High-pressure refolding of bikunin: efficacy and thermodynamics

Connecting high-temperature and low-temperature protein stability and aggregation

Protein aggregation is a long-standing problem for preservation of proteins in both laboratory settings and for commercial biotechnology products. It is well established that heating (cooling) can accelerate (slow) aggregation by populating (depopulating) unfolded or partially unfolded monomer states that are key intermediates in aggregation processes. However, there is a long-standing question of whether the same mechanism(s) that lead to aggregation under high-temperature stress are relevant for low-temperature stress such as in refrigerated or supercooled liquids. This report shows the first direct comparison of “hot” and “cold” aggregation kinetics and folding/unfolding thermodynamics, using bovine hemoglobin as a model system. The results suggest that the same mechanism for non-native aggregation holds from “hot” to “cold” temperatures, with an aggregation temperature-of-maximum-stability slightly below 0°C. This highlights that sub-zero temperatures can induce cold-mediated aggregation, even in the absence of freezing stresses. From a practical perspective, the results suggests the possibility that cold-stress may be a useful alternative to heat-stress for extrapolating predictions of protein shelf life at refrigerated conditions, as well as providing a foundation for more mechanistic studies of cold-stress conditions in future work. A comparison between isochoric and isobaric methods is also briefly discussed.

Chaotropic heat treatment resolves native-like aggregation of a heterologously produced hyperthermostable laminarinase

Production of hyperthermostable enzymes in mesophilic hosts frequently causes undesired aggregation of these proteins. During production of Pyrococcus furiosus endo-β-1,3 glucanase (LamA) in Escherichia coli, soluble and insoluble species form. Here, the authors address the composition of this mixture, including the nature of LamA conformers, and establish a method to increase the yield of native monomer. With gel electrophoresis, size-exclusion chromatography, light scattering, circular dichroism and enzyme kinetics the authors show that approximately 50 % of heterologously produced LamA is soluble, and that 40 % of this fraction constitutes native-like oligomers and non-native monomers. Soluble oligomers display, like native LamA monomer, substrate inhibition, although with poor activity. Treatment of soluble oligomers with 3 M guanidinium hydrochloride at 80 °C yields up to 75 % properly active monomer. Non-native monomer shows low specific activity without substrate inhibition. Incubating non-native monomer with 3 M guanidinium hydrochloride at 80 °C causes formation of 25 % native LamA. Also, a large amount of insoluble LamA aggregates can be converted into soluble native monomer by application of this procedure. Thus, chaotropic heat treatment can improve the yield and quality of hyperthermostable proteins that form aberrant species during production in E. coli.

High-yield reactivation of anionic tobacco peroxidase overexpressed in Escherichia coli

Anionic tobacco peroxidase (TOP) is extremely active in chemiluminescence reaction of luminol oxidation without addition of enhancers and more stable than horseradish peroxidase under antibody conjugation conditions. In addition, recombinant TOP (rTOP) produced in Escherichia coli is known to be a perfect direct electron transfer catalyst on electrodes of various origin. These features make the task of development of a high-yield reactivation protocol for rTOP practically important. Previous attempts to reactivate the enzyme from E. coli inclusion bodies were successful, but the reported reactivation yield was only 14%. In this work, we thoroughly screened the refolding conditions for dilution protocol and compared it with gel-filtration chromatography. The impressive reactivation yield in the dilution protocol (85%) was achieved for 8 μg/mL solubilized rTOP protein and the refolding medium containing 0.3 mM oxidized glutathione, 0.05 mM dithiothreitol, 5 mM CaCl2, 5% glycerol in 50 mM Tris-HCl buffer, pH 9.6, with 1 μM hemin added at the 24th hour of incubation. A practically important discovery was a 30-40% increase in the reactivation yield upon delayed addition of hemin. The reactivation yield achieved is one of the highest reported in the literature on protein refolding by dilution. The final yield of purified active non-glycosylated rTOP was ca. 60 mg per L of E. coli culture, close to the yield reported before for tomato and tobacco plants overexpressing glycosylated TOP (60 mg/kg biomass) and much higher than for the previously reported refolding protocol (2.6 mg per L of E. coli culture).

Protein aggregation and its impact on product quality

Protein pharmaceutical products are typically active as folded monomers that are composed of one or more protein chains, such as the heavy and light chains in monoclonal antibodies that are a mainstay of current drug pipelines. There are numerous possible aggregated states for a given protein, some of which are potentially useful, while most of which are considered deleterious from the perspective of pharmaceutical product quality and performance. This review provides an overview of how and why different aggregated states of proteins occur, how this potentially impacts product quality and performance, fundamental approaches to control aggregate formation, and the practical approaches that are currently used in the pharmaceutical industry.

High-pressure fluorescence applications

Fluorescence is the most widely used technique to study the effect of pressure on biochemical systems. The use of pressure as a physical variable sheds light into volumetric characteristics of reactions. Here we focus on the effect of pressure on protein solutions using a simple unfolding example in order to illustrate the applications of the methodology. Topics covered in this review include the relationships between practical aspects and technical limitations; the effect of pressure and the study of protein cavities; the interpretation of thermodynamic and relaxation kinetics; and the study of relaxation amplitudes. Finally, we discuss the insights available from the combination of fluorescence and other methods adapted to high pressure, such as SAXS or NMR. Because of the simplicity and accessibility of high-pressure fluorescence, the technique is a starting point that complements appropriately multi-methodological approaches related to understanding protein function, disfunction, and folding from the volumetric point of view.

Inactivation of caliciviruses

The Caliciviridae family of viruses contains clinically important human and animal pathogens, as well as vesivirus 2117, a known contaminant of biopharmaceutical manufacturing processes employing Chinese hamster cells. An extensive literature exists for inactivation of various animal caliciviruses, especially feline calicivirus and murine norovirus. The caliciviruses are susceptible to wet heat inactivation at temperatures in excess of 60 °C with contact times of 30 min or greater, to UV-C inactivation at fluence ≥30 mJ/cm2, to high pressure processing >200 MPa for >5 min at 4 °C, and to certain photodynamic inactivation approaches. The enteric caliciviruses (e.g.; noroviruses) display resistance to inactivation by low pH, while the non-enteric species (e.g.; feline calicivirus) are much more susceptible. The caliciviruses are inactivated by a variety of chemicals, including alcohols, oxidizing agents, aldehydes, and β-propiolactone. As with inactivation of viruses in general, inactivation of caliciviruses by the various approaches may be matrix-, temperature-, and/or contact time-dependent. The susceptibilities of the caliciviruses to the various physical and chemical inactivation approaches are generally similar to those displayed by other small, non-enveloped viruses, with the exception that the parvoviruses and circoviruses may require higher temperatures for inactivation, while these families appear to be more susceptible to UV-C inactivation than are the caliciviruses.

Refolding by high pressure of a toxin containing seven disulfide bonds: bothropstoxin-1 from Bothrops jararacussu

Aggregation is a serious obstacle for recovery of biologically active heterologous proteins from inclusion bodies (IBs) produced by recombinant bacteria. E. coli transformed with a vector containing the cDNA for Bothropstoxin-1 (BthTx-1) expressed the recombinant product as IBs. In order to obtain the native toxin, insoluble and aggregated protein was refolded using high hydrostatic pressure (HHP). IBs were dissolved and refolded (2 kbar, 16 h), and the effects of protein concentration, as well as changes in ratio and concentration of oxido-shuffling reagents, guanidine hydrochloride (GdnHCl), and pH in the refolding buffer, were assayed. A 32% yield (7.6 mg per liter of bacterial culture) in refolding of the native BthTx-1 was obtained using optimal conditions of the refolding buffer (Tris-HCl buffer, pH 7.5, containing 3 mM of a 2:3 ratio of GSH/GSSG, and 1 M GdnHCl). Scanning electron microscopy (SEM) showed that that disaggregation of part of IBs particles occurred upon compression and that the morphology of the remaining IBs, spherical particles, was not substantially altered. Dose-dependent cytotoxic activity of high-pressure refolded BthTx-1 was shown in C2C12 muscle cells.

Strategies for successful recombinant expression of disulfide bond-dependent proteins in Escherichia coli

Bacteria are simple and cost effective hosts for producing recombinant proteins. However, their physiological features may limit their use for obtaining in native form proteins of some specific structural classes, such as for instance polypeptides that undergo extensive post-translational modifications. To some extent, also the production of proteins that depending on disulfide bridges for their stability has been considered difficult in E. coli.

Both eukaryotic and prokaryotic organisms keep their cytoplasm reduced and, consequently, disulfide bond formation is impaired in this subcellular compartment. Disulfide bridges can stabilize protein structure and are often present in high abundance in secreted proteins. In eukaryotic cells such bonds are formed in the oxidizing environment of endoplasmic reticulum during the export process. Bacteria do not possess a similar specialized subcellular compartment, but they have both export systems and enzymatic activities aimed at the formation and at the quality control of disulfide bonds in the oxidizing periplasm.

This article reviews the available strategies for exploiting the physiological mechanisms of bactera to produce properly folded disulfide-bonded proteins.

Confronting high-throughput protein refolding using high pressure and solution screens

Over-expression of heterologous proteins in Escherichia coli is commonly hindered by the formation of inclusion bodies. Nevertheless, refolding of proteins in vitro has become an essential requirement in the development of structural genomics (proteomics) and as a means of recovering functional proteins from inclusion bodies. Many distinct methods for protein refolding are now in use. However, regardless of method used, developing a reliable protein refolding protocol still requires significant optimization through trial and error. Many proteins fall into the category of "Challenging" or "Difficult to Express" and are problematic to refold using traditional chaotrope-based refolding techniques. This review discusses new methods for improving protein refolding, such as implementing high hydrostatic pressure, using small molecule additives to enhance traditional protein refolding strategies, as well as developing practical methods for performing refolding studies to maximize their reliability and utility. The strategies examined here focus on high-throughput, automated refolding screens, which can be applied to structural genomic projects.

Specific volume and adiabatic compressibility measurements of native and aggregated recombinant human interleukin-1 receptor antagonist: Density differences enable pressure-modulated refolding

High hydrostatic pressures have been used to dissociate non-native protein aggregates and foster refolding to the native conformation. In this study, partial specific volume and adiabatic compressibility measurements were used to examine the volumetric contributions to pressure-modulated refolding. The thermodynamics of pressure-modulated refolding from non-native aggregates of recombinant human interleukin-1 receptor antagonist (IL-1ra) were determined by partial specific volume and adiabatic compressibility measurements. Aggregates of IL-1ra formed at elevated temperatures (55 degrees C) were found to be less dense than native IL-1ra and refolded at 31 degrees C under 1,500 bar pressure with a yield of 57%. Partial specific adiabatic compressibility measurements suggest that the formation of solvent-free cavities within the interior of IL-1ra aggregates cause the apparent increase in specific volume. Dense, pressure-stable aggregates could be formed at 2,000 bar which could not be refolded with additional high pressure treatment, demonstrating that aggregate formation conditions and structure dictate pressure-modulated refolding yields.

Kinetic analysis of amyloid protofibril dissociation and volumetric properties of the transition state

We present here the first detailed kinetic analysis of the dissociation reaction of amyloid protofibrils by utilizing pressure as an accelerator of the reaction. The experiment is carried out on an excessively diluted typical protofibril solution formed from an intrinsically denatured disulfide-deficient variant of hen lysozyme with Trp fluorescence as the reporter in the pressure range 3-400 MPa. From the analysis of the time-dependent fluorescence decay and the length distribution of the protofibrils measured on atomic force microscopy, we conclude that the protofibril grows or decays by attachment or detachment of a monomer at one end of the protofibril with a monomer dissociation rate independent of the length of the fibril. Furthermore, we find that the dissociation reaction is strongly dependent on pressure, characterized with a negative activation volume DeltaV(odouble dagger) = -50.5 +/- 1.60 ml mol(-1) at 0.1 MPa and with a negative activation compressibility Deltakappa(double dagger) = -0.013 +/- 0.001 ml mol(-1) bar(-1) or -0.9 x 10(-6) ml g(-1) bar(-1). These results indicate that the protofibril is a highly compressible high-volume state, but that it becomes less compressible and less voluminous in the transition state, most probably due to partial hydration of the existing voids. The system eventually reaches the lowest-volume state with full hydration of the monomer in the dissociated state.

Micronization of Insulin by High Pressure Homogenization

PURPOSE

The aim of this study was to establish the high pressure homogenization of proteins in non-aqueous suspension as an alternative method for classical micronization strategies and to investigate the effect of high pressure on protein stability and bioactivity.

METHODS

The influence of drug loading, homogenization pressure and cycles on particle size reduction was investigated by experimental design using a Box Behnken matrix with insulin as a model compound. Particle size measurements were performed by laser light scattering. Protein stability was investigated by HPLC and HPLC-MS analysis and the bioactivity of insulin was tested in a chondrocyte proliferation assay. For investigations into the effect of temperature on protein stability, insulin was micronized in molten lipid at 75 degrees C in one cycle at 1,000 bar.

RESULTS

Within one homogenization cycle at 1,500 bar, the particle size of insulin could be reduced from 15.8 to 7.3 microm, six cycles resulted in a particle size of 3.7 microm d(0.5) (50% of the particles are smaller than the indicated value). Evaluation of the response surface diagram revealed that the homogenization pressure had the highest impact on micronization efficiency, followed by the number of homogenization cycles. Protein stability was maintained during the micronization process as well as bioactivity. Micronization at elevated temperature (75 degrees C) had no effect on protein stability.

CONCLUSION

High pressure homogenization of protein suspensions can be used as an alternative method for the micronization of proteins without affecting protein stability or bioactivity.

High pressure–low temperature processing of food proteins

High pressure-low temperature (HP-LT) processing is of interest in the food field in view of: (i) obtaining a "cold" pasteurisation effect, the level of microbial inactivation being higher after pressurisation at low or sub-zero than at ambient temperature; (ii) limiting the negative impact of atmospheric pressure freezing on food structures. The specific effects of freezing by fast pressure release on the formation of ice I crystals have been investigated on oil in water emulsions stabilized by proteins, and protein gels, showing the formation of a high number of small ice nuclei compared to the long needle-shaped crystals obtained by conventional freezing at 0.1 MPa. It was therefore of interest to study the effects of HP-LT processing on unfolding or dissociation/aggregation phenomena in food proteins, in view of minimizing or controlling structural changes and aggregation reactions, and/or of improving protein functional properties. In the present studies, the effects of HP-LT have been investigated on protein models such as (i) beta-lactoglobulin, i.e., a whey protein with a well known 3-D structure, and (ii) casein micelles, i.e., the main milk protein components, the supramolecular structure of which is not fully elucidated. The effects of HP-LT processing was studied up to 300 MPa at low or sub-zero temperatures and after pressure release, or up to 200 MPa by UV spectroscopy under pressure, allowing to follow reversible structural changes. Pressurisation of approximately 2% beta-lactoglobulin solutions up to 300 MPa at low/subzero temperatures minimizes aggregation reactions, as measured after pressure release. In parallel, such low temperature treatments enhanced the size reduction of casein micelles.

What lies in the future of high-pressure bioscience?

Without being comprehensive in this mini-review, I will address perspectives, some speculative, for the development and use of high pressure to explore biochemical phenomena. This will be illustrated with several examples.

Effects of solutes on solubilization and refolding of proteins from inclusion bodies with high hydrostatic pressure

High hydrostatic pressure (HHP)-mediated solubilization and refolding of five inclusion bodies (IBs) produced from bacteria, three gram-negative binding proteins (GNBP1, GNBP2, and GNBP3) from Drosophila, and two phosphatases from human were investigated in combination of a redox-shuffling agent (2 mM DTT and 6 mM GSSG) and various additives. HHP (200 MPa) combined with the redox-shuffling agent resulted in solubilization yields of approximately 42%-58% from 1 mg/mL of IBs. Addition of urea (1 and 2 M), 2.5 M glycerol, L-arginine (0.5 M), Tween 20 (0.1 mM), or Triton X-100 (0.5 mM) significantly enhanced the solubilization yield for all proteins. However, urea, glycerol, and nonionic surfactants populated more soluble oligomeric species than monomeric species, whereas arginine dominantly induced functional monomeric species (approximately 70%-100%) to achieve refolding yields of approximately 55%-78% from IBs (1 mg/mL). Our results suggest that the combination of HHP with arginine is most effective in enhancing the refolding yield by preventing aggregation of partially folded intermediates populated during the refolding. Using the refolded proteins, the binding specificity of GNBP2 and GNBP3 was newly identified the same as with that of GNBP1, and the enzymatic activities of the two phosphatases facilitates their further characterization.

Tuning amyloidogenic conformations through cosolvents and hydrostatic pressure: when the soft matter becomes even softer

Compact packing, burial of hydrophobic side-chains, and low free energy levels of folded conformations contribute to stability of native proteins. Essentially, the same factors are implicated in an even higher stability of mature amyloid fibrils. Although both native insulin and insulin amyloid are resistant to high pressure and influence of cosolvents, intermediate aggregation-prone conformations are susceptible to either condition. Consequently, insulin fibrillation may be tuned under hydrostatic pressure or-- through cosolvents and cosolutes-- by preferential exclusion or binding. Paradoxically, under high pressure, which generally disfavors aggregation of insulin, an alternative "low-volume" aggregation pathway, which leads to unique circular amyloid is permitted. Likewise, cosolvents are capable of preventing, or altering amyloidogenesis of insulin. As a result of cosolvent-induced perturbation, distinct conformational variants of fibrils are formed. Such variants, when used as templates for seeding daughter generations, reproduce initial folding patterns regardless of environmental biases. By the close analogy, this suggests that the "prion strains" phenomenon may mirror a generic, common feature in amyloids. The susceptibility of amyloidogenic conformations to pressure and cosolvents is likely to arise from their "frustration", as unfolding results in less-densely packed side-chains, void volumes, and exposure of hydrophobic groups. The effects of cosolvents and pressure are discussed in the context of studies on other amyloidogenic protein models, amyloid polymorphism, and "strains".

High‐Pressure Studies on Protein Aggregates and Amyloid Fibrils

High hydrostatic pressure (HHP) modulates protein-protein and protein-solvent interactions through volume changes and thereby affects the equilibrium of protein conformational species between native and denatured forms as well as monomeric, oligomeric, and aggregated forms without the addition of chemicals or use of high temperature. Because of this unique property, HHP has provided deep insights into the thermodynamics and kinetics of protein folding and aggregation, including amyloid fibril formation. In particular, HHP is a useful tool to stabilize and populate specific folding intermediates, the characterization of which provides thorough understanding of protein folding and aggregation pathways. Furthermore, recent application of HHP for dissociation of protein aggregates, such as inclusion bodies (IBs), into native proteins in a single step facilitates protein preparation for structural and functional studies. This chapter overviews recent HHP studies on the population and characterization of folding intermediates associated with protein aggregation and protein refolding from protein aggregates of amyloid fibrils and IBs. Finally, we describe overall experimental procedures of HHP-mediated protein refolding and provide a detailed discussion of each operating parameter to optimize the refolding.

High-pressure studies of aggregation of recombinant human interleukin-1 receptor antagonist: thermodynamics, kinetics, and application to accelerated formulation studies

Recombinant human interleukin-1 receptor antagonist (IL-1ra) in aqueous solutions unfolds and aggregates when subjected to hydrostatic pressures greater than about 180 MPa. This study examined the mechanism and thermodynamics of pressure-induced unfolding and aggregation of IL-1ra. The activation free energy for growth of aggregates (DeltaG-/+(aggregation)) was found to be 37 +/- 3 kJ/mol, whereas the activation volume (DeltaV-/+(aggregation)) was -120 +/- 20 mL/mol. These values compare closely with equilibrium values for denaturation: The free energy for denaturation, DeltaG(denaturation), was 20 +/- 5 kJ/mol, whereas the partial specific volume change for denaturation, DeltaV(denaturation), was -110 +/- 30 mL/mol. When IL-1ra begins to denature at pressures near 140 MPa, cysteines that are normally buried in the native state become exposed. Under oxidizing conditions, this results in the formation of covalently cross-linked aggregates containing nonnative, intermolecular disulfide bonds. The apparent activation free energy for nucleation of aggregates, DeltaG-/+(nuc), was 42 +/- 4 kJ/mol, and the activation volume for nucleation, DeltaV-/+(nuc),was -175 +/- 37 mL/mol, suggesting that a highly solvent-exposed conformation is needed for nucleation. We hypothesize that the large specific volume of IL-1ra, 0.752 +/- 0.004 mL/g, coupled with its relatively low conformational stability, leads to its susceptibility to denaturation at relatively low pressures. The positive partial specific adiabatic compressibility of IL-1ra, 4.5 +/- 0.7 +/- 10(-12) cm2/dyn, suggests that a significant component of the DeltaV(denaturation) is attributable to the elimination of solvent-free cavities. Lastly, we propose that hydrostatic pressure is a useful variable to conduct accelerated formulation studies of therapeutic proteins.