Multiple advantages of capillary zone electrophoresis for exploring protein conformational stability

Conformational Stability and Denaturation Processes of Proteins Investigated by Electrophoresis under Extreme Conditions

The functional structure of proteins results from marginally stable folded conformations. Reversible unfolding, irreversible denaturation, and deterioration can be caused by chemical and physical agents due to changes in the physicochemical conditions of pH, ionic strength, temperature, pressure, and electric field or due to the presence of a cosolvent that perturbs the delicate balance between stabilizing and destabilizing interactions and eventually induces chemical modifications. For most proteins, denaturation is a complex process involving transient intermediates in several reversible and eventually irreversible steps. Knowledge of protein stability and denaturation processes is mandatory for the development of enzymes as industrial catalysts, biopharmaceuticals, analytical and medical bioreagents, and safe industrial food. Electrophoresis techniques operating under extreme conditions are convenient tools for analyzing unfolding transitions, trapping transient intermediates, and gaining insight into the mechanisms of denaturation processes. Moreover, quantitative analysis of electrophoretic mobility transition curves allows the estimation of the conformational stability of proteins. These approaches include polyacrylamide gel electrophoresis and capillary zone electrophoresis under cold, heat, and hydrostatic pressure and in the presence of non-ionic denaturing agents or stabilizers such as polyols and heavy water. Lastly, after exposure to extremes of physical conditions, electrophoresis under standard conditions provides information on irreversible processes, slow conformational drifts, and slow renaturation processes. The impressive developments of enzyme technology with multiple applications in fine chemistry, biopharmaceutics, and nanomedicine prompted us to revisit the potentialities of these electrophoretic approaches. This feature review is illustrated with published and unpublished results obtained by the authors on cholinesterases and paraoxonase, two physiologically and toxicologically important enzymes.

Monitoring of Deamidation and Lanthionine Formation in Recombinant Mugwort Allergen by Capillary Zone Electrophoresis (CZE)-UV and Transient Capillary Isotachophoresis-CZE-Electrospray Ionization-TOF-MS

The response to thermal stress is an important parameter relevant for characterizing the biological activity and long-term stability of recombinant proteins, which may show irreversible, pH dependent structural changes under these conditions. We selected the recombinant pollen allergen of mugwort ( Artemisia vulgaris) rArt v 3.0201 as a relevant model to study structural changes due to thermal and pH stress by means of capillary zone electrophoresis (CZE)-UV and capillary zone electrophoresis (CZE)-electrospray ionization (ESI)-TOF-MS. Therefore, this recombinant protein was exposed to 95 °C under acidic (pH 3.4) and slightly alkaline (pH 7.3) conditions for up to 120 min. CZE-UV data showed a continuous degradation of the allergen accompanied by the gradual formation of several reaction products. Characterization of novel allergen variants occurring at longer migration times was done via CZE-ESI-TOF-MS using in-capillary transient capillary isotachophoresis (tCITP) preconcentration to facilitate the identification of minor variants. MS data revealed various modifications of rArt v 3.0201 in response to heating. Variants with deamidations and sulfur-related modifications including both yield and loss of sulfur were identified at increased migration times. Desulfurization produced allergen variants with up to four lanthionines that replaced initial disulfide bonds. In addition, mass spectra revealed shifts in the charge state distribution which indicate concomitant conformational alterations. Moreover, several low-abundant oxidized variants were identified. With extended thermal stress, the portfolio of variants increased and progressively shifted toward rArt v 3.0201 with high lanthionine content. The kinetics of conversion and the complexity of variant composition were pH dependent and increased under alkaline conditions.

Capillary electrophoresis analysis of different variants of the amyloidogenic protein β2 -microglobulin as a simple tool for misfolding and stability studies

Free solution capillary electrophoresis with UV detection is here used to retrieve information on the conformational changes of wild-type β2 -microglobulin and a series of naturally and artificially created variants known to have different stability and amyloidogenic potential. Under nondenaturing conditions, the resolution of at least two folding conformers at equilibrium is obtained and a third species is detected for the less stable isoforms. Partial denaturation by using chaotropic agents such as acetonitrile or trifluoroethanol reveals that the separated peaks are at equilibrium, as the presence of less structured species is either enhanced or induced at the expenses of the native form. Reproducible CE data allow to obtain an interesting semiquantitative correlation between the peak areas observed and the protein stability. Thermal unfolding over the range 25-42°C is induced inside the capillary for the two pathogenic proteins (wtβ2 -microglobulin and D76N variant): the large differences observed upon small temperature variation draw attention on the robustness of analytical methods when dealing with proteins prone to misfolding and aggregation.

Advances in hyphenated analytical techniques for shotgun proteome and peptidome analysis--a review

Proteomics is defined as the analysis of part or all of the protein components of a complex biological system (a cell, organ or tissue) at a given moment. Due to the huge number of proteins encoded by the genome, novel analytical techniques must be developed to meet the need of large scale analysis. This has led to the hyphenation of multiple techniques to achieve this object. Here current status of the hyphenated analytical techniques of one-dimensional and multidimensional liquid chromatography-mass spectrometry for shotgun proteomic analysis is reviewed, and on-line techniques for automated sample preparation and injection are also covered. In addition, the hyphenated techniques for peptidome analysis are also covered.

On the application of CZE to the study of protein denaturation

Experimental mobilities obtained from CZE are used to study protein denaturation through a model based on known physicochemical theories. This model is able to provide additional information concerning the folded and unfolded protein states from mobility data. Its use comprises first the evaluation of relevant parameters of the protein microstates like the electrostatic free energy, apart from the classical conformational free energy, and second the expression of raw experimental data concerning the folding-unfolding transition into more specific physicochemical parameters like protein hydrodynamic radius, net charge number, and hydration. Spurious effects that are intrinsic to the experimental evaluation of the mobility of protein states, like BGE viscosity, pH, and ionic strength variations accompanying the changes of the denaturant agent intensity are eliminated. In order to illustrate the proposal of this work, two case studies are considered here. The first one concerns thermal and urea denaturations of horse heart ferricytochrome c and the second one involves thermal denaturation of hen egg-white lysozyme. Thus, relevant theoretical thermodynamic considerations of the folded-unfolded protein transition are presented, where the electrostatic free energy is included explicitly in the effective free energy. It is found that this transition involves sharp increases of hydrodynamic radius and protein hydration.

Stability of highly purified human paraoxonase (PON1): Association with human phosphate binding protein (HPBP) is essential for preserving its active conformation(s)

The biological role of human paraoxonase (PON1) remains unclear, whilst there is a consensus that the enzyme has a protective influence. A toxicological role, protecting from environmental poisoning by organophosphate derivatives drove earlier works, and more recently, clinical interest has focused on a protective role in vascular disease. PON1 resides essentially on HDL particles, a complex and dynamic molecular environment. Our recent discovery of the human phosphate binding protein (HPBP), displaying a firm propensity to associate with PON1, has steered new directions for characterizing PON1 functional state. Here, we report investigations on the effect of HPBP on oligomerization, storage and thermal stability of PON1. We found that purified PON1 is as a mixture of at least two states, and that the absence of HPBP favors homo-oligomerization of PON1 into state(s) of higher molecular size. We showed that HPBP allows stabilizing active conformation(s) of PON1 disencumbered of its natural environment. We also showed that PON1 exhibits intrinsically a remarkable thermal stability, and that the association of HPBP strongly contributes to slow the denaturation rate. A hybrid recombinant PON1 was shown more thermostable than the human enzyme, and its stability was unaffected by the presence of HPBP. Altogether, the results strongly encourage further study of the human enzyme.

Analysis of recombinant human growth hormone by capillary electrophoresis with bilayer-coated capillaries using UV and MS detection

The characterization of recombinant human growth hormone (rhGH; somatropin) by capillary electrophoresis (CE) with UV-absorbance and mass spectrometric (MS) detection using capillaries noncovalently coated with polybrene (PB) and poly(vinyl sulfonic acid) (PVS) is demonstrated. Compared with bare fused-silica capillaries, PB-PVS coated capillaries yielded more favorable migration-time reproducibilities and higher separation efficiencies. Optimal separation conditions for the bilayer-coated capillaries comprised a background electrolyte (BGE) of 400 mM Tris phosphate (pH 8.5) yielding migration-time R.S.D.s of less than 1.0% and plate numbers above 300,000 for intact rhGH. The protein was also analyzed using the CE method described in the European Pharmacopoeia (Ph. Eur.) monograph. The pharmacopoeial method gave much longer analysis times (22 min versus 8 min), lower resolution and plate numbers, and consecutive shifts in migration time for rhGH, indicating possible interactions between the protein and the inner capillary wall. Due to stable migration times obtained with the coated capillaries, reliable profiling and quantification of rhGH and its byproducts in time was possible. Analysis of thermally degraded rhGH revealed the formation of two main degradation products. CE-mass spectrometry (MS) of this sample, using a PB-PVS coated capillary and a BGE of 75 mM ammonium formate (pH 8.5), suggests that these products are desamido forms of rhGH. Analyses of expired rhGH preparations with CE-UV and CE-MS indicated the presence of both deamidation and oxidation products.

Dynamic unfolding of a regulatory subunit of cAMP-dependent protein kinase by capillary electrophoresis: Impact of cAMP dissociation on protein stability

Characterization of the unfolding dynamics of a recombinant type IA regulatory subunit (RIalpha) of cyclic adenosine monophosphate (cAMP)-dependent protein kinase (cAPK) was examined by CE with UV detection. Electrophoretic separation of RIalpha by CE in a buffer devoid of cAMP resulted in rapid dissociation of the complex from the original sample due to the high negative mobility of the ligand relative to receptor. This process enabled in-capillary generation of cAMP-stripped RIalpha, which was used to estimate the apparent dissociation constant (Kd) of 0.6 +/- 0.2 microM. A comparison of RIalpha dynamic unfolding processes with urea denaturation was performed by CE with (i.e., RIalpha-cAMP) and without (i.e., cAMP-stripped RIalpha) excess cAMP in the buffer during electromigration. The presence of cAMP in the buffer confirmed greater stabilization of the protein, as reflected by a higher standard free energy change (DeltaG(U) degrees) of 10.1 +/- 0.5 kcal x mol(+1) and greater cooperativity in unfolding (m) of -2.30 +/- 0.11 kcal x mol(-1) M(-1). CE offers a rapid, yet versatile platform for probing the thermodynamics of cAPK and other types of receptor-ligand complexes in free solution.

Capillary electrophoresis versus differential scanning calorimetry for the analysis of free enzyme versus enzyme-ligand complexes: in the search of the ligand-free status of cholinesterases

Cholinesterases (ChEs) are highly efficient biocatalysts whose active site is buried in a deep, narrow gorge. The talent of CE to discover inhibitors in the gorge of highly purified preparations has fairly altered the meaning of a ChE ligand-free status. To attempt at a description of this one, we investigated the stability of Bungarus fasciatus acetylcholinesterase (AChE), alone or complexed with different inhibitors. Determination of mid-transition temperature for thermal denaturation, using differential scanning calorimetry (DSC) and CE, provided conflicting results. Discrepancies strongly question the reality of a ligand-free AChE state. DSC allowed estimation of the stability of AChE-ligands complexes, and to rank the stabilizing effect of different inhibitors. CE acted as a detector of hidden ligands, provided that they were charged, reversibly bound, and thus dissociable upon action of electric fields. Then, CE allowed quantification of the stability of ligand-free AChE. CE and DSC providing each fractional and nonredundant information, cautious attention must be paid for actual estimation of the conformational stability of ChEs. Because inhibitors used in purification of ChEs by affinity chromatography are charged, CE remains a leading method to estimate enzyme stability and detect the presence of bound hidden ligands.

Negative pressure pinched sample injection for microchip-based electrophoresis

A simple method for injecting well-defined non-biased sample plugs into the separation channel of a microfluidic chip-based capillary electrophoresis system was developed by a combination of flows generated by negative pressure, electrokinetic and hydrostatic forces. This was achieved by using only a single syringe pump and a single voltage supply at constant voltage. In the loading step, a partial vacuum in the headspace of a sealed sample waste reservoir was produced using a syringe pump equipped with a 3-way valve. Almost instantaneously, sample was drawn from the sample reservoir across the injection intersection to the sample waste reservoir by negative pressure. Simultaneously, buffer flow from the remaining two buffer reservoirs pinched the sample flow to form a well-defined sample plug at the channel intersection. In the subsequent separation stage, the vacuum in headspace of the sample waste reservoir was released to terminate all flows generated by negative pressure, and the sample plug at the channel intersection was electrokinetically injected into the separation channel under the potential applied along the separation channel. The liquid levels of the four reservoirs were optimized to prevent sample leakage during the separation stage. The approach considerably simplified the operations and equipment for pinched injection in chip-based CE, and improved the throughput. Migration time precisions of 3.3 and 1.5% RSD for rhodamine123 (Rh123) and fluorescein sodium (Flu) in the separation of a mixture of Flu and Rh123 were obtained for 56 consecutive determinations with peak height precisions of 6.2% and 4.4% RSD for Rh123 and Flu, respectively.