Probing the toxic mechanism of Ag⁺ with lysozyme

Nanoplastics composite norfloxacin induced changes in conformation and function of lysozyme and differential effects of co-exposure contamination

The concurrent environmental contamination by nanoplastics (NPs) and norfloxacin (NOR) is a burgeoning concern, with significant accumulations in various ecosystems and potential ingress into the human body via the food chain, posing threats to both public health and ecological balance. Despite the gravity of the situation, studies on the co-exposure contamination effects of these substances are limited. Moreover, the response mechanisms of key functional proteins to these pollutants are yet to be fully elucidated. In this work, we conducted a comprehensive assessment of the interaction mechanisms of NPs and NOR with lysozyme under both single and co-exposure condition, utilizing dynamic light scattering, ζ-potential measurements, multi-spectroscopy methods, enzyme activity assays and molecular docking, to obtain a relationship between the compound effects of NPs and NOR. Our results indicate that NPs adsorb NOR on their surface, forming more stable aggregates. These aggregates influence the conformation, secondary structure (α-Helix ratio decreased by 3.1 %) and amino acid residue microenvironment of lysozyme. And changes in structure affect the activity of lysozyme (reduced by 39.9 %) with the influence of composited pollutants exerting stronger changes. Molecular simulation indicated the key residues Asp 52 for protein function located near the docking site, suggesting pollutants preferentially binds to the active center of lysozyme. Through this study, we have found the effect of increased toxicity on lysozyme under the compounded conditions of NPs and NOR, confirming that the increased molecular toxicity of NPs and NOR is predominantly realized through the increase in particle size and stability of the aggregates under weak interactions, as well as induction of protein structural looseness. This study proposes a molecular perspective on the differential effects and mechanisms of NPs-NOR composite pollution, providing new insights into the assessment of in vitro responses to composite pollutant exposure.

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.

Cytotoxicity and Oxidative Stress Effects of Indene on Coelomocytes of Earthworm (Eisenia foetida): Combined Analysis at Cellular and Molecular Levels

Indene (IND) is a kind of important aromatic hydrocarbon that is extracted from coal tar and has important applications in industry and biology. In the process of production and utilization, it is easy to enter the soil and produce toxic effects on the soil or organisms. The earthworm is an important organism in the soil. The toxicity of indene on earthworm coelomocytes is rarely studied, and the oxidative stress effects of IND on earthworm coelomocytes remain unclear. In this study, coelomocytes from earthworms and antioxidant enzymes were selected as the research targets. In addition, IND caused oxidative stress, and its related toxic effects and mechanisms were systematically studied and evaluated at the cellular and molecular levels. The results showed that IND destroyed the redox balance in earthworm coelomocytes, and the large accumulation of reactive oxygen species (ROS) significantly inhibited the activities of the antioxidant system, including superoxide dismutase (SOD), catalase (CAT), and glutathione (GSH), and caused lipid peroxidation and membrane permeability changes, resulting in a decrease in cell viability to 74.5% of the control group. At the molecular level, IND was bound to SOD by the arene-H bond, and the binding constant was 4.95 × 10³. IND changed the secondary structure of the SOD and led to a loosening of the structure of the SOD peptide chain. Meanwhile, IND caused SOD fluorescence sensitization, and molecular simulation showed that IND was mainly bound to the junction of SOD subunits. We hypothesized that the changes in SOD structure led to the increase in SOD activity. This research can provide a scientific basis for IND toxicity evaluation.

Silver nanoparticle-protein interactions and the role of lysozyme as an antagonistic antibacterial agent

The photoreductive synthesis and antibacterial activity of silver nanoparticles (AgNP) prepared in the presence of bovine serum albumin (BSA) and lysozyme (LZ) were evaluated. AgNP@BSA showed similar antibacterial activity to those stabilized with citrate (AgNP@CIT) and to an AgNO₃ solution, suggesting the releases of Ag⁺ as the mechanism of death. In contrast, AgNP@LZ solutions showed no activity, although LZ behaves as a moderately antibacterial peptide. Furthermore, the addition of LZ to the AgNP@CIT or AgNP@BSA solutions induced their agglomeration and suppressed their original antibacterial efficacy. This antagonistic antibacterial effect exerted by LZ on AgNPs is associated with electrostatic interactions exerted by LZ. Specific metal-LZ interactions produce a harder protein corona on AgNP@LZ that retains Ag⁺, while LZ acts as a glue for AgNP@CIT or AgNP@LZ due to its opposite electrical charge, besides strong binding to Ag⁺avoiding the bactericide effect. Therefore, bactericidal effects of AgNP in biological media may be modulated by specific protein interactions.

Responses of catalase and superoxide dismutase to low-dose quantum dots on molecular and cellular levels

With the wider application of cadmium-containing quantum dots (Cd-QDs) in biomedical fields, it is easier for people to be exposed. Studies have suggested that Cd-QDs could release cadmium ion and induce oxidative effects due to the disruption of redox equilibrium. Antioxidant enzymes catalase (CAT) and superoxide dismutase (SOD), play an important role in organisms to resist the negative impact of exogenous substances. Molecular mechanisms of antioxidant enzymes with Cd-QDs remain unclear, however. In this study, structural and functional changes of CAT and SOD have been investigated under low dose Cd-QDs exposure. Cell viability, malondialdehyde (MDA) level, CAT and SOD activities were influenced by Cd-QDs in hepatocytes of mice. To further investigate the responses of CAT and SOD to Cd-QDs, multiple spectroscopic, calorimetric and activity measurements were carried out. Similar interaction patterns were observed that result in interaction force, structural and functional changes: Cd-QDs combine with CAT and SOD through hydrophobic forces; Intrinsic fluorescence of proteins was statically quenched by Cd-QDs and new complexes were formed; Also, the skeleton and secondary structure (with α-helix decrease) of CAT and SOD was influenced. Taken together, we suggest that Cd-QDs chosen in this study induce oxidative stress effects to hepatocytes but have not caused serious oxidative stress damage at concentrations below 10 μg/mL. MPA-CdSe/ZnS QDs caused the lowest level of oxidative stress which is associated with the induction of antioxidant proteins. This paper presents responses of CAT and SOD to low-dose Cd-QDs, and provides a reference for evaluating health damages caused by Cd-QDs.

The antimicrobial spectrum of lysozyme broadened by reductive modification

In order to broaden the antimicrobial action spectrum of lysozyme against Gram-negative bacteria, different partially reduced lysozyme derivatives were obtained by reducing the R-S-S-R bonds of lysozyme using the reducing agent Na2SO3. The circular dichroism behavior analysis showed that the tertiary structure of the partially reduced lysozyme molecules became more flexible, leading to an obvious increase in the surface hydrophobicity and the tryptophan fluorescence intensity. Zeta potential analysis indicated that the introduction of SO32- led to a decrease in the surface charges of the reduced lysozyme molecules. Compared with the native lysozyme, the reduced lysozymes not only showed a 1 log increase in the antimicrobial activity against Escherichia coli ATCC 25922 and Salmonella enteritidis ATCC 13076 (P <0.05), but also maintained nearly the same antimicrobial activity against Staphylococcus aureus ATCC 29213 (P >0.05). Overall, the reductive modification with the food-friendly, compatible, and safe reducing agent Na2SO3 has broadened the antimicrobial action spectrum of the modified lysozymes against Gram-negative bacteria, with the enhancement extent depending on the reduction degree and the type of bacterial strains. The integrated results suggest that the Na2SO3-reduced lysozyme can be used as a novel safe potential bactericidal additive for food-processing industry.

Study on the interactions between toxic nitroanilines and lysozyme by spectroscopic approaches and molecular modeling

Being exogenous environmental pollutants, nitroanilines (NAs) are highly toxic and have mutagenic and carcinogenic activity. Being lack of studies on interactions between NAs and lysozyme at molecular level, the binding interactions of lysozyme with o-nitroaniline (oNA), m-nitroaniline (mNA) and p-nitroaniline (pNA) were investigated by means of steady-state fluorescence, synchronous fluorescence, UV-vis absorption spectroscopy, as well as molecular modeling. The experimental results revealed that the fluorescence of lysozyme is quenched by oNA and mNA through a static quenching, while the fluorescence quenching triggered by pNA is a combined dynamic and static quenching. The number of binding sites (n) and the binding constant (K_b) corresponding thermodynamic parameters ΔH^⊖, ΔS^⊖, ΔG^⊖ at different temperatures were calculated. The reactions between NAs and lysozyme were spontaneous and entropy driven and the binding of NAs to lysozyme induced conformation changes of lysozyme. The difference of the position of -NO₂ group affected the binding and the binding constants K_b decreased in the following pattern: K_b (pNA) >K_b (mNA) >K_b (oNA). Molecular docking studies were performed to reveal the most favorable binding sites of NAs on lysozyme. Our recently results could offer mechanistic insights into the nature of the binding interactions between NAs and lysozyme and provide information about the toxicity risk of NAs to human health.

Probing the toxic mechanism of bisphenol A with acid phosphatase at the molecular level

As an endocrine-disrupting chemical, bisphenol A (BPA), can affect normal endocrine function of hormone. This paper studied the toxic effect of BPA on acid phosphatase at the molecular level by multi-spectroscopic measurements, molecular docking, and enzyme activity experiment. BPA could enhance the fluorescence intensity, change the structure, and cause an increased hydrophobicity of acid phosphatase. Hydrogen bond interaction and van der Waals forces were the main forces to generate the BPA-acid phosphatase complex on account of the negative ΔH (- 36.92 kJ mol^-1) and ΔS (- 50.78 J mol^-1 K^-1). BPA led to the loosening and unfolding of protein structure and extending the peptide strands, as revealed by UV-vis absorption and CD spectra. Based on the enzyme activity experiment, BPA could decrease the activity of the acid phosphatase by entering the active site of the enzyme. The molecular docking model showed that BPA could bind into the cavity of acid phosphatase and interact with Tyr A252 and a hydrogen bond (1.47 Å) was formed in the binding process. This work suggested the structures and functions of acid phosphatase were both affected by BPA.

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.

The study on interactions between levofloxacin and model proteins by using multi-spectroscopic and molecular docking methods

The interactions of levofloxacin (LEV) with lysozyme (LYZ), trypsin and bovine hemoglobin (BHb) were investigated, respectively, by using multi-spectral techniques and molecular docking in vitro. Fluorescence studies showed that LEV quenched LYZ/trypsin fluorescence in a combined quenching ways and BHb fluorescence in a static quenching with binding constants of .14, .51 and .20 × 10⁵ L mol^-1 at 298 K, respectively. The thermodynamic parameters demonstrated that hydrophobic forces, hydrogen bonds, and van der Waals forces played the major role in the binding process. The binding distances between LEV and the inner tryptophan residues of LYZ, trypsin, and BHb were calculated to be 4.04, 3.38, and 4.52 nm, respectively. Furthermore, the results of circular dichroism spectra (CD), UV-vis, and three-dimensional fluorescence spectra indicated that the secondary structures of LYZ, trypsin, and BHb were partially changed by LEV with the α-helix percentage of LYZ-LEV system increased while that of BHb-LEV system was decreased, the β-sheet percentage of trypsin-LEV system increased from 41.3 to 42.9%. UV-vis spectral results showed that the binding interactions could cause conformational and some micro-environmental changes of LYZ, trypsin, and BHb. The results of molecular docking revealed that in LYZ and trypsin systems, LEV bound to the active sites residues GLU 35 and ASP 52 of LYZ and trypsin at the active site SER 195, and in BHb system, LEV was located in the central cavity, which was consistent with the results of synchronous fluorescence experiment. Besides, LEV made the activity of LYZ decrease while the activity of trypsin increased.

Multi-spectral and thermodynamic analysis of the interaction mechanism between Cu2+ and α-amylase and impact on sludge hydrolysis

An increasing amount of heavy metals (e.g., Cu²⁺) is being discharged into sewage treatment plants and is accumulating in sludge, which is toxic to the enzyme in sludge or soil when the sludge is used as fertilizer, resulting in unfavorable effect on the biological treatment of sludge and the circulation and conversion of materials in soil. In this research, effect of Cu²⁺ on sludge hydrolysis by α-amylase is studied from the respect of concentration and components of soluble organic matter in sludge, using three-dimensional fluorescence spectra. Results show that Cu²⁺ exposure not only inhibits the hydrolysis of sludge due to the denaturation of α-amylase but also affects the components of soluble organic matter in sludge. In order to illuminate the interaction mechanism between Cu²⁺ and α-amylase (a model of hydrolase in sludge), multi-spectra and isothermal titration microcalorimetry techniques are applied. Results show that the secondary structure of α-amylase is changed as that the α-helical content increases and the structure loosens. The microenvironment of amino acid residue in α-amylase is changed that the hydrophobicity decreases and the polarity increases with Cu²⁺ exposure. Isothermal titration calorimetry results show that Van der Waals force and hydrogen bond exist in the interaction between Cu²⁺ and α-amylase. Results from this research would favor the development of advanced process for the biological treatment of sludge containing heavy metals.

Interaction rule and mechanism of perfluoroalkyl sulfonates containing different carbon chains with human serum albumin

All three PFASs bind to HSA mainly through electrostatic forces and the toxicity decreases with the shortening of the carbon chain.