Review: Multipoint binding and heterogeneity in immobilized metal affinity chromatography

Competitive protein adsorption in isocratic anion exchange chromatography

Protein chromatography is the dominant method of purification of biopharmaceuticals. Although all practical chromatography involves competitive absorption and separation of M. species, competitive protein absorption has remained inadequately understood. We previously introduced the measurement of equilibrium protein adsorption isotherms with all intensive variables held constant, including competitor concentration. In this work, we introduce isocratic chromatographic retention measurements of dynamic protein adsorption in the presence of a constant concentration of a competitor protein. These measurements are achieved by establishing a dynamic equilibrium with a constant concentration of competitor (insulin) in the mobile phase flowing through an ion exchange adsorbent column and following the behavior of a test protein (α-lactalbumin) injected into this environment. We observed decreased retention times for α-lactalbumin in presence of the competitor. The presence of competitor also reduces the heterogeneity of the sites available for adsorption of the test protein. This investigation provides an approach to fundamental understanding of competitive dynamics of multicomponent protein chromatography.

Human Immunoglobulin G Adsorption in Epoxy Chitosan/Alginate Adsorbents: Evaluation of Isotherms by Artificial Neural Networks

This work aimed to evaluate the interaction of human IgG in non-conventional adsorbents based on chitosan and alginate in the absence and presence of Reactive Green, Reactive Blue and Cibacron Blue immobilized as ligands. The adsorption was evaluated at 277, 288, 298 and 310 K using sodium phosphate buffer, pH 7.6, at 25 mmol L−1. The highest adsorption capacity was observed in the experiments performed with no immobilized dye, although all showed adsorption capacity higher than 120 mg g−1. Data modeling was done using Langmuir, Langmuir-Freundlich and Temkin classical nonlinear models, and artificial neural networks (ANN) for comparison. According to the parameters obtained, a possible adsorption in multilayers was observed due to protein-adsorbent and protein-protein interactions, concluding that IgG adsorption process is favorable and spontaneous. Using an ANN structure with 3 hidden neurons (single hidden layer), the MSE (RMSE) for training, test and validation were 13.698 (3.701), 11.206 (3.347) and 7.632 (2.763), respectively, achieving correlation coefficients of 0.999 in all steps. ANN modeling proved to be effective in predicting the adsorption isotherms in addition to overcoming the difficulties caused by experimental errors and/or arising from adsorption phenomenology.

Multivalent Chelators for In Vivo Protein Labeling

With the advent of single-molecule methods, chemoselective and site-specific labeling of proteins evolved to become a central aspect in chemical biology as well as cell biology. Protein labeling demands high specificity, rapid as well as efficient conjugation, while maintaining low concentration and biocompatibility under physiological conditions. Generic methods that do not interfere with the function, dynamics, subcellular localization of proteins, and crosstalk with other factors are crucial to probe and image proteins in vitro and in living cells. Alternatives to enzyme-based tags or autofluorescent proteins are short peptide-based recognition tags. These tags provide high specificity, enhanced binding rates, bioorthogonality, and versatility. Here, we report on recent applications of multivalent chelator heads, recognizing oligohistidine-tagged proteins. The striking features of this system has facilitated the analysis of protein complexes by single-molecule approaches.

Construction of Escherichia coli BL21/A-53 producing histidine-tagged carboxymethylcellulase and comparison of its characteristics with CMCase without histidine-tag

To enhance recovery yield of carboxymethylcellulase (CMCase), E. coli BL21/A-53 producing the histidine-tagged CMCase was constructed in this study. The recovery yield of the histidine-tagged CMCase using the His-tag affinity chromatography was 39.8%. The predicted molecular weight of the histidine-tagged CMCase was determined as 56,260 Da. Its K_m and V_max were 9.3 g l^-1 and 76.3 g l^-1·min^-1, respectively. The histidine-tagged CMCase hydrolyzed avicel, carboxymethylcellulose (CMC), filter paper, pullulan, xylan, but there was no detectable activity on cellobiose, p-Nitrophenyl-β-D-glucopyranoside (pNPG). The optimal temperature and pH for the enzymatic reaction of the histidine-tagged CMCase was 50 °C and 5.0. The histidine-tagged CMCase was enhanced by CoCl₂ until the concentration of 100 mM, but inhibited by EDTA, HgCl₂, MnCl₂, NiCl₂, and RbCl₂. The characteristics of the histidine-tagged CMCase produced by E. coli BL21/A-53 were compared with those of CMCase without the histidine-tag of Bacillus subtilis subsp. subtilis A-53. The little changed characteristics of the histidine-tagged CMCase compared to the CMCase without a His-tag seemed to be the conformational change in the structure due to a His-tag.

Enhanced purification of histidine-tagged carboxymethylcellulase produced by Escherichia coli BL21/LBH-10 and comparison of its characteristics with carboxymethylcellulase without histidine-tag

To enhance purification yield of the carboxymethylcellulase (CMCase) of P. aquimaris LBH-10, E. coli BL21/LBH-10 was constructed to produce the six histidine-tagged CMCase (CMCase with a His-tag). The purification yield of the CMCase with a His-tag produced by E. coli BL21/LBH-10 was 44.4%. The molecular weight of the CMCase with a His-tag was determined as 56 kDa. Its K_m and V_max were 7.4 g/L and 70.9 g/L min, respectively. The CMCase with a His-tag hydrolyzed avicel, carboxymethylcellulose (CMC), filter paper, pullulan, and xylan but did not hydrolyze cellobiose and p-nitrophenyl-β-D-glucopyranoside. The optimal temperature for reaction was 50 °C and more than 75% of its original activity was maintained at broad temperatures ranging from 20 to 70 °C after 24 h. The optimal pH was 4.0 and more than 60% of its original activity was maintained at pH ranging from 4.0 to 7.0. The activity of the CMCase with a His-tag was enhanced by CoCl₂, KCl, PbCl₂, RbCl₂, and SrCl₂ until the concentration of 100 mM, but inhibited by EDTA, HgCl₂, MnCl₂, and NiCl₂. The characteristics of the CMCase with a His-tag produced by E. coli BL21/LBH-10 were little different from the CMCase without a His-tag, which seemed to resulted from the conformational change in the structure due to a His-tag. The purification yield of the CMCase with a His-tag using affinity chromatography from the cell broth after cell breakdown was proven to be more economic than that from the supernatant with its low concentration of cellulase.

Extraction and stability of bovine serum albumin (BSA) using cholinium-based Good's buffers ionic liquids

Good's buffers ionic liquids (GB-ILs), composed of cholinium-based cations and Good's buffers anions, display self-buffering characteristics in the biological pH range, and their polarity and hydrophobicity can be easily tuned by a proper manipulation of their ions chemical structures. In this work, the extraction ability for bovine serum albumin (BSA) of aqueous biphasic systems (ABS) formed by polypropylene glycol 400 (PPG 400) and several GB-ILs was evaluated. ABS formed by PPG 400 and cholinium chloride ([Ch]Cl), GBs, and sucrose were also investigated for comparison purposes. It is shown that BSA preferentially migrates for the GB-IL-rich phase, with extraction efficiencies of 100%, achieved in a single-step. Dynamic light scattering, and circular dichroism (CD) and Fourier transform infrared (FTIR) spectroscopies were employed to evaluate the effect of the investigated cholinium-based GB-ILs on the BSA stability, and compared with results obtained for the respective GBs precursors, [Ch]Cl and sucrose, a well-known protein stabilizer. Molecular docking studies were also carried out to investigate on the binding sites of GB-IL ions to BSA. The experimental results confirm that BSA has a higher stability in GB-ILs than in any of the other compounds investigated.

High ionic strength narrows the population of sites participating in protein ion-exchange adsorption: a single-molecule study

The retention and elution of proteins in ion-exchange chromatography is routinely controlled by adjusting the mobile phase salt concentration. It has repeatedly been observed, as judged from adsorption isotherms, that the apparent heterogeneity of adsorption is lower at more-eluting, higher ionic strength. Here, we present an investigation into the mechanism of this phenomenon using a single-molecule, super-resolution imaging technique called motion-blur Points Accumulation for Imaging in Nanoscale Topography (mbPAINT). We observed that the number of functional adsorption sites was smaller at high ionic strength and that these sites had reduced desorption kinetic heterogeneity, and thus narrower predicted elution profiles, for the anion-exchange adsorption of α-lactalbumin on an agarose-supported, clustered-charge ligand stationary phase. Explanations for the narrowing of the functional population such as inter-protein interactions and protein or support structural changes were investigated through kinetic analysis, circular dichroism spectroscopy, and microscopy of agarose microbeads, respectively. The results suggest the reduction of heterogeneity is due to both electrostatic screening between the protein and ligand and tuning the steric availability within the agarose support. Overall, we have shown that single molecule spectroscopy can aid in understanding the influence of ionic strength on the population of functional adsorbent sites participating in the ion-exchange chromatographic separation of proteins.

Unified superresolution experiments and stochastic theory provide mechanistic insight into protein ion-exchange adsorptive separations

Chromatographic protein separations, immunoassays, and biosensing all typically involve the adsorption of proteins to surfaces decorated with charged, hydrophobic, or affinity ligands. Despite increasingly widespread use throughout the pharmaceutical industry, mechanistic detail about the interactions of proteins with individual chromatographic adsorbent sites is available only via inference from ensemble measurements such as binding isotherms, calorimetry, and chromatography. In this work, we present the direct superresolution mapping and kinetic characterization of functional sites on ion-exchange ligands based on agarose, a support matrix routinely used in protein chromatography. By quantifying the interactions of single proteins with individual charged ligands, we demonstrate that clusters of charges are necessary to create detectable adsorption sites and that even chemically identical ligands create adsorption sites of varying kinetic properties that depend on steric availability at the interface. Additionally, we relate experimental results to the stochastic theory of chromatography. Simulated elution profiles calculated from the molecular-scale data suggest that, if it were possible to engineer uniform optimal interactions into ion-exchange systems, separation efficiencies could be improved by as much as a factor of five by deliberately exploiting clustered interactions that currently dominate the ion-exchange process only accidentally.

Cellulose aerogel regenerated from ionic liquid solution for immobilized metal affinity adsorption

Surface morphology of cellulosic adsorbents is expected to influence the adsorption behavior of biomacromolecules. In the present study, cellulose aerogel regenerated from ionic liquid solution was prepared for use as a polymer support for protein adsorption. Iminodiacetic acid groups were introduced to the aerogel for immobilized metal affinity adsorption of proteins. A Cu(II)-immobilized iminodiacetic acid cellulose aerogel (Cu(II)-IDA-CA), which has a large specific surface area, showed a higher adsorption capacity than Cu(II)-immobilized iminodiacetic acid bacterial cellulose (Cu(II)-IDA-BC) and Cu(II)-immobilized iminodiacetic acid plant cellulose (Cu(II)-IDA-PC). In contrast, the Cu(II)-immobilized cellulosic adsorbents showed similar adsorption capacities for smaller amino acid and peptides. The results show that cellulose aerogels are useful as polymer supports with high protein adsorption capacities.

High-Capacity Purification of His-tagged Proteins by Affinity Membranes Containing Functionalized Polymer Brushes

Porous membrane absorbers are attractive for increasing the rate of protein purification, but their binding capacity is low relative to porous beads. Modification of membranes with functionalized polymer brushes, however, can greatly enhance capacity. This work demonstrates that membrane modification with poly(2-hydroxyethyl methacrylate) (PHEMA) brushes derivatized with nitrilotriacetate-Ni2+ (NTA-Ni2+) complexes allows purification of polyhistidine-tagged ubiquitin (HisU) in less than 30 min with a binding capacity of 120 mg of HisU/cm3 of porous alumina membrane. Adsorption isotherms show that saturation of the brushes occurs at HisU concentrations as low as 0.04 mg/mL and that these brushes can bind up to 23 monolayers of HisU. Gel electrophoresis reveals that the purity of eluted HisU is more than 99%, even when the initial feed solution contains 10% bovine serum or a 20-fold excess of BSA. Thus, reusable porous membranes modified by PHEMA-NTA-Ni2+ brushes are attractive candidates for rapid purification of polyhistidine-tagged proteins.

The effectiveness of three multi-component binding models in describing the binary competitive equilibrium adsorption of two cytochrome b(5) mutants

Competitive adsorption isotherms for two conservative surface charge-neutralizing mutants of cytochrome b(5), E11Q and E44Q, previously measured with competitor concentration held constant over the range of the isotherm, were used to test three widely-used multi-component isotherm models. The extended Langmuir-Freundlich, Langmuir and Jovanovic-Freundlich models each adequately described the weaker infinite dilution adsorption of the E44Q protein in the presence of the strong binding E11Q. The extended Langmuir-Freundlich model generally gave the lowest errors at higher concentrations, and the Jovanovic-Freundlich model gave the best fits when using empirically optimized maximal loading values based on multi-component as well as pure-component isotherm data.

The affinity concept in bioseparation: Evolving paradigms and expanding range of applications

The meaning of the word affinity in the context of protein separation has undergone evolutionary changes over the years. The exploitation of molecular recognition phenomenon is no longer limited to affinity chromatography modes. Affinity based separations today include precipitation, membrane based purification and two-phase/three-phase extractions. Apart from the affinity ligands, which have biological relationship (in vivo) with the target protein, a variety of other ligands are now used in the affinity based separations. These include dyes, chelated metal ions, peptides obtained by phage display technology, combinatorial synthesis, ribosome display methods and by systematic evolution of ligands by exponential enrichment (SELEX). Molecular modeling techniques have also facilitated the designing of biomimetic ligands. Fusion proteins obtained by recombinatorial methods have emerged as a powerful approach in bioseparation. Overexpression in E. coli often result in inactive and insoluble inclusion bodies. A number of interesting approaches are used for simultaneous refolding and purification in such cases. Proteomics also needs affinity chromatography to reduce the complexity of the system before analysis by electrophoresis and mass spectrometry are made. At industrial level, validation, biosafety and process hygiene are also important aspects. This overview looks at these evolving paradigms and various strategies which utilize affinity phenomenon for protein separations.

Supermacroporous cryogel matrix for integrated protein isolation

A new type of supermacroporous, monolithic, cryogel affinity adsorbent was developed, allowing the specific capture of urokinase from conditioned media of human fibrosarcoma cell line HT1080. The affinity adsorbent was designed with the objective of using it as a capture column in an integrated perfusion/protein separation bioreactor setup. A comparative study between the utility of this novel cryogel based matrix and the conventional Sepharose based affinity matrix for the continuous capture of urokinase in an integrated bioreactor system was performed. Cu(II)-ion was coupled to epoxy activated polyacrylamide cryogel and Sepharose using iminodiacetic acid (IDA) as the chelating ligand. About 27-fold purification of urokinase from the conditioned culture media was achieved with Cu(II)-IDA-polyacrylamide cryogel column giving specific activity of about 814 Plough units (PU)/mg protein and enzyme yields of about 80%. High yields (95%) were obtained with Cu(II)-IDA-Sepharose column by virtue of its high binding capacity. However, the adsorbent showed lower selectivity as compared to cryogel matrix giving specific activity of 161 PU/mg protein and purification factor of 5.3. The high porosity, selectivity and reasonably good binding capacity of Cu(II)-IDA-polyacrylamide cryogel column make it a promising option for use as a protein capture column in integrated perfusion/separation processes. The urokinase peak pool from Cu(II)-IDA-polyacrylamide cryogel column could be further resolved into separate fractions for high and low molecular weight forms of urokinase by gel filtration chromatography on Sephacryl S-200. The selectivity of the cryogel based IMAC matrix for urokinase was found to be higher as compared to that of Cu(II)-IDA-Sepharose column.

Synthesis of magnetic chelator for high-capacity immobilized metal affinity adsorption of protein by cerium initiated graft polymerization

A novel magnetic chelator with high adsorption capacity of protein by immobilized metal affinity adsorption was prepared by cerium (IV) initiated graft polymerization of tentacle-type polymer chains with iminodiacetic acid (IDA) chelating group on magnetic particles with hydroxyl groups. The micron-sized magnetic poly(vinyl acetate-divinylbenzene) (PVAc-DVB) particles were prepared by a modified suspension polymerization in the presence of oleic acid-coated magnetite nanoparticles and subsequently modified by ester exchange reaction to introduce functional hydroxyl groups. Bovine hemoglobin (BHb) was selected as a model protein to investigate the adsorption capacity of these magnetic particles. The magnetic particles were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), superconducting quantum interference device (SQUID) magnetometry, diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and X-ray diffraction (XRD). The results showed that the magnetic particles had an average size of 5 microm and superparamagnetism with saturation magnetization of 20.0 emu/g at room temperature. The protein adsorption indicated that the graft polymerization of tentacle type polymer chains on the magnetic particles could produce magnetic adsorbents with high adsorption capacity (1428.21 mg/g) and low nonspecific adsorption of protein. The magnetic particles with grafted tentacle polymer chains have potential application in large-scale affinity separation of proteins.

Competitive ion-exchange adsorption of proteins: Competitive isotherms with controlled competitor concentration

The competitive adsorption processes inevitably present in chromatographic separations of complex mixtures have not been extensively studied. This is partly due to the difficulty of measuring true competitive isotherms, in which all system parameters (including competitor concentrations) are held constant. We report a novel approach to determining competitive protein adsorption isotherms in which the competitor concentration is held constant across the entire isotherm. By using the heme prosthetic group in cytochrome b5 as a quantitative spectrophotometric label, competitive isotherms between cytochrome b5 and alpha-lactalbumin can be constructed. Similarly, manganese-substituted protoporphyrin IX heme replacement allows the non-perturbing labeling of individual cytochrome b5 conservative surface charge mutants by replacement of a single atom in the interior of the protein. This labeling allows the study of competition between cytochrome b5 charge mutants of identical size and shape, which differ only in charge arrangement. Using these techniques, the effect of competing species on equilibrium behavior and the apparent heterogeneity of anion-exchange adsorbents in the presence of competitors can be quantitatively studied by fitting the data to two popular single-component binding models, the Temkin and the Langmuir-Freundlich (L-F) isotherms.

Designing selective sites in templated polymers utilizing coordinative bonds

This review gives a survey over recent achievements on the design of selective sites in templated polymers. Particularly, coordinative bonds as driving force for the interaction between a substrate and a templated polymer are discussed. Recent achievements on the selective recognition of larger molecules, such as dipeptides and disaccharides, are highlighted that promise a fast development on biomolecule templated material towards enzyme-like catalysis in the up-coming years. Additionally, the achievements on the incorporation of catalytic centers based on transition metal complexes are summarized.

One-step metal-affinity purification of histidine-tagged proteins by temperature-triggered precipitation

The feature of elastin-like proteins (ELPs) to reversibly precipitate above their transition temperature was exploited as a general method for the purification of histidine (His)-tagged proteins. The principle of the single-step metal-affinity method is based on coordinated ligand-bridging between the modified ELPs and the target proteins. ELPs with repeating sequences of [(VPGVG)(2)(VPGKG)(VPGVG)(2)](21) were synthesized and the free amino groups on the lysine residues were modified by reacting with imidazole-2-carboxyaldehyde to incorporate the metal-binding ligands into the ELP bio- polymers. Biopolymers charged with Ni(2+) were able to interact with a His tag on the target proteins based on metal coordination chemistry. Purifications of two His-tagged enzymes, beta-D-galactosidase and chloramphenicol acetyltransferase, were used to demonstrate the utility of this general method and over 85% recovery was observed in both cases. The bound enzymes were easily released by addition of either EDTA or imidazole. The recovered ELPs were reused four times with no observable decrease in the purification performance.

Multivalent binding interaction of alcohol dehydrogenase on dye-metal affinity matrix

The optimization of a chromatographic process using immobilized metal affinity chromatography requires an understanding of the factors that govern the interaction between proteins and immobilized metal ions. Factors, such as concentrations of protein, NaCl and imidazole were investigated to elucidate kinetics of adsorption of alcohol dehydrogenase (ADH) onto a dye-iminodiacetic acid matrix (dye-IDA matrix). The results indicate that the adsorption of ADH onto a dye-IDA matrix occurs in the mode of multiple-site binding interactions between ADH and zinc ions immobilized on the dye-IDA matrix. The estimated average number of interaction sites was 4.5 and the association constant was 6 x 10(-9) mM(-n). The isotherm of ADH adsorption was well represented by a multivalent model of protein-zinc ion interactions. For the adsorption of ADH from clarified yeast homogenate, addition of imidazole as a protein competitor to adsorption buffer increased the adsorption specificity of ADH, thereby suppressing contaminant protein adsorption. It was also observed that the adsorption of ADH was better performed at high initial protein concentrations in the yeast homogenate. Consequently, these results may have important implications on the optimization of the strategy for immobilized metal affinity adsorption in packed and expanded bed systems.

Development of new dye-metal agarose-coated alumina matrix and elution strategy for purification of alcohol dehydrogenase

A high-density matrix was prepared by coating an alumina particle with agarose using an emulsion technique. Iminodiacetic acid and Cibacron Blue 3GA were immobilized onto this matrix. Charging this matrix with zinc created a useful chromatography matrix for purification of yeast alcohol dehydrogenase. The elution strategy was then investigated to obtain a high recovery of this enzyme and a high purification factor. One-step elution using 4 mM EDTA containing 0.5 M NaCl resulted in 66% enzyme recovery and a purification factor of 4.7. Two-step elution using imidazole containing NaCl resulted in a higher purification factor. The first-step elution using 5 mM imidazole containing 1 M NaCl released most contaminant proteins. The second-step elution using 150 mM imidazole containing 1.5 M NaCl resulted in high-performance purification with a purification factor of 6.5 and an enzyme recovery of 40.7%. Equilibration of the matrix with imidazole prior to sample application increased the purification factor and the enzyme recovery to 8.4 and 76.8%, respectively.

One-step purification, covalent immobilization, and additional stabilization of poly-His-tagged proteins using novel heterofunctional chelate-epoxy supports

Epoxy supports covalently immobilize proteins following a two-step mechanism; that is, the protein is physically adsorbed and then the covalent reaction takes place. This mechanism has been exploited to combine the selectivity of metal chelate affinity chromatography with the covalent immobilization capacity of epoxy supports. In this way, it has been possible to accomplish, in a simple manner, the purification, immobilization, and stabilization of a poly-His-tagged protein. To fulfill this objective we developed a new kind of multifunctional epoxy support (chelate epoxy support [CES]), which was tested using a poly-His-tagged glutaryl acylase as a model protein (an alphabeta-heterodimeric enzyme of significant industrial interest). The selectivity of the immobilization in CES toward poly-His-tagged proteins was dependent to a large extent on the density and nature of the chelated metal. The highest selectivity was achieved by using low-density chelate groups (e.g., 5 micromol/g) and metals with a low affinity (e.g., Co). However, the rate of covalent immobilization of the protein by its reaction with the epoxy groups on the support significantly increased at alkaline pH values. The multipoint attachment to the CES also depended on the reaction time. The immobilization of both glutaryl acylase subunits was achieved by incubation of the enzyme derivative at pH 10 for 24 h, with the best enzyme derivative 100-fold more stable than the soluble enzyme. By taking advantage of the selectivity properties of the novel support, we were able to immobilize up to 30 mg of protein per gram of modified Eupergit 250 using either pure enzyme or a very crude enzyme extract.

Perspectives of immobilized-metal affinity chromatography

Immobilized Metal-Affinity Chromatography (IMAC) represents a relatively new separation technique that is primarily appropriate for the purification of proteins with natural surface-exposed histidine residues and for recombinant proteins with engineered histidine tags or histidine clusters. Because the method has gained broad popularity in recent years, the main recent developments in the field of new sorbents, techniques and possible applications are discussed in this article. Advantages of the method and new prospects are described as well as the problems and concerns that appear when the method is to be used for production of pharmaceutical-grade proteins.

Affinity chromatography of polyhistidine tagged enzymes. New dextran-coated immobilized metal ion affinity chromatography matrices for prevention of undesired multipoint adsorptions

New immobilized metal ion affinity chromatography (IMAC) matrices containing a high concentration of metal-chelate moieties and completely coated with inert flexible and hydrophilic dextrans are here proposed to improve the purification of polyhistidine (poly-His) tagged proteins. The purification of an interesting recombinant multimeric enzyme (a thermoresistant beta-galactosidase from Thermus sp. strain T2) has been used to check the performance of these new chromatographic media. IMAC supports with a high concentration (and surface density) of metal chelate groups promote a rapid adsorption of poly-His tagged proteins during IMAC. However, these supports also favor the promotion of undesirable multi-punctual adsorptions and problems may arise for the simple and effective purification of poly-His tagged proteins: (a) more than 30% of the natural proteins contained in crude extracts from E. coli become adsorbed, in addition to our target recombinant protein, on these IMAC supports via multipoint weak adsorptions; (b) the multimeric poly-His tagged enzyme may become adsorbed via several poly-His tags belonging to different subunits. In this way, desorption of the pure enzyme from the support may become quite difficult (e.g., it is not fully desorbed from the support even using 200 mM of imidazole). The coating of these IMAC supports with dextrans greatly reduces these undesired multi-point adsorptions: (i) less than 2% of natural proteins contained in crude extracts are now adsorbed on these novel supports; and (ii) the target multimeric enzyme may be fully desorbed from the support using 60 mM imidazole. In spite of this dramatic reduction of multi-point interactions, this dextran coating hardly affects the rate of the one-point adsorption of poly-His tagged proteins (80% of the rate of adsorption compared to uncoated supports). Therefore, this dextran coating of chromatographic matrices seems to allow the formation of strong one-point adsorptions that involve small areas of the protein and support surface. However, the dextran coating seems to have dramatic effects for the prevention of weak or strong multipoint interactions that should involve a high geometrical congruence between the enzyme and the support surface.

Interactions of proteins with immobilized metal ions: a comparative analysis using various isotherm models

Immobilized metal ion affinity chromatography (IMAC) is now a widely accepted technique for the purification of natural and recombinant therapeutic products and is beginning to find industrial applications. The design, optimization, and scale-up of a chromatographic process using IMAC demands a thorough understanding to be developed regarding the fundamental factors governing the various interactions between immobilized metal ions and proteins. Consequently, there is an immediate need to find out a theory that is able to account for these interactions most efficiently in a qualitative as well as a quantitative manner. In view of this requirement, the interactions of several model proteins (lysozyme, ovalbumin, bovine serum albumin, conalbumin, and wheat germ agglutinin) with metal (Cu(II), Ni(II))-chelated IDA (iminodiacetate) and tris(2-aminoethyl)amine were investigated. The adsorption data were analyzed using four isotherm models, viz., the general affinity interaction theory/Langmuir model, the Freundlich model, the Temkin model, and the Langmuir-Freundlich model, and the sorption parameters were computed. Although the first three models were applicable to some protein-IMA-M(II) systems, the Langmuir-Freundlich model appeared to be the most efficient model for explaining the interactions of proteins with IMA-M(II) gels. Also, this model was able to explain cooperativity and binding heterogeneity in quantitative terms. It is envisaged that this analysis would be useful in developing an improved understanding of protein-immobilized metal ion interactions and providing guidelines for designing preparative-scale separations using IMAC.

Histidines in affinity tags and surface clusters for immobilized metal-ion affinity chromatography of trimeric tumor necrosis factor alpha

In order to achieve efficient IMAC (immobilized metal-ion affinity chromatography) purification of tumor necrosis factor alpha (TNF-alpha) and its analogs by a common chromatographic procedure, we tested four histidine-rich affinity tags attached to the N-termini of the trimeric TNF-alpha molecule. Using low cultivation temperature and appropriate protease deficient E. coli strains, it was possible to obtain intact, full-length proteins with NHis2Xa and HisArg tags, which could be purified to over 95% purity in a single step. However, in comparison to model proteins bearing a surface histidine cluster, accumulation of the histidine-tagged proteins in E. coli was significantly reduced, even in protease deficient strains. In addition, the histidine tagged TNF-alpha proteins never displayed good chromatographic behavior, which was otherwise easily achieved with model proteins. Although the most popular hexa-histidine tag is generally recognized as very convenient for single step isolation of monomeric proteins, our results with trimeric TNF-alpha indicate that oligomeric proteins may require further optimization of the tag, with respect to its length, composition, and location. Histidines, relatively rigidly inserted in the structure, as in our model proteins, display superior chromatographic characteristics vis a vis flexible tags with the same total number of histidines.

Selective adsorption of poly-His tagged glutaryl acylase on tailor-made metal chelate supports

A poly-His tag was fused in the glutaryl acylase (GA) from Acinetobacter sp. strain YS114 cloned in E. coli yielding a fully active enzyme. Biochemical analyses showed that the tag did not alter the maturation of the chimeric GA (poly-His GA) that undergoes a complex post-translational processing from an inactive monomeric precursor to the active heterodimeric enzyme. This enzyme has been used as a model to develop a novel and very simple procedure for one-step purification of poly-His proteins via immobilized metal-ion affinity chromatography on tailor-made supports. It was intended to improve the selectivity of adsorption of the target protein on tailor-made chelate supports instead of performing a selective desorption. The rate and extent of the adsorption of proteins from a crude extract from E. coli and of pure poly-His tagged GA on different metal chelate supports was studied. Up to 90% of proteins from E. coli were adsorbed on commercial chelate supports having a high density of ligands attached to the support through long spacer arms, while this adsorption becomes almost negligible when using low ligand densities, short spacer arms and Zn2+ or Co2+ as cations. On the contrary, poly-His GA adsorbs strongly enough on all supports. A strong affinity interaction between the poly-His tail and a single chelate moiety seems to be the responsible for the adsorption of poly-His GA. By contrast, multipoint weak interactions involving a number of chelate moieties seem to be mainly responsible for adsorption of natural proteins. By using tailor-made affinity supports, a very simple procedure for one-step purification of GA with minimal adsorption of host proteins could be performed. Up to 20 mg of GA were adsorbed on each ml of chelate support while most of accompanying proteins were hardly adsorbed on such supports. Following few washing steps, the target enzyme was finally recovered (80% yield) by elution with 50 mM imidazole with a very high increment of specific activity (up to a 120 purification factor).

Microcalorimetric Studies of the Interactions of Lysozyme with Immobilized Metal Ions: Effects of Ion, pH Value, and Salt Concentration

This study extends previous research on the interaction of biomaterials with immobilized Cu(II) by isothermal titration calorimetry (ITC) on Fe(III). The difference of the binding behavior of protein with that of the immobilized metal ions is also discussed. For the immobilized Fe(III), ITC results show that the adsorption enthalpy at a constant pH value decreased as the NaCl concentration increased and also decreased with the pH values at constant NaCl concentrations. The adsorption enthalpy become negative under higher pH values or higher salt concentrations indicating the adsorption process is partly driven by the enthalpy. The enthalpy of lysozyme with Fe(III) is higher than that with Cu(II) implying that the heat required for the dehydration of Cu(II) is lower than for the dehydration of Fe(III) and/or that the heat generated from the formation of the coordination with Cu(II) is higher than with Fe(III). In addition, the comparison of different immobilized metal ions corresponding to the equilibrium binding affinity suggests that the binding force of lysozyme with Cu(II) is higher than with Fe(III). This study presents the chemical differences between the binding affinity and the adsorption enthalpy of lysozyme interacting with the immobilized metal ions. The binding and thermodynamic data presented in this study elucidate the mechanism and process of lysozyme binding with immobilized metal ions. In addition, the thermodynamic characteristic functions provide valuable information enabling a more thorough understanding of protein adsorption at the immobilized metal ion affinity surface. Copyright 1999 Academic Press.

Examination of the protein binding behaviour of immobilised copper (II)-2,6-diaminomethylpyridine and its application in the immobilised metal ion affinity chromatographic separation of several human serum proteins

A new metal ion chelator has been developed for use in the immobilised metal ion affinity chromatography (IMAC) of proteins. The aromatic tridentate ligand 2,6-diaminomethylpyridine (bisampyr), 1, was prepared as the dihydrochloride salt, via a two step synthesis from 2,6-pyridinedimethanol, 2, and immobilised onto Sepharose CL-4B through an epoxide coupling procedure. The resulting sorbent was chelated with Cu2+ ions to a density of 420 micromol Cu2+ ions per g gel and then characterised by frontal analysis using the protein, horse heart myoglobin (HMYO), at pH 7.0 and 9.0. From the resulting adsorption isotherms, the adsorption capacity, qm, for HMYO at pH 7.0 and pH 9.0 with the immobilised Cu2+-bisampyr Sepharose sorbent was found to be 1.27 micromol protein/g gel and 1.43 micromol protein/g gel, whilst the corresponding dissociation constants, K(D)s, were 18.0 x 10(-6) M and 16.0 x 10(-6) M respectively. The results confirm that the HMYO-Cu2+-bisampyr complex had similar stability at these pH values. This finding is in contrast with the situation observed with some other commonly used IMAC chelating ligates such as Cu2+-iminodiacetic acid (Cu2+-IDA) or Cu2+-nitrilotriacetic acid (Cu2+-NTA). Using human serum proteins, the interactive properties of the immobilised Cu2+-bisampyr Sepharose sorbent were further characterised at pH 5.0, 7.0 and 9.0 with specific reference to the binding behaviour of albumin, transferrin, and alpha2-macroglobulin.

Protein purification with vapor-phase carbon dioxide

Gaseous CO2 was used as an antisolvent to induce the fractional precipitation of alkaline phosphatase, insulin, lysozyme, ribonuclease, trypsin, and their mixtures from dimethylsulfoxide (DMSO). Compressed CO2 was added continuously and isothermally to stationary DMSO solutions (gaseous antisolvent, GAS). Dissolution of CO2 was accompanied by a pronounced, pressure-dependent volumetric expansion of DMSO and a consequent reduction in solvent strength of DMSO towards dissolved proteins. View cell experiments were conducted to determine the pressures at which various proteins precipitate from DMSO. The solubility of each protein in CO2-expanded DMSO was different, illustrating the potential to separate and purify proteins using gaseous antisolvents. Polyacrylamide gel electrophoresis in sodium dodecyl sulfate (SDS-PAGE) was used to quantify the separation of lysozyme from ribonuclease, alkaline phosphatase from insulin, and trypsin from catalase. Lysozyme biological activity assays were also performed to determine the composition of precipitates from DMSO initially containing lysozyme and ribonuclease. SDS-PAGE characterizations suggest that the composition and purity of solid-phase precipitated from a solution containing multiple proteins may be accurately controlled through the antisolvent's pressure. Insulin, lysozyme, ribonuclease, and trypsin precipitates recovered substantial amounts of biological activity upon redissolution in aqueous media. Alkaline phosphatase, however, was irreversibly denaturated. Vapor-phase antisolvents, which are easily separated and recovered from proteins and liquid solvents upon depressurization, appear to be a reliable and effective means of selectively precipitating proteins.