Probing the interaction of a therapeutic flavonoid, pinostrobin with human serum albumin: multiple spectroscopic and molecular modeling investigations.

Interaction of a pharmacologically important flavonoid, pinostrobin (PS) with the major transport protein of human blood circulation, human serum albumin (HSA) has been examined using a multitude of spectroscopic techniques and molecular docking studies. Analysis of the fluorescence quenching data showed a moderate binding affinity (1.03 × 10(5) M(-1) at 25°C) between PS and HSA with a 1∶1 stoichiometry. Thermodynamic analysis of the binding data (ΔS = +44.06 J mol(-1) K(-1) and ΔH = -15.48 kJ mol(-1)) and molecular simulation results suggested the involvement of hydrophobic and van der Waals forces, as well as hydrogen bonding in the complex formation. Both secondary and tertiary structural perturbations in HSA were observed upon PS binding, as revealed by intrinsic, synchronous, and three-dimensional fluorescence results. Far-UV circular dichroism data revealed increased thermal stability of the protein upon complexation with PS. Competitive drug displacement results suggested the binding site of PS on HSA as Sudlow's site I, located at subdomain IIA, and was well supported by the molecular modelling data

Distance of the predicted hydrogen bonds formed between interacting residues of HSA and PS.

<p>Distance of the predicted hydrogen bonds formed between interacting residues of HSA and PS.</p

Interaction of a tyrosine kinase inhibitor, vandetanib with human serum albumin as studied by fluorescence quenching and molecular docking

<p>Interaction of a tyrosine kinase inhibitor, vandetanib (VDB), with the major transport protein in the human blood circulation, human serum albumin (HSA), was investigated using fluorescence spectroscopy, circular dichroism (CD) spectroscopy, and molecular docking analysis. The binding constant of the VDB–HSA system, as determined by fluorescence quenching titration method was found in the range, 8.92–6.89 × 10³M⁻¹ at three different temperatures, suggesting moderate binding affinity. Furthermore, decrease in the binding constant with increasing temperature revealed involvement of static quenching mechanism, thus affirming the formation of the VDB–HSA complex. Thermodynamic analysis of the binding reaction between VDB and HSA yielded positive Δ<i>S</i> (52.76 J mol⁻¹ K⁻¹) and negative Δ<i>H</i> (−6.57 kJ mol⁻¹) values, which suggested involvement of hydrophobic interactions and hydrogen bonding in stabilizing the VDB–HSA complex. Far-UV and near-UV CD spectral results suggested alterations in both secondary and tertiary structures of HSA upon VDB-binding. Three-dimensional fluorescence spectral results also showed significant microenvironmental changes around the Trp residue of HSA consequent to the complex formation. Use of site-specific marker ligands, such as phenylbutazone (site I marker) and diazepam (site II marker) in competitive ligand displacement experiments indicated location of the VDB binding site on HSA as Sudlow’s site I (subdomain IIA), which was further established by molecular docking results. Presence of some common metal ions, such as Ca²⁺, Zn²⁺, Cu²⁺, Ba²⁺, Mg²⁺, and Mn²⁺ in the reaction mixture produced smaller but significant alterations in the binding affinity of VDB to HSA.</p> <p>Binding orientation of the VDB in the Sudlow’s binding site I (subdomain IIA) of HSA.</p

Binding and thermodynamic parameters for the interaction between PS and HSA, studied at different temperatures, pH

<p>Binding and thermodynamic parameters for the interaction between PS and HSA, studied at different temperatures, pH</p

Analysis of fluorescence quenching data.

<p>(A) Stern–Volmer and (B) against plots of PS–HSA system at different temperatures. Inset of (B) shows the van’t Hoff plot for PS–HSA interaction.</p

Cluster analysis of PS docking to binding sites I and II of HSA.

<p>The different crystal structures of HSA used in the analysis were (A) 1BM0, (B) 2BXD and (C) 2BXF. A total of 100 runs were performed for each binding locus.</p

Characteristics of three-dimensional fluorescence spectra of native HSA and its complexes with PS at pH 7.4, 25°C.

<p>Characteristics of three-dimensional fluorescence spectra of native HSA and its complexes with PS at pH 7.4, 25°C.</p

3-D fluorescence spectral projections and corresponding contour maps of HSA and various PS–HSA complexes.

<p>(A and A′) Free HSA, (B and B′) 1∶1 PS–HSA, (C and C′) 2∶1 PS–HSA and (D and D′) 3∶1 PS–HSA. The spectra were recorded in 10 mM Tris-HCl buffer, pH 7.4, 25°C using a protein concentration of 3 µM.</p

Synchronous fluorescence spectra of HSA obtained in the absence and presence of increasing PS concentrations.

<p>[HSA] = 3 µM, [PS] = 0–22.5 µM with 1.5 µM intervals (1–16) studied in 10 mM Tris-HCl buffer, pH 7.4, 25°C. The difference between excitation and emission wavelengths (Δλ) was (A) 15 nm and (B) 60 nm. Arrows depict the position of the emission maximum of HSA in presence of increasing PS concentrations.</p