A mechanistic analysis of the increase in the thermal stability of proteins in aqueous carboxylic acid salt solutions

Addition of sodium malonate alters the morphology and increases the critical flux during tangential flow filtration of precipitated immunoglobulins

Recent studies have demonstrated that one can control the packing density, and in turn the filterability, of protein precipitates by changing the pH and buffer composition of the precipitating solution to increase the structure/order within the precipitate. The objective of this study was to examine the effect of sodium malonate, which is known to enhance protein crystallizability, on the morphology of immunoglobulin precipitates formed using a combination of ZnCl2 and polyethylene glycol. The addition of sodium malonate significantly stabilized the precipitate particles as shown by an increase in melting temperature, as determined by differential scanning calorimetry, and an increase in the enthalpy of interaction, as determined by isothermal titration calorimetry. The sodium malonate also increased the selectivity of the precipitation, significantly reducing the coprecipitation of DNA from a clarified cell culture fluid. The resulting precipitate had a greater packing density and improved filterability, enabling continuous tangential flow filtration with minimal membrane fouling relative to precipitates formed under otherwise identical conditions but in the absence of sodium malonate. These results provide important insights into strategies for controlling precipitate morphology to enhance the performance of precipitation-filtration processes for the purification of therapeutic proteins.

Elucidating the mechanisms of additive effects at high concentrations on hydrophobic interaction chromatography

Hydrophobic interaction chromatography (HIC) is a commonly used chromatography technique for purifying proteins. It utilizes salting-out salts to facilitate the binding of native proteins to weakly hydrophobic ligands. There have been three proposed mechanisms for the promoting effects of salting-out salts, which include the dehydration of proteins by salts, cavity theory, and salt exclusion. To evaluate the above three mechanisms, an HIC study was conducted on Phenyl Sepharose using four different additives. These additives included a salting-out salt (NH₄)₂SO₄, sodium phosphate that increases the surface tension of water, a salting-in salt MgCl₂, and an amphiphilic protein-precipitant polyethylene glycol (PEG). Results indicated that the first two salts resulted in protein binding, while MgCl₂ and PEG led to flow-through. These findings were then used to interpret the three proposed mechanisms, which showed that MgCl₂ and PEG deviated from the dehydration mechanism, and MgCl₂ also deviated from the cavity theory. The observed effects of these additives on HIC were reasonably well explained for the first time by their interactions with proteins.

Pharmacological targeting of endoplasmic reticulum stress in disease

The accumulation of misfolded proteins in the endoplasmic reticulum (ER) leads to ER stress, resulting in activation of the unfolded protein response (UPR) that aims to restore protein homeostasis. However, the UPR also plays an important pathological role in many diseases, including metabolic disorders, cancer and neurological disorders. Over the last decade, significant effort has been invested in targeting signalling proteins involved in the UPR and an array of drug-like molecules is now available. However, these molecules have limitations, the understanding of which is crucial for their development into therapies. Here, we critically review the existing ER stress and UPR-directed drug-like molecules, highlighting both their value and their limitations.

Recombinant expression and molecular characterization of buffalo sperm lysozyme-like protein 1

Several sperm lysozyme-like genes evolved from lysozyme by successive duplications and mutations; however their functional role in the reproduction of farm animals is not well understood. To understand the function and molecular properties of buffalo sperm lysozyme-like protein 1 (buSLLP1), it was expressed in E. coli; however, it partitioned to inclusion bodies. Lowering of temperature and inducer concentration did not help in the recovery of the expressed protein in the biologically active form. Therefore, buSLLP1 was cloned and expressed in Pichiapink system based on auxotrophic Pichia pastoris in a labscale fermenter. The expressed protein was obtained in flow-through by using a 30 kDa ultrafiltration membrane followed by MonoQ anion exchange chromatography, resulting in a homogenous preparation of 40 mg recombinant buSLLP1 per liter of initial spent culture-supernatant. Circular dichroism spectroscopy showed that recombinant buSLLP1 possessed a native-like secondary structure. The recombinant buSLLP1 also showed thermal denaturation profile typical of folded globular proteins; however, the thermal stability was lower than the hen egg white lysozyme. Binding of buSLLP1 to chitin and zona pellucida of buffalo oocytes showed that the recombinant buSLLP1 possessed a competent binding pocket, therefore, the produced protein could be used to study its functional role in the reproduction of farm animals.

A single point mutation converts a proton-pumping rhodopsin into a red-shifted, turn-on fluorescent sensor for chloride

The visualization of chloride in living cells with fluorescent sensors is linked to our ability to design hosts that can overcome the energetic penalty of desolvation to bind chloride in water. Fluorescent proteins can be used as biological supramolecular hosts to address this fundamental challenge. Here, we showcase the power of protein engineering to convert the fluorescent proton-pumping rhodopsin GR from Gloeobacter violaceus into GR1, a red-shifted, turn-on fluorescent sensor for chloride in detergent micelles and in live Escherichia coli. This non-natural function was unlocked by mutating D121, which serves as the counterion to the protonated retinylidene Schiff base chromophore. Substitution from aspartate to valine at this position (D121V) creates a binding site for chloride. The binding of chloride tunes the pK a of the chromophore towards the protonated, fluorescent state to generate a pH-dependent response. Moreover, ion pumping assays combined with bulk fluorescence and single-cell fluorescence microscopy experiments with E. coli, expressing a GR1 fusion with a cyan fluorescent protein, show that GR1 does not pump ions nor sense membrane potential but instead provides a reversible, ratiometric readout of changes in extracellular chloride at the membrane. This discovery sets the stage to use natural and laboratory-guided evolution to build a family of rhodopsin-based fluorescent chloride sensors with improved properties for cellular applications and learn how proteins can evolve and adapt to bind anions in water.

The Role of Buffers in Wild-Type HEWL Amyloid Fibril Formation Mechanism

Amyloid fibrils, highly ordered protein aggregates, play an important role in the onset of several neurological disorders. Many studies have assessed amyloid fibril formation under specific solution conditions, but they all lack an important phenomena in biological solutions-buffer specific effects. We have focused on the formation of hen egg-white lysozyme (HEWL) fibrils in aqueous solutions of different buffers in both acidic and basic pH range. By means of UV-Vis spectroscopy, fluorescence measurements and CD spectroscopy, we have managed to show that fibrillization of HEWL is affected by buffer identity (glycine, TRIS, phosphate, KCl-HCl, cacodylate, HEPES, acetate), solution pH, sample incubation (agitated vs. static) and added excipients (NaCl and PEG). HEWL only forms amyloid fibrils at pH = 2.0 under agitated conditions in glycine and KCl-HCl buffers of high enough ionic strength. Phosphate buffer on the other hand stabilizes the HEWL molecules. Similar stabilization effect was achieved by addition of PEG12000 molecules to the solution.

Mechanism-based rescue of Munc18-1 dysfunction in varied encephalopathies by chemical chaperones

Heterozygous de novo mutations in the neuronal protein Munc18-1 are linked to epilepsies, intellectual disability, movement disorders, and neurodegeneration. These devastating diseases have a poor prognosis and no known cure, due to lack of understanding of the underlying disease mechanism. To determine how mutations in Munc18-1 cause disease, we use newly generated S. cerevisiae strains, C. elegans models, and conditional Munc18-1 knockout mouse neurons expressing wild-type or mutant Munc18-1, as well as in vitro studies. We find that at least five disease-linked missense mutations of Munc18-1 result in destabilization and aggregation of the mutant protein. Aggregates of mutant Munc18-1 incorporate wild-type Munc18-1, depleting functional Munc18-1 levels beyond hemizygous levels. We demonstrate that the three chemical chaperones 4-phenylbutyrate, sorbitol, and trehalose reverse the deficits caused by mutations in Munc18-1 in vitro and in vivo in multiple models, offering a novel strategy for the treatment of varied encephalopathies.

Munc18-1 is an evolutionary conserved gene whose mutations are linked to various neurological diseases in human. In order to better understand the exact nature of the mutations, the authors here utilize several model systems to show mutant Munc18-1 can aggregate and deplete functional pool of Wt protein, and that chemical chaperones can reverse the cellular deficits.

Role of Buffers in Protein Formulations

Buffers comprise an integral component of protein formulations. Not only do they function to regulate shifts in pH, they also can stabilize proteins by a variety of mechanisms. The ability of buffers to stabilize therapeutic proteins whether in liquid formulations, frozen solutions, or the solid state is highlighted in this review. Addition of buffers can result in increased conformational stability of proteins, whether by ligand binding or by an excluded solute mechanism. In addition, they can alter the colloidal stability of proteins and modulate interfacial damage. Buffers can also lead to destabilization of proteins, and the stability of buffers themselves is presented. Furthermore, the potential safety and toxicity issues of buffers are discussed, with a special emphasis on the influence of buffers on the perceived pain upon injection. Finally, the interaction of buffers with other excipients is examined.

In vitro assessment of choline dihydrogen phosphate (CDHP) as a vehicle for recombinant human interleukin-2 (rhIL-2)

Choline dihydrogen phosphate (CDHP) is an ionic liquid reported to increase thermal stability of model proteins. The current work investigated CDHP effect on structural integrity and biological activity of recombinant human interleukin-2 (rhIL-2), a therapeutic protein used for treating advanced melanoma. In vitro CDHP biocompatibility was also evaluated using primary cell cultures, or B16-F10 cell line, chronically exposed to the ionic liquid. Formulation of rhIL-2 in an aqueous 680mM CDHP pH 7.4 solution resulted in a 12.5°C increase in the T_m of rhIL-2 compared to a basic buffer formulation, and provided conformational rhIL-2 stabilization when the solution was heated to 23.3°C above the T_m. CDHP solutions (≤80mM), exhibited no cytotoxic activity toward primary splenocytes or B16-F10 cells in culture. However, a 10-fold loss in biological activity was observed when rhIL-2 was used in a 30mM CDHP aqueous solution with NaHCO₃ (pH≥7.2) compared to controls without CDHP. While increased T_m is associated with a diminished rhIL-2 biological activity, the therapeutic protein remains structurally intact and functional.

Structure and function of proteins in hydrated choline dihydrogen phosphate ionic liquid

Ionic liquids are being intensely studied as promising media for the stabilization of proteins and other biomolecules. Choline dihydrogen phosphate (CDHP) has been identified as one of the most promising candidates for this application. In this work we have probed in more detail the effects that CDHP may have on the thermodynamics, structure, and stability of proteins, including one of therapeutic interest. Microcalorimetry and circular dichroism spectropolarimetry (CD) were used to assess the thermal stability of protein solutions in CDHP/water mixtures at various concentrations. Increasing thermal stability of lysozyme and interleukin-2 in proportion to CDHP concentration was observed. Isothermal titration calorimetry (ITC) was used to quantify binding interactions, and indicate that the mechanism for stability does not appear to be dependent upon CDHP binding to protein. CD and small angle X-ray scattering (SAXS) analyses were used to probe for structural changes due to the presence of CDHP. SAXS indicates charge effects on the surface of the protein play a role in protein stability in ionic liquids, and no significant alteration of the overall tertiary conformation of lysozyme was observed at 25 °C. However, after incubation at 37 °C or at higher concentrations of CDHP, small changes in protein structure were seen. Effects on protein activity were monitored using turbidity assays, and CDHP decreases protein activity but does not eliminate it. Protein solubility was also monitored using a turbidity assay and was found to be inversely proportional to the concentration of CDHP in solution.

Interactions of formulation excipients with proteins in solution and in the dried state

A variety of excipients are used to stabilize proteins, suppress protein aggregation, reduce surface adsorption, or to simply provide physiological osmolality. The stabilizers encompass a wide variety of molecules including sugars, salts, polymers, surfactants, and amino acids, in particular arginine. The effects of these excipients on protein stability in solution are mainly caused by their interaction with the protein and the container surface, and most importantly with water. Some excipients stabilize proteins in solution by direct binding, while others use a number of fundamentally different mechanisms that involve indirect interactions. In the dry state, any effects that the excipients confer to proteins through their interactions with water are irrelevant, as water is no longer present. Rather, the excipients stabilize proteins through direct binding and their effects on the physical properties of the dried powder. This review will describe a number of mechanisms by which the excipients interact with proteins in solution and with various interfaces, and their effects on the physical properties of the dried protein structure, and explain how the various interaction forces are related to their observed effects on protein stability.

Buffer-dependent fragmentation of a humanized full-length monoclonal antibody

During storage stability studies of a monoclonal antibody (mAb) it was determined that the primary route of degradation involved fragmentation into lower molecular weight species. The fragmentation was characterized with size-exclusion high performance liquid chromatography (SE-HPLC), SDS-PAGE, and matrix-assisted laser desorption/ionization time of flight (MALDI-TOF) mass spectrometry. Fragmentation proceeded via hydrolysis, likely catalyzed by trace metal ions, of a peptide bond in the hinge region of the mAb's heavy chain, which produced two prominent low molecular weight species during storage: a single, free Fab fragment and a Fab + Fc fragment. The fragmentation is observed in phosphate-buffered solutions at two ionic strengths but not in histidine-buffered solutions at identical ionic strengths. Chaotrope-induced and thermally induced unfolding studies of the mAb indicated differences in the unfolding pathways between the two buffer solutions. The folding intermediate observed during chaotrope-induced unfolding was further characterized by intrinsic fluorescence quenching, which suggested that a small portion of the molecule is resistant to chaotrope-induced unfolding in histidine buffer systems. The thermally induced unfolding indicates a reduction in cooperativity of the unfolding process in the presence of histidine relative to phosphate. A relationship between the histidine-induced effects on unfolding pathway and the relative resistance to fragmentation is suggested.

Application of hanging drop technique to optimize human IgG formulations

Objectives

The purpose of this work is to assess the hanging drop technique in screening excipients to develop optimal formulations for human immunoglobulin G (IgG).

Methods

A microdrop of human IgG and test solution hanging from a cover slide and undergoing vapour diffusion was monitored by a stereomicroscope. Aqueous solutions of IgG in the presence of different pH, salt concentrations and excipients were prepared and characterized.

Key findings

Low concentration of either sodium/potassium phosphate or McIlvaine buffer favoured the solubility of IgG. Addition of sucrose favoured the stability of this antibody while addition of NaCl caused more aggregation. Antimicrobial preservatives were also screened and a complex effect at different buffer conditions was observed. Dynamic light scattering, differential scanning calorimetry and size exclusion chromatography studies were performed to further validate the results.

Conclusions

In conclusion, hanging drop is a very easy and effective approach to screen protein formulations in the early stage of formulation development.

Inhibition of beta-amyloid peptide aggregation and neurotoxicity by alpha-d-mannosylglycerate, a natural extremolyte

The aggregation of soluble beta-amyloid (Abeta) peptide into oligomers/fibrils is one of the key pathological features in Alzheimer's disease (AD). The use of naturally occurring small molecules for inhibiting protein aggregation has recently attracted many interests due to their effectiveness for treating protein folding diseases such as AD, Parkinson's, Huntington's disease, and other amyloidosis diseases. alpha-d-Mannosylglycerate (MG), a natural extremolyte identified in microorganisms growing under extremely high temperatures up to 100 degrees C, had been shown to protect proteins against various stress conditions such as heat, freezing, thawing, and drying. Here, we report the effectiveness of MG on the suppression of Alzheimer's Abeta aggregation and neurotoxicity to human neuroblastoma cells. According to our study--carried out by using thioflavin-T induced fluorescence, atomic force microscopy, and cell viability assay--MG had significant inhibitory effect against Abeta amyloid formation and could reduce the toxicity of amyloid aggregates to human neuroblastoma cells while MG itself was innocuous to cells. On the other hand, the structural analogs of MG such as alpha-d-mannosylglyceramide, mannose, methylmannoside, glycerol, showed negligible effect on Abeta aggregate formation. The results suggest that MG could be a potential drug candidate for treating Alzheimer's disease.

Effects of salts on protein–surface interactions: applications for column chromatography

Development of protein pharmaceuticals depends on the availability of high quality proteins. Various column chromatographies are used to purify proteins and characterize the purity and properties of the proteins. Most column chromatographies require salts, whether inorganic or organic, for binding, elution or simply better recovery and resolution. The salts modulate affinity of the proteins for particular columns and nonspecific protein-protein or protein-surface interactions, depending on the type and concentration of the salts, in both specific and nonspecific manners. Salts also affect the binding capacity of the column, which determines the size of the column to be used. Binding capacity, whether equilibrium or dynamic (under an approximation of a slow flow rate), depends on the binding constant, protein concentration and the number of the binding site on the column as well as nonspecific binding. This review attempts to summarize the mechanism of the salt effects on binding affinity and capacity for various column chromatographies and on nonspecific protein-protein or protein-surface interactions. Understanding such salt effects should also be useful in preventing nonspecific protein binding to various containers.

Stabilization of yeast hexokinase A by polyol osmolytes: Correlation with the physicochemical properties of aqueous solutions

Osmolytes of the polyol series are known to accumulate in biological systems under stress and stabilize the structures of a wide variety of proteins. While increased surface tension of aqueous solutions has been considered an important factor in protein stabilization effect, glycerol is an exception, lowering the surface tension of water. To clarify this anomalous effect, the effect of a series of polyols on the thermal stability of a highly thermolabile two domain protein yeast hexokinase A has been investigated by differential scanning calorimetry and by monitoring loss in the biological activity of the enzyme as a function of time. A larger increase in the T(m) of domain 1 compared with that of domain 2, varying linearly with the number of hydroxyl groups in polyols, has been observed, sorbitol being the best stabilizer against both thermal as well as urea denaturation. Polyols help retain the activity of the enzyme considerably and a good correlation of the increase in T(m) (DeltaT(m)) and the retention of activity with the increase in the surface tension of polyol solutions, except glycerol, which breaks this trend, has been observed. However, the DeltaT(m) values show a linear correlation with apparent molal heat capacity and volume of aqueous polyol solutions including glycerol. These results suggest that while bulk solution properties contribute significantly to protein stabilization, interfacial properties are not always a good indicator of the stabilizing effect. A subtle balance of various weak binding and exclusion effects of the osmolytes mediated by water further regulates the stabilizing effect. Understanding these aspects is critical in the rational design of stable protein formulations.

Compatible solutes of the hyperthermophile Palaeococcus ferrophilus: osmoadaptation and thermoadaptation in the order thermococcales

The effect of salinity and growth temperature on the accumulation of intracellular organic solutes was examined in the hyperthermophilic archaeon Palaeococcus ferrophilus. The genus Palaeococcus represents a deep-branching lineage of the order Thermococcales, which diverged before Thermococcus and Pyrococcus. Palaeococcus ferrophilus accumulated mannosylglycerate, glutamate, and aspartate as major compatible solutes. Unlike members of the genera Pyrococcus and Thermococcus, Palaeococcus ferrophilus did not accumulate di-myo-inositol phosphate, a canonical solute of hyperthermophiles. The level of mannosylglycerate increased in response to both heat and salt stress; glutamate increased at supraoptimal growth temperatures, whereas aspartate increased at supraoptimal salt concentration. Proline, alanine, and trehalose were also found in lesser amounts, but their levels did not respond significantly to any of the stresses imposed. Additionally, the genes involved in the synthesis of mannosylglycerate in Palaeococcus ferrophilus and Thermococcus litoralis were identified. In both organisms the synthesis proceeds via the two-step pathway comprising mannosyl-3-phosphoglycerate synthase (MPGS) (EC 2.4.1.217) and mannosyl-3-phosphoglycerate phosphatase (MPGP) (EC 3.1.3.70). The mpgS and mpgP genes of Palaeococcus ferrophilus were expressed in Escherichia coli and the proteins were characterized. MPGS had maximal activity at 90 degrees C and pH near 7.0, and was strictly dependent on Mg2+. MPGP had optimal activity at 90 degrees C and pH 6.0 and was barely dependent on Mg2+. The half-life values for inactivation of MPGS and MPGP at 83 degrees C were 18 and 25 min, respectively. A comparative discussion of the osmo- and thermoadaptation strategies in these three genera of the Thermococcales is presented.

Stress response by solute accumulation in archaea

The accumulation of organic solutes is a prerequisite for osmotic adjustment of all organisms. Archaea synthesize unusual solutes such as beta-amino acids, Nepsilon-acetyl-beta-lysine, mannosylglycerate and di-myo-inositol phosphate but, as in other cells, uptake of solutes such as glycine betaine is preferred over de novo synthesis. Study of the molecular basis of osmoadaptation and its regulation in archaea is still in its infancy, but genomics and functional genome analyses combined with classical biochemistry shed light on the processes that confer osmoadaptation in archaea. Most interestingly, some solutes are not only produced in response to salt but also to temperature stress.

Effective elution of antibodies by arginine and arginine derivatives in affinity column chromatography

It has been shown that the recovery of monomeric antibodies from protein A affinity chromatography is enhanced significantly by using arginine as an eluent. To extend the applications of arginine to antibody purification and obtain an insight into the mechanism of arginine elution, we compared arginine with citrate, guanidine hydrochloride (GdnHCl), arginine derivatives, and other amino acids in protein A chromatography. We also applied arginine to elution of polyclonal antibodies (pAbs) in antigen affinity chromatography. As described previously, arginine was effective in eluting monoclonal antibodies IgG1 and IgG4. Two arginine derivatives, acetyl-arginine and agmatine, resulted in efficient elution at pH 4.0 or higher, and this was comparable to arginine. On the other hand, other amino acids, such as glycine, proline, lysine, and histidine, are much less effective than arginine under identical pH conditions. Whereas elution increased with arginine concentration, elution with citrate was insignificant in excess of 1 M at pH 4.3. Arginine was also effective in fractionation of pAbs using antigen-conjugated affinity columns. Although GdnHCl was also effective under similar conditions, the eluted material showed more aggregation than did the protein eluted by arginine.

Effects of pH, salt, and macromolecular crowding on the stability of FK506-binding protein: an integrated experimental and theoretical study

Environmental variables can exert significant influences on the folding stability of a protein, and elucidating these influences provides insight on the determinants of protein stability. Here, experimental data on the stability of FKBP12 are reported for the effects of three environmental variables: pH, salt, and macromolecular crowding. In the pH range of 5-9, contribution to the pH dependence of the unfolding free energy from residual charge-charge interactions in the unfolded state was found to be negligible. The negligible contribution was attributed to the lack of sequentially nearest neighboring charged residues around groups that titrate in the pH range. KCl lowered the stability of FKBP12 and the E31Q/D32N double mutant at small salt concentrations but raised stability after approximately 0.5 M salt. Such a turnover behavior was accounted for by the balance of two opposing types of protein-salt interactions: the Debye-Hückel type, modeling the response of the ions to protein charges, favors the unfolded state while the Kirkwood type, accounting for the disadvantage of the ions moving toward the low-dielectric protein cavity from the bulk solvent, disfavors the unfolded state. Ficoll 70 as a crowding agent was found to have a modest effect on protein stability, in qualitative agreement with a simple model suggesting that the folded and unfolded states are nearly equally adversely affected by macromolecular crowding. For any environmental variable, it is the balance of its effects on the folded and unfolded states that determines the outcome on the folding stability.

Modulation of tetracycline-phospholipid interactions by tuning of pH at the water-air interface

This paper is part of a systematic study of the interactions of tetracycline antibiotics with phospholipid monolayers at the water-air interface. Tetracyclines are widespread antibiotics that undergo a series of protonation equilibria in solution, depending on the pH. The surface activity of tetracyclines was determined by means of surface tension measurements for three different systems, i.e. water, TRIS and McIlvaine-EDTA buffer. Surface pressure-molecular area and surface potential-molecular area isotherms were acquired for dipalmitoylphosphatidic acid monolayers on TRIS buffer (pH=7.0) and McIlvaine-EDTA buffer (pH=4.0) solution as a function of tetracycline concentration in the subphase. Comparative analysis of surface potential data, with the molecular dipole moment of tetracycline obtained from semiempirical calculations, provided information on the orientation of tetracycline at the interface. Surface pressure measurements as a function of monolayer compression were described, applying either a continuous partition model or Langmuir adsorption isotherms. The results obtained in the case of buffer solutions were compared to those obtained for tetracycline in water subphases. The analysis of the results indicated that electrostatic interactions dictate the migration of tetracycline to the monolayer interface. Penetration of the molecule to the lipophilic portion of the monolayer was unlikely, especially at high surface pressures. The results showed that stronger interactions are established between the zwitterionic tetracycline and the deprotonated phosphatidic group in TRIS buffer solution; in this case, tetracycline binds at the monolayer interface following a Langmuir type adsorption. In the case of water, where the monodeprotonated acid and the tetracycline zwitterions are the only species involved, the data can be described by continuous partition of tetracycline between interfacial and bulk phases. The same holds for McIlvaine-EDTA buffer subphases, although the high concentrations of citrate ions in this buffer competitively interfere with tetracycline association at the monolayer interface.

Protein Stabilization by Osmolytes from Hyperthermophiles

2-O-alpha-Mannosylglycerate, a negatively charged osmolyte widely distributed among (hyper)thermophilic microorganisms, is known to provide notable protection to proteins against thermal denaturation. To study the mechanism responsible for protein stabilization, pico-second time-resolved fluorescence spectroscopy was used to characterize the thermal unfolding of a model protein, Staphylococcus aureus recombinant nuclease A (SNase), in the presence or absence of mannosylglycerate. The fluorescence decay times are signatures of the protein state, and the pre-exponential coefficients are used to evaluate the molar fractions of the folded and unfolded states. Hence, direct determination of equilibrium constants of unfolding from molar fractions was carried out. Van't Hoff plots of the equilibrium constants provided reliable thermodynamic data for SNase unfolding. Differential scanning calorimetry was used to validate this thermodynamic analysis. The presence of 0.5 m potassium mannosylglycerate caused an increase of 7 degrees C in the SNase melting temperature and a 2-fold increase in the unfolding heat capacity. Despite the considerable degree of stabilization rendered by this solute, the nature and population of protein states along unfolding were not altered in the presence of mannosylglycerate, denoting that the unfolding pathway of SNase was unaffected. The stabilization of SNase by mannosylglycerate arises from decreased unfolding entropy up to 65 degrees C and from an enthalpy increase above this temperature. In molecular terms, stabilization is interpreted as resulting from destabilization of the denatured state caused by preferential exclusion of the solute from the protein hydration shell upon unfolding, and stabilization of the native state by specific interactions. The physiological significance of charged solutes in hyperthermophiles is discussed.

Gelatin degradation at elevated temperature

Four gelatin types (A, B, C and AB), two different samples of each, were subjected to temperature treatments with the incubation temperature, incubation time, gelatin concentration and solvent (type and concentration of salt ions and pH) as variables. Degradation was studied by means of fast protein liquid chromatography and sodium dodecylsulphate polyacrylamide gel electrophoresis. All the variables tested seemed to be critical. Addition of a protease inhibitor cocktail confirmed that the observed degradation was not due to the action of proteases.Fluorescence measurements indicated that during the temperature treatment pentosidine and pyridinoline cross-links can be broken, while the cleavage of peptide bonds was verified by ninhydrin tests and N-terminal amino-acid analyses with phenyl isothiocyanate.

Why is trehalose an exceptional protein stabilizer? An analysis of the thermal stability of proteins in the presence of the compatible osmolyte trehalose

Trehalose, a naturally occurring osmolyte, is known to be an exceptional stabilizer of proteins and helps retain the activity of enzymes in solution as well as in the freeze-dried state. To understand the mechanism of action of trehalose in detail, we have conducted a thorough investigation of its effect on the thermal stability in aqueous solutions of five well characterized proteins differing in their various physico-chemical properties. Among them, RNase A has been used as a model enzyme to investigate the effect of trehalose on the retention of enzymatic activity upon incubation at high temperatures. 2 m trehalose was observed to raise the transition temperature, Tm of RNase A by as much as 18 degrees C and Gibbs free energy by 4.8 kcal mol-1 at pH 2.5. There is a decrease in the heat capacity of protein denaturation (DeltaCp) in trehalose solutions for all the studied proteins. An increase in the DeltaG and a decrease in the DeltaCp values for all the proteins points toward a general mechanism of stabilization due to the elevation and broadening of the stability curve (DeltaG versus T). A direct correlation of the surface tension of trehalose solutions and the thermal stability of various proteins has been observed. Wyman linkage analysis indicates that at 1.5 m concentration 4-7 molecules of trehalose are excluded from the vicinity of protein molecules upon denaturation. We further show that an increase in the stability of proteins in the presence of trehalose depends upon the length of the polypeptide chain. The pH dependence data suggest that even though the charge status of a protein contributes significantly, trehalose can be expected to work as a universal stabilizer of protein conformation due to its exceptional effect on the structure and properties of solvent water compared with other sugars and polyols.

Biphasic reductive unfolding of ribonuclease A is temperature dependent

The kinetics of the reversible thermal unfolding, irreversible thermal unfolding, and reductive unfolding processes of bovine pancreatic ribonuclease A (RNase A) were investigated in NaCl/Pi solutions. Image parameters including Shannon entropy, Hamming distance, mutual information and correlation coefficient were used in the analysis of the CD and 1D NMR spectra. The irreversible thermal unfolding transition of RNase A was not a cooperative process, pretransitional structure changes occur before the main thermal denaturation. Different dithiothreitol (dithiothreitolred) concentration dependencies were observed between 303 and 313 K during denaturation induced by a small amount of reductive reagent. The protein selectively follows a major unfolding kinetics pathway with the selectivity can be altered by temperature and reductive reagent concentration. Two possible explanations of the selectivity mechanism were discussed.

Gln277 and Phe554 residues are involved in thermal inactivation of protective antigen of Bacillus anthracis

Protective antigen (PA) is the main component of all the vaccines against anthrax. The currently available vaccines have traces of other proteins that contribute to its reactogenicity. Thus, purified PA is recommended for human vaccination. PA loses its biological activity within 48h at 37 degrees C and its thermolability has been a cause of concern as accidental exposure to higher temperatures during transportation or storage could decrease its efficacy. In the present study, we have used protein engineering approach to increase the thermostability of PA by mutating amino acid residues on the surface as well as the interior of the protein. After screening several mutants, the mutants Gln277Ala and Phe554Ala have been found to be more thermostable than the wild-type PA. Gln277Ala retains approximately 45% and Phe554Ala retains approximately 90% activity, even after incubation at 37 degrees C for 48h while in the same period wild-type PA loses its biological activity completely. It is the first report of increasing thermostability of PA using site-directed mutagenesis. Generation of such mutants could pave the way for better anthrax vaccines with longer shelf life.

The unusually slow relaxation kinetics of the folding-unfolding of pyrrolidone carboxyl peptidase from a hyperthermophile, Pyrococcus furiosus

In order to understand the thermodynamic and kinetic basis of the intrinsic stability of proteins from hyperthermophiles, the folding-unfolding reactions of cysteine-free pyrrolidone carboxyl peptidase (Cys142/188Ser) (PCP-0SH) from Pyrococcus furiosus were examined using circular dichroism (CD) and differential scanning calorimetry (DSC) at pH 2.3, where PCP-0SH exists in monomeric form. DSC showed a strong dependence of the shape and position of the unfolding profiles on the scan rate, suggesting the stability of PCP-0SH under kinetic control. On DSC timescales, even at a scan rate of 1 deg. C/hour, heat denaturation of PCP-0SH was non-equilibrium. However, over a long period of incubation of the heat-denatured PCP-0SH at pre-transition temperatures, it refolded completely, indicating reversibility with very slow relaxation kinetics. The rates of refolding of the heat-denatured PCP-0SH determined from the time-resolved DSC and CD spectroscopic progress curves were found to be similar within experimental error, confirming the mechanism of refolding to be a two-state process. The equilibrium established with a relaxation time of 5080 seconds (at t(m)=46.5 degrees C), which is unusually higher than the relaxation times observed for mesophilic and hyperthermophilic proteins. The long relaxation time may lead to the apparent irreversibility of an unfolding process occurring on the DSC experiment timescale. The refolding rate (9.8 x 10(-5) s(-1)) peaked near the t(m) (=46.5 degrees C), whereas the stability profile reached maxima (11.8 kJ mol(-1)) at 17 degrees C. The results clearly indicate the unusual mode of protein destabilization via a drastic decrease in the rate of folding at low pH and still maintaining a high activation energy barrier (284 kJ mol(-1)) for unfolding, which provides an effective kinetic advantage to unusually stable proteins from hyperthermophiles.

Thermodynamic analysis of biomolecular interactions

Direct measurement of the thermodynamics of biomolecular interactions is now relatively easy. Interpretation of these thermodynamics in simple molecular terms is not. Recent work shows how the multiplicity of weak noncovalent interactions, and the inevitable enthalpy/entropy compensation that these interactions engender, lead to difficulties in teasing out the different components.