Self-assembly of a peptide sequence, EKKE, composed of exclusively charged amino acids: Role of charge in morphology and lead binding.

The self-assembly of peptides is influenced by their amino acid sequence and other factors including pH, charge, temperature, and solvent. Herein, we explore whether a four-residue sequence, EKKE, consisting of exclusively charged amino acids shows the propensity to form self-assembled ordered nanostructures and whether the overall charge plays any role in morphological and functional properties. From a combination of experimental data provided by Thioflavin T fluorescence, Congo red absorbance, circular dichroism spectroscopy, dynamic light scattering, field emission-scanning electron microscopy, atomic force microscopy, and confocal microscopy, it is clear that the all-polar peptide and charged EKKE sequence shows a pH-dependent tendency to form amyloid-like structures, and the self-assembled entities under acidic, basic and neutral conditions exhibit morphological variation. Additionally, the ability of the self-assembled amyloid nanostructures to bind to the toxic metal, lead (Pb2+ ), was demonstrated from the analysis of the ultraviolet absorbance and X-ray photoelectron spectroscopy data. The modulation at the sequence level for the amyloid-forming EKKE scaffold can further extend its potential role not only in the remediation of other toxic metals but also towards biomedical applications.

Crowding by Poly(ethylene glycol) Destabilizes Chemotaxis Protein Y (CheY).

The prevailing understanding of various aspects of biochemical processes, including folding, stability, intermolecular interactions, and the binding of metals, substrates, and inhibitors, is derived from studies carried out under dilute and homogeneous conditions devoid of a crowding-related environment. The effect of crowding-induced modulation on the structure and stability of native and magnesium-dependent Chemotaxis Y (CheY), a bacterial signaling protein, was probed in the presence and absence of poly(ethylene glycol) (PEG). A combined analysis from circular dichroism, intrinsic and extrinsic fluorescence, and tryptophan fluorescence lifetime changes indicates that PEG perturbs the structure but leaves the thermal stability largely unchanged. Intriguingly, while the stability of the protein is enhanced in the presence of magnesium under dilute buffer conditions, PEG-induced crowding leads to reduced thermal stability in the presence of magnesium. Nuclear magnetic resonance (NMR) chemical shift perturbations and resonance broadening for a subset of residues indicate that PEG interacts specifically with a subset of hydrophilic and hydrophobic residues found predominantly in α helices, ß strands, and in the vicinity of the metal-binding region. Thus, PEG prompted conformational perturbation, presumably provides a different situation for magnesium interaction, thereby perturbing the magnesium-prompted stability. In summary, our results highlight the dominance of enthalpic contributions between PEG and CheY via both hydrophilic and hydrophobic interactions, which can subtly affect the conformation, modulating the metal-protein interaction and stability, implying that in the context of cellular situation, structure, stability, and magnesium binding thermodynamics of CheY may be different from those measured in dilute solution.

Rapid NMR Assignments of Proteins by Using Optimized Combinatorial Selective Unlabeling.

A new approach for rapid resonance assignments in proteins based on amino acid selective unlabeling is presented. The method involves choosing a set of multiple amino acid types for selective unlabeling and identifying specific tripeptides surrounding the labeled residues from specific 2D NMR spectra in a combinatorial manner. The methodology directly yields sequence specific assignments, without requiring a contiguously stretch of amino acid residues to be linked, and is applicable to deuterated proteins. We show that a 2D [(15) N,(1) H] HSQC spectrum with two 2D spectra can result in ∼50 % assignments. The methodology was applied to two proteins: an intrinsically disordered protein (12 kDa) and the 29 kDa (268 residue) α-subunit of Escherichia coli tryptophan synthase, which presents a challenging case with spectral overlaps and missing peaks. The method can augment existing approaches and will be useful for applications such as identifying active-site residues involved in ligand binding, phosphorylation, or protein-protein interactions, even prior to complete resonance assignments.

Integrated multi-omics analysis of Alzheimer's disease shows molecular signatures associated with disease progression and potential therapeutic targets.

Alzheimer's disease (AD) is a progressive neurodegenerative disease characterized by the formation of amyloid plaques implicated in neuronal death. Genetics, age, and sex are the risk factors attributed to AD. Though omics studies have helped to identify pathways associated with AD, an integrated systems analysis with the available data could help to understand mechanisms, potential biomarkers, and therapeutic targets. Analysis of transcriptomic data sets from the GEO database, and proteomic and metabolomic data sets from literature was performed to identify deregulated pathways and commonality analysis identified overlapping pathways among the data sets. The deregulated pathways included those of neurotransmitter synapses, oxidative stress, inflammation, vitamins, complement, and coagulation pathways. Cell type analysis of GEO data sets showed microglia, endothelial, myeloid, and lymphoid cells are affected. Microglia are associated with inflammation and pruning of synapses with implications for memory and cognition. Analysis of the protein-cofactor network of B2, B6, and pantothenate shows metabolic pathways modulated by these vitamins which overlap with the deregulated pathways from the multi-omics analysis. Overall, the integrated analysis identified the molecular signature associated with AD. Treatment with anti-oxidants, B2, B6, and pantothenate in genetically susceptible individuals in the pre-symptomatic stage might help in better management of the disease.

Structural features and oligomeric nature of human podocin domain.

Podocytes are crucial cells of the glomerular filtration unit and plays a vital role at the interface of the blood-urine barrier. Podocyte slit-diaphragm is a modified tight junction that facilitates size and charge-dependent permselectivity. Several proteins including podocin, nephrin, CD2AP, and TRPC6 form a macromolecular assembly and constitute the slit-diaphragm. Podocin is an integral membrane protein attached to the inner membrane of the podocyte via a short transmembrane region (101-125). The cytosolic N- and C-terminus help podocin to attain a hook-like structure. Podocin shares 44% homology with stomatin family proteins and similar to the stomatin proteins, podocin was shown to associate into higher-order oligomers at the site of slit-diaphragm. However, the stoichiometry of the homo-oligomers and how it partakes in the macromolecular assemblies with other slit-diaphragm proteins remains elusive. Here we investigated the oligomeric propensity of a truncated podocin construct (residues:126-350). We show that the podocin domain majorly homo-oligomerizes into a 16-mer. Circular dichroism and fluorescence spectroscopy suggest that the 16-mer oligomer has considerable secondary structure and moderate tertiary packing.

ßαß Super-Secondary Motifs: Sequence, Structural Overview, and Pursuit of Potential Autonomously Folding ßαß Sequences from (ß/α)8/TIM Barrels.

ßαß super-secondary structures constitute the basic building blocks of (ß/α)8 class of proteins. Despite the success in designing super-secondary structures, till date, there is not a single example of a natural ßαß sequence known to fold in isolation. In this chapter, to address the finding the "needles" in the haystack scenario, we have combined the sequence preferences and structural features of independent ßαß motifs, dictated by natural selection, with rationally derived parameters from a designed ßαß motif adopting stable fold in solution. Guided by this approach, a set of potential ßαß sequences from (ß/α)8/TIM barrels are proposed as likely candidates for autonomously folding based on the assessment of their foldability.

A tightly packed hydrophobic cluster directs the formation of an off-pathway sub-millisecond folding intermediate in the alpha subunit of tryptophan synthase, a TIM barrel protein.

Protein misfolding is now recognized as playing a crucial role in both normal and pathogenic folding reactions. An interesting example of misfolding at the earliest state of a natural folding reaction is provided by the alpha-subunit of tryptophan synthase, a (beta/alpha)(8) TIM barrel protein. The molecular basis for the formation of this off-pathway misfolded intermediate, I(BP), and a subsequent on-pathway intermediate, I1, was probed by mutational analysis of 20 branched aliphatic side-chains distributed throughout the sequence. The elimination of I(BP) and the substantial destabilization of I1 by replacement of a selective set of the isoleucine, leucine or valine residues (ILV) with alanine in a large ILV cluster external-to-the-barrel and spanning the N and C termini (cluster 2) implies tight-packing at most sites in both intermediates. Differential effects on I(BP) and I1 for replacements in alpha3, beta4 and alpha8 at the boundaries of cluster 2 suggest that their incorporation into I1 but not I(BP) reflects non-native folds at the edges of the crucial (beta/alpha)(1-2)beta(3) core in I(BP). The retention of I(BP) and the smaller and consistent destabilization of both I(BP) and I1 by similar replacements in an internal-to-the-barrel ILV cluster (cluster 1) and a second external-to-the-barrel ILV cluster (cluster 3) imply molten globule-like packing. The tight packing inferred, in part, for I(BP) or for all of I1 in cluster 2, but not in clusters 1 and 3, may reflect the larger size of cluster 2 and/or the enhanced number of isoleucine, leucine and valine self-contacts in and between contiguous elements of secondary structure. Tightly packed ILV-dominated hydrophobic clusters could serve as an important driving force for the earliest events in the folding and misfolding of the TIM barrel and other members of the (beta/alpha)(n) class of proteins.

Diversity in αß and ßα Loop Connections in TIM Barrel Proteins: Implications for Stability and Design of the Fold.

The (ßα)8/TIM barrel is one of the most common folds of known protein structures facilitating diverse catalytic functions. The fold is formed by the repetition of the basic ßαß building block in which the ß-strands are followed by α-helices eight times alternating in sequence and structure. αß and ßα loops connecting α-helices to the ß-strands and the ß-strands to the α-helices contribute to stability and function, respectively, an inherent imposition by the TIM barrel architecture itself. In this study, αß and ßα loops from a data set of 430 non-redundant, high-resolution triosephosphate isomerase (TIM) barrels bearing sequence homology of <30% were analyzed for their amino acid propensities, sequence profiles, and positional preferences of amino acids. While the distribution of short connections is significantly higher in αß loops, there appears to be no such preference in ßα loops. Glycine, proline, lysine, and arginine tend to show greater preference to occur in αß loops, whereas serine, threonine, cysteine, tryptophan, and histidine occur more frequently in ßα loops. In addition, striking dissimilarities in sequence and positional preferences of amino acids, especially, in short, αß and ßα loops are observed. Together, the analysis suggests the role for short loops and charged residues in promoting both non-polar and polar interactions and in ß strand registry. The observed diversity, perhaps, dictates the distinct role of αß and ßα loops in stability and function, respectively. In summary, the overall observations and reasoning, in addition to steering protein engineering efforts on TIM barrel design and stabilization can provide the basis for incorporating consensus loop sequences for designing independently folding ßαß modules.

Long-range side-chain-main-chain interactions play crucial roles in stabilizing the (betaalpha)8 barrel motif of the alpha subunit of tryptophan synthase.

The role of hither-to-fore unrecognized long-range hydrogen bonds between main-chain amide hydrogens and polar side chains on the stability of a well-studied (betaalpha)8, TIM barrel protein, the alpha subunit of tryptophan synthase (alphaTS), was probed by mutational analysis. The F19-D46 and I97-D124 hydrogen bonds link the N terminus of a beta-strand with the C terminus of the succeeding antiparallel alpha-helix, and the A103-D130 hydrogen bond links the N terminus of an alpha-helix with the C terminus of the succeeding antiparallel beta-strand, forming clamps for the respective betaalpha or alphabeta hairpins. The individual replacement of these aspartic acid side chains with alanine leads to what appear to be closely related partially folded structures with significantly reduced far-UV CD ellipticity and thermodynamic stability. Comparisons with the effects of eliminating another main-chain-side-chain hydrogen bond, G26-S33, and two electrostatic side-chain-side-chain hydrogen bonds, D38-H92 and D112-H146, all in the same N-terminal folding unit of alphaTS, demonstrated a unique role for the clamp interactions in stabilizing the native barrel conformation. Because neither the asparagine nor glutamic acid variant at position 46 can completely reproduce the spectroscopic, thermodynamic, or kinetic folding properties of aspartic acid, both size and charge are crucial to its unique role in the clamp hydrogen bond. Kinetic studies suggest that the three clamp hydrogen bonds act in concert to stabilize the transition state leading to the fully folded TIM barrel motif.

Identification of abelson tyrosine kinase inhibitors as potential therapeutics for Alzheimer's disease using multiple e-pharmacophore modeling and molecular dynamics.

Efforts to combat Alzheimer's disease are focused predominantly on inhibiting the activity of the enzyme(s) that have been identified to be responsible for the production of the amyloid-forming peptide. However, the inherent complexity associated with the network of pathways leading to the disease may involve additional targets for designing effective therapies. Recent experimental findings have identified abelson tyrosine kinase, a non-receptor kinase as a new target for Alzheimer's. In this work, we employed energy optimized multiple pharmacophore modeling strategy from multiple c-Abl structures bound with ligands in the inactive ATP binding conformation (DFG-out). Virtual screening followed by docking of molecules from ChemBridge resulted in the identification of 10 best scoring molecules. MD simulations of the top three complexes revealed that Compound A, C are the most stable complexes with the most persistent protein-ligand interactions consistent with the calculated binding affinities for the top three compounds. Given the implied role of c-Abl not only in AD but in Parkinson's disease, the identified compounds may serve as leads for effective neurotherapeutics.

Pharmacophore based 3D-QSAR modeling, virtual screening and docking for identification of potential inhibitors of ß-secretase.

The enzyme ß-secretase-1 is responsible for the cleavage of the amyloid precursor protein, a vital step in the process of the formation of amyloid-ß peptides which are known to lead to neurodegeneration causing Alzheimer's disease. Challenges associated with toxicity and blood brain permeation inability of potential inhibitors, continue to evade a successful therapy, thus demanding the search and development of highly active and effective inhibitors. Towards these efforts, we used a ligand based pharmacophore model generation from a dataset of known inhibitors whose activities against ß-secretase hovered in the nano molar range. The identified 5 feature pharmacophore model, AHHPR, was validated via three dimensional quantitative structure activity relationship as indicated by r2, q2 and Pearson R values of 0.9013, 0.7726 and 0.9041 respectively. For a dataset of compounds with nano molar activity, the important pharmacophore features present in the current model appear to be similar with those observed in the models resulting from much wider activity range of inhibitors. Virtual screening of the ChemBridge CNS-Set™, a database having compounds with a better suitability for central nervous system based disorders followed by docking and analysis of the ligand protein interactions resulted in the identification of eight prospective compounds with considerable diversity. The current pharmacophore model can thus be useful for the identification, design and development of potent ß-secretase inhibitors which by optimization can be potential therapeutics for Alzheimer's disease.

LoopX: A Graphical User Interface-Based Database for Comprehensive Analysis and Comparative Evaluation of Loops from Protein Structures.

Due to their crucial role in function, folding, and stability, protein loops are being targeted for grafting/designing to create novel or alter existing functionality and improve stability and foldability. With a view to facilitate a thorough analysis and effectual search options for extracting and comparing loops for sequence and structural compatibility, we developed, LoopX a comprehensively compiled library of sequence and conformational features of ∼700,000 loops from protein structures. The database equipped with a graphical user interface is empowered with diverse query tools and search algorithms, with various rendering options to visualize the sequence- and structural-level information along with hydrogen bonding patterns, backbone φ, ψ dihedral angles of both the target and candidate loops. Two new features (i) conservation of the polar/nonpolar environment and (ii) conservation of sequence and conformation of specific residues within the loops have also been incorporated in the search and retrieval of compatible loops for a chosen target loop. Thus, the LoopX server not only serves as a database and visualization tool for sequence and structural analysis of protein loops but also aids in extracting and comparing candidate loops for a given target loop based on user-defined search options.

Specific structure appears at the N terminus in the sub-millisecond folding intermediate of the alpha subunit of tryptophan synthase, a TIM barrel protein.

Competing views of the products of sub-millisecond folding reactions observed in many globular proteins have been ascribed either to the formation of discrete, partially folded states or to the random collapse of the unfolded chain under native-favoring conditions. To test the validity of these alternative interpretations for the stopped-flow burst-phase reaction in the (betaalpha)8, TIM barrel motif, a series of alanine replacements were made at five different leucine or isoleucine residues in the alpha subunit of tryptophan synthase (alphaTS) from Escherichia coli. This protein has been proposed to fold, in the sub-millisecond time range, to an off-pathway intermediate with significant stability and approximately 50% of the far-UV circular dichroism (CD) signal of the native conformation. Individual alanine replacements at any of three isoleucine or leucine residues in either alpha1, beta2 or beta3 completely eliminate the off-pathway species. These variants, within 5 ms, access an intermediate whose properties closely resemble those of an on-pathway equilibrium intermediate that is highly populated at moderate urea concentrations in wild-type alphaTS. By contrast, alanine replacements for leucine residues in either beta4 or beta6 destabilize but preserve the off-pathway, burst-phase species. When considered with complementary thermodynamic and kinetic data, this mutational analysis demonstrates that the sub-millisecond appearance of CD signal for alphaTS reflects the acquisition of secondary structure in a distinct thermodynamic state, not the random collapse of an unfolded chain. The contrasting results for replacements in the contiguous alpha1/beta2/beta3 domain and the C-terminal beta4 and beta6 strands imply a heterogeneous structure for the burst-phase species. The alpha1/beta2/beta3 domain appears to be tightly packed, and the C terminus appears to behave as a molten-globule-like structure whose folding is tightly coupled to that of the alpha1/beta2/beta3 domain.

An obligatory intermediate controls the folding of the alpha-subunit of tryptophan synthase, a TIM barrel protein.

The proposed kinetic folding mechanism of the alpha-subunit of tryptophan synthase (alphaTS), a TIM barrel protein, displays multiple unfolded and intermediate forms which fold through four parallel pathways to reach the native state. To obtain insight into the secondary structure that stabilizes a set of late, highly populated kinetic intermediates, the refolding of urea-denatured alphaTS from Escherichia coli was monitored by pulse-quench hydrogen exchange mass spectrometry. Following dilution from 8 M urea, the protein was pulse-labeled with deuterium, quenched with acid and mass analyzed by electrospray ionization mass spectrometry (ESI-MS). Hydrogen bonds that form prior to the pulse of deuterium offer protection against exchange and, therefore, retain protons at the relevant amide bonds. Consistent with the proposed refolding model, an intermediate builds up rapidly and decays slowly over the first 100 seconds of folding. ESI-MS analysis of the peptic fragments derived from alphaTS mass-labeled and quenched after two seconds of refolding indicates that the pattern of protection of the backbone amide hydrogens in this transient intermediate is very similar to that observed previously for the equilibrium intermediate of alphaTS highly populated at 3 M urea. The protection observed in a contiguous set of beta-strands and alpha-helices in the N terminus implies a significant role for this sub-domain in directing the folding of this TIM barrel protein.

Crowding interactions perturb structure and stability by destabilizing the stable core of the α-subunit of tryptophan synthase.

The consequences of crowding derived from relatively small and intrinsically disordered proteins are not clear yet. We report the effect of ficoll-70 on the structure and stability of native and partially folded states of the 29 kDa alpha subunit of tryptophan synthase (αTS). Overall, combining the changes in the circular dichroism and fluorescence spectra, in conjunction with the gradual loss of cooperativity under urea denaturation in the presence of increasing amounts of ficoll, it may be concluded that the crowding agent perturbs not only the native state but also the partially folded state of αTS. Importantly, NMR data indicate that ficoll interacts with the residues that constitute the stable core of the protein thus shedding light on the origin of the observed perturbation.

Multi-state unfolding of the alpha subunit of tryptophan synthase, a TIM barrel protein: insights into the secondary structure of the stable equilibrium intermediates by hydrogen exchange mass spectrometry.

The urea-induced unfolding of the alpha subunit of tryptophan synthase (alphaTS) from Escherichia coli, an eight-stranded (beta/alpha)(8) TIM barrel protein, has been shown to involve two stable equilibrium intermediates, I1 and I2, well populated at approximately 3 M and 5 M urea, respectively. The characterization of the I1 intermediate by circular dichroism (CD) spectroscopy has shown that I1 retains a significant fraction of the native ellipticity; the far-UV CD signal for the I2 species closely resembles that of the fully unfolded form. To obtain detailed insight into the disruption of secondary structure in the urea-induced unfolding process, a hydrogen exchange-mass spectrometry study was performed on alphaTS. The full-length protein was destabilized in increasing concentration of urea, the amide hydrogen atoms were pulse-labeled with deuterium, the labeled samples were quenched in acid and the products were analyzed by electrospray ionization mass spectrometry. Consistent with the CD results, the I1 intermediate protects up to approximately 129 amide hydrogen atoms against exchange while the I2 intermediate offers no protection. Electrospray ionization mass spectrometry analysis of the peptic fragments derived from alphaTS labeled at 3 M urea indicates that most of the region between residues 12-130, which constitutes the first four beta strands and three alpha helices, (beta/alpha)(1-3)beta(4), is structured. The (beta/alpha)(1-3)beta(4) module appears to represent the minimum sub-core of stability of the I1 intermediate. A 4+2+2 folding model is proposed as a likely alternative to the earlier 6+2 folding mechanism for alphaTS.

Partial NMR assignments and secondary structure mapping of the isolated alpha subunit of Escherichia coli tryptophan synthase, a 29-kD TIM barrel protein.

The alpha subunit of tryptophan synthase (alphaTS) from S. typhimurium belongs to the triosephosphate isomerase (TIM) or the (beta/alpha)(8) barrel fold, one of the most common structures in biology. To test the conservation of the global fold in the isolated Escherichia coli homolog, we have obtained a majority of the backbone assignments for the 29-kD alphaTS by using standard heteronuclear multidimensional NMR methods on uniformly (15)N- and (15)N/(13)C-labeled protein and on protein selectively (15)N-labeled at key hydrophobic residues. The secondary structure mapped by chemical shift index, nuclear Overhauser enhancements (NOEs), and hydrogen-deuterium (H-D) exchange, and several abnormal chemical shifts are consistent with the conservation of the global TIM barrel fold of the isolated E. coli alphaTS. Because most of the amide protons that are slow to exchange with solvent correspond to the beta-sheet residues, the beta-barrel is likely to play an important role in stabilizing the previously detected folding intermediates for E. coli alphaTS. A similar combination of uniform and selective labeling can be extended to other TIM barrel proteins to obtain insight into the role of the motif in stabilizing what appear to be common partially folded forms.

Multiple e-Pharmacophore Modeling Combined with High-Throughput Virtual Screening and Docking to Identify Potential Inhibitors of ß-Secretase(BACE1).

ß-Secretase (BACE1) is an aspartate protease involved in the production of amyloid-ß a major peptide responsible for the pathogenesis of Alzheimer's disease. Given its role in the formation of amyloids leading to Alzheimer's disease, it has been a major therapeutic target for intervention and has been a challenge in the past and the progress has been very slow. More than hundred crystal structures with inhibitors are available in the protein data bank. Many strategies for drug design have been employed in the design of numerous diverse ligands for this target and many have failed due to undesirable drug properties primarily the inability to cross the blood-brain barrier. In the present work we attempted to consider multiple crystal structures with bound inhibitors showing affinity in the range of 2-210 nM efficacy and optimize the pharmacophoric requirement based on the energy involved in binding termed as e-pharmacophore mapping. A high throughput screening combined with molecular docking, ADMET predictions, logP values and in vitro assay led to the identification of 7 potential compounds showing inhibition at 10µM which could be further developed as novel inhibitors for ß-secretase.

Betaalpha-hairpin clamps brace betaalphabeta modules and can make substantive contributions to the stability of TIM barrel proteins.

Non-local hydrogen bonding interactions between main chain amide hydrogen atoms and polar side chain acceptors that bracket consecutive betaalpha or alphabeta elements of secondary structure in alphaTS from E. coli, a TIM barrel protein, have previously been found to contribute 4-6 kcal mol(-1) to the stability of the native conformation. Experimental analysis of similar betaalpha-hairpin clamps in a homologous pair of TIM barrel proteins of low sequence identity, IGPS from S. solfataricus and E. coli, reveals that this dramatic enhancement of stability is not unique to alphaTS. A survey of 71 TIM barrel proteins demonstrates a 4-fold symmetry for the placement of betaalpha-hairpin clamps, bracing the fundamental betaalphabeta building block and defining its register in the (betaalpha)(8) motif. The preferred sequences and locations of betaalpha-hairpin clamps will enhance structure prediction algorithms and provide a strategy for engineering stability in TIM barrel proteins.