Circular dichroic and fluoropolarimetric studies on tryptophyl residues in acid-induced isomerization of bovine plasma albumin

Spectroscopic and mechanistic analysis of the interaction between Jack bean urease and polypseudorotaxane fabricated with bis-thiolated poly(ethylene glycol) and α-cyclodextrin

Self-assembled polypseudorotaxanes (PPRXs) fabricated with α-cyclodextrin and poly(ethylene glycol) (PEG) or its thiolated derivatives were candidate functional materials for enzyme soft-immobilization, encapsulation and controlled-release. The study of their interaction with Jack bean urease (JBU) indicated that they inconspicuously influenced the activity and stability of JBU during long storage, up to 30 days. The macro-species were inaccessible to JBU's active site and the steric effect might play a significant role in the stabilization of JBU, when compared with the small-molecular sulfhydryl inhibitor thioglycolic acid. Circular dichroism and fluorescence spectra analyses revealed that thiolated PEG₄₀₀-(SH)₂ and its assembly PPRX₄₀₀(SH) brought in perturbations to certain α-helical or β-sheet domains of JBU, making JBU's conformation more flexible. The resulting partial unfolding of domains exposed several hydrophobic clusters and varied JBU's surface hydrophobicity. It also rendered the chromophores more hydrophilic and more bared to the polar environment, leading to the typical bathochromic-shift and quenching in intrinsic and synchronous fluorescence spectra. Moreover, the surface hydrophobicity profile of JBU was depicted by fluorescent probe monitoring and the unique "hydrophobic cave" motif was proposed by analyzing JBU's structural data from the Protein Data Bank. It should be pointed out that conformational variations mainly occurred at the surface region of JBU, while the buried active bi-nickel center was not markedly influenced by the macro-species. The results demonstrated that the PPRXs might act as a proper carrier for JBU encapsulation or soft-immobilization.

Probing the Acid-Induced Packing Structure Changes of the Molten Globule Domains of a Protein near Equilibrium Unfolding

Using simultaneously scanning small-angle X-ray scattering (SAXS) and UV-vis absorption with integrated online size exclusion chromatography, supplemental with molecular dynamics simulations, we unveil the long-postulated global structure evolution of a model multidomain protein bovine serum albumin (BSA) during acid-induced unfolding. Our results differentiate three global packing structures of the three molten globule domains of BSA, forming three intermediates I₁, I₂, and E along the unfolding pathway. The I₁-I₂ transition, overlooked in all previous studies, involves mainly coordinated reorientations across interconnected molten globule subdomains, and the transition activates a critical pivot domain opening of the protein for entering into the E form, with an unexpectedly large unfolding free energy change of -9.5 kcal mol^-1, extracted based on the observed packing structural changes. The revealed local packing flexibility and rigidity of the molten globule domains in the E form elucidate how collective motions of the molten globule domains profoundly influence the folding-unfolding pathway of a multidomain protein.

Interaction of virstatin with human serum albumin: spectroscopic analysis and molecular modeling

Virstatin is a small molecule that inhibits Vibrio cholerae virulence regulation, the causative agent for cholera. Here we report the interaction of virstatin with human serum albumin (HSA) using various biophysical methods. The drug binding was monitored using different isomeric forms of HSA (N form ∼pH 7.2, B form ∼pH 9.0 and F form ∼pH 3.5) by absorption and fluorescence spectroscopy. There is a considerable quenching of the intrinsic fluorescence of HSA on binding the drug. The distance (r) between donor (Trp214 in HSA) and acceptor (virstatin), obtained from Forster-type fluorescence resonance energy transfer (FRET), was found to be 3.05 nm. The ITC data revealed that the binding was an enthalpy-driven process and the binding constants K(a) for N and B isomers were found to be 6.09×10(5 )M(-1) and 4.47×10(5) M(-1), respectively. The conformational changes of HSA due to the interaction with the drug were investigated from circular dichroism (CD) and Fourier Transform Infrared (FTIR) spectroscopy. For 1:1 molar ratio of the protein and the drug the far-UV CD spectra showed an increase in α- helicity for all the conformers of HSA, and the protein is stabilized against urea and thermal unfolding. Molecular docking studies revealed possible residues involved in the protein-drug interaction and indicated that virstatin binds to Site I (subdomain IIA), also known as the warfarin binding site.

Do hydration dynamics follow the structural perturbation during thermal denaturation of a protein: a terahertz absorption study

We investigate the thermal denaturation of human serum albumin and the associated solvation using terahertz (THz) spectroscopy in aqueous buffer solution. Far- and near-ultraviolet circular dichroism spectroscopy reveal that the protein undergoes a native (N) to extended (E) state transition at temperature ≤55°C with a marginal change in the secondary and tertiary structure. At 70°C, the protein transforms into an unfolded (U) state with significant irreversible disruption of its structures. We measure the concentration- and temperature-dependent THz absorption coefficient (α) of the protein solution using a p-Ge THz difference spectrometer (2.1-2.8 THz frequency range), thereby probing the collective protein-water network dynamics. When the solvated protein is heated up to 55°C and cooled down again, a reversible change in THz absorption is observed. When increasing the temperature up to 70°C, we find a dramatic irreversible change of THz absorption. The increase in THz absorption compared to bulk water is attributed to a blue shift in the spectrum of the solvated protein compared to bulk water. This is supported by measurements of THz absorption coefficients using THz time-domain spectroscopy (0.1-1.2 THz frequency range). We also use picosecond-resolved fluorescence spectroscopy of the tryptophan 214 moiety of human serum albumin. All experimental observations can be explained by a change in the hydration dynamics of the solvated protein due to the additional exposure of hydrophobic residues upon unfolding.

Conformational Changes in Human Serum Albumin Induced by Sodium Perfluorooctanoate in Aqueous Solutions

Conformational changes in the bulk solution and at the air-aqueous interface of human serum albumin (HSA) induced by changes in concentration of sodium perfluorooctanoate (C(7)F(15)COO(-)Na(+)) were studied by difference spectroscopy, zeta-potential data, and axisymmetric drop shape analysis. zeta-potential was used to monitor the formation of the HSA-C(7)F(15)COO(-)Na(+) complex and the surface charge of the complex. The conformational transition of HSA in the bulk solution was followed as a function of denaturant concentration by absorbance measurements at 280 nm. The data were analyzed to obtain values for the Gibbs energies of the transition in water (DeltaG(0)(W)) and in a hydrophobic environment (DeltaG(0)(hc)) pertaining to saturated protein-surfactant complexes. The conformational changes that surfactants induce in HSA molecules alter its absorption behavior at the air-water interface. Dynamic surface measurements were used to evaluate this behavior. At low [C(7)F(15)COO(-)Na(+)], proteins present three adsorption regimes: induction time, monolayer saturation, and interfacial gelation. When surfactant concentration increases and conformational changes in the bulk solution occur, the adsorption regimes disappear. HSA molecules in an intermediate conformational state migrate to the air-water interface and form a unique monolayer. At high [C(7)F(15)COO(-)Na(+)], the adsorption of denatured molecules exhibits a behavior analogous to that of dilute solutions.

Isolation and characterization of denatured serum albumin from rats with endotoxicosis

Due to its rapid breakdown in the body, denatured serum albumin has not been identified in biological samples. In this study we attempted to determine whether denatured albumin could be identified in rats with endotoxicosis. Male Wistar rats were injected with lipopolysaccharide (LPS; 5 mg/kg body weight). Plasma albumin concentration decreased to one-third the normal level at 2 days after the injection. By using the purified IgG against the specific epitope of chemically denatured albumin, two immunoreactive plasma proteins (bands D2 and D3) were identified by native PAGE followed by Western blot analysis. The plasma concentration of these two proteins increased significantly at 1 and 1.5 days after LPS injection. Peptide mass fingerprinting using matrix-assisted laser desorption/ionization-time of flight mass spectrometry (MALDI/TOF-MS) identified these two proteins as serum albumin. In order to characterize their conformational nature, ion-exchange chromatography was used to isolate D2 and D3 albumins from rats injected with LPS. Far- and near-UV circular dichroism (CD), tryptophan and 1-anilino-8-naphthalenesulfonate (ANS) fluorescence, and proteolytic susceptibility showed conformational alterations in the D2 and D3 albumins as compared with native albumin. These data indicate the presence of denatured albumin in circulating rat plasma, and this fact may contribute to a further understanding of the molecular mechanisms of albumin breakdown in physiological and pathophysiological conditions.

Salt-induced refolding in different domains of partially folded bovine serum albumin

In our earlier communication on urea denaturation of bovine serum albumin (BSA), we showed significant unfolding of domain III along with domain I prior to intermediate formation around 4.6-5.2 M urea based on the binding results of domain specific ligands:chloroform, bilirubin and diazepam for domains I, II and III, respectively. Here, we present our results on the salt-induced refolding of the two partially folded states of BSA obtained at 4.5 M urea and at pH 3.5, respectively. Both these states were characterized by significant unfolding of both domains I and III as indicated by decreased binding of chloroform and diazepam, respectively. Salt-induced stabilization of partially folded states of BSA was accompanied by nearly complete refolding of both domains I and III as the binding isotherms of chloroform and diazepam obtained in presence of approximately 1.0 M KCl were nearly identical to that obtained with native BSA at pH 7.4. From these observations, it can be concluded that the anion binding sites on serum albumin are not only confined to domain III (C-terminal region) but few sites are also present on domain I (or N-terminal region) of the protein.

Understanding the role of internal lysine residues of serum albumins in conformational stability and bilirubin binding

The role of internal lysine residues of different serum albumins, viz. from human, rabbit, goat, sheep and buffalo (HSA, RbSA, GSA, SSA and BuSA), in conformational stability and bilirubin binding was investigated after blocking them using acetylation, succinylation and guanidination reactions. No significant change in the secondary structure was noticed whereas the tertiary structure of these proteins was slightly altered upon acetylation or succinylation as revealed by circular dichroism (CD), fluorescence and gel filtration results. Guanidination did not affect the native protein conformation to a measurable extent. Scatchard analysis, CD and absorption spectroscopic results showed marked reductions (5-21-fold decrease in K(a) and approximately 50% decrease in the CD Cotton effect intensity) in the affinity of albumins for bilirubin upon acetylation or succinylation whereas guanidination produced a small change. Interestingly, monosignate CD spectra of bilirubin complexed with GSA, SSA and BuSA were transformed to bisignate CD spectra upon acetylation or succinylation of internal lysine residues whereas spectra remained bisignate in the case of bilirubin bound to acetylated or succinylated derivatives of HSA and RbSA. When probed by CD spectroscopy, bilirubin bound to acetylated or succinylated derivatives of GSA and SSA rapidly switched over to native albumins and not vice versa. These results suggested that salt linkage(s) contributed by internal lysine residue(s) play an important role in the high-affinity binding of bilirubin to albumin and provide stability to the native three-dimensional conformation of the bound pigment. Chloroform severely decreased the intensity of both positive and negative CD Cotton effects of bilirubin complexed with acetylated or succinylated derivatives of all albumins which otherwise increased significantly in the case of bilirubin complexed with native and guanidinated albumin derivatives, except the bilirubin-RbSA complex which showed a small decrease in intensity. These results suggest that the presence of salt linkage(s) in bilirubin-albumin complexation is(are) crucial to bring about effective and efficient stereochemical changes in the bound pigment by co-binding of chloroform which seems to have at least one conserved binding site on these albumins that is shared with bilirubin.

Conformational transitions of the three recombinant domains of human serum albumin depending on pH

Human serum albumin (HSA) is a protein of 66.5 kDa that is composed of three homologous domains, each of which displays specific structural and functional characteristics. HSA is known to undergo different pH-dependent structural transitions, the N-F and F-E transitions in the acid pH region and the N-B transition at slightly alkaline pH. In order to elucidate the structural behavior of the recombinant HSA domains as stand-alone proteins and to investigate the molecular and structural origins of the pH-induced conformational changes of the intact molecule, we have employed fluorescence and circular dichroic methods. Here we provide evidence that the loosening of the HSA structure in the N-F transition takes place primarily in HSA-DOM III and that HSA-DOM I undergoes a structural rearrangement with only minor changes in secondary structure, whereas HSA-DOM II transforms to a molten globule-like state as the pH is reduced. In the pH region of the N-B transition of HSA, HSA-DOM I and HSA-DOM II experience a tertiary structural isomerization, whereas with HSA-DOM III no alterations in tertiary structure are observed, as judged from near-UV CD and fluorescence measurements.

Circular dichroic and 1H-NMR studies on the aged form of bovine plasma albumin

Bovine plasma albumin (BPA) has 17 disulfide bonds and approximately one SH group at Cys-34 which catalyzes the intramolecular SH, S-S exchange reaction in the alkaline region at low ionic strength, resulting in the formation of the aged form (A-form). 1) Fractions of alpha-helix (f alpha) and beta-form (f beta) of iodoacetamide-blocked non-aged BPA (IA-BPA) at pH 6.5 (the N-form) and 9.0 (the B-form) in the absence of added salt were 0.70, 0.12 and 0.62, 0.18, respectively (Era et al. (1990]. However, there were no changes in f alpha and f beta of the iodoacetamide-blocked A-form (IA-A-form) over the pH range from 5.5 to 9.1 in the absence of added salt or in 0.10 M KCl (f alpha approximately 0.60, f beta approximately 0.20), indicating that the secondary structure of the IA-A-form might be similar to that of non-aged IA-BPA at pH 9.0 (B-form) in the absence of added salt, that is, the frozen B-form, stabilized covalently by the repairing of disulfide bonds. 2) The rigidity of the A- and IA-A-forms, as monitored by cross-relaxation times between irradiated and observed protein protons, was similar to or slightly higher than that of non-aged IA-BPA or BMA.(ABSTRACT TRUNCATED AT 250 WORDS)

Structural transition of bovine plasma albumin in the alkaline region--the N-B transition

Bovine plasma albumin (BPA) has approximately one SH group (Cys-34) which catalyzes the intramolecular SH, S-S exchange reaction in the alkaline region at low ionic strength, resulting in the formation of the aged form. So, the N-B transition at ionic strength above 0.20 and below 0.10 was studied using BPA and iodoacetamide-blocked BPA (IA-BPA), respectively. (1) pH profiles of [theta]262 and [theta]268 of BPA in 0.20 M KCl showed the characteristic changes in the pH region 7.0-9.0, corresponding to the N-B transition. On going from pH 7.0 to 9.0 in 0.10 M KCl or NaCl, IA-BPA did not show significant changes in rotational relaxation times of tryptophyl fluorophors, CD-resolved secondary structures, spin-echo 1H-n.m.r. spectra and cross-relaxation times (TIS) between irradiated and observed protein protons, which might reflect the rigidity of the domains and/or subdomains. On the other hand, rotational relaxation times of 1-anilino-8-naphthalenesulfonate-IA-BPA complex (IA-BPA-ANS0.9, molar ratio of ANS to IA-BPA = 0.9/1) showed significant decreases from 131 to 114 ns on going from the N- to the B-forms in 0.10 M KCl. The above results and reported experimental evidence might indicate that on going from the N- to the B-forms in 0.10 M KCl or NaCl, the mutual movement of subdomains, connected with a flexible hinge region (Brown & Shockley (1982)) might increase without loss in the helicity and the rigidity of subdomains. (2) The N-B transition of IA-BPA in the absence of salt was quite different from those in 0.10 M KCl or NaCl. Decreases in the helicity and the intramolecular rigidity, as monitored by TIS-measurements, were observed on going from the N- to the B-forms.

Fluorescence lifetime and spectral study of the acid expansion of bovine serum albumin

The fluorescence lifetimes of the tryptophan residues of bovine serum albumin were measured in the native and acid-expanded conformation. A three-exponential process is required to fit the fluorescence decay data. The results are interpreted empirically in terms of two emitting species. The emission at longer wavelength (360 nm) has slower rates of decay than that at shorter wavelength (325 nm). For both emitting species the average lifetime decreases when the N-F transition occurs and shortens further when the protein expands. Rotational correlation times, derived from the decay of the fluorescence anisotropy of the tryptophan residues, suggest that longer emission wavelengths are associated with somewhat shorter correlation times. There is no certain indication of any independent motion of the tryptophans in any conformation, although some very fast process, perhaps Raman scattering, appears to occur. On acid expansion the long correlation times decrease to around 10 ns in the fully expanded form. Static quenching experiments using I- or acrylamide suggest a greater average exposure of the tryptophans when the protein is most greatly expanded. This is despite the fact that the fluorescence emission maximum shifts to shorter wavelength under these conditions. Also, there is no difference in accessibility to quenching between the longer and shorter wavelength emissions.

Circular dichroic and fluorometric studies on the acid-induced isomerization of bovine plasma albumin--1 -anilino-8-naphthalenesulfonate complex

The acid-induced isomerization (the N-F transition) and expansion of bovine plasma albumin--1-anilino-8-naphthalenesulfonate complex, BPA-ANS1.0 complex (molar ratio of added ANS to BPA = 1.0) were studied by measuring fluorescence and induced CD spectra of ANS. Decrease in the reciprocal of fluorescence polarization, increase in fluorescence intensity and blue shift of fluorescence of ANS in BPA-ANS1.0 complex were correlated with the initial part of the N-F transition and/or the N-F1 transition. Induced CD spectra of ANS showed positive bands at 250-258 and 320-350 nm and one negative band at 280 nm. Most of changes (decreases) in -[theta]280 were also correlated with the initial part of the N-F transition and/or the N-F1 transition. Changes in fluorescence parameters and induced CD spectra of ANS (-[theta]280) might indicate the conformational changes around a strong ANS binding site in the N-terminal domain (Reed et al. (1975), Jonas & Weber (1970) and Brown & Shockley (1982].

Resolution of human mercapt- and nonmercaptalbumin by high-performance liquid chromatography

Gel-exclusion high-performance liquid chromatographic (HPLC) analysis of human serum albumin (HSA) on PGP 2000 column (0.10 M sodium phosphate buffer, 0.30 M NaCl, pH 6.86) showed at least two peaks, the principal component corresponding to human mercaptalbumin (HMA) and the second one to human nonmercaptalbumin (HNA). Mechanism for the separation of HMA and HNA might be due to weak resin-HSA interaction. HPLC analysis of bovine plasma albumin (BPA) showed a single peak on PGP 2000 column. The elution volume of HSA was larger than that of BPA, resulting in a clear resolution of HSA and BPA.

Effect of ligand binding and conformational changes in proteins on oxygen quenching and fluorescence depolarization of tryptophan residues

The rotational freedom of tryptophan residues in protein-ligand complexes was studied by measuring steady-state fluorescence anisotropies under conditions of oxygen quenching. There was a decrease in the oxygen bimolecular quenching constant upon complexation of trypsin and alpha-chymotrypsin with proteinaceous trypsin inhibitors, of lysozyme with N-acetylglucosamine (NAG) and di(N-acetyl-D-glucosamine) ((NAG)2) and of hexokinase with glucose. Binding of the bisubstrate analogue N-phosphonacetyl-L-aspartate (PALA) to aspartate transcarbamylase (ATCase) and binding of biotin to avidin resulted in increased oxygen quenching constants. The tryptophan of human serum albumin (HSA) in the F state was more accessible to oxygen quenching than that in the N state. With the exception of ATCase, the presence of subnanosecond motions of the tryptophan residues in all the proteins is suggested by the short apparent correlation times for fluorescence depolarization and by the low apparent anisotropies obtained by extrapolation to a lifetime of zero. Complex formation evidently resulted in more rigid structures in the case of trypsin, alpha-chymotrypsin and lysozyme. The effects of glucose binding on hexokinase were not significant. Binding of biotin to avidin resulted in a shorter correlation time for the tryptophan residues. The N --> F transition in HSA resulted in a more rigid environment for the tryptophan residue. Overall, these changes in the dynamics of the protein matrix and motional freedom of tryptophan residues due to complex formation and subsequent conformational changes are in the same direction as those observed by other techniques, especially hydrogen exchange. Significantly, the effects of complex formation on protein dynamics are variable. Among the limited number of cases we examined, the effects of complex formation were to increase, decrease or leave unchanged the apparent dynamics of the protein matrix.

CD-resolved secondary structure of bovine plasma albumin in acid-induced isomerization

Bovine plasma albumin (BPA) showed the acid-induced two-step transition, the N-F transition and acid-expansion. Changes in fractions of alpha-helix (f alpha), beta-form (f beta) and unordered form (fR) in the acid-induced isomerization of BPA were studied using the method of Chen et al. (1972) with two constraints: sigma fi = 1, 0 less than or equal to fi less than or equal to 1. pH-profiles of f alpha and fR showed the two-step change, one corresponding to the N-F transition and the other to the acid-expansion in 0.10 M KCl and in 0.02 M NaClO4. pH-profile of f beta showed one-step change, correlating to the later part (lower pH side) of the N-F transition. The N-F transition might thus involve the helix leads to beta and helix leads to coil transitions.

Conformational changes of bovine plasma albumin prior to the salting-out of protein in concentrated salt solution

By working at very low protein concentration (ca. 0.003%), it is possible to measure tryptophyl fluorescence intensity at 350 nm (F350) of bovine plasma albumin (BPA) as a function of pH under precipitating conditions (acidic concentrated salt solutions). Under such conditions, distinct changes in F350 were seen before the starting of precipitation of BPA and no further changes in F350 over the precipitating pH range. Comparison of pH-profiles monitored by F350 with those by solubility in the presence of various salts at various concentrations indicated that the change of solubility is observed after definite changes in conformation of the protein.