Urea-induced denaturation of human calcium/calmodulin-dependent protein kinase IV: a combined spectroscopic and MD simulation studies

Molecular Basis of P131 Cryptosporidial-IMPDH Selectivity-A Structural, Dynamical and Mechanistic Stance

Cryptosporidiosis accounts for a surge in infant (<5 years) mortality and morbidity. To date, several drug discovery efforts have been put in place to develop effective therapeutic options against the causative parasite. Based on a recent report, P131 spares inosine monophosphate dehydrogenase (IMPDH) in a eukaryotic model (mouse IMPDH (mIMPDH)) while binding selectively to the NAD⁺ site in Cryptosporidium parvum (CpIMPDH). However, no structural detail exists on the underlining mechanisms of P131-CpIMPDH selective targeting till date. To this effect, we investigate the selective inhibitory dynamics of P131 in CpIMPDH relative to mIMPDH via molecular biocomputation methods. Pairwise sequence alignment revealed prominent variations at the NAD⁺ binding regions of both proteins that accounted for disparate P131 binding activities. The influence of these variations was further revealed by the MM/PBSA energy estimations coupled with per-residue energy decomposition which monitored the systematic binding of the compound. Furthermore, relative high-affinity interactions occurred at the CpIMPDH NAD⁺ site which were majorly mediated by SER22, VAL24, PRO26, SER354, GLY357, and TYR358 located on chain D. These residues are unique to the parasite IMPDH form and not in the eukaryotic protein, highlighting variations that account for preferential P131 binding. Molecular insights provided herein corroborate previous experimental reports and further underpin the basis of CpIMPDH inhibitor selectivity. Findings from this study could present attractive prospects toward the design of novel anticryptosporidials with improved selectivity and binding affinity against parasitic targets.

Impact of amino acid substitution in the kinase domain of Bruton tyrosine kinase and its association with X-linked agammaglobulinemia

X-linked agammaglobulinemia (XLA) is a rare disease that affects the immune system, characterized by a serial development of bacterial infection from the onset of infantile age. Bruton tyrosine kinase (BTK) is a non-receptor cytoplasmic kinase that plays a crucial role in the B-lymphocyte maturation. The altered expression, mutation and/or structural variations of BTK are responsible for causing XLA. Here, we have performed extensive sequence and structure analyses of BTK to find deleterious variations and their pathogenic association with XLA. First, we screened the pathogenic variations in the BTK from a pool of publicly available resources, and their pathogenicity/tolerance and stability predictions were carried out. Finally, two pathogenic variations (E589G and M630K) were studied in detail and subjected to all-atom molecular dynamics simulation for 200 ns. Intramolecular hydrogen bonds (H-bonds), secondary structure, and principal component analysis revealed significant conformational changes in variants that support the structural basis of BTK dysfunction in XLA. The free energy landscape analysis revealed the presence of multiple energy minima, suggests that E589G brings a large destabilization and consequently unfolding behavior compared to M630K. Overall, our study suggests that amino acid substitutions, E589G, and M630K, significantly alter the structural conformation and stability of BTK.

Mechanistic insights into the urea-induced denaturation of human sphingosine kinase 1

Sphingosine kinase 1 (SphK1) plays a significant role in various cellular processes, including cell proliferation, apoptosis, and angiogenesis. SphK1 is considered as an attractive target for drug development owing to its connection with several diseases, including cancer. In the current work, the urea-induced unfolding of SphK1 was performed at pH 8.0 and 25 °C using CD and fluorescence spectroscopy. SphK1 follows a biphasic unfolding transition (N ⇌ I ⇌ D) with an intermediate (I) state populated around 4.0 M urea concentration. The circular dichroism ([θ]₂₂₂) and fluorescence emission spectra (λ_max) of SphK1 with increasing concentrations of urea were analyzed to calculate Gibbs free energy (ΔG⁰) for both the transitions (N ⇌ I and I ⇌ D). A significant overlap of both the transitions obtained by two spectroscopic properties ([θ]₂₂₂ and λ_max) was observed, indicating that both N ⇌ I and I ⇌ D transition follow two-step equilibrium unfolding pattern. Also, we performed 100 ns molecular dynamics (MD) simulations to get atomistic insights into the structural changes in SphK1 with increasing urea concentrations. Our results showed a consistent pattern of the SphK1 unfolding with increasing urea concentrations. Together, spectroscopic and MD simulation findings provide deep insights into the unfolding mechanism and conformational features of SphK1.

Identification of Potential Inhibitors of Calcium/Calmodulin-Dependent Protein Kinase IV from Bioactive Phytoconstituents

Calcium/calmodulin-dependent protein kinase IV (CaMKIV) is an upstream regulator of CaMKK-CaMKIV signaling cascade that activates various transcription factors, thereby regulating several cellular activities including, neuronal communication and immune response. Owing to the abnormal expression in cancer and neurodegenerative diseases, the CaMKIV has been considered a potential drug target. In the present study, we checked the binding affinity of plant-derived natural compounds viz., quercetin, ellagic acid (EA), simvastatin, capsaicin, ursolic acid, DL-α-tocopherol acetate, and limonin towards CaMKIV. Molecular docking and fluorescence binding studies showed that EA and quercetin bind to the CaMKIV with a considerable affinity in comparison to other compounds. Enzyme inhibition assay revealed that both EA and quercetin inhibit CaMKIV activity with their IC₅₀ values in the micromolar range. To get atomistic insights into the mode of interactions, inhibition mechanism, and the stability of the CaMKIV-ligand complex, a 100 ns MD simulation analysis was performed. Both EA and quercetin bind to the catalytically important residues of active site pocket of CaMKIV forming enough stabilizing interactions presumably inhibiting enzyme activity. Moreover, no significant structural change in the CaMKIV was observed upon binding of EA and quercetin. In conclusion, this study illustrates the application of phytoconstituents in the development of therapeutic molecules targeting CaMKIV having implications in cancer and neurodegenerative diseases after in vivo validation.

Unravelling the unfolding pathway of human Fas-activated serine/threonine kinase induced by urea

Fas-activated serine/threonine kinase (FASTK) is a mitochondria-associated nuclear protein that inhibits Fas- and UV-induced apoptosis. This protein is generally activated during Fas-mediated apoptosis by phosphorylating a nuclear RNA-binding protein T-cell intracellular antigen-1 and thus considered as a modulator of apoptosis. In the present study, we have examined the equilibrium unfolding and conformational stability of the kinase domain of FASTK (FASTK_353-444). The kinase domain of FASTK_353-444 was cloned, expressed, and purified. The folding ↔ unfolding transitions of urea-induced denaturation was monitored with the help of circular dichroism, intrinsic fluorescence, and UV absorption spectroscopies. Analysis of transition curves obtained from different probes revealed a coincidence of denaturation curves, suggesting that folding/unfolding of FASTK follows a two-state process with the midpoint (C_m) value at 3.50 ± 0.1 M. Urea-induced denaturation curves were further analyzed to estimate change in the Gibbs free energy in the absence of urea (ΔG_D⁰) associated with the equilibrium of denaturation. To get atomistic insights into the urea-induced denaturation of FASTK, we performed an all-atom molecular dynamics simulation for 100 ns. A close agreement was noticed between experimental and computational studies. This study will help to understand the unfolding mechanism and structural stability of the kinase domain of FASTK.Communicated by Ramaswamy H. Sarma.

Effect of pH on the structure and function of pyruvate dehydrogenase kinase 3: Combined spectroscopic and MD simulation studies

Pyruvate dehydrogenase kinase-3 (PDK3) plays important role in the glucose metabolism and is associated with cancer progression, and thus being considered as an attractive target for cancer therapy. In this study, we employed spectroscopic techniques to study the structural and conformational changes in the PDK3 at varying pH conditions ranging from pH 2.0 to 12.0. UV/Vis, fluorescence and circular dichroism spectroscopic measurements revealed that PDK3 maintains its native-like structure (both secondary and tertiary) in the alkaline conditions (pH 7.0-12.0). However, a significant loss in the structure was observed under acidic conditions (pH 2.0-6.0). The propensity of aggregate formation at pH 4.0 was estimated by thioflavin T fluorescence measurements. To further complement structural data, kinase activity assay was performed, and maximum activity of PDK3 was observed at pH 7.0-8.0 range; whereas, its activity was lost under acidic pH. To further see conformational changes at atomistic level we have performed all-atom molecular dynamics at different pH conditions for 150 ns. A well defined correlation was observed between experimental and computational studies. This work highlights the significance of structural dependence of pH for wide implications in protein-protein interaction, biological function and drug design procedures.

Identification of High-Affinity Inhibitors of Cyclin-Dependent Kinase 2 Towards Anticancer Therapy

Cyclin-dependent kinase 2 (CDK2) is an essential protein kinase involved in the cell cycle regulation. The abnormal activity of CDK2 is associated with cancer progression and metastasis. Here, we have performed structure-based virtual screening of the PubChem database to identify potent CDK2 inhibitors. First, we retrieved all compounds from the PubChem database having at least 90% structural similarity with the known CDK2 inhibitors. The selected compounds were subjected to structure-based molecular docking studies to investigate their pattern of interaction and estimate their binding affinities with CDK2. Selected compounds were further filtered out based on their physicochemical and ADMET properties. Detailed interaction analysis revealed that selected compounds interact with the functionally important residues of the active site pocket of CDK2. All-atom molecular dynamics simulation was performed to evaluate conformational changes, stability and the interaction mechanism of CDK2 in-complex with the selected compound. We found that binding of 6-N,6-N-dimethyl-9-(2-phenylethyl)purine-2,6-diamine stabilizes the structure of CDK2 and causes minimal conformational change. Finally, we suggest that the compound (PubChem ID 101874157) would be a promising scaffold to be further exploited as a potential inhibitor of CDK2 for therapeutic management of cancer after required validation.

Identification and evaluation of glutathione conjugate gamma-l-glutamyl-l-cysteine for improved drug delivery to the brain

Glutathione (GU), an endogenous antioxidant tripeptide, is frequently transferred in the human brain through N-methyl-d-aspartate receptor (NMDAR), profusely expressed at the blood-brain barrier (BBB) junction. GU, also modifies the characteristics of tight junction proteins (occludin and claudin) at the site of BBB by depolarizing the enzyme, protein tyrosine phosphatase that manifests its usefulness for passive delivery of nanocarriers to the brain. GU, thus, represents itself as an ideal ligand for the surface decoration of nanocarriers to successfully administer them across the brain via receptor-mediated drug delivery pathway. Hence, we have employed here, in-silico approaches to identify the potential GU-like molecules, as appropriate ligand(s) for surface engineering of nanoconstruct with the purpose of attaining targeted drug delivery to the brain. Structure-based virtual screening methods was used to filter PubChem database for the identification of bioactive compounds with >95% structure similarity with GU. We have further screened the compounds against NMDAR using molecular docking approach. Top hits were selected based on their high binding affinities and selectivity towards NMDAR, and their binding pattern was analysed in detail. Finally, all atom molecular dynamics simulation for 100 ns was carried out on free NMDAR and in-presence of the selected GU-like compound, gamma-l-glutamyl-l-cysteine to evaluate complex stability and structural dynamics. In conclusion, gamma-l-glutamyl-l-cysteine may act as potential binding partner of NMDAR which can further be evaluated in drug delivery system to brain across the BBB.Communicated by Ramaswamy H. Sarma.

Effects of Pro1266Leu mutation on structure and function of glycoprotein Ib binding domain of von Willebrand factor

Glycoprotein Ibα (GpIbα) binding ability of A1 domain of von Willebrand factor (vWF) facilitates platelet adhesion that plays a crucial role in maintaining hemostasis and thrombosis at the site of vascular damage. There are both "loss as well as gain of function" mutations observed in this domain. Naturally occurring "gain of function" mutations leave self-activating impacts on the A1 domain which turns the normal binding to characteristic constitutive binding with GPIbα. These "gain of function" mutations are associated with the von Willebrand disease type 2B. In recent years, studies focused on understanding the mechanism and conformational patterns attached to these phenomena have been conducted, but the conformational pathways leading to such binding patterns are poorly understood as of now. To obtain a microscopic picture of such events for the better understanding of pathways, we used molecular dynamics (MD) simulations along with principal component analysis and normal mode analysis to study the effects of Pro1266Leu (Pro503Leu in structural context) mutation on the structure and function of A1 domain of vWF. MD simulations have provided atomic-level details of intermolecular motions as a function of time to understand the dynamic behavior of A1 domain of vWF. Comparative analysis of the trajectories obtained from MD simulations of both the wild type and Pro503Leu mutant suggesting appreciable conformational changes in the structure of mutant which might provide a basis for assuming the "gain of function" effects of these mutations on the A1 domain of vWF, resulting in the constitutive binding with GpIbα.

Hesperidin-CAMKIV interaction and its impact on cell proliferation and apoptosis in the human hepatic carcinoma and neuroblastoma cells

Calcium/calmodulin-dependent protein kinase IV (CAMKIV) is a key regulatory molecule of cell signaling, and thereby controls its growth and proliferation, including expression of certain genes. The overexpression of CAMKIV is directly associated with the development of different types of cancers. Hesperidin is abundantly found in citrus fruits and exhibits wide range of pharmacological activities including anti-inflammatory, antibacterial and anticancerous effects. We have investigated binding mechanism of hesperidin with the CAMKIV using molecular docking methods followed by fluorescence quenching and isothermal titration calorimetric assays. An appreciable binding affinity of hesperidin was observed with CAMKIV during fluorescence quenching and isothermal titration calorimetric studies. Efficacy of hesperidin to inhibit the growth of human hepatic carcinoma (HepG2) and neuroblastoma (SH-SY5Y) cancer cell lines were investigated. Hesperidin has significantly reduced the proliferation of HepG2 and SH-SY5Y cells and induces apoptosis by activating the caspase-3-dependent intrinsic pathway through the upregulation of proapoptotic Bax protein. Hesperidin treatment reduces the mitochondrial membrane potential of HepG2 and SH-SY5Y cells. All these observations clearly anticipated hesperidin a potent inhibitor of CAMKIV which may be further exploited a newer therapeutic approach for the management of different cancer types.

High throughput screening, docking, and molecular dynamics studies to identify potential inhibitors of human calcium/calmodulin-dependent protein kinase IV

Calcium/calmodulin-dependent protein kinase IV (CAMKIV) is associated with many diseases including cancer and neurodegenerative disorders and thus being considered as a potential drug target. Here, we have employed the knowledge of three-dimensional structure of CAMKIV to identify new inhibitors for possible therapeutic intervention. We have employed virtual high throughput screening of 12,500 natural compounds of Zinc database to screen the best possible inhibitors of CAMKIV. Subsequently, 40 compounds which showed significant docking scores (-11.6 to -10.0 kcal/mol) were selected and further filtered through Lipinski rule and drug likeness parameter to get best inhibitors of CAMKIV. Docking results are indicating that ligands are binding to the hydrophobic cavity of the kinase domain of CAMKIV and forming a significant number of non-covalent interactions. Four compounds, ZINC02098378, ZINC12866674, ZINC04293413, and ZINC13403020, showing excellent binding affinity and drug likeness were subjected to molecular dynamics simulation to evaluate their mechanism of interaction and stability of protein-ligand complex. Our observations clearly suggesting that these selected ligands may be further employed for therapeutic intervention to address CAMKIV associated diseases. Communicated by Ramaswamy H. Sarma.

Unravelling the unfolding mechanism of human integrin linked kinase by GdmCl-induced denaturation

Integrin-linked kinase (ILK) is a ubiquitously expressed Ser/Thr kinase which plays significant role in the cell-matrix interactions and growth factor signalling. In this study, guanidinium chloride (GdmCl)-induced unfolding of kinase domain of ILK (ILK_193-446) was carried out at pH 7.5 and 25 °C. Eventually, denaturation curves of mean residue ellipticity at 222 nm ([θ]₂₂₂) and fluorescence emission spectrum were analysed to estimate stability parameters. The optical properties maximum emission (λ_max) and difference absorption coefficient at 292 nm (Δε₂₉₂) were analysed. The denaturation curve was measured only in the GdmCl molar concentration ranging 3.0-4.2 M because protein was aggregating below 3.0 M of GdmCl concentrations. The denaturation process of ILK_193-446 was found as reversible at [GdmCl] ≥ 3.0 M. Moreover, a coincidence of normalized denaturation curves of optical properties ([θ]₂₂₂, Δε₂₉₂ and λ_max) suggesting that GdmCl-induced denaturation of ILK_193-446 is a two-state process. In addition, 100 ns molecular dynamics simulations were performed to see the effects of GdmCl on the structure and stability of ILK_193-446. Both the spectroscopic and molecular dynamics approaches provided clear insights into the stability and conformational properties of ILK_193-446.

Evidence of vanillin binding to CAMKIV explains the anti-cancer mechanism in human hepatic carcinoma and neuroblastoma cells

Human calcium/calmodulin-dependent protein kinase IV (CAMKIV) is a member of Ser/Thr kinase family, and is associated with different types of cancer and neurodegenerative diseases. Vanillin is a natural compound, a primary component of the extract of the vanilla bean which possesses varieties of pharmacological features including anti-oxidant, anti-inflammatory, anti-bacterial and anti-tumor. Here, we have investigated the binding mechanism and affinity of vanillin to the CAMKIV which is being considered as a potential drug target for cancer and neurodegenerative diseases. We found that vanillin binds strongly to the active site cavity of CAMKIV and stabilized by a large number of non-covalent interactions. We explored the utility of vanillin as anti-cancer agent and found that it inhibits the proliferation of human hepatocyte carcinoma (HepG2) and neuroblastoma (SH-SY5Y) cells in a dose-dependent manner. Furthermore, vanillin treatment resulted into the significant reduction in the mitochondrial membrane depolarization and ROS production that eventually leads to apoptosis in HepG2 and SH-SY5Y cancer cells. These findings may offer a novel therapeutic approach by targeting the CAMKIV using natural product and its derivative with a minimal side effect.

Biophysical insight into structure-function relation of Allium sativum Protease Inhibitor by thermal, chemical and pH-induced modulation using comprehensive spectroscopic analysis

In this study, we have analyzed the structural and functional changes in the nature of Allium sativum Protease Inhibitor (ASPI) on undergoing various denaturation with variable range of pH, temperature and urea (at pH 8.2). ASPI being anti-tryptic in nature has native molecular mass of ∼15kDa. The conformational stability, functional parameters and their correlation were estimated under different conditions using circular dichroism, fluorescence and activity measurements. ASPI was found to fall in belongs to α+β protein. It demonstrated structural and functional stability in the pH range 5.0-12.0 and up to70°C temperature. Further decrease in pH and increase in temperature induces unfolding followed by aggregation. Chemical induced denaturation was found to be cooperative and transitions were reversible and sigmoid. T_m (midpoint of denaturation), ΔC_p (constant pressure heat capacity change) and ΔH_m (van't Hoff enthalpy change at T_m were calculated to be 41.25±0.2°C, 1.3±0.07kcalmol^-1K^-1 and 61±2kcalmol^-1 respectively for thermally denatured ASPI earlier. The reversibility of the protein was confirmed for both thermally and chemically denatured ASPI. The results obtained from trypsin inhibitory activity assay and structural studies are found to be in a significant correlation and hence established structure-function relationship of ASPI.

Design, synthesis, and biological evaluation of pyrimidine derivatives as potential inhibitors of human calcium/calmodulin-dependent protein kinase IV

Calcium/calmodulin-dependent protein kinase IV (CAMKIV) is a multifunctional Ser/Thr kinase, associated with cerebral hypoxia, cancer, and neurodegenerative diseases. Here, we report design, synthesis, and biological evaluation of seven pyrimidine-substituted novel inhibitors of CAMKIV. We successfully synthesized and extensively characterized (ESI-MS, ¹ H NMR, and ¹³ C NMR studies) seven compounds that are showing appreciable binding affinity to the CAMKIV. Molecular docking and fluorescence binding studies revealed that compound 1 is showing very high binding free energy (ΔG = -11.52 kcal/mol) and binding affinity (K = 9.2 × 10¹⁰ m^-1 ) to the CAMKIV. We further performed MTT assay to check the cytotoxicity and anticancer activity of these compounds. An appreciable IC₅₀ (39 μm) value of compound 1 was observed on human hepatoma cell line and nontoxic till the 400 μm on human embryonic kidney cells. To ensure anticancer activity of all these compounds, we further performed propidium iodide assay to evaluate cell viability and DNA content during the cell cycle. We found that compound 1 is again showing a better anticancer activity on both human hepatoma and human embryonic kidney cell lines.

Effect of pH on the stability of hemochromatosis factor E: a combined spectroscopic and molecular dynamics simulation-based study

Hereditary hemochromatosis is an iron overburden condition, which is mainly governed by hereditary hemochromatosis factor E (HFE), a member of major histocompatibility complex class I. To understand the effect of pH on the structure and stability of HFE, we have cloned, expressed, and purified the HFE in the bacterial system and performed circular dichroism, fluorescence, and absorbance measurements at a wide pH range (pH 3.0-11.0). We found that HFE remains stable in the pH range 7.5-11.0 and gets completely acid denatured at low pH values. In this work, we also analyzed the contribution of salt bridges to the stability of HFE. We further performed molecular dynamics simulations for 80 ns at different pH values. An excellent agreement was observed between results from biophysical and MD simulation studies. At lower pH, HFE undergoes denaturation and may be driven toward a degradation pathway, such as ubiquitination. Hence, HFE is not available to bind again with transferrin receptor1 to negatively regulate iron homeostasis. Further we postulated that, might be low pH of cancerous cells helps them to meet their high iron requirement.

GdmCl-induced unfolding studies of human carbonic anhydrase IX: a combined spectroscopic and MD simulation approach

Carbonic anhydrase IX (CAIX) is a transmembrane glycoprotein, associated with tumor, acidification which leads to the cancer, and is considered as a potential biomarker for hypoxia-induced cancers. The overexpression of CAIX is linked with hypoxia condition which is mediated by the transcription of hypoxia-induced factor (HIF-1). To understand the biophysical properties of CAIX, we have carried out a reversible isothermal denaturation of CAIX-induced by GdmCl at pH 8.0 and 25°C. Three different spectroscopic probes, the far-UV CD at 222 nm ([θ]₂₂₂), Trp fluorescence emission at 342 nm (F₃₄₂) and difference molar absorption coefficient at 287 nm (Δε₂₈₇) were used to estimate stability parameters, [Formula: see text] (Gibbs free energy change in the absence of GdmCl; C_m (midpoint of the denaturation curve), i.e. molar GdmCl concentration ([GdmCl]) at which ΔG_D = 0; and m, the slope (=∂ΔG_D/∂[GdmCl])). GdmCl induces a reversible denaturation of CAIX. Coincidence of the normalized transition curves of all optical properties suggests that unfolding/refolding of CAIX is a two-state process. We further performed molecular dynamics simulation of CAIX for 40 ns to see the dynamics of protein structure in different GdmCl concentrations. An excellent agreement was observed between in silico and in vitro studies.