Investigation of binding mechanism of novel 8-substituted coumarin derivatives with human serum albumin and α-1-glycoprotein

Development of quinazolinone and vanillin acrylamide hybrids as multi-target directed ligands against Alzheimer's disease and mechanistic insights into their binding with acetylcholinesterase

In view of Multi-Target Directed Ligand (MTDL) approach in treating Alzheimer's Disease (AD), a series of novel quinazolinone and vanillin cyanoacetamide based acrylamide derivatives (9a-z) were designed, synthesized, and assessed for their activity against a panel of selected AD targets including acetylcholinesterase (AChE), butyrylcholinesterase (BChE), amyloid β protein (Aβ), and also 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging and neuroprotective activities. Five of the target analogs 9e, 9h, 9 l, 9t and 9z showed elevated AChE inhibitory activity with IC50 values of 1.058 ± 0.06, 1.362 ± 0.09, 1.434 ± 0.10, 1.015 ± 0.10, 1.035 ± 0.02 µM respectively, high inhibition selectivity against AChE over BChE and good DPPH radical scavenging activity. Enzyme kinetic studies of the potent hybrids in the series disclosed their mixed inhibition approach. Active analogs were found to be non-toxic on SK-N-SH cell lines and have excellent neuroprotective effects against H2O2-induced cell death. Strong modulating affinities on Aβ aggregation process were observed for most active compounds since; they irretrievably interrupted the morphology of Aβ42 fibrils, increased the aggregates and declined the Aβ-induced toxicity in neurons. From the fluorescence emission studies, the binding constants (K) were determined as 2.5 ± 0.021x103, 2.7 ± 0.015x103, 3.7 ± 0.020x103, 2.4 ± 0.013x104, and 5.0 ± 0.033x103 M-1 and binding free energies as -5.82 ± 0.033, -6.07 ± 0.042, -6.26 ± 0.015, -7.71 ± 0.024, and -6.29 ± 0.026 kcal M-1 for complexes of AChE-9e, 9h, 9 l, 9t and 9z, respectively. Moreover, the CD analysis inferred the limited modifications in the AChE secondary structure when it binds to 9e, 9h, 9 l, 9t and 9z. On the basis of docking studies against AChE, the most active congeners were well oriented in the enzyme's active site by interacting with both catalytic active site (CAS) and peripheral anionic site (PAS). In summary, these quinazolinone and vanillin acrylamide hybrid analogs can be used as promising molecular template to further explore their in vivo efficiency in the development of lead compound to treat AD.Communicated by Ramaswamy H. Sarma.

Exploring the binding mechanism and adverse toxic effects of persistent organic pollutant (dicofol) to human serum albumin: A biophysical, biochemical and computational approach

The organochlorine pesticide dicofol (DCF), a persistent organic pollutant, is used as acaricide worldwide. Considering its large consumption in the agriculture sector and potential toxic effects such as endocrine disruption, carcinogenicity, and environmental persistence are detrimental to human health. To take an extensive evaluation of its potential toxicity, the current study was aimed to explore the binding mechanism and adverse effect of DCF on human serum albumin (HSA) by using an array of biophysical techniques (UV-visible, fluorescence, 3D fluorescence, and circular dichroism spectroscopy), isothermal titration calorimetric (ITC), computational methods and biochemical approaches. Fluorescence quenching and UV-Visible spectra of the HSA-DCF system confirmed static quenching mechanism and complex formation between HSA and DCF. The thermodynamics results from ITC revealed DCF-HSA interaction was exothermic and spontaneous and involved hydrophobic interactions and hydrogen bonding. The esterase activity of HSA displayed constant V_max and elevated K_m values confirming DCF-HSA competitive interaction. Circular dichroism spectra results revealed structural changes in HSA protein on interaction with DCF. Furthermore, molecular-specific site marker and molecular modelling results affirmed that the binding Site of DCF is Site I of HSA. A significant carbonyl content level in DCF-HSA system suggested protein structure damage. This work is likely to add a better understanding of DCF toxicity in human health and helpful in fortifying the check on food safety.

Deciphering the AChE-binding mechanism with multifunctional tricyclic coumarin anti-Alzheimer's agents using biophysical and bioinformatics approaches and evaluation of their modulating effect on Amyloidogenic peptide assembly

Investigating the drug-AChE binding mechanism is vital in understanding its cogent use in medical practice against Alzheimer's disease (AD). The production and accumulation of oligomers of β-amyloid is a central event in the neuropathology of AD. Beside the inhibition of assembly process, modulation of the aggregation process of these proteins towards minimally toxic pathways may be a possible therapeutic strategy for AD. Hence, the present study aims to examine the effect of multifunctional fused tricyclic 7-hydroxy 4-methyl coumarin analogs (HMC_1-5) on the self-induced aggregation of β-amyloid using Thioflavin T (ThT) assay, scanning electron microscopic study, AlamarBlue and immune blotting assays and also the binding mechanism with AChE by fluorescence emission, conformational, molecular docking and molecular dynamic simulation studies under physiological pH 7.4. The ThT assay, FE-SEM study, cell line and western blots establish that the HMC_1-5 molecules could irreversibly disrupt preformed Aβ₄₂ fibrils, accelerate the aggregates into micro size co-assembled structures, and effectively eliminate the cytotoxicity of Aβ_1-42. Fluorescence emission studies indicating a strong binding affinity between HMC_1-5 and AChE with the binding constants of 1.04 × 10⁵, 3.57 × 10⁴, 1.97 × 10⁴, 3.07 × 10⁴ and 2.95 × 10⁴ M^-1, respectively and binding sites number found to be 1. CD studies disclosed a partial unfolding in the secondary structure of AChE upon binding with HMC_1-5. Docking analysis inferred that the HMC_1-5 were bound through hydrophobic and hydrophilic interactions to the AChE active site. Molecular dynamics simulations emphasized the stability of AChE-HMC_1-5 complexes throughout the 100 ns simulations, and the local conformational changes of the residues of AChE validate the stability of complexes. These results provide new and unique complementary approach for modulating the biological effects of the Aβ aggregates by coumarin analogs and new insights for further in vivo investigations as novel anti AD agents.

Addressing the Exigent Role of a Coumarin Fluorophore toward Finding the Suitable Microenvironment of Biomimicking and Biomolecular Systems: Steering to Project the Drug Designing and Drug Delivery Study

The photophysics of 4-azidocoumarin (4-AC), a novel fluorescent coumarin derivative, is well established by the investigation of the alteration of the microheterogeneous environment comprising two types of systems: supramolecular systems, cyclodextrins (CDs), and biomolecular systems, serum albumins (SAs). The enhanced emission of the ligand with the organized assemblies like α-CD, β-CD, and γ-CD by steady-state and time-resolved fluorescence and fluorescence anisotropy at 298 K is compared with those of bovine serum albumin (BSA) and human serum albumin (HSA). The remarkable enhancement of the emission of ligand 4-AC along with the blue shift of the emission for both the systems are visualized as the incorporation of 4-AC into the hydrophobic core of the CDs and proteins mainly due to reduction of nonradiative decay process in the hydrophobic interior of CDs and SAs. The binding constants at 298 K and the single binding site are estimated using enhanced emission and anisotropy of the bound ligand in both the systems. The marked enhancement of the fluorescence anisotropy indicates that the ligand molecule experiences a motionally constrained environment within the CDs and SAs. Rotational correlation time (θ_c) of the bound ligand 4-AC is calculated in both the categories of the confined environment using time-resolved anisotropy at 298 K. Molecular docking studies for both the variety of complexes of the ligand throw light to assess the location of the ligand and the microenvironment around the ligand in the ligand-CD and ligand-protein complexes. Solvent variation study of the probe 4-AC molecule in different polar protic and aprotic solvents clearly demonstrates the polarity and hydrogen-bonding ability of the solvents, which supports the alteration of the microenvironments around 4-AC due to binding with the biomimicking as well as biomolecular systems. Dynamic light scattering is employed to determine the hydrodynamic diameter of free BSA/HSA and complexes of BSA/HSA with the ligand 4-AC.

Synthesis of two novel pyrazolo[1,5-a]pyrimidine compounds with antibacterial activity and biophysical insights into their interactions with plasma protein

Two novel water-soluble pyrazolo[1,5-a]pyrimidine derivatives, 5-chloro-7-(4-methyl-piperazin -1-yl)-pyrazolo[1,5-a]pyrimidine (CMPS) and N'-(5-chloro-pyrazolo[1,5-a]pyrimidin-7-yl)-N,N-dimethyl -propane-1,3-diamine (NCPS), were synthesized and characterized with antibacterial activity. Then, the interactions of these compounds with bovine serum albumin (BSA) were studied by fluorescence, time-resolved fluorescence, circular dichroism (CD) spectroscopy and molecular docking. The results indicate that both CMPS and NCPS could effectively quench the intrinsic fluorescence of BSA via a static quenching process. The energy transfer from BSA to CMPS and NCPS may occur with high probability. Both CMPS and NCPS bind in the site I of BSA. The hydrophobic force and hydrogen bonds play major roles in the complex formation. Binding constants for both systems show that the affinity of CMPS binding to BSA is stronger than that of NCPS. The results of three-dimensional fluorescence and CD spectra reveal that the binding of CMPS and NCPS to BSA can induce conformational changes of BSA, and the influence of CMPS is slightly stronger than that of NCPS.

Probing the interaction of a coumarin-di(2-picolyl)amine hybrid drug-like molecular entity with human serum albumin: Multiple spectroscopic and molecular modeling techniques

HSA is an important plasma protein responsible for transport of drug molecules. Coumarin derivatives play critical role as anticancer, antidiabetic and antiparkinson agents. In our lab we have synthesized coumarin-based pharmacophore, di(2-picolyl)amine-3(bromoacetyl) coumarin (ligand-L) endowed with anticancer activity. Anticancer agents binding mode of HSA provides valuable pharmacological information and is a structural guidance in synthesizing new drugs with greater efficacy. Thus, binding mechanism of ligand-L with HSA was explored using spectroscopic and molecular docking techniques. UV-Vis spectroscopy demonstrates hyperchromism in the absorbance spectra of HSA on addition of ligand-L suggesting interaction of ligand-L with HSA. Fluorescence spectroscopy indicates quenching in the fluorescence of HSA in the presence of ligand-L confirming the complex formation and this binding follows static mechanism. Steady state fluorescence spectroscopy revealed high binding affinity between ligand-L and HSA with a 1:1 stoichiometry. Thermodynamic parameters obtained by ITC suggest that the interaction between ligand-L and HSA is mainly driven by van der Waals forces and hydrogen bonds, and the negative value of ΔG is an indication of spontaneous binding process. Competitive binding and molecular docking experiments showed that the binding site of ligand-L mainly resides in sub-domain IIA of HSA. CD experiments revealed no significant conformational changes in the secondary structure of HSA on binding of ligand-L. We also found that esterase-like activity of HSA was not affected by ligand-L. In conclusion, this study demonstrates binding mechanism of ligand-L with HSA, and the binding did not induce conformational changes in HSA. This study is likely to provide better understanding of transport and delivery of ligand-L via HSA. Overall, it will provide insights into pharmacokinetic properties of ligand-L and designing new ligand-L based derivatives with greater efficacy.

An overview of albumin and alpha-1-acid glycoprotein main characteristics: highlighting the roles of amino acids in binding kinetics and molecular interactions

Although Albumin (ALB) and alpha-1-acid glycoprotein (AGP) have distinctive structural and functional characteristics, they both play a key role in binding a large variety of endogenous and exogenous ligands. An extensive binding to these plasma proteins could have a potential impact on drugs disposition (e.g. bioavailability, distribution and clearance), on their innocuity and their efficacy. This review summarizes the common knowledge about the structural and molecular characteristics of both ALB and AGP in humans, and about the most involved amino acids in their high-affinity binding pockets. However, the variability in residues found in binding pockets, for the same species, allows each plasma protein to interact differently with the ligands. The protein-ligand interaction influences differently the disposition of drugs that bind to either of these plasma proteins. The content of this review is useful for the design of new drug entities with high-binding characteristics, in qualitative and quantitative modelling (e.g. in vitro-in vivo extrapolations, 3D molecular docking, interspecies extrapolations), and for other interdisciplinary research.

Metabolism and disposition of pyrotinib in healthy male volunteers: covalent binding with human plasma protein

Pyrotinib is a novel irreversible EGFR/HER2 dual tyrosine kinase inhibitor that is used to treat HER2-positive breast cancer. In this study we investigated the metabolism and disposition of pyrotinib in six healthy Chinese men after a single oral dose of 402 mg of [¹⁴C]pyrotinib. At 240 h postdose, the mean cumulative excretion of the dose radioactivity was 92.6%, including 1.7% in urine and 90.9% in feces. In feces, oxidative metabolites were detected as major drug-related materials and the primary metabolic pathways were O-depicoline (M1), oxidation of pyrrolidine (M5), and oxidation of pyridine (M6-1, M6-2, M6-3, and M6-4). In plasma, the major circulating entities identified were pyrotinib, SHR150980 (M1), SHR151468 (M2), and SHR151136 (M5), accounting for 10.9%, 1.9%, 1.0%, and 3.0%, respectively, of the total plasma radioactivity based on the AUC_0-∞ ratios. Approximately 58.3% of the total plasma radioactivity AUC_0-∞ was attributed to covalently bound materials. After incubation of human plasma with [¹⁴C]pyrotinib at 37 °C for 2, 5, 8, and 24 h, the recovery of radioactivity by extraction was 97.4%, 91.8%, 69.6%, and 46.7%, respectively, revealing covalent binding occurred independently of enzymes. A group of pyrotinib adducts, including pyrotinib-lysine and pyrotinib adducts of the peptides Gly-Lys, Lys-Ala, Gly-Lys-Ala, and Lys-Ala-Ser, was identified after HCl hydrolysis of the incubated plasma. Therefore, the amino acid residue Lys190 of human serum albumin was proposed to covalently bind to pyrotinib via Michael addition. Finally, the covalently bound pyrotinib could dissociate from the human plasma protein and be metabolized by oxidation and excreted via feces.

Determination of interactions between human serum albumin and niraparib through multi-spectroscopic and computational methods

The interactions between 2-{4-[(3S)-piperidin-3-yl] phenyl}-2H-indazole-7-carboxamide (niraparib) and human serum albumin (HSA) were investigated through fluorescence and computational studies. Fluorescence experiments showed that the static quenching mechanism and the binding constant of the HSA-niraparib system at a single binding site was approximately 4 × 10⁴ L mol^-1. Thermodynamic constants indicated that the binding of niraparib to HSA was mainly driven by electrostatic interactions. Competition experiments and molecular docking simulations revealed that niraparib bound to site III of HSA. Synchronous fluorescence and Fourier transform infrared spectroscopy (FT-IR) results suggested that interactions between niraparib and HSA could affect the conformation and microenvironment of HSA. Circular dichroism (CD) measurements revealed that the α-helix contents of HSA negligibly increased after binding with niraparib. Molecular dynamics simulations demonstrated the stability of the binary HSA-niraparib system and confirmed that electrostatic forces accounted for the dominant contribution to system energy between HSA and niraparib.

Elucidating the active interaction mechanism of phytochemicals withanolide and withanoside derivatives with human serum albumin

Withania somnifera (Ashwagandha) is an efficient medicinal plant known in Ayurveda and Chinese medicine since ancient times, whose extracts are consumed orally as food supplement or as a health tonic owing to its several restorative properties for various CNS disorders, inflammation, tumour, stress, rheumatism etc. In this study, we have analyzed the binding interaction of four derivatives of Withania somnifera (Withanolide A, Withanolide B, Withanoside IV and Withanoside V) with HSA because of their important pharmacological properties. To unravel the binding between derivatives of Withania somnifera and HSA, fluorescence spectroscopy was used. Binding studies were further studied by molecular docking and dynamics and results confirmed greater stability upon binding of derivatives with HSA. Circular dichroism data illustrated change in the secondary structure of protein upon interaction with these derivatives, particularly the helical structure was increased and β-sheets and random coils were decreased. Furthermore, morphological and topological changes were observed using AFM and TEM upon binding of ligands with HSA indicating that HSA-withnoside/withanolide complexes were formed. All the results cumulatively demonstrate strong binding of withanosides and withanolides derivatives with serum albumin, which should further be explored to study the pharmacokinetics and pharmacodynamics of these derivatives.

Decrease of the affinity of theophylline bind to serum proteins induced by flavonoids and their synergies on protein conformation

In this study, the single and simultaneous interactions of theophylline and flavonoids with human serum albumin (HSA) were studied by multi-spectroscopic and molecular docking approaches. The influences of flavonoids on the binding constant (K_b) and the binding distance (r) of theophylline bind to HSA were determined and the changes of HSA conformation caused by the synergies of theophylline and flavonoids were investigated. Because theophylline, rutin and baicalin are all bond to the same binding site, the competitive bind of theophylline and flavonoids to HSA leads to the reduction of the K_a value of theophylline binding to HSA. The addition of rutin and baicalin can increase the value of r of theophylline binding to HSA, which further confirm the existence of the competitive bind of theophylline and flavonoids to HSA. Additionally, the results of synchronous fluorescence, three-dimensional fluorescence and circular dichroism spectra indicate that the presence of rutin and baicalin can give rise to the further changes of HSA conformation. These results suggest that the intake of flavonoid-rich food and beverages can increase the serum concentrations of theophylline and induce a high incidence of toxic symptom in clinic.