Received Date: May 26, 2017 Accepted Date: June 01, 2017 Published Date: June 07, 2017
Citation: Davoudmanesh S, Hadichegeni S, Goliaei B, Taghizadeh M, Tackallou SH, et al. (2017) Spectroscopic and Molecular Docking Approaches for Investigating the Interaction of Fenvalerate with Human Serum Albumin. J Phys Chem Biophys 7: 246. doi: 10.4172/2161-0398.1000246
Copyright: © 2017 Davoudmanesh S, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
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Fenvalerate is an insecticide which is widely utilized in agriculture. In this research, the interaction of fenvalerate with HSA, which is a blood carrier of small molecules such as drugs and toxins, is investigated. Four different methods, UV-Vis, FT-IR spectroscopy, Fluorescence spectroscopy, and molecular modeling were used to characterize the binding properties of fenvalerate with HSA at the molecular level under physiological conditions. The binding constant, which was obtained via UV-Vis spectroscopy, was computed to be KHSA/Fen=3.78 × 10+4 M-1, which indicated a relatively strong binding interactions between ligands and receptors. FT-IR results indicated a decrease in α-helixes from 55% to 50.23% and an increase in β-sheet from 13.96% to 16.82%, β-antiparallel from 6% to 8.93%, were observed on first and thirtieth day and a major decrease of α-helix from 42.99% to 38.82% and an increase in β-sheet from 1.9% to 13.9%, β-antiparallel from 2.21% to 2.53% were observed during ligand binding especially at high concentrations of ligand. The fluorescence intensity of HSA decreased regularly with the gradually increasing concentration of fenvalerate. These results also could be an evidence for binding ligands to the receptors and they were in good agreement with UV-Vis results. On the other hand, a potential binding site in the region III-B of HSA protein was determined via docking calculations. In addition, the obtained results indicate a binding site for interaction of fenvalerate with HSA, which is a chance for excreting this toxin by utilizing HSA protein.
Fenvalerate; Fluorescence spectroscopy; HAS; Molecular modeling
FT-IR: Fourier Transform Infrared; HAS: Human Serum Albumin; UV-Vis: Ultraviolet-Visible
Nowadays, the population growth increase in demands for food products and agricultural crops, which leads to the wide overuse of toxins all around the word. But the point is that the overuses of these toxins bring about many global problems, especially in developing countries . Some of these problems are environmental pollution , outbreak of extended risks regarding human hygiene and health [3,4], some short-term chronic effects like headache and nausea , some severe diseases like cancer (leukemia, breast cancer, ovarian cancer, prostate cancer [5,6], even Central Nervous System (CNS) involving cancers , pregnancy defects  and endocrine system malfunctions . The chronic effects can occur even years after exposure to the minimum residue amount of these toxins in water, food or environment. Children living in regions with common usages of these toxins are more vulnerable to afflicted with cancer comparing with non-exposed children . A new class of agricultural pesticides, called synthetic pyrethroids, are utilized as an alternative for organophosphorus and organochloride toxins as insecticides and among them fenvalerate can be mentioned . This insecticide can be absorbed via skin, digestive and respiratory system (inhaling) and it is mainly excreted from body via kidneys . Fenvalerate (C25H22ClNO3) is a tricyclic compound with one cyanide group. Figure 1 shows the molecular structure of fenvalerate . HSA is the most abundant blood plasma protein which has a great tendency to bind to the endogenic or exogenic compounds and it acts as a carrier for many different chemicals like metabolites, drugs and toxins to the target sites. Therefore, HAS can usually be used as a protein model for biophysical study of ligand binding . This globular protein is a 585 residue polypeptide with 3 equivalent structural domains (I, II, II) . Albumin has the potential to change its conformation at the presence of specific ligands . Numerous studies on interaction of HSA with different macromolecules illuminated the effects of them on HSA structure and function, as the most important blood ligand binder . In this study the interaction of fenvalerate with HSA is investigated in physiologic conditions using experimental and computational techniques at a constant concentration of HSA in the presence of different concentrations of fenvalerate. All assays were performed at two different times, first day and thirtieth days after incubating the HSA and Fenvalerate complex.
Soluble HSA (200 mg/ml) with molecular weight of 66500 Da was purchased from Sigma-Aldrich. To perform experiments on simulated physiological environment, stoke HSA with concentration of (30 mg/ml) or (0.45 mM) equivalent to 3% W/V was diluted with phosphate buffer solution (PBS) 50 mM. Fenvalerate (96%) was diluted with acetonitrile solvent to 9 mM and then, this solution was utilized to prepare eleven concentrations of fenvalerate via dilution, and the concentrations are summarized in Table 1. To keep the HSA-fenvalerate complex solutions stable, EDTA 1 mM and NaN3 0.1 mM solutions were added to them and all of the pH solutions were adjusted to 7.4. The samples of the thirtieth day were incubated at 37°C in darkness. All of the experiments were done for three times.
Table 1: Eleven concentration of fenvalerate was prepared at pH 7.4 for the experiments.
UV-Vis spectroscopy: The UV-Vis studies were done by a dual beam spectrometer (T90+UV / Vis spectrometer PG Instruments Ltd) which had two quartz cuvettes, 1 cm. The main spectrum was within 250-290 nm, but by increasing wavelength up to 400 nm, the studies of another spectrum were provided. For UV-Vis studies, all of the solutions were diluted due to their high concentrations. For this purpose, the ratio of free proteins was decreased to 0.2 mg/ml (3 × 10- 6M), therefore UV-Vis spectrum could be recorded. The dilution was performed by taking 6 μL of the initial stock solution of HSA and 1 mL of PBS (pH 7.4) was added to it. This dilution process was also done for the stoke solutions of HSA-fenvalerate and after this step the prepared solutions were utilized for spectroscopy studies.
Fluorescence spectroscopy: The effect of fenvalerate on HSA structure and the changes caused by it on microenvironment of tryptophan residue (Trp214) amino acid were investigated by measuring the intrinsic fluorescence intensity of this amino acid at the presence of fenvalerate with the fluorescence spectrophotometer Eclipse Cary Model 100. In all assays, 5 μL of the samples were diluted with PBS (pH 7.4) and they were excited at 290 nm, and then the emission spectrums of the excited species were recorded at 300-600 nm. The width of the exciting beam gap was adjusted to 5 nm and the smoothing factor was chosen .
FT-IR spectroscopy: FT-IR spectroscopy is one of the techniques which establishes for determining protein secondary structure at different physiological systems . A Thermo Nicolet FT- IR Spectrometer was used for structural analysis. The spectral range was 400-4000 nm and the numbers of scans was from 100 to 500. The power of resolution of 4 was appropriate for the test. The aim of this research was not the identification of the chemical formula of the studied molecules, but it was focused on the secondary structure of albumin protein and its changes which was resulted from conformation percent of secondary structure. Therefore, the region of two spectral bands of amide I and II were the most important obtained data [17,18]. The difference spectra [(protein solution+ Fenvalerate solutions)-(protein solution)] were generated using the water combination mode around 2300 cm-1, as the standard . When producing different spectra, this band was adjusted to the baseline level, in order to normalize different spectra.
Analysis of protein conformation: The analysis of secondary structure of the protein and the complexes of its drug was performed on the basis of the procedure which was reported previously . The FT-IR spectra were smoothed, and their baselines were corrected automatically by Grams AI software. Thus, the root-mean square (rms) noise of every spectrum was calculated. By means of the second derivative in the spectral region 1700-1600 cm-1, the major peaks in protein and the complexes were resolved. The above spectral region was deconvoluted by curve-fitting method with the Levenberg Marquadt algorithm and the peaks corresponding to a-helix (1660-1650 cm-1), β -sheet (1638-1610 cm-1), turn (1680-1660 cm-1), random coil (1648- 1638 cm-1), and β-antiparallel (1692-1680 cm-1) were adjusted and the area was measured with Gaussian function. The area of all the component bands assigned to a given conformation was then summed up and divided by the total area [21,22]. The curve fitting analysis was done using the GRAMS/AI Version 7.01 software of the Galactic Industries Corporation.
Molecular docking: The crystal structure of HSA was downloaded from PDB database (PDB-ID: 1E78(23). The three-dimensional structure of ligand was obtained from PubChem database and AutoDock Tools software was utilized to convert it to PDBQT format. Docking study was carried out with AutoDock Vina (24). In order to set the charges of HSA residues, Kollman charges were added and Gasteiger partial charges were computed for ligand. Web Lab Viewer software (Accelrys Software Inc) and AutoDock Tools software were used for visualization.
The chromophore group in HSA structure has only one Trp124  and because of this, the light absorption property of HSA is relatively lower than the other proteins. The absorption of HSA was measured at constant concentration of HAS and different the concentrations of the ligands (F1-F11). The binding constants of the fenvalerate-HSA complexes were calculated as reported . It was assumed that the interaction between the ligand L and the substrate S was 1:1; for this reason a single complex SL (1:1) formed. The relationship between the observed absorbance change per centimeter and the system variables and parameters are as follows:
Eq. (1) is the binding isotherm, which shows the hyperbolic dependence of the free ligand concentration. In this equation, ΔA and St are (A0-A) and ((S)+(SL)) respectively, and (S)=St /(1+Ka (L)). The double-reciprocal form of plotting the rectangular hyperbola is based on the linearization of the following equation:
Thus, the three reciprocal plots of 1/ΔA versus 1/ [Q] were linear and the binding constant was estimated from the following equation:
Figure 2 is a two-dimensional plot which illustrates 1/(A0-A) vs. 1/ [Q], in which [Q], A0 and A are the ligand concentration, the absorption in the absence of fenvalerate and the absorptions in the presence of different concentrations of fenvalerate, respectively. By dividing xintercept to the slope the association coefficient (ka) was achieved. One binding site was observed for each fenvalerate-HSA complex with the overall binding constants of KFen-HSA=3.78 × 10+4M-1
Figure 3, Shows the three replications of HSA absorption scans in the presence of fenvalerate for first day and thirtieth days samples at wavelength 200-400 nm (a and b respectively).
In fluorescence spectroscopy the effect of hydrophobic interaction on the protein structure is important. Therefore, the exposed hydrophobic residues and their emitting lights were studied. Figure 4a, shows the Stern-Volmer curve (F0/F vs. [Q]) for fluorescence emission of HSA-fenvalerate complexes in emitted wavelength of 290 nm, at temperature of 298 K and pH 7.4. Figure 4b, illustrates the changes of log((F0-F)/F) vs. log[Q] during the interaction of fenvalerate, at its different concentrations, with HSA at exiting wavelength of 290 nm, temperature of 298 K and pH 7.4.
Figure 5 shows the fluorescence emission spectra of HSA in the absence and presence of fenvalerate at its different concentrations at pH 7.4 in the first day (Figure 5a) and thirtieth day (Figure 5b).
The complexation of fenvalerate with HSA was characterized by infrared spectroscopy and the derivative of the IR spectra was investigated by Jusco and Ominic software. Since there was no significant spectral shifting for the amide I band at 1656 cm-1 (mainly C=O stretching band) and amide II band at 1552 cm-1 (C-N stretching band coupled with N-H bending modes) [25,26]. After the interaction of HSA with fenvalerate, the differential spectra of [(protein solution+fenvalerate solutions)-(protein solution)] at the first and thirtieth day was calculated to investigate the intensity of the vibrational changes (Figures 6 and 7). Similarly, the infrared self deconvolution with second derivative resolution enhancement and curve-fitting procedures  were used to determine the protein secondary structures in the presence of fenvalerate-HSA complexes (Figure 7; Tables 2 and 3). Table 2 shows the percent of conformation of HSA secondary structure before and after interacting fenvalerate in the first and thirtieth day, and Table 3 shows the conformational data at day thirty. These conformational data were provided from the curves which were shown in Figure 7.
|Type second structures||Area||Free HSA||Complex HSA+F1||Complex HSA+F2||Complex HSA+F3||Complex HSA+F7|
Table 2: The percent of conformation of HSA secondary structure before and after interacting with fenvalerate in the first day.
|Type second structures||Area||Free
|Complex HSA+F1||Complex HSA+F2||Complex HSA+F3||Complex HSA+F7|
Table 3: The conformation percent of HSA secondary structure before and after interacting with fenvalerate in the thirtieth day.
HSA-fenvalerate docking study
The prediction of interaction of HSA with fenvalerate, illustrated a binding site with -9.5 kcal mol-1 as free energy of binding. This binding site was not the first or the second classic binding sites of HSA. All residues around fenvalerate within this non-classic binding pocket were included of Arg-117, Arg-145, Arg-186, Glu-141, Leu-182, Tyr-138, Tyr-161, Ile-142, His-146, Gly-189 and Phe-149. Based on these residues which were in contact with the ligands, electrostatic and hydrophobic interactions were involved in fenvalerate-HSA binding. This binding site was nearly the III-B binding site of HSA. Figure 8 illustrates this non-classic binding site. In Figure 8, the panel a displays the position of predicted fenvalerate-HSA binding site in a whole 3D structure of HSA protein. The green CPK model in this panel is fenvalerate. Panel b demonstrates the residues around fenvalerate in its binding pocket. The -9.5 kcal mol-1 is the free energy binding ligand to receptor.
The most evident phenomenon in absorption curves of HSA and fenvalerate-HSA complexes in first and thirtieth day (Figure 3a and 3b, respectively) is the hyperchromic effect of fenvalerate which is reflected in Figure 2 and this can be related to the mobility of ligand around HSA molecule. The increase in absorption ratio could be caused by elevating interaction between ligands and proteins. The intensity of the absorption, increased in thirtieth day (Figure 3b).
By fluorescence spectroscopy, the intrinsic fluorescence of proteins, which was caused by Trp124 and tyrosine residues, were studied and both emitting intensities were depended on the environment. When a hydrophobic group is present at the surface of the protein it can be prepared as a site for small molecules binding and due to that, the exposed hydrophobic regions in the protein structure are the goal of many structural studies of proteins . When the protein was excited at wavelength of 290 nm, the Try124 and tyrosine residues were also affected , as it was recorded in Figure 4, the quenching yield decreased at presence of fenvalerate, which happened because of that Try124 and tyrosine residues left the hydrophilic environments and transmitted to the more inner hydrophobic environment. This hypochromic phenomenon and the decrease of intrinsic fluorescence confirmed the interaction of fenvalerate with HSA. By utilizing the data obtained from Figure 4b and Eq. 4, the Stern-Volmer constant was measured as the slop of the curve [28-30].
In this equation, F0 is the intensity of protein fluorescence emitting in the absence of ligand (Fenvalerate) and (F) is the fluorescence emission intensity of protein in the presence of ligands. [Q] and τ0 are the ligand concentration and the fluorophore half-life in the absence of ligand, respectively. kq is the velocity constant (the quenching rate constant of biological macromolecule) and Ksv is the Stern-Volmer constant. By plotting the diagram of F0/F vs. [Q], the Stern-Volmer constant was achieved which was equal to the slope of the plot. Finally, the Stern-Volmer constant was computed to be Ksv=3.112 × 10+4 M-1. To find the number of probable binding sites of fenvalerate on the HSA structure, equation (5), the Hill was used .
In this equation Ka, [Q], F0 and F are the association constant of protein-ligand complex, the concentration of ligand, the protein emission in the absence of ligand and the protein emission in the presence of ligand, respectively. Therefore, by plotting log (F0 − F)/F vs. log[Q], using its slope and rearrangement of Eq. 5, the amount of Ka was obtained to be 3.787 × 10+4 M-1 and the number of binding sites was calculated to be n=1. The thermodynamic parameter of binding fenvalerate to HSA was calculated using Eq. (6) .
In this equation, R, ΔG0 and T are the gas constant (1.987 kcal mol-1), the free Gibbs energy for ligand binding and the absolute temperature (298.15 k), respectively. The fenvalerate-HSA binding free Gibbs energy was calculated to be -26.11 kj mol-1. It was confirmed that, by increasing the concentration of fenvalerate, the intensity of the fluorescence decreased at first and thirtieth days (Figures 5a and 5b).
As it can be seen from Figure 6, the intensity of the amide I and II band decreased by increasing the ligand concentration at 1653 cm-1 (0.000595 mM, HSA-F1), 1650 cm-1 (0.00119 mM, HSA-F2), 1640 cm-1 (0.00238 mM, HSA-F3) and 1639 cm-1 (0.00714 mM, HSA-F7) for amide I and at 1672 cm-1 (0.000595 mM, HSA-F1), 1671 cm-1 (0.00119 mM, HSA-F2), 1669 cm-1 (0.00238 mM, HSA-F3) and 1668 cm-1 (0.00714 mM, HSA-F7) for amide II in first day and at 1652 cm-1 (0.000595 mM, HSA-F1), 1651 cm-1 (0.00119 mM, HSA-F2), 1650 cm-1 (0.00238 mM, HSA-F3) and 1648 cm-1 (0.00714 mM, HSA-F7) for amide I and at 1638 cm-1 (0.000595 mM, HSA-F1), 1636 cm-1 (0.00119 mM, HSA-F2), 1635 cm-1 (0.00238 mM, HSA-F3) and 1634 cm-1 (0.00714 mM, HSA-F7) for amide II in thirtieth day. This decrease was more evident at thirtieth day which could be caused by changes in protein conformation that was related to the probable binding of fenvalerate to C=O, C-N and N-H groups. This binding led to considerable changes of protein secondary structure (mostly α-helix). The alterations of protein secondary structure were more severe in thirtieth day. A quantitative analysis of the protein secondary structure for the free HSA and its fenvalerate adducts was carried out by the Caviar method at the first and thirtieth day, and then the results were shown in Figure 7 and Tables 2 and 3. The free HSA in first day was 55% α -helix (1658.7 cm-1), 1396% β-sheet (1626 cm-1), 15.04% turn structure (1676 cm-1), 6% β -antiparallel (1688 cm-1), and 13% random coil (1644 cm-1) (Figure 7 and Table 2). The free HSA in thirtieth day was 42.99% α -helix (1662 cm-1), 1.9% β-sheet (1632 cm-1), 40.39% turn structure (1680 cm-1), 2.21% β-antiparallel (1692 cm-1), and 12.52% random coil (1648 cm-1) (Figure 7 and Table 3). Upon fenvalerate interaction, a major decrease of α-helix from 55%(free HSA) to 50.23% (F7-HSA, 0.00714 mM) with changes in β-sheet from 13.96% (free HSA) to 16.82% (F7-HSA, 0.00714 mM) and turn structure from 15.04% (free HSA) to 11.22% (F7-HSA, 0.00714 mM) were observed (Figure 7 and Table 2). A similar increase was also observed for β-antiparallel from 6% (free HSA) to 8.93% (FEN-HSA, 0.00714 mM) with changes in random coil from 13% (free HSA) to 14.08% (F7-HSA, 0.00714 mM) were observed on first day (Figure 7 and Table 2). In thirtieth day a major decrease of α-helix from 42.99% (free HSA) to 38.82% (F7-HSA, 0.00714 mM) with changes in β-sheet from 1.9% (free HSA) to 13.9% (F7-HSA, 0.00714 mM) and turn structure from 40.39% (free HSA) to 41.46% (FEN-HAS, 0.00714 mM) were observed (Figure 7 and Table 2). A similar increase was also observed for β-antiparallel from 2.21% (free HSA) to 2.53% (F7-HSA, 0.00714 mM) with changes in random coil from 12.52% (free HSA) to 5% (F7- HSA, 0.00714 mM) were observed (Figure 7 and Table 3). These results were consistent with the decrease in intensity of the protein amide I band discussed above. The decrease in α-helix structure and increase in β-sheet, turn structures and β-antiparallel were indicative for protein destabilization upon fenvalerate interaction.
Docking studies of fenvalerate-HSA
Docking studies also predicted just one relatively strong binding site for fenvalerate on HSA. Based on residues around the ligand in its pocket the interaction of fenvalerate with HSA protein was mostly included of both hydrophobic and electrostatic types.
The results of secondary and tertiary structural changes of HSA showed that fenvalerate-HSA binding site had good vicinity. This study has been suggested that human prolonged exposure to fenvalerate can create problems for HSA.
Our special thanks to Institute of Biochemistry and Biophysics (IBB) of Tehran University and Islamic Azad University, Science and Research, Tehran Branch for support of this work.