Thyroxine binding to human serum albumin immobilized on sepharose and effects of nonprotein albumin-binding plasma constituents

Modulation of the Protein-Ligand Interaction in the Presence of Graphene Oxide: a Detailed Spectroscopic Study

Several applications of graphene oxide (GO) have been established over the years, and it has the potential to be used as a biomedical material. Studying the effect of GO on protein-ligand (small molecules/drugs) complex systems are vital as the mechanisms involved are not well understood. The interaction of GO on the protein-ligand binding is also vital for the preparation of an effective drug carrier in the bloodstream. In this work, we have tried to explore in details the effect of GO on the interaction between a hydrophilic molecule, namely, 7-(N,N'-diethylamino) coumarin-3-carboxylic acid (7-DCA) with human serum albumin (HSA) by employing multispectroscopic, microscopic, calorimetric, and molecular docking studies. We find out that protein-ligand complexes were placed on the GO surface, and GO gives stability to the protein-ligand complex via hydrogen bonding, electrostatic interactions, hydrophobic interactions, and so forth. Due to the presence of a large surface area in GO, it offers a hydrophobic environment, and as a result, the emission maxima of 7-DCA in the ternary complex is more blue-shifted, and the average lifetime becomes higher compared to the binary system. Circular dichroism spectral studies give information about the conformational changes of HSA in the absence and presence of GO when it forms complex with 7-DCA. The fluorescence lifetime imaging study shows the presence of the 7-DCA/HSA complex on the GO sheet. Molecular docking simulation shows that the closest distance between 7-DCA and HSA is 11.9 Å, and the protein interacted with the ligand through hydrogen bonding, hydrophobic interaction, and so forth.

Monitoring the endocytosis of bovine serum albumin based on the fluorescence lifetime of small squaraine dye in living cells

Bovine serum albumin (BSA) has a wide range of physiological functions involving the binding, transportation, and delivery of fatty acids, porphyrins, bilirubin, steroids, etc. In the present study, we prepared a small squaraine dye (SD), which can selectively detect BSA using fluorescence lifetime imaging microscopy (FLIM), to monitor the endocytosis of BSA in live cultured cells in real time. This approach revealed that BSA uptake is concentration-dependent in living cells. Furthermore, we used paclitaxel (PTX), a chemotherapeutic drug, to influence the endocytosis of BSA in living cells. The results demonstrated that the endocytic rate was clearly reduced after pretreatment with 0.4 µM PTX for 2 h. The present study demonstrates the potential value of using the fluorescence lifetime of SD to detect BSA concentration and study the physiological mechanism of chemotherapeutic drugs.

Binding of Fatty Acid Amide Amphiphiles to Bovine Serum Albumin: Role of Amide Hydrogen Bonding

The study of protein-surfactant interactions is important because of the widespread use of surfactants in industry, medicine, and pharmaceutical fields. Sodium N-lauroylsarcosinate (SL-Sar) is a widely used surfactant in cosmetics, shampoos. In this paper, we studied the interactions of bovine serum albumin (BSA) with SL-Sar and sodium N-lauroylglycinate (SL-Gly) by use of a number of techniques, including fluorescence and circular dichroism spectroscopy and isothermal titration calorimetry. The binding strength of SL-Sar is stronger than that of structurally similar SL-Gly, which differs only by the absence of a methyl group in the amide nitrogen atom. Also, these two surfactants exhibit different binding patterns with the BSA protein. The role of the amide bond and hence the surfactant headgroup in the binding mechanism is discussed in this paper. It was observed that while SL-Sar destabilized, SL-Gly stabilized the protein structure, even at concentrations less than the critical micelle concentration (cmc) value. The thermodynamics of surfactant binding to BSA was studied by use of ITC. From the ITC results, it is concluded that three molecules of SL-Sar in contrast to only one molecule of SL-Gly bind to BSA in one set of binding sites at room temperature. However, on increasing temperature four molecules of SL-Gly bind to the BSA through H-bonding and van der Waals interactions, due to loosening of the BSA structure. In contrast, with SL-Sar the binding process is enthalpy driven, and very little structural change of BSA was observed at higher temperature.

Modulated photophysics of a cationic DNA-staining dye inside protein bovine serum albumin: study of binding interaction and structural changes of protein

The binding affinity of cationic DNA-staining dye, propidium iodide, with transport protein, bovine serum albumin, has been explored using UV-vis absorption, fluorescence, and circular dichroism spectroscopy. Steady state and time resolved fluorescence studies authenticate that fluorescence quenching of bovine serum albumin by propidium iodide is due to bovine serum albumin-propidium iodide complex formation. Thermodynamic parameters obtained from temperature dependent spectral studies cast light on binding interaction between the probe and protein. Site marker competitive binding has been encountered using phenylbutazone and flufenamic acid for site I and site II, respectively. Energy transfer efficiency and distance between bovine serum albumin and propidium iodide have been determined using Förster mechanism. Structural stabilization or destabilization of protein by propidium iodide has been investigated by urea denaturation study. The circular dichroism study as well as FT-IR measurement demonstrates some configurational changes of the protein in presence of the dye. Docking studies support the experimental data thereby reinforcing the binding site of the probe to the subdomain IIA of bovine serum albumin.

Mechanistic investigation of domain specific unfolding of human serum albumin and the effect of sucrose

This study is devoted to understand the unfolding mechanism of a multidomain protein, human serum albumin (HSA), in absence and presence of the sucrose by steady-state and time-resolved fluorescence spectroscopy with domain specific marker molecules and is further being substantiated by molecular dynamics (MD) simulation. In water, the domain III of HSA found to unfold first followed by domains I and II as the concentration of GnHCl is increased in the medium. The sequential unfolding behavior of different domains of HSA remains same in presence of sucrose; however, a higher GnHCl concentration is required for unfolding, suggesting stabilizing effect of sucrose on HSA. Domain I is found to be most stabilized by sucrose. The stabilization of domain II is somewhat similar to domain I, but the effect of sucrose on domain III is found to be very small. MD simulation also predicted a similar behavior of sucrose on HSA. The stabilizing effect of sucrose is explained in terms of the entrapment of water molecules in between HSA surface and sucrose layer as well as direct interaction between HSA and sucrose.

Structural Investigations of a New Familial Dysalbuminemic Hyperthyroxinemia Genotype

Background: In a previous study, we found that the amino acid substitution R218H in human serum albumin (HSA) was the cause of familial dysalbuminemic hyperthyroxinemia (FDH) in several Caucasian patients. Subsequently the substitution R218P was shown to be the cause of FDH in several members of a Japanese family. This study attempts to resolve discrepancies in the only other study of R218P HSA and identifies two new Japanese R218P FDH patients unrelated to those described previously.

Methods and Results: Recombinant R218H, R218P, and wild-type HSA were synthesized in yeast, and the affinities of these HSA species for l- and d-thyroxine were determined using fluorescence spectroscopy. The dissociation constants for the binding of wild-type, R218P, and R218H HSA to l-thyroxine were 1.44 × 10−6, 2.64 × 10−7, and 2.49 × 10−7 mol/L, respectively. The circular dichroism spectra of thyroxine bound to R218H and R218P HSA were markedly different, indicating that the structure of the thyroxine/HSA complex is different for either protein.

Conclusions: The Kd values for l-thyroxine bound to R218P and R218H HSA determined in this study were similar. The extremely high serum total-thyroxine concentrations reported previously for R218P FDH patients (10-fold higher than those reported for R218H FDH patients) are not consistent with the Kd values determined in this study. Possible explanations for these discrepancies are discussed.

Mutations in a specific human serum albumin thyroxine binding site define the structural basis of familial dysalbuminemic hyperthyroxinemia

The familial dysalbuminemic hyperthyroxinemia (FDH) phenotype results from a natural human serum albumin (HSA) mutant with histidine instead of arginine at amino acid position 218. This mutation results in an enhanced affinity for thyroxine. Site-directed mutagenesis and a yeast protein expression system were used to synthesize wild type HSA and FDH HSA as well as several other HSA mutants. Studies on the binding of thyroxine to these HSA species using equilibrium dialysis and quenching of tryptophan 214 fluorescence suggest that the FDH mutation affects a single thyroxine binding site located in the 2A subdomain of HSA. Site-directed mutagenesis of HSA and thyroxine analogs were used to obtain information about the mechanism of thyroxine binding to both wild type and FDH HSA. These studies suggest that the guanidino group of arginine at amino acid position 218 in wild type HSA is involved in an unfavorable binding interaction with the amino group of thyroxine, whereas histidine at amino acid position 218 in FDH HSA is involved in a favorable binding interaction with thyroxine. Neither arginine at amino acid position 222 nor tryptophan at amino acid position 214 appears to favorably influence the binding of thyroxine to wild type HSA.

Drug-protein binding sites. New trends in analytical and experimental methodology

In the last few years, continuous progress in instrumental analytical methodology has been achieved with a substantial increase in the number of new, more specific and more flexible methods for ligand-protein assays. In general, the methods used for drug-protein binding studies can be divided into two main groups: separation methods (enabling the calculation of binding parameters, i.e. the number of binding sites and their respective affinity constants) and non-separation methods (describing predominantly qualitative parameters of the ligand-protein complex). This review will be focussed particularly on recent trends in the development of drug-protein binding methods including stereoselective and non-stereoselective aspects using chromatography, capillary electrophoresis and microdialysis as compared to the "conventional approach" using equilibrium dialysis, ultrafiltration or size exclusion chromatography. The advantages and limitations of various methods will be discussed including a focus on "optimal" experimental strategies taking into account in vitro, ex vivo and/or in vivo studies. Furthermore, the importance of some particular aspects concerning the drug binding to proteins (covalent binding of drugs and metabolites, stereoselective interactions and evaluation of binding data) will be outlined in more detail.

A point mutation in the human serum albumin gene results in familial dysalbuminaemic hyperthyroxinaemia

Using DNA samples obtained from two unrelated patients, diagnosed as having familial dysalbuminaemic hyperthyroxinaemia (FDH), exons 1-14 which span the entire coding region of the human serum albumin (HSA) gene were amplified by the polymerase chain reaction. The sequence of each of the 14 DNA fragments was then determined. In each case a point mutation was identified at nucleotide 653 which causes an Arg to His substitution at amino acid position 218. The substitution was confirmed by amino acid sequencing of a mutant peptide resulting from tryptic digestion of the protein. Abnormal affinity of FDH HSA for a thyroxine (T4) analogue was verified by an adaptation of the procedure used in routine free T4 measurement. The location of the mutation is discussed in relation to other studies on the binding properties of HSA.