement has been altered to that of long chain acyl-CoA dehydrogenase (LCADH), Glu376Gly/Thr255Glu, have been determined by X-ray crystallography at 2.75 and 2.4 A resolution, respectively. The catalytic base responsible for the alpha-proton abstraction from the thioester substrate is Glu376 in MCADH, while that in LCADH is Glu255 (MCADH numbering), located over 100 residues away in its primary amino acid sequence. The structures of the mutant complexed with C8-, C12, and C14-CoA have also been determined. The human enzyme structure is essentially the same as that of the pig enzyme. The structure of the mutant is unchanged upon ligand binding except for the conformations of a few side chains in the active site cavity. The substrate with chain length longer than C12 binds to the enzyme in multiple conformations at its omega-end. Glu255 has two conformations, "active" and "resting" forms, with the latter apparently stabilized by forming a hydrogen bond with Glu99. Both the direction in which Glu255 approaches the C alpha atom of the substrate and the distance between the Glu255 carboxylate and the C alpha atom are different from those of Glu376; these factors are responsible for the intrinsic differences in the kinetic properties as well as the substrate specificity. Solvent accessible space at the "midsection" of the active site cavity, where the C alpha-C beta bond of the thioester substrate and the isoalloxazine ring of the FAD are located, is larger in the mutant than in the wild type enzyme, implying greater O2 accessibility in the mutant which might account for the higher oxygen reactivity.