Bromophenol Blue Binding as a Probe to Study Urea and Guanidine Hydrochloride Denaturation of Bovine Serum Albumin

Insight into the binding of glycerol with myoglobin: Spectroscopic and MD simulation approach

Stability of proteins plays a significant role not only in their biological function but also in medical science and protein engineering. Since proteins are only stable in special conditions, maintaining their stability and function in biological and biotechnological applications may pose serious challenges. Osmolytes provide a general method of shielding proteins from the unfolding and aggregation caused by extreme stress on the environment. In such studies, the researchers used spectroscopic and simulation approaches to study the alterations of the myoglobin structure and stability in glycerol presence. Experimental results showed a stability improvement of the complex myoglobin-glycerol. After the addition of glycerol resulting in the initiation of hydrogen bonds and higher levels of hydrophobicity, the increase of the T_m was observed. The static mode quenching observed in this study. Van der Waals forces and hydrogen bindings had a decisive and significant role concerning the stability of protein which was consistent with the modeling results. Molecular dynamics simulation showed that the glycerol presence could enhance myoglobin stability. The consistency between the theoretical studies and experimental findings demonstrates that the method proposed in this study could provide a useful method for protein-ligand complex investigations.

On-chip measurements of protein unfolding from direct observations of micron-scale diffusion

The unfolding process of BSA in solution as a function of pH was studied by microfluidic diffusional sizing device.

Investigations of protein folding, unfolding and stability are critical for the understanding of the molecular basis of biological structure and function. We describe here a microfluidic approach to probe the unfolding of unlabelled protein molecules in microliter volumes. We achieve this objective through the use of a microfluidic platform, which allows the changes in molecular diffusivity upon folding and unfolding to be detected directly. We illustrate this approach by monitoring the unfolding of bovine serum albumin in solution as a function of pH. These results show the viability of probing protein stability on chip in small volumes.

Deciphering the binding behaviours of BSA using ionic AIE-active fluorescent probes

The binding behaviours of a transport protein, bovine serum albumin (BSA), in its native, unfolding and refolding states have been probed by monitoring the emission changes of two exogenous AIE-active fluorescent probes, M2 and M3, which are designed to be anionic and cationic, respectively. Due to their AIE properties, both M2 and M3 display emission enhancement when bound to the hydrophobic cavity of BSA. The binding site of M2 and M3 is found to be subdomain IIA. Then, the BSA + M2 and BSA + M3 systems are utilized to fluorescently signal the conformation changes of BSA caused by various external stimuli, including thermally or chemically induced denaturation. The data confirmed the multi-step unfolding process and the existence of a molten-globule intermediate state. The unfolding process consists of the rearrangement of subdomain IIA, the exposure of a negatively charged binding site in domain I that prefers interacting with cationic species, and the transformation of the molten-globule intermediate into the final random coil. The anionic and cationic modifications of the probes enable us to observe that electrostatic interactions play a role in the folding and unfolding of BSA.

Selective Uptake and Refolding of Globular Proteins in Coacervate Microdroplets

Intrinsic differences in the molecular sequestration of folded and unfolded proteins within poly(diallyldimethylammonium) (PDDA)/poly(acrylate) (PAA) coacervate microdroplets are exploited to establish membrane-free microcompartments that support protein refolding, facilitate the recovery of secondary structure and enzyme activity, and enable the selective uptake and exclusion of folded and unfolded biomolecules, respectively. Native bovine serum albumin, carbonic anhydrase, and α-chymotrypsin are preferentially sequestered within positively charged coacervate microdroplets, and the unfolding of these proteins in the presence of increasing amounts of urea results in an exponential decrease in the equilibrium partition constants as well as the kinetic release of unfolded molecules from the droplets into the surrounding continuous phase. Slow refolding in the presence of positively charged microdroplets leads to the resequestration of functional proteins and the restoration of enzymatic activity; however, fast refolding results in protein aggregation at the droplet surface. In contrast, slow and fast refolding in the presence of negatively charged PDDA/PAA droplets gives rise to reduced protein aggregation and misfolding by interactions at the droplet surface to give increased levels of protein renaturation. Together, our observations provide new insights into the bottom-up design and construction of self-assembling microcompartments capable of supporting the selective uptake and refolding of globular proteins.

The influence of putrescine on the structure, enzyme activity and stability of α-chymotrypsin

Information on protein stability is essential to study protein structure, activity, and interactions with ligands.

Quantitation of Albumin in Serum Using "Turn-on" Fluorescent Probe with Aggregation-Enhanced Emission Characteristics

An aggregation-enhanced emission active luminogen named as sodium 4,4'4″-(3,4-diphenyl-1H-pyrrole-1,2,5-triyl)tribenzoate (DP-TPPNa) with propeller construction was synthesized and developed as a "turn on" fluorescent probe for in situ quantitation of albumin in blood serum. The DP-TPPNa fluorescence intensity was linearly correlated with the concentration of two serum albumins, bovine serum albumin (BSA) and human serum albumin (HSA), in pure PBS buffer in the ranges of 2.18-70 and 1.68-100 μg/mL, respectively. The detection limits were as low as 2.18 μg/mL for BSA and 1.68 μg/mL for HSA. The response time of fluorescence to serum albumin (SA) was very short (below 6 s), which achieved real-time detection. It also showed high selectivity to SA because other components in serum barely interfere with the detection of DP-TPPNa to SA, enabling in situ quantitative detection of SA without isolation from serum. DP-TPPNa was successfully applied for the quantitative detection of BSA in fetal bovine serum. The mechanism of fluorescent turn-on behavior was elucidated utilizing an unfolding process induced by guanidine hydrochloride, which revealed a capture process via selective hydrophobic interaction and hydrogen bonding between luminogen and SA.

Structural stability as a probe for molecular evolution of homologous albumins studied by spectroscopy and bioinformatics

Equilibrium unfolding by guanidinium hydrochloride (GuHCl) and urea as well as evolutionary trends of two homologous albumins, pig serum albumin (PSA) and rabbit serum albumin (RSA), has been studied with circular dichroism, tryptophanyl fluorescence and bioinformatics. GuHCl cannot distinguish the contribution of electrostatic interactions to the proteins which were otherwise effectively monitored by urea. Higher differences in free energy changes due to urea than GuHCl show electrostatic interactions among charged amino acids are possibly responsible for higher structural stability of RSA in comparison to PSA. From the sequence of HSA and RSA, deletion of arginine at position 117 and the presence of one extra tryptophan at position 135 may possess some clue for lesser stability of PSA. Here, for comparison, chemical unfolding data of HSA and BSA had been taken into consideration. We found that thermodynamically RSA and PSA are closer to HSA and BSA, respectively, in accordance with their sequence homologies. Taxonomically, rabbit belongs to lagomorph which is closer to hominids than ungulates. Hence, on the basis of these thermodynamic data of protein denaturation of different species we can use this new approach to analyze the phylogenetic relationship among the major clades of eutherian mammals to obtain their evolutionary trends.

Quantitation, visualization, and monitoring of conformational transitions of human serum albumin by a tetraphenylethene derivative with aggregation-induced emission characteristics

Human serum albumin (HSA) is a major protein component of blood plasma, and its assay is of obvious value to biological research. We, herein, present a readily accessible fluorescent bioprobe for HSA detection and quantitation. A nonemissive tetraphenylethene derivative named sodium 1,2-bis[4-(3-sulfonatopropoxyl)phenyl]-1,2-diphenylethene (BSPOTPE) is induced to emit by HSA, showing a novel phenomenon of aggregation-induced emission (AIE). The AIE bioprobe enjoys a broad working range (0-100 nM), a low detection limit (down to 1 nM), and a superior selectivity to albumins. The fluorescent bioassay is unperturbed by the miscellaneous bioelectrolytes in the artificial urine. The AIE luminogen can also be used as a rapid and sensitive protein stain in gel electrophoresis for HSA visualization. Utilizing the AIE feature of BSPOTPE and the Forster resonance energy transfer from HSA to BSPOTPE, the unfolding process of HSA induced by guanidine hydrochloride is monitored, which reveals a multistep transition with the involvement of molten globule intermediates. Computational modeling suggests that the AIE luminogens dock in the hydrophobic cleft between subdomains IIA and IIIA of HSA with the aid of hydrophobic effect, charge neutralization, and hydrogen bonding interactions, offering mechanistic insight into the microenvironment inside the hydrophobic cavity.

Bromophenol blue binding to mammalian albumins and displacement of albumin-bound bilirubin

Interaction of bromophenol blue (BPB) with serum albumins from different mammalian species, namely, human (HSA), bovine (BSA), goat (GSA), sheep (SSA), rabbit (RbSA), porcine (PSA) and dog (DSA) was studied using absorption and absorption difference spectroscopy. BPB-albumin complexes showed significant differences in the spectral characteristics, i.e., extent of bathochromic shift and hypochromism relative to the spectral features of free BPB. Absorption difference spectra of these complexes also showed variations in the position of maxima and absorption difference (deltaAbs.) values. Absorption difference spectra of different bilirubin (BR)-albumin complexes showed a significant blue shift accompanied by decrease in deltaAbs. values in presence of BPB which were indicative of the displacement of bound BR from its binding site in BR-albumin complexes. These changes in the difference spectral characteristics of BR-albumin complexes were more marked at higher BPB concentration. However, the extent of these changes was different for different BR-albumin complexes. Taken together, all these results suggest that BPB partially shares BR binding site on albumin and different mammalian albumins show differences in the microenvironment of the BR/BPB binding site.