Multi-site binding of epigallocatechin gallate to human serum albumin measured by NMR and isothermal titration calorimetry

Computational Insights Into the Mechanism of EGCG's Binding and Inhibition of the TDP-43 Aggregation

Misfolding and aggregation of TAR DNA-binding protein, TDP-43, is linked to devastating proteinopathies such as ALS. Therefore, targeting TDP-43's aggregation is significant for therapeutics. Recently, green tea polyphenol, EGCG, was observed to promote non-toxic TDP-43 oligomer formation disallowing TDP-43 aggregation. Here, we investigated if the anti-aggregation effect of EGCG is mediated via EGCG's binding to TDP-43. In silico molecular docking and molecular dynamics (MD) simulation suggest a strong binding of EGCG with TDP-43's aggregation-prone C-terminal domain (CTD). Three replicas, each having 800 ns MD simulation of the EGCG-TDP-43-CTD complex, yielded a high negative binding free energy (ΔG) inferring a stable complex formation. Simulation snapshots show that EGCG forms close and long-lasting contacts with TDP-43's Phe-313 and Ala-341 residues, which were previously identified for monomer recruitment in CTD's aggregation. Notably, stable physical interactions between TDP-43 and EGCG were also detected in vitro using TTC staining and isothermal titration calorimetry which revealed a high-affinity binding site of EGCG on TDP-43 (Kd, 7.8 μM; ΔG, -6.9 kcal/mol). Additionally, TDP-43 co-incubated with EGCG was non-cytotoxic when added to HEK293 cells. In summary, EGCG's binding to TDP-43 and blocking of residues important for aggregation can be a possible mechanism of its anti-aggregation effects on TDP-43.

Food Antioxidants and Their Interaction with Human Proteins

Common to all biological systems and living organisms are molecular interactions, which may lead to specific physiological events. Most often, a cascade of events occurs, establishing an equilibrium between possibly competing and/or synergistic processes. Biochemical pathways that sustain life depend on multiple intrinsic and extrinsic factors contributing to aging and/or diseases. This article deals with food antioxidants and human proteins from the circulation, their interaction, their effect on the structure, properties, and function of antioxidant-bound proteins, and the possible impact of complex formation on antioxidants. An overview of studies examining interactions between individual antioxidant compounds and major blood proteins is presented with findings. Investigating antioxidant/protein interactions at the level of the human organism and determining antioxidant distribution between proteins and involvement in the particular physiological role is a very complex and challenging task. However, by knowing the role of a particular protein in certain pathology or aging, and the effect exerted by a particular antioxidant bound to it, it is possible to recommend specific food intake or resistance to it to improve the condition or slow down the process.

The measurement of binding affinities by NMR chemical shift perturbation

We have carried out chemical shift perturbation titrations on three contrasting proteins. The resulting chemical shifts have been analysed to determine the best way to fit the data, and it is concluded that a simultaneous fitting of all raw shift data to a single dissociation constant is both the most accurate and the most precise method. It is shown that the optimal weighting of ¹⁵N chemical shifts to ¹H chemical shifts is protein dependent, but is around the consensus value of 0.14. We show that chemical shift changes of individual residues can be fit to give residue-specific affinities. Residues with affinities significantly stronger than average are found in close contact with the ligand and are suggested to form a rigid contact surface, but only when the binding involves little conformational change. This observation may be of value in analysing binding and conformational change.

A discussion on the affecting factors of the fitting procedures' reliability in isothermal titration calorimetry analysis

The standard isothermal titration calorimetry (ITC) curve, characterized as a typical sigmoid is strictly confined by the so-called c value, which is a ratio of titrand concentration to K_D. The proper c value with a range from 5 to 500 is commonly recommended as a standard protocol in routine detection process for acquiring the reliable fitting results in 1:1 binding mode. However, if the c value is less than "1" due to the weak binding or low concentration of analyte, fitting precision gets unstable and susceptible to the data noise. Herein, we first got a deep discussion into the reliability of the fitting procedure for 1:1 binding mode by data simulation, then quantized the effect of several affecting factors on the precision of parameters estimation through mathematical analysis. Finally, we proposed the value of 2~4 times K_D for final ligand concentration is optimal for the ITC titration in low c system (c < 1). All the theoretical derivations were further verified by a practical experiment of Magnesium-EDTA binding test.

Ligand binding constants for human serum albumin evaluated by ratiometric analysis of DSC thermograms

This paper describes an expanded application of our recently reported method (Eskew et al., Analytical Biochemistry 621,1 2021) utilizing thermogram signals for thermal denaturation measured by differential scanning calorimetry. Characteristic signals were used to quantitatively evaluate ligand binding constants for human serum albumin. In our approach the ensemble of temperature dependent calorimetric responses for various protein-ligand mixtures and native HSA were compared, in a ratiometric manner, to extract binding constants and stoichiometries. Protein/ligand mixtures were prepared at various ligand concentrations and subjected to thermal denaturation analysis by calorimetry. Measurements provided the melting temperature, T_m, and free-energy ΔG_cal(37°C) for melting ligand-bound Albumin as a function of ligand concentration. Concentration dependent behaviors of these parameters derived from protein/ligand mixtures were used to construct dose-response curves. Fitting of dose-response curves yielded quantitative evaluation of the ligand binding constant and semi-quantitative estimates of the binding stoichiometry. Many of the ligands had known binding affinity for Albumin with binding constants reported in the literature. Evaluated binding parameters for the ligands impressively agreed with reported literature values determined using other standard experimental methods. Results are reported for 29 drug ligands binding to Albumin. These validate our calorimetry-based process for applications in pre-clinical drug screening.

How epigallocatechin gallate binds and assembles oligomeric forms of human alpha-synuclein

The intrinsically disordered human protein α-synuclein (αSN) can self-associate into oligomers and amyloid fibrils. Several lines of evidence suggest that oligomeric αSN is cytotoxic, making it important to devise strategies to either prevent oligomer formation and/or inhibit the ensuing toxicity. (−)-epigallocatechin gallate (EGCG) has emerged as a molecular modulator of αSN self-assembly, as it reduces the flexibility of the C-terminal region of αSN in the oligomer and inhibits the oligomer's ability to perturb phospholipid membranes and induce cell death. However, a detailed structural and kinetic characterization of this interaction is still lacking. Here, we use liquid-state NMR spectroscopy to investigate how EGCG interacts with monomeric and oligomeric forms of αSN. We find that EGCG can bind to all parts of monomeric αSN but exhibits highest affinity for the N-terminal region. Monomeric αSN binds ∼54 molecules of EGCG in total during oligomerization. Furthermore, kinetic data suggest that EGCG dimerization is coupled with the αSN association reaction. In contrast, preformed oligomers only bind ∼7 EGCG molecules per protomer, in agreement with the more compact nature of the oligomer compared with the natively unfolded monomer. In previously conducted cell assays, as little as 0.36 EGCG per αSN reduce oligomer toxicity by 50%. Our study thus demonstrates that αSN cytotoxicity can be inhibited by small molecules at concentrations at least an order of magnitude below full binding capacity. We speculate this is due to cooperative binding of protein-stabilized EGCG dimers, which in turn implies synergy between protein association and EGCG dimerization.

Effects of zinc binding on the binding of epigallocatechin gallate (green tea) to bovine serum albumin and myoglobin

Serum albumin as a zinc carrier binds 80% plasma zinc to facilitate zinc absorption. Epigallocatechin gallate (EGCG, green tea) is reported to bind to serum albumin to perform its biological functions in vivo. However, the available information on how zinc binding affects the binding of EGCG to proteins or how EGCG binding affects the binding of zinc to proteins are very limited. In the present study, the effects of zinc binding on the binding of EGCG to bovine serum albumin and myoglobin were investigated using isothermal titration calorimetry, fluorescence quenching and molecular docking. The obtained results suggested that binding of zinc to bovine serum albumin and myoglobin could increase the EGCG binding affinity of proteins as indicated by the thermodynamic parameters. In addition, the formation of protein/zinc complex shifted the EGCG binding site of proteins. For myoglobin, the electron transfer from EGCG to myoglobin was facilitated by zinc binding induced stronger EGCG binding to myoglobin. Such study provides very fundamental and useful knowledge for zinc and EGCG nutrition.

EGCG impedes human Tau aggregation and interacts with Tau

Tau aggregation and accumulation is a key event in the pathogenesis of Alzheimer’s disease. Inhibition of Tau aggregation is therefore a potential therapeutic strategy to ameliorate the disease. Phytochemicals are being highlighted as potential aggregation inhibitors. Epigallocatechin-3-gallate (EGCG) is an active phytochemical of green tea that has shown its potency against various diseases including aggregation inhibition of repeat Tau. The potency of EGCG in altering the PHF assembly of full-length human Tau has not been fully explored. By various biophysical and biochemical analyses like ThS fluorescence assay, MALDI-TOF analysis and Isothermal Titration Calorimetry, we demonstrate dual effect of EGCG on aggregation inhibition and disassembly of full-length Tau and their binding affinity. The IC50 for Tau aggregation by EGCG was found to be 64.2 μM.

Efficient synergistic combination effect of Quercetin with Curcumin on breast cancer cell apoptosis through their loading into Apo ferritin cavity

Combination of natural agents has received a great attention in cancer treatment because of synergistically increased apoptotic effect on cancer cell lines by triggering several apoptotic signaling pathways. However, the hydrophobic nature, poor bioavailability and low cellular uptake of most natural agents limit their therapeutic effectiveness. The purpose of this study was to design Apoferritin nanoparticles loaded with Quercetin and Curcumin (Que-Cur-HoS-Apo NPs) and to test their synergistic antitumor properties on a breast cancer cell line (MCF7). The physico-chemical characterization of the Que-Cur-HoS-Apo NPs by Size Exclusion Chromatography (FPLC) and Dynamic Light Scattering (DLS) confirmed the encapsulation of the compounds in the protein cage with narrow size distribution in the range 17.4 ± 1.2 nm. Cell viability study indicated that Que-Cur-HoS-Apo NPs were able to exert a more pronounced effect at lower dose on the MCF7 cell line when compared to the free combination of the drugs. The Que-Cur-HoS-Apo system allowed cellular uptake of natural agents thus triggering enhanced apoptosis. These effects were confirmed by Annexin-V/7-AAD Staining Assay and intracellular Reactive Oxygen Species (ROS) quantitative detection. These results suggest the potential of Que-Cur-HoS-Apo NPs as a promising anti-cancer agent in breast cancer therapy and pave the way to examine Que-Cur-HoS-Apo NPs effect in vivo.

Oxidization increases the binding of EGCG to serum albumin revealed by kinetic data from label-free optical biosensor with reference channel

Schematic illustration of the biosensor to measure the EGCG–BSA interaction and the concentration dependent bound amount of EGCG and oxidized EGCG.

Conformation and Aggregation of Human Serum Albumin in the Presence of Green Tea Polyphenol (EGCg) and/or Palmitic Acid

Polyphenols such as epigallocatechin gallate (EGCg) may have roles in preventing some chronic diseases when they are ingested as components of plant-based foods and beverages. Human serum albumin (HSA) is a multi-domain protein that binds various ligands and aids in their transport, distribution, and metabolism in the circulatory system. In the present study, the HSA-EGCg interaction in the absence or presence of fatty acid has been investigated. Förster resonance energy transfer (FRET) was used to determine inter- and intra-domain distances in the protein with and without EGCg and palmitic acid (PA). By labeling Cys-34 with 7-(diethyl amino)-4-methylcoumarin 3-maleimide (CPM), the distance between Trp-214 at domain IIA and CPM-Cys-34 at domain IA could be established. A small amount of PA decreased the distance, while a large amount increased the distance up to 5.4 Å. EGCg increased the inter-domain distance in HSA and HSA-PA up to 2.8 and 7.6 Å, respectively. We concluded that PA affects protein conformation more significantly compared to EGCg. Circular dichroism (CD) established that EGCg affects protein secondary structure more significantly than PA. PA had little effect on the α-helix content of HSA, while EGCg decreased the α-helix content in a dose-dependent fashion. Moreover, EGCg decreased α-helix content in HSA and HSA-PA to the same level. Dynamic light scattering (DLS) data revealed that both PA and EGCg increased HSA aggregation. EGCg increased HSA aggregation more significantly and promoted formation of aggregates that were more heterogenous. Any of these effects could impact the ability of serum albumin to transport and stabilize ligands including EGCg and other polyphenols.

Epigallocatechin Gallate Modulates Muscle Homeostasis in Type 2 Diabetes and Obesity by Targeting Energetic and Redox Pathways: A Narrative Review

Obesity is associated with the hypertrophy and hyperplasia of adipose tissue, affecting the healthy secretion profile of pro- and anti-inflammatory adipokines. Increased influx of fatty acids and inflammatory adipokines from adipose tissue can induce muscle oxidative stress and inflammation and negatively regulate myocyte metabolism. Muscle has emerged as an important mediator of homeostatic control through the consumption of energy substrates, as well as governing systemic signaling networks. In muscle, obesity is related to decreased glucose uptake, deregulation of lipid metabolism, and mitochondrial dysfunction. This review focuses on the effect of epigallocatechin-gallate (EGCG) on oxidative stress and inflammation, linked to the metabolic dysfunction of skeletal muscle in obesity and their underlying mechanisms. EGCG works by increasing the expression of antioxidant enzymes, by reversing the increase of reactive oxygen species (ROS) production in skeletal muscle and regulating mitochondria-involved autophagy. Moreover, EGCG increases muscle lipid oxidation and stimulates glucose uptake in insulin-resistant skeletal muscle. EGCG acts by modulating cell signaling including the NF-κB, AMP-activated protein kinase (AMPK), and mitogen-activated protein kinase (MAPK) signaling pathways, and through epigenetic mechanisms such as DNA methylation and histone acetylation.

Epigallocatechin-3-gallate remodels apolipoprotein A-I amyloid fibrils into soluble oligomers in the presence of heparin

Amyloid deposits of WT apolipoprotein A-I (apoA-I), the main protein component of high-density lipoprotein, accumulate in atherosclerotic plaques where they may contribute to coronary artery disease by increasing plaque burden and instability. Using CD analysis, solid-state NMR spectroscopy, and transmission EM, we report here a surprising cooperative effect of heparin and the green tea polyphenol (-)-epigallocatechin-3-gallate (EGCG), a known inhibitor and modulator of amyloid formation, on apoA-I fibrils. We found that heparin, a proxy for glycosaminoglycan (GAG) polysaccharides that co-localize ubiquitously with amyloid in vivo, accelerates the rate of apoA-I formation from monomeric protein and associates with insoluble fibrils. Mature, insoluble apoA-I fibrils bound EGCG (K_D = 30 ± 3 μm; B_max = 40 ± 3 μm), but EGCG did not alter the kinetics of apoA-I amyloid assembly from monomer in the presence or absence of heparin. EGCG selectively increased the mobility of specific backbone and side-chain sites of apoA-I fibrils formed in the absence of heparin, but the fibrils largely retained their original morphology and remained insoluble. By contrast, fibrils formed in the presence of heparin were mobilized extensively by the addition of equimolar EGCG, and the fibrils were remodeled into soluble 20-nm-diameter oligomers with a largely α-helical structure that were nontoxic to human umbilical artery endothelial cells. These results argue for a protective effect of EGCG on apoA-I amyloid associated with atherosclerosis and suggest that EGCG-induced remodeling of amyloid may be tightly regulated by GAGs and other amyloid co-factors in vivo, depending on EGCG bioavailability.