Molecular mechanisms of polystyrene nanoplastics and alpha-amylase interactions and their binding model: A multidimensional analysis

Plastic fragments are widely distributed in different environmental media and has recently drawn special attention due to its difficulty in degradation and serious health and environmental problems. Among, nanoplastics (NPs) are smaller in size, larger in surface/volume ratio, and more likely to easily adsorb ambient pollutants than macro plastic particles. Moreover, NPs can be easily absorbed by wide variety of organisms and accumulate in multiple tissues/organs and cells, thus posing a more serious threat to living organisms. Alpha-amylase (α-amylase) is a hydrolase, which can be derived from various sources such as animals, plants, and microorganisms. Currently, no studies have concentrated on the binding of NPs with α-amylase and their interaction mechanisms by employing a multidimensional strategy. Hence, we explored the interaction mechanisms of polystyrene nanoplastics (PS-NPs) with α-amylase by means of multispectral analysis, in vitro enzymatic activity analysis, and molecular simulation techniques under in vitro conditions. The findings showed that PS-NPs had the capability to bind with the intrinsic fluorescence chromophores, leading to fluorescence changes of these specific amino acids. This interaction also caused the alterations in the micro-environment of the fluorophore residues mainly tryptophan (TRP) and tyrosine (TYR) residues of α-amylase. PS-NPs interaction promoted the unfolding and partial expansion of polypeptide chains and the loosening of protein skeletons, and destroyed the secondary structure (increased random coil contents and decreased α-helical contents) of this protein, forming a larger particle size of the PS-NPs-α-amylase complex. Moreover, the enzymatic activity of α-amylase in vitro was found to be inhibited in a concentration dependent manner, thereby impairing its physiological functions. Further molecular simulation found that PS-NPs had a higher tendency to bind to the active site of α-amylase, which is the cause for its structural and functional changes. Additionally, the hydrophobic force played a major role in mediating the binding interactions between PS-NPs and α-amylase. Taken together, our study indicated that PS-NPs interaction can initiate the abnormal physiological functions of α-amylase through PS-NPs-induced structural and conformational alternations.

Regulation mechanisms of ferric ions release from iron-loaded transferrin protein caused by nano-sized polystyrene plastics-induced conformational and structural changes

Currently, the binding of iron-binding protein transferrin (TF) with NPs and their interaction mechanisms have not been completely elucidated yet. Here, we probed the conformation-dependent release of Fe ions from TF induced by nano-sized polystyrene plastics (PS-NPs) using dialysis, ICP-MS, multi-spectroscopic techniques, and computational simulation. The results showed that the release of free Fe ions from TF was activated after PS-NPs binding, which displayed a clear dose-effect correlation. PS-NPs binding can induce the unfolding and loosening of polypeptide chain and backbone of TF. Alongside this we found that the TF secondary structure was destroyed, thereby causing TF protein misfolding and denaturation. In parallel, PS-NPs interacted with the chromophores, resulting in the occurrence of fluorescence sensitization effects and the disruption of the surrounding micro-environment of aromatic amino acids. Also, the binding of PS-NPs induced the formation of new aggregates in the PS-NPs-TF system. Further simulations indicated that PS-NPs exhibited a preference for binding to the hinge region that connects the C-lobe and N-lobe, which is responsible for the Fe ions release and structural alterations of TF. This finding provides a new understanding about the regulation of the release of Fe ions of iron-loaded TF through NPs-induced conformational and structural changes.

A Study on the Interactions of Proteinase K with Myricetin and Myricitrin by Multi-Spectroscopy and Molecular Modeling

Myricetin (MYR) and myricitrin (MYT) are well recognized for their nutraceutical value, such as antioxidant, hypoglycemic, and hypotensive effects. In this work, fluorescence spectroscopy and molecular modeling were adopted to investigate the conformational and stability changes of proteinase K (PK) in the presence of MYR and MYT. The experimental results showed that both MYR and MYT could quench fluorescence emission via a static quenching mechanism. Further investigation demonstrated that both hydrogen bonding and van der Waals forces play significant roles in the binding of complexes, which is consistent with the conclusions of molecular modeling. Synchronous fluorescence spectroscopy, Förster resonance energy transfer, and site-tagged competition experiments were performed to prove that the binding of MYR or MYT to PK could alter its micro-environment and conformation. Molecular docking results revealed that either MYR or MYT spontaneously interacted with PK at a single binding site via hydrogen bonding and hydrophobic interactions, which is consistent with the results of spectroscopic measurements. A 30 ns molecular dynamics simulation was conducted for both PK-MYR and PK-MYT complexes. The calculation results showed that no large structural distortions or interaction changes occurred during the entire simulation time span. The average RMSD changes of PK in PK-MYR and PK-MYT were 2.06 and 2.15 Å, respectively, indicating excellent stability of both complexes. The molecular simulation results suggested that both MYR and MYT could interact with PK spontaneously, which is in agreement with spectroscopic results. This agreement between experimental and theoretical results indicates that the method herein could be feasible and worthwhile for protein–ligand complex studies.

Spectroscopic investigations on the interaction between nano plastic and catalase on molecular level

As an emerging pollutant that is easily bonded with some functional proteins and the effects of their physiological expressions, nano plastics (NPs) have been widely detected in various environmental mediums, even in human blood. Compared to microplastics, less information on the interactions between NPs and proteins has been reported. Here, the interaction mechanism between common polystyrene nano plastics (PSNPs) and catalase (CAT) under two typical physiological conditions, pH 7.4 and 4.0, was investigated by UV-visible spectroscopy, circular dichroism (CD), and dynamic light scattering (DLS). Compared with the enhanced catalytic effects when increasing PSNPs at pH 7.4, a trend of initial inhibition and enhanced activity was observed at pH 4.0. Spectroscopic analysis and calculation results indicated that their binding was static, with only one binding site and stronger interactions under acidic conditions. UV-visible and CD spectra analysis demonstrated that the difference in enzymatic activity could be mainly attributed to the conformational alternation of CAT in the presence of PSNPs, which is obviously affected by solution chemistry. The change was also revealed by the hydrodynamic diameter and zeta potentials of the complexes supplied by DLS analysis. This study will help understand the health risks of nano plastic pollution and provide a theoretical basis for studying their toxicological effects.

Potential toxicity of bisphenol A to α-chymotrypsin and the corresponding mechanisms of their binding

Bisphenol A (BPA) is an endocrine disruptor widely existing in plastics and resins, which can accumulate in animals and human bodies, posing a potential threat to the physiological and biochemical reactions of human beings or other organisms. α-Chymotrypsin is a kind of proteolytic enzyme existing in humans and animals, which can cause diseases when its activity is excessive. However, there is a lack of research on the mechanism of endocrine disruptors affecting α-chymotrypsin activity. In this study, the interaction between BPA and α-chymotrypsin was proved via multiple spectroscopic approaches, enzyme activity change, isothermal titration calorimetry and molecular docking. Results showed that α-chymotrypsin's polypeptide chains were unfolded, and protein skeletons were loosened with the exposure to BPA. α-Helix content increased and β-sheet content was decreased. The particle size of the BPA-α-chymotrypsin complex became smaller. Fluorescence sensitization may also be explained by a perturbation of the chromophore Trp 141. The thermodynamic parameters of the binding reaction were measured by isothermal titration calorimetry (ITC), which showed that there was hydrophobic interaction between BPA and α-chymotrypsin, which was consistent with the results of molecular docking. Moreover, BPA may stop near the active center of α-chymotrypsin and interact with the key residues His 57 and Ser 195. The above phenomenon explained the result that the activity of α-chymotrypsin increased to 139% when exposed to high dose BPA (40 μM). Taken together, the effects of BPA on the structure and function of α-chymotrypsin were clarified at the molecular level, which made up the gap in the mechanism of BPA on the proteolytic enzyme, and provided a reliable basis for disease avoidance and prevention.

The toxic effects of alizarin red S on catalase at the molecular level

Alizarin red S (ARS) is a widespread mordant dye derived from alizarin. However, it was reported to be mutagenic and carcinogenic probably because it could induce oxidative damages in organisms. Catalase (CAT) is an important antioxidant enzyme defensing oxidative damages induced by xenobiotics. The underlying mechanisms of ARS interacting with CAT have not been clarified yet. This study is conducted to characterize the functional and conformational changes on CAT by ARS and the binding details to further investigate their interaction mechanisms. Under exposure of ARS at 5 μM, CAT activity was significantly decreased to 76.2%. Inhibition of CAT probably resulted in promotion of intracellular oxidative stress and pro-oxidant property of ARS. The interaction between ARS and CAT was proved to be spontaneous and exothermic. However, limited structural changes were observed according to spectroscopic results. Results showed that ARS prefers to bind with residues buried in the active site and could alter the activity of CAT, which were agree with the molecular docking results. This work proves the adverse effects of ARS on CAT mainly at molecular level and further highlights its potential risks to heath.

Alizarin red S is confirmed to be toxic to catalase at molecular level attributing to the structural and functional changes of catalase.

Catalytic activity, structure and stability of proteinase K in the presence of biosynthesized CuO nanoparticles

Here, CuO nanoparticles were synthesized using Sambucus nigra (elderberry) fruit extract. Further, the binding of proteinase K, as a model enzyme with green synthesized nanoparticles was investigated. The results demonstrated that the structural changes in enzyme were induced by the binding of nanoparticles. These changes were accompanied by the decrease in the Michaelis-Menten constant at 298 K. This means that the enzyme affinity for the substrate was increased. Thermodynamic parameters of protein stability and protein-ligand binding were estimated from the spectroscopic measurements at 298-333 K. Depending on the temperature, CuO nanoparticles showed a dual effect on the thermodynamic stability and binding affinity of enzyme. Nanoparticles increase the stability of the native state of enzyme at room temperature. On the other hand, nanoparticles stabilize the unfolded state of enzyme at 310-333 K. An overall favorable Gibbs energy change was observed for the binding process at 298-333 K. The enzyme-nanoparticle binding is enthalpically driven at room temperature. It was concluded that hydrogen bonding plays a key role in the interaction of enzyme with nanoparticles at 298-310 K. At higher temperatures, the protein-ligand binding is entropically driven. This means that hydrophobic association plays a major role in the proteinase K-CuO binding at 310-333 K.

Characterizing the binding interactions of PFOA and PFOS with catalase at the molecular level

Perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS) have effects on human health by inducing oxidative stress and catalase (CAT) is a vital enzyme involved in protection against oxidative damage. The interactions of PFOA and PFOS with CAT were investigated by using biophysical methods including spectroscopic techniques, molecular docking and enzyme activity measurements. UV-visible, circular dichroism (CD) and resonance light scattering (RLS) spectroscopy results showed that the structure and conformation of CAT were changed by PFOA and PFOS. PFOA could loosen and unfold the skeleton of CAT but PFOS affected the microenvironment around the aromatic amino acid residues and heme groups. Both PFOA and PFOS altered the secondary structure of CAT by decreasing α-helix and increasing β-sheet content. The size of CAT was smaller and CAT became dispersed when it was bound by perfluorinated compounds (PFCs). Furthermore, enzyme activity test showed that PFOS decreased the activity of CAT because the binding site of PFOS was close to the active center of CAT, but PFOA had little effect on the activity because PFOA bound at the surface of the enzyme. These results indicated that PFCs could damage the structures and conformations of CAT but the changes were not always related to the activity and function of CAT.

Spectroscopic investigations on the conformational changes of lysozyme effected by different sizes of N-acetyl-l-cysteine-capped CdTe quantum dots

The effect of N-acetyl-l-cysteine-capped CdTe quantum dots (NAC-CdTe QDs) with different sizes on lysozyme was investigated by isothermal titration calorimetry (ITC), enzyme activity assays, and multi-spectroscopic methods. ITC results proved that NAC-CdTe QDs can spontaneously bind with lysozyme and hydrophobic force plays a major role in stabilizing QDs-lysozyme complex. Multi-spectroscopic measurements revealed that NAC-CdTe QDs caused strong quenching of the lysozyme's fluorescence in a size-dependent quenching manner. Moreover, the changes of secondary structure and microenvironment in lysozyme caused by the NAC-CdTe QDs were higher with a bigger size. The results of enzyme activity assays showed that the interaction between lysozyme and NAC-CdTe QDs inhibited the activity of lysozyme and the inhibiting effect was in a size-dependent manner. Based on these results, we conclude that NAC-CdTe QDs with larger particle size had a larger impact on the structure and function of lysozyme.

Spectroscopic investigation of the effects of aqueous-phase prepared CdTe quantum dots on protein hemoglobin at the molecular level

3-Mercaptopropionic Acid-modified CdTe quantum dots (QDs) were synthesized and characterized by infrared, fluorescence, and ultraviolet-visible absorption spectra and Nano-ZetaSizer measurements. Then the interaction between QDs and hemoglobin was studied to investigate the effects of QDs on the structure and function of hemoglobin by using a variety of spectroscopy methods and isothermal titration calorimetry. The results showed van der Waals forces and hydrogen bonding predominantly played major roles in the binding. The intrinsic fluorescence of hemoglobin was quenched with changes to the microenvironment of tyrosine and tryptophan residues and complex conformational changes of hemoglobin were induced with the loosening and unfolding skeleton. However, the heme in hemoglobin was still stable, indicating that the main physiological function of hemoglobin might not be significantly inhibited. This study will provide a new strategy to study the biological toxicity of QDs at the molecular level.