Secondary structure changes of disulfide bridge-cleaved bovine serum albumin in solutions of urea, guanidine hydrochloride, and sodium dodecyl sulfate

Removal of Dodecyl Sulfate Ions Bound to Human and Bovine Serum Albumins Using Sodium Cholate

The secondary structures of human serum albumin (HSA) and bovine serum albumin (BSA) were disrupted in the solution of sodium dodecyl sulfate (SDS), while being hardly damaged in the solution of the bile salt, sodium cholate (NaCho). In the present work, the removal of dodecyl sulfate (DS) ions bound to these proteins was attempted by adding various amounts of NaCho. The extent of removal was estimated by the restoration of α-helical structure of each protein disrupted by SDS. Increases and decreases in α-helical structure were examined using the mean residue ellipticity at 222 nm, [θ]₂₂₂, which was frequently used as a measure of α-helical structure content. The magnitudes of [θ]₂₂₂ of HSA and BSA, weakened by SDS, were restrengthened upon the addition of NaCho. This indicated that the α-helical structures of HSA and BSA that were disrupted by the binding of DS ions were nearly reformed by the addition of NaCho. The NaCho concentration at which the maximum restoration of [θ]₂₂₂ of each protein was attained increased nearly linearly with SDS concentration. These results indicated that most of the bound DS ions were removed from the proteins but the removal was incomplete. The removal of DS ions, examined by means of the equilibrium dialysis, was also incomplete. The α-helical structure restoration and the DS ion removal by NaCho were considered to be due to the ability of cholate anions to strip the surfactant ions bound to HSA and BSA. These stripped DS ions appeared to be more likely to form SDS-NaCho mixed micelles in bulk rather than SDS-NaCho mixed aggregates on the proteins.

Interactions between surfactants and the skin - Theory and practice

One of the primary causes of skin irritation is the use of body wash cosmetics and household chemicals, since they are in direct contact with the skin, and they are widely available and frequently used. The main ingredients of products of this type are surfactants, which may have diverse effects on the skin. The skin irritation potential of surfactants is determined by their chemical and physical properties resulting from their structure, and specific interactions with the skin. Surfactants are capable of interacting both with proteins and lipids in the stratum corneum. By penetrating through this layer, surfactants are also able to affect living cells in deeper regions of the skin. Further skin penetration may result in damage to cell membranes and structural components of keratinocytes, releasing proinflammatory mediators. By causing irreversible changes in cell structure, surfactants can often lead to their death. The paper presents a critical review of literature on the effects of surfactants on the skin. Aspects discussed in the paper include the skin irritation potential of surfactants, mechanisms underlying interactions between compounds of this type and the skin which have been proposed over the years, and verified methods of reducing the skin irritation potential of surfactant compounds. Basic research conducted in this field over many years translate into practical applications of surfactants in the cosmetic and household chemical industries. This aspect is also emphasized in the present study.

Homogeneous and heterogeneous dynamics in native and denatured bovine serum albumin

Quasielastic incoherent neutron spectroscopy experiments reveal that chemical denaturation significantly modifies the internal dynamics of bovine serum albumin.

Structural aspects of a protein-surfactant assembly: native and reduced States of human serum albumin

The inherently present seventeen disulfide bonds of the circulatory protein, human serum albumin (HSA) provide the necessary structural stability. Various spectroscopic approaches were used to investigate the effect of reduction of these disulfide bonds and its binding with the anionic surfactant, sodium dodecyl sulfate (SDS). Based on several spectroscopic analyses, our investigations highlight the following interesting aspects: (1) HSA on reduction loses not only its tertiary structure but also a significant amount of secondary structure as well. However, the reduced state of the protein is not like the molten-globule, (2) this structural loss of the protein due to reduction is more prominent than that caused by higher SDS concentrations alone and can certainly be attributed to the role of disulfide bonds, (3) lower surfactant concentrations provide marginal structural rigidity to the native state of the protein, whereas, higher concentrations of SDS induces secondary structure to the reduced state of HSA, (4) the binding of SDS with both the native and reduced states of HSA, occurred in three distinct stages which was followed by a saturation stage. However, the nature of such binding is different for both the states as investigated by using the Stern-Volmer equations and estimating the thermodynamic parameters. Besides, in contrast to the native state, the reduced state of HSA shows that the lone tryptophan residue gets more buried. However, there occurs a sudden decrement in the lifetime of the tryptophan and the hydrodynamic diameter increases by twofold.

Protein structural change in a mixed system of ionic and zwitterionic surfactants

The secondary structure of bovine serum albumin (BSA) in the binary surfactant system of anionic sodium dodecyl sulfate (SDS) and zwitterionic N-dodecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (DDAPS) was examined at 25°C. The helicity of BSA decreased from 66% to 55% in a solution of DDAPS alone and decreased to 50% in a solution of SDS alone. However, the late addition of DDAPS reformed the helical structure of BSA, which was initially disrupted by SDS. The reformation required higher DDAPS concentrations as the initial SDS concentration increased. A maximum helicity of 63% was attained by this reformation. On the other hand, the helical structure of the protein, which was first affected by DDAPS denaturation, was also reformed to the same degree by the late addition of certain amounts of SDS. Although attention was paid to the additive order of these two surfactants to BSA, the final helicity of the protein depended on the final concentrations of these two surfactants, irrespective of the additive order. These phenomena may be attributed to the predominance of mixed micelle formation over complex formation between BSA and the two surfactants below the mixing ratio of DDAPS ([DDAPS]/([DDAPS]+[SDS])) of 0.95. The predominance of the mixed micelle formation distinctly appeared in mixing ratios between 0.50 and 0.75. In this range, the mixed micelle formation accompanied the removal of dodecyl sulfate (DS) ions bound to BSA upon the late addition of DDAPS to the BSA-SDS mixture, whereas, upon the late addition of SDS to the BSA-DDAPS mixture, the mixed micelle formation was accelerated by the coexistence of DDAPS which disturbed the binding of DS ions to the protein.

Role of ApoE in conformation-prone diseases and atherosclerosis

Three isoforms of human plasma apolipoprotein E (apoE) are ligands to lipoprotein receptors and influence in different manner the synthesis and catabolism of pro-atherogenic triglyceride-rich lipoproteins. Among three isoforms, the apoE4 isoform is associated with increased frequency of atherosclerosis and Alzheimer's disease (AD). The conformational transitions of beta-amyloid (Abeta) influenced by apoE and serum amyloid P (SAP) component are key events in AD development, the accumulation of intermediate diffusible and soluble oligomers of Abeta being of particular significance. SAP and apoE, in a different manner for the three isoforms, serve as "pathological" chaperones during the aggregation of Abeta considered as a conformation-prone process. In turn, apoE consisting of two domains self-associates in solution and intermediate structures differently populated for the three isoforms exist. The different structures of the three isoforms determine their different distribution among various plasma lipoproteins. The structural and metabolic consideration of the common apoE pathway(s) in two pathologies assumes four molecular targets for AD correction: (i) inhibition of the accumulation of diffusible soluble Abeta oligomers; (ii) inhibition of apoE synthesis and secretion by astrocytes, in particular, under lipid-lowering therapy; (iii) inhibition of the binding of apoE and/or SAP to Abeta; (iv) stimulation of the expression of cholesterol transporter ABCA1.

Protein Secondary Structure Controlled with Light and Photoresponsive Surfactants

The interaction of a light-responsive azobenzene surfactant with bovine serum albumin (BSA) has been investigated as a means to examine photoreversible changes in protein secondary structure. The cationic azobenzene surfactant undergoes a reversible photoisomeriztion upon exposure to the appropriate wavelength of light, with the visible-light (trans) form being more hydrophobic and, thus, inducing a greater degree of protein unfolding than the UV-light (cis) form. Fourier transform infrared (FT-IR) spectroscopy is used to provide quantitative information on the secondary structure elements in the protein (alpha-helices, beta-strands, beta-turns, and unordered domains). Comparing the secondary structure changes induced by light illumination in the presence of the photoresponsive surfactant with previous measurements of the tertiary structure of BSA obtained from small-angle neutron scattering (SANS) allows the three discrete conformation changes in BSA to be fully characterized. At low surfactant concentrations, an alpha-helix --> beta-structure rearrangement is observed as the tertiary structure of BSA changes from a heart-shaped to a distorted heart-shaped conformation. Intermediate surfactant concentrations lead to a dramatic decrease in the alpha-helix fraction in favor of unordered structures, which is accompanied by an unfolding of the C-terminal portion of the protein as evidenced from SANS. Further increases in photosurfactant concentration lead to a beta --> unordered transition with the protein adopting a highly elongated conformation in solution. Each of these protein conformational changes can be precisely and reversibly controlled with light illumination, as revealed through FT-IR spectra collected during repeated visible-light <--> UV-light cycles.

Protective effects of small amounts of bis(2-ethylhexyl)sulfosuccinate on the helical structures of human and bovine serum albumins in their thermal denaturations

The protective effect of an anionic double-tailed surfactant, sodium bis(2-ethylhexyl)sulfosuccinate (AOT), on the structures of human serum albumin (HSA) and bovine serum albumin (BSA) in their thermal denaturations was examined by means of circular dichroism measurements. The structural changes of these albumins were reversible in the thermal denaturation below 50 degrees C, but became partially irreversible above this temperature. The effect was observed in the thermal denaturation above 50 degrees C. Although the helicity of HSA decreased from 66% to 44% at 65 degrees C in the absence of the surfactant, the decrement of it was restrained in the coexistence of AOT of extremely low concentrations. When the HSA concentration was 10 muM, the maximal protective effect appeared at 0.15 mM AOT. In the coexistence of the surfactant of this concentration, the helicity was maintained at 58% at 65 degrees C, increasing to the original value upon cooling to 25 degrees C. Beyond 0.15 mM AOT, the helicity sharply decreased until 3 mM AOT. A particular AOT concentration required to induce the maximal protective effect ([AOT]REQ) was examined at different HSA concentrations. [AOT]REQ shifted to higher values with an increase of the protein concentration. From the protein concentration dependences of [AOT]REQ, the maximal protection was estimated to require 8.0 and 5.0 AOT ions per a molecule of HSA and BSA, respectively. The AOT concentration, where the protective effect was observed, was too low to form its micelle-like aggregate. Then the protein structures might be stabilized by a cross-linking of surfactant monomers bound to specific sites. These specific sites might exist between a group of nonpolar residues and a positively charged residue located on several sets of amphiphilic helical rods in the proteins. Such a unique function of the double-tailed ionic surfactant is first presented by its characteristic nature as an amphiphilic material.

Protective effect of small amounts of sodium dodecyl sulfate on the helical structure of bovine serum albumin in thermal denaturation

In the presence of sodium dodecyl sulfate (SDS), the secondary structure of bovine serum albumin (BSA) was almost protected against thermal denaturation above 50 degrees C, where the structural change became irreversible. Beyond 30 degrees C, the helicity (66%) of the protein sharply decreased with rise of temperature. In response to this, the proportions of beta-structure and random coil increased. The helicity and the beta-structural proportion were 44% and 13% at 65 degrees C, respectively. The protective effect was observed upon the coexistence of SDS of extremely low concentrations: the molar ratio of [SDS]/[BSA] of 15 was enough to induce the maximal protective effect on the helical structure of the protein. The maximal protected helicity was 58% at 65 degrees C, increasing to 64% upon cooling down to 25 degrees C. This protective effect became greater with an increase of chain length of alkyl sulfate ion. On the other hand, a cationic surfactant did not protect the BSA structure at all against the thermal denaturation. This protective effect was characterized by the specific amphiphilic nature of anionic surfactant. Such an anionic surfactant is considered to protect the protein structure by building bridges between particular nonpolar residues and particular positively charged residues located on different loops of the protein.

The formulation of biopharmaceutical products

Biopharmaceutical products represent a diverse group of products that includes proteins, peptides, nucleic acids, whole cells, viral particles and vaccines. The conformation of the macromolecule or cell must be maintained to retain biological activity, and animal models for biological activity and characterization assays are often developed in tandem with initial formulation studies. This presents the formulation scientist with a unique set of challenges when compared to those for small molecules. This review focuses on approaches to the formulation of macromolecules into biopharmaceutical products, and provides examples of studies that have been undertaken within the authors' laboratories.

Fluorescence behavior of tryptophan residues of bovine and human serum albumins in ionic surfactant solutions: a comparative study of the two and one tryptophan(s) of bovine and human albumins

The fluorescence behavior of two tryptophans (Trp-134, Trp-213) in bovine serum albumin (BSA) and a single tryptophan (Trp-214) in human serum albumin (HSA) was examined. The maximum emission wavelength (lambda max) was 340.0 nm for both proteins. In a solution of sodium dodecyl sulfate (SDS), the lambda max of BSA abruptly shifted to 332 nm at 1 mM SDS and then reversed to 334 nm at 3 mM SDS. The lambda max of HSA gradually shifted to 330 nm below 3 mM SDS, although it returned to 338 nm at 10 mM SDS. In contrast to this, in a solution of dodecltrimethylammonium bromide, the lambda max positions of BSA and HSA gradually shifted to 334.0 and 331.5 nm, respectively. Differences in the fluorescence behavior of the proteins are attributed to the fact that Trp-134 exists only in BSA, with the assumption that Trp-213 of BSA behaves the same as Trp-214 of HSA. The Trp-134 behavior appears to relate to the disruption of the helical structure in the SDS solution.

Osmotic pressure method to measure salt induced folding/unfolding of bovine serum albumin

A new approach has been developed to monitor protein folding by utilizing osmotic pressure and a range of salt concentrations in a well characterized protein, bovine serum albumin (BSA). It is hypothesized that both the 'effective' osmotic molecular weight, Ae, and the solute/solvent interaction parameter, I, in the empirical relation Msolvent/Msolute = (RT rho/Ae)1/pi + I [1] can be used as measures of protein folding. I is a measure of solvent perturbed by the solute and is thought to depend directly upon the solvent accessible surface area (ASA). It is reasoned that larger solvent accessible surface area of an unfolded or denatured protein should perturb more water and produce larger I-values. Thus I-values allow calculation of a unfolded protein fraction, fua, due to changes in relative solvent accessible surface area. It has been observed that Ac decreases for filamentous, denatured proteins due to segmental motion of the molecule [2]. This allows calculation of unfolded protein fraction from the effective molecular weight, fum. Colloid osmotic pressure of BSA was measured in a range of salt concentrations at 25 degrees C, and pH = 7 (above the isoelectric point of BSA at pH = 5.4). Both S and I were used to monitor protein folding as the salt concentration was varied. In general, larger and variable I-values and smaller Ae were observed at salt concentrations less than 50 mmolal NaCl (Imax = 8.9), while constant I = 4.1 and Ae = 66,500 were observed above 50 mmolal NaCl. The two expressions for fractional unfolding (fua and fum) are in general agreement. Small differences in the parameters below 50 mmolal salt concentration are explained with well known shifts in the relative amounts of alpha-helix, beta-sheet and random coil in denatured BSA. The relative amounts of these shifts agree with predictions in the literature attributed to continuous BSA expansion rather than an 'all-or-none' conversion.

The interaction of human serum albumin in the native and fully reduced states with apolipoprotein E, serum amyloid protein and very low density lipoproteins from human plasma

1. Complex formation in a solution of apolipoprotein E (apoE) isolated from human plasma very low density lipoproteins (VLDL) and human serum albumin (HSA) in both native and fully reduced states was studied. The existence of a kinetically unstable complex of apoE and native albumin was shown. The complex became more stable with the reduction of the S--S links in the albumin molecules capable of forming aggregates under these conditions. 2. The interaction between native HSA as opposed to a fully reduced one and isolated VLDL particles was more pronounced, probably, due to the existence of amphipathic alpha-helical regions. 3. Dissociation of the serum amyloid protein (SAP) oligomeric form in solution and the interaction of the protein with fully reduced HSA owing to the provision with the additional hydrophobic surface was shown. ApoE displaced SAP from the complex with fully reduced albumin. 4. It is suggested that the ability of the apolipoprotein to interact with albumin is determined by internal stability of the molecular structure of the latter and the complexes detected in vitro may be a new transport form of apolipoproteins in lipid-free form in serum. It is assumed that competitive interactions in the HSA-SAP-apoE system may be involved in the development of secondary amyloidosis.

Conformational stability of alpha-lactalbumin missing a peptide bond between Asp66 and Pro67

The peptide bond between Asp66-Pro67 of alpha-lactalbumin was cleaved with formic acid (cleaved alpha-lactalbumin). Secondary structural changes of the cleaved alpha-lactalbumin, in which the two separated polypeptides were joined by disulfide bridges, were examined in solutions of sodium dodecyl sulfate (SDS), urea, and guanidine hydrochloride. The structural changes of the cleaved alpha-lactalbumin were compared with those of the intact protein. The relative proportions of secondary structures were determined by curve fitting of the circular dichroism. The cleaved alpha-lactalbumin contained 29% alpha-helical structure as against 34% for the intact protein. Some helices of the cleaved alpha-lactalbumin which had been disrupted by the cleavage appeared to be reformed upon the addition of SDS of very low concentration (0.5 mM). In the SDS solution, the helicities of both the intact and cleaved proteins increased, attaining 44% at 4 mM SDS. On the other hand, the helical structures of the cleaved alpha-lactalbumin began to be disrupted at low concentrations of guanidine hydrochloride and urea compared with that of the intact protein. However, no difference was observed in the thermal denaturations of the intact and cleaved proteins, except for the difference in the original helicities. The helicities of both proteins decreased with an increase of temperature up to 65 degrees C and recovered upon cooling.

Conformational changes of alpha-lactalbumin and its fragment, Phe31-Ile59, induced by sodium dodecyl sulfate

Conformational changes of bovine alpha-lactalbumin in sodium dodecyl sulfate (SDS) solution were studied with the circular dichroism (CD) method using a dilute phosphate buffer of pH 7.0 and ionic strength 0.014. The proportions of alpha-helix and beta-structure in alpha-lactalbumin were 34% and 12%, respectively, in the absence of SDS. In the SDS solution, the helicity increased to 44%, while the beta-structure disappeared. In order to verify the structural change from beta-structure to alpha-helix, the moiety, assuming the beta-structure in the alpha-lactalbumin, was isolated by a chymotryptic digestion. The structure of this alpha-lactalbumin fragment, Phe31-Ile59, was almost disordered. However, the fragment adopted a considerable amount of alpha-helical structure in the SDS solution. On the other hand, the tertiary structure of alpha-lactalbumin, detected by changes of CD in the near-ultraviolet region, began to be disrupted before the secondary structural change in the surfactant solution. Dodecyl sulfate ions of 80 mol were cooperatively bound to alpha-lactalbumin. Although the removal of the bound dodecyl sulfate ions was tried by the dialysis against the phosphate buffer for 5 days, 4 mol dodecyl sulfates remained per mole of the protein. The remaining amount agreed with the number of stoichiometric binding site, determined by the Scatchard plot, indicating that the stoichiometric binding was so tight.

Secondary structural changes of large and small fragments of bovine serum albumin in thermal denaturation and in sodium dodecyl sulfate denaturation

The helicities in various fragments of bovine serum albumin (BSA) were examined in the thermal denaturation and in sodium dodecyl sulfate (SDS) denaturation. The thermal denaturation was examined in a temperature range between 2 and 65 degrees C. The helicity decreased with a rise of temperature and it recovered to some degree upon cooling temperature. A rather high reversibility was observed in the BSA fragments, which were located in the N-terminal of the parent protein and then contained the first large loop with no disulfide bridge. The high reversibility was available also for the helicity in the first large loop of the fragment, disulfide bridges of which were reduced. The fragments, which were smaller than one domain, became unstable in the SDS denaturation. The helicities of such fragments decreased in lower SDS concentrations compared with those of the intact BSA and the large fragments, which contained one or more domains. A resistance to the SDS denaturation appeared in the helices of every large loop even after the fragmentation. On the other hand, helicities of the fragments decreased to 20-25% upon the reduction of disulfide bridges. However, the helicities of these fragments increased to 35-40% in the SDS denaturation.

Phosphorescence lifetime studies of interactions between serum albumins and sodium dodecyl sulfate

Binding of sodium dodecyl sulfate (SDS) to bovine serum albumin (BSA) and human serum albumin (HSA) in aqueous solutions at room temperature induces significant changes in the phosphorescence lifetime of tryptophan (Trp) residues. A steep rise of the phosphorescence lifetime from 1.9 ms to 10.0 ms for BSA and from 1.9 ms to 5.5 ms for HSA is observed when the total SDS concentration increases from 0.0 mM to 0.22 mM at 1 mg/mL protein concentration. As the total SDS concentration is further increased to 2.2 mM, a slower increase in the phosphorescence lifetime is observed, from 10.0 ms to 19.5 ms for BSA and from 5.5 ms to 7.2 ms for HSA. It appears that the phosphorescence lifetime modifications are mainly due to an increase of protein matrix rigidity around Trp residues. The observed differences (between HSA and BSA) allow us to distinguish the contribution of the two Trp residues to the BSA phosphorescence.

Secondary structural changes of metmyoglobin and apomyoglobin in anionic and cationic surfactant solutions: effect of the hydrophobic chain length of the surfactants on the structural changes

Secondary structural changes of metmyoglobin and apomyoglobin were examined in solutions of sodium alkylsulfates with hydrocarbon numbers of 8 and 12, and alkyltrimethylammonium bromides with hydrocarbon numbers of 10, 12, 14, and 16. The relative proportion of alpha-helical structure was estimated by the curve-fitting method of circular dichroic spectrum. The helical proportions of metmyoglobin and apomyoglobin were 82 and 63%, respectively. The shorter the hydrocarbon chain the surfactant had, the higher the concentration necessary to disrupt the secondary structures of these proteins. However, the helical proportion had a tendency to decrease down to lower values in solutions of the cationic surfactants with short hydrophobic groups. On the other hand, the alpha-helical structure of apomyoglobin was disrupted in lower concentrations of each cationic surfactant than that of metmyoglobin, although the disruptions of the same structures in both the proteins occurred in the same concentration range of each anionic surfactant. It appeared likely that the removal of the heme group unstabilized the myoglobin conformation only in the cationic surfactant solutions.

Dependence of reaction rate of 5,5'-dithiobis-(2-nitrobenzoic acid) to free sulfhydryl groups of bovine serum albumin and ovalbumin on the protein conformations

The effect of protein conformations on the reaction rate of Ellman's reagent, 5,5'-dithiobis (2-nitrobenzoic acid) (DTNB) with sulfhydryl (SH) groups of proteins was examined. The stopped-flow method was applied to follow the reaction of DTNB with SH group of two proteins, bovine serum albumin (BSA) and ovalbumin (OVA), at various concentrations of guanidine hydrochloride and urea. The rates for both the proteins were faster in guanidine than in urea. The rate sharply depended on the protein conformations, which were monitored by changes of helix contents on the basis of the circular dichroism measurements. The reaction rate of DTNB with SH groups of BSA was maximal around 2 M guanidine and 5 M urea. On the other hand, the reaction rate of DTNB with OVA was maximal at 3.5 M guanidine, while it gradually increased with an increase in the urea concentration. The amount of reactive SH group participating in the reaction with DTNB was also estimated by the absorbance change at 412 nm. The magnitudes of absorbance change for the reaction with free SH groups of OVA at low concentrations of the denaturants were appreciably smaller than those for BSA with one free SH group. Most of the four SH groups of OVA might react with DTNB above 5 M guanidine, although only a part of them did even at 9 M urea.

Changes of fluorescence lifetime and rotational correlation time of bovine serum albumin labeled with 1-dimethylaminonaphthalene-5-sulfonyl chloride in guanidine and thermal denaturations

The nanosecond fluorescence depolarization method was applied to measure the fluorescence lifetime (tau) and the rotational correlation time (phi) of bovine serum albumin (BSA) labeled with 1-dimethylaminonaphthalene-5-sulfonyl chloride (dansyl-Cl). Changes of tau and phi of dansyl BSA in the guanidine denaturation and in the thermal denaturation were examined. In parallel, the secondary structural change of dansyl BSA was followed by circular dichroism measurements. The magnitude of tau was almost unchanged between 1 and 2M guanidine, where the secondary structure of the protein was predominantly disrupted; whereas that of phi began to increase before the disruption of secondary structure in the guanidine denaturation. In the thermal denaturation, in contrast, changes of both tau and phi occurred in a temperature range where the secondary structure was predominantly disrupted. The volume of equivalent sphere (Ve) and the axial ratio (rho) for the BSA were 3.6-3.8 x 10(-19) cm3 and 3.6 at 2 M guanidine as against 2.1 x 10(-19) cm3 and 2.2 in the absence of guanidine (25 degrees C), respectively. The magnitudes of Ve and rho were 4.9 x 10(-19) cm3 and 4.5 at 65 degrees C, respectively. Although the secondary structural change of dansyl BSA was irreversible in the thermal denaturation, Ve and rho were reversible.

Circular dichroism studies on helical structure preferences of amino acid residues of proteins caused by sodium dodecyl sulfate

The extent of helical structure of 19 intact proteins and of 15 proteins with no disulfide bridges in the absence and presence of 10 mM sodium dodecyl sulfate (SDS) was determined using the curve-fitting method of circular dichroic spectra. The change in helicity caused by the addition of SDS was examined as a function of each amino acid fraction. An increase in the helicity upon the addition of SDS occurred in most of the proteins with no disulfide bridges (C proteins) and containing more than 0.06 Lys fraction. In most of the intact proteins (B proteins), most of which contained disulfide bridges, helicity in SDS decreased with an increase in Lys fraction. The helicity of the C proteins in SDS also tended to increase with an increase in the Leu and Phe fractions, while it decreased with an increase in the Gly fraction. For the helicity of the B proteins in SDS, there was a tendency to increase with increased Asn fraction and decrease with increased His fraction. On the other hand, amino acids were divided into eight groups according to their side-chain properties and the conformational preference for each of the amino acid groups of C proteins was calculated using a simple assumption.

Fluorescence lifetime and rotational correlation time of bovine serum albumin-sodium dodecyl sulfate complex labeled with 1-dimethylaminonaphthalene-5-sulfonyl chloride: effect of disulfide bridges in the protein on these fluorescence parameters

A fluorescent dye, 1-dimethylaminonaphthalene-5-sulfonyl chloride, was used to label bovine serum albumin (BSA), intact and disulfide bridges-cleaved. The fluorescence lifetime and fluorescence anisotropy of the adducts in sodium dodecyl sulfate (SDS) solutions were studied by the nanosecond fluorescence depolarization method. The volume of equivalent sphere (Ve) was calculated to be 2.1 x 10(-19) cm3 for BSA with the intact disulfide bridges from the rotational correlation time. The value of Ve was 4.4 x 10(-19) cm3 for the disulfide bridges-cleaved BSA. With an increase in SDS concentration, the rotational correlation time of the intact BSA became longer, while that of the disulfide bridges-cleaved BSA became shorter. This suggests that upon the binding of SDS, the total volume of the intact BSA increases while the expanded state of the protein, caused by the cleavage of the disulfide bridges, becomes compact.

Conformational change of bovine serum albumin by heat treatment

The thermal denaturation of bovine serum albumin (BSA) was studied at pH 2.8 and 7.0 in the range of 2-65 degrees C. The relative proportions of alpha-helix, beta-structure, and disordered structure in the protein conformation were determined as a function of temperature, by the curve-fitting method of circular dichroism spectra. With the rise of temperature at pH 7.0, the proportion of alpha-helix decreased above 30 degrees C and those of beta-structure and disordered structure increased in the same temperature range. The structural change was reversible in the temperature range below 45 degrees C. However, the structural change was partially reversible upon cooling to room temperature subsequent to heating at 65 degrees C. On the other hand, the structural change of BSA at pH 2.8 was completely reversible in the temperature range of 2-65 degrees C, probably because the interactions between domains and between subdomains might disappear due to the acid expansion. The secondary structure of disulfide bridges-cleaved BSA remained unchanged during the heat treatment up to 65 degrees C at pH 2.8 and 7.0.

Circular dichroic study of conformational changes in ovalbumin induced by modification of sulfhydryl groups and disulfide reduction

Sulfhydryl groups of ovalbumin were chemically modified under denaturing conditions in the absence and presence of dithiothreitol, and effects on the secondary structure of the protein were investigated by circular dichroic (CD) measurements. The contents of alpha-helix, beta-structure, and "random coil" (unordered, nonrepetitive structure) were estimated by simulation of the CD spectra and using the parameters established by Chen et al. The principal findings were these: (1) Modification of the four free sulfhydryl groups [with 5,5'-dithiobis(2-nitrobenzoic acid), iodoacetate, or iodoacetamide] caused ovalbumin molecule to unfold partially and to undergo primarily helix-to-beta structure transition. (2) Cleavage of the disulfide bond did not lead to a further conformational change in the sulfhydryl-modified ovalbumin. (3) The remaining helical structure existed in a destabilized state with increased chain flexibility, as the modified protein was very susceptible to denaturation by guanidine and urea. (4) Further evidence for increased chain flexibility was provided by the finding that sodium dodecyl sulfate (SDS) induced helix formation in the sulfhydryl-modified, but not native, ovalbumin. And (5), since both nonreduced and reduced proteins, with their sulfhydryl groups blocked, displayed similar transitions in solutions of guanidine, urea, and SDS suggested that the single disulfide bond did not physically constrain the ovalbumin molecule.

Secondary structural changes in the intact and the disulfide bridges cleaved beta-lactoglobulin A and B in solutions of urea, guanidine hydrochloride, and sodium dodecyl sulfate

The relative proportions of alpha-helix, beta-sheet, and unordered form in beta-lactoglobulin A and B were examined in solutions of urea, guanidine, and sodium dodecyl sulfate (SDS). In the curve-fitting method of circular dichroism (CD) spectra, the reference spectra of the corresponding structures determined by Chen et al. (1974) were modified essentially according to the secondary structure of beta-lactoglobulin B predicted by Creamer et al. (1983), i.e., that the protein has 17% alpha-helix and 41% beta-sheet. The two variants showed no appreciable difference in structural changes. The reduction of disulfide bridges in the proteins increased beta-sheet up to 48% but did not affect the alpha-helical proportion. The alpha-helical proportions of nonreduced beta-lactoglobulin A and B were not affected below 2 M guanidine or below 3 M urea, but those of the reduced proteins began to decrease in much lower concentrations of these denaturants. By contrast, the alpha-helical proportions of the nonreduced and reduced proteins increased to 40-44% in SDS. The beta-sheet proportions of both nonreduced and reduced proteins which remained unaffected even in 6 M guanidine and 9 M urea, decreased to 24-25% in SDS.

Circular dichroic study of the conformational stability of sulfhydryl-blocked bovine serum albumin

1. The blockage of the single sulfhydryl-group of bovine serum albumin does not alter the secondary structure, although the alpha-helical structure is destabilized since lower concentrations of guanidine and of urea unfold the protein. 2. What happens to the previously helical structure depends upon the reagent used to block the sulfhydryl-group. Bovine serum albumin derivatized with 5,5'-dithiobis-(2-nitrobenzoic acid) and iodoacetate preferentially acquire the beta-structure in high concentrations of guanidine and urea, whereas iodoacetamide-derivatized bovine serum albumin acquires primarily the random coil structure. 3. Part of the helical structure is also lost in 5-6 mM sodium dodecyl sulfate; thionitrobenzoate-bovine serum albumin shows an increase in the random coil, whereas the two alkylated proteins display the increase both in beta-structure and random coil. 4. Carboxymethylation or carboxamidomethylation of fully reduced bovine serum albumin results in a drastic change in the secondary structure of the protein with a substantial decrease in alpha-helix and a corresponding increase in both beta-structure and random coil. These extensively alkylated proteins also display differences in denaturation profiles in solutions of guanidine and urea.

Secondary structural changes of non-reduced and reduced ribonuclease A in solutions of urea, guanidine hydrochloride and sodium dodecyl sulfate

The secondary structures of ribonuclease A (RNAase A) before and after reduction of the disulfide bridges and blockage of the thiol groups with iodoacetamide were examined in solutions of urea, guanidine hydrochloride, and sodium dodecyl sulfate (SDS). The relative proportions of alpha-helix, beta-structure, and disordered structure were estimated by the curve-fitting method of circular dichroism (Chen, Y.H., Yang, J.T. and Chau, K.H. (1974) Biochemistry 13, 3350-3359). The native RNAase A, with the disulfide bridges intact, contained 19% helix and 38% beta-structure. Reduction of its disulfide bridges led to a decrease in the proportion of these structures to 9% for the alpha-helix and 17% for the beta-structure. The non-reduced RNAase A resisted unfolding in low concentrations of urea and guanidine hydrochloride. The beta-structure which remained after reduction appeared to be stable even in solutions of 6 M guanidine and 9 M urea. A considerable amount of the beta-structure in both the non-reduced and the reduced RNAase A remained unaffected by high concentrations of SDS.