A novel approach to characterize the binding orientation of lysozyme on ion-exchange resins

All-atom modeling of methacrylate-based multi-modal chromatography resins for Langmuir constant prediction of peptides

In downstream processing, the intricate nature of the interactions between biomolecules and adsorbent materials presents a significant challenge in the prediction of their binding and elution behaviors. This complexity is further heightened in multi-modal chromatography (MMC), which employs two distinct binding mechanisms. To gain a deeper understanding of the involved interactions, simulating the adsorption of biomolecules on resin surfaces is a focal point of ongoing research. However, previous studies often simplified the adsorbent surface, modeling it as a flat or slightly curved plane without including a realistic backbone structure. Here, we introduce and validate two novel workflows aimed at predicting peptide binding behaviors in MMC, specifically targeting methacrylate-based resins. Our first achievement was the development of an all-atom model of a commercial MMC resin surface, incorporating its polymethacrylic backbone. Furthermore, we established and tested a workflow for rapid calculations of binding free energies (ΔG) with 10 linear peptides as target molecules. These ΔG calculations were effectively used to predict Langmuir constants, achieving a high coefficient of determination (R²) of 0.96. In subsequent benchmarking tests, our model outperformed established, simpler resin surface models in terms of predictive capabilities.

Characterization of the Charge Heterogeneity of a Monoclonal Antibody That Binds to Both Cation Exchange and Anion Exchange Columns under the Same Binding Conditions

Therapeutic antibodies play an important role in the public healthcare system to treat patients with a variety of diseases. Protein characterization using an array of analytical tools provides in-depth information for drug quality, safety, efficacy, and the further understanding of the molecule. A therapeutic antibody candidate MAB1 exhibits unique binding properties to both cation and anion exchange columns at neutral pH. This uniqueness disrupts standard purification processes and necessitates adjustments in manufacturing. This study identifies that the charge heterogeneity of MAB1 is primarily due to the N-terminal cyclization of glutamine to pyroglutamine and, to a lesser extent, succinimide intermediate, deamidation, and C-terminal lysine. Using three approaches, i.e., deferential chemical labeling, H/D exchange, and molecular modeling, the binding to anion exchange resins is attributed to negatively charged patches on the antibody's surface, involving specific carboxylic acid residues. The methodologies shown here can be extended to study protein binding orientation in column chromatography.

On the Performance of a Ready-to-Use Electrospun Sulfonated Poly(Ether Ether Ketone) Membrane Adsorber

Motivated by the need for efficient purification methods for the recovery of valuable resources, we developed a wire-electrospun membrane adsorber without the need for post-modification. The relationship between the fiber structure, functional-group density, and performance of electrospun sulfonated poly(ether ether ketone) (sPEEK) membrane adsorbers was explored. The sulfonate groups enable selective binding of lysozyme at neutral pH through electrostatic interactions. Our results show a dynamic lysozyme adsorption capacity of 59.3 mg/g at 10% breakthrough, which is independent of the flow velocity confirming dominant convective mass transport. Membrane adsorbers with three different fiber diameters (measured by SEM) were fabricated by altering the concentration of the polymer solution. The specific surface area as measured with BET and the dynamic adsorption capacity were minimally affected by variations in fiber diameter, offering membrane adsorbers with consistent performance. To study the effect of functional-group density, membrane adsorbers from sPEEK with different sulfonation degrees (52%, 62%, and 72%) were fabricated. Despite the increased functional-group density, the dynamic adsorption capacity did not increase accordingly. However, in all presented cases, at least a monolayer coverage was obtained, demonstrating ample functional groups available within the area occupied by a lysozyme molecule. Our study showcases a ready-to-use membrane adsorber for the recovery of positively charged molecules, using lysozyme as a model protein, with potential applications in removing heavy metals, dyes, and pharmaceutical components from process streams. Furthermore, this study highlights factors, such as fiber diameter and functional-group density, for optimizing the membrane adsorber's performance.

Mono-PEGylated lysozyme purification with increased productivity and isomer differentiation through heparin monolith chromatography

PEGylated protein purification with the required quality attributes has represented a bioengineering challenge and Affinity Monolith Chromatography (AMC) has never been exploited for this goal. This work reports the generation of a heparin-modified affinity monolith disk by reductive alkylation with raised ligand density for its use as chromatographic support in the separation of lysozyme PEGylation reactions (LPRs) with three different PEG sizes (1, 20 and 40 kDa). For immobilized heparin determination a modified toluidine colorimetric assay adapted to microplate format was proposed. The heparin modified-disk was able to differentiate positional isomers of 20 kDa mono-PEGylated lysozyme at neutral pH using a salt linear gradient. Identity of PEG-conjugates was verified by SDS-PAGE and positional isomers were partially characterized by peptide mapping mass spectrometry. 20 kDa mono-PEGylated lysozyme conjugate purity (99.69 ± 0.05%) was comparable with traditional chromatographic methods while productivity (0.0964 ± 0.0001 mg/mL*min) was increased up to 6.1 times compared to that obtained in heparin packed-bed affinity chromatography procedures. The proposed AMC method represents a reliable, efficient, easy-handling, fast and single-step operation for the analysis or preparative isolation of PEGylated proteins containing a heparin binding domain.

Probing IgG1 FC-Multimodal Nanoparticle Interactions: A Combined Nuclear Magnetic Resonance and Molecular Dynamics Simulations Approach

In this study, NMR and molecular dynamics simulations were employed to study IgG1 F_C binding to multimodal surfaces. Gold nanoparticles functionalized with two multimodal cation-exchange ligands (Capto and Nuvia) were synthesized and employed to carry out solution-phase NMR experiments with the F_C. Experiments with perdeuterated ¹⁵N-labeled F_C and the multimodal surfaces revealed micromolar residue-level binding affinities as compared to millimolar binding affinities with these ligands in free solution, likely due to cooperativity and avidity effects. The binding of F_C with the Capto ligand nanoparticles was concentrated near an aliphatic cluster in the C_H2/C_H3 interface, which corresponded to a focused hydrophobic region. In contrast, binding with the Nuvia ligand nanoparticles was more diffuse and corresponded to a large contiguous positive electrostatic potential region on the side face of the F_C. Results with lower-ligand-density nanoparticles indicated a decrease in binding affinity for both systems. For the Capto ligand system, several aliphatic residues on the F_C that were important for binding to the higher-density surface did not interact with the lower-density nanoparticles. In contrast, no significant difference was observed in the interacting residues on the F_C to the high- and low-ligand density Nuvia surfaces. The binding affinities of F_C to both multimodal-functionalized nanoparticles decreased in the presence of salt due to the screening of multiple weak interactions of polar and positively charged residues. For the Capto ligand nanoparticle system, this resulted in an even more focused hydrophobic binding region in the interface of the C_H2 and C_H3 domains. Interestingly, for the Nuvia ligand nanoparticles, the presence of salt resulted in a large transition from a diffuse binding region to the same focused binding region determined for Capto nanoparticles at 150 mM salt. Molecular dynamics simulations corroborated the NMR results and provided important insights into the molecular basis of F_C binding to these different multimodal systems containing clustered (observed at high-ligand densities) and nonclustered ligand surfaces. This combined biophysical and simulation approach provided significant insights into the interactions of F_C with multimodal surfaces and sets the stage for future analyses with even more complex biotherapeutics.

Adsorption of Mussel Protein on Polymer Antifouling Membranes: A Molecular Dynamics Study

Biofouling is one of the most difficult problems in the field of marine engineering. In this work, molecular dynamics simulation was used to study the adsorption process of mussel protein on the surface of two antifouling films-hydrophilic film and hydrophobic film-trying to reveal the mechanism of protein adsorption and the antifouling mechanism of materials at the molecular level. The simulated conclusion is helpful to design and find new antifouling coatings for the experiments in the future.

Adsorption and Conformation Behavior of Lysozyme on a Gold Surface Determined by QCM-D, MP-SPR, and FTIR

The physicochemical properties of protein layers at the solid–liquid interface are essential in many biological processes. This study aimed to link the structural analysis of adsorbed lysozyme at the water/gold surface at pH 7.5 in a wide range of concentrations. Particular attention was paid to the protein’s structural stability and the hydration of the protein layers formed at the interface. Complementary methods such as multi-parameter surface plasmon resonance (MP-SPR), quartz crystal microbalance with energy dissipation (QCM-D), and infrared spectroscopy (FTIR) were used for this purpose. The MP-SPR and QCM-D studies showed that, during the formation of a monolayer on the gold surface, the molecules’ orientation changes from side-on to end-on. In addition, bilayer formation is observed when adsorbing in the high-volume concentration range >500 ppm. The degree of hydration of the monolayer and bilayer varies depending on the degree of surface coverage. The hydration of the system decreases with filling the layer in both the monolayer and the bilayer. Hydration for the monolayer varies in the range of 50–70%, because the bilayer is much higher than 80%. The degree of hydration of the adsorption layer has a crucial influence on the protein layers’ viscoelastic properties. In general, an increase in the filling of a layer is characterized by a rise in its rigidity. The use of infrared spectroscopy allowed us to determine the changes taking place in the secondary structure of lysozyme due to its interaction with the gold surface. Upon adsorption, the content of II-structures corresponding to β-turn and random lysozyme structures increases, with a simultaneous decrease in the content of the β-sheet. The increase in the range of β-turn in the structure determines the lysozyme structure’s stability and prevents its aggregation.

Identification of preferred multimodal ligand-binding regions on IgG1 FC using nuclear magnetic resonance and molecular dynamics simulations

In this study, the binding of multimodal chromatographic ligands to the IgG1 F_C domain were studied using nuclear magnetic resonance and molecular dynamics simulations. Nuclear magnetic resonance experiments carried out with chromatographic ligands and a perdeuterated ¹⁵ N-labeled F_C domain indicated that while single-mode ion exchange ligands interacted very weakly throughout the F_C surface, multimodal ligands containing negatively charged and aromatic moieties interacted with specific clusters of residues with relatively high affinity, forming distinct binding regions on the F_C . The multimodal ligand-binding sites on the F_C were concentrated in the hinge region and near the interface of the C_H 2 and C_H 3 domains. Furthermore, the multimodal binding sites were primarily composed of positively charged, polar, and aliphatic residues in these regions, with histidine residues exhibiting some of the strongest binding affinities with the multimodal ligand. Interestingly, comparison of protein surface property data with ligand interaction sites indicated that the patch analysis on F_C corroborated molecular-level binding information obtained from the nuclear magnetic resonance experiments. Finally, molecular dynamics simulation results were shown to be qualitatively consistent with the nuclear magnetic resonance results and to provide further insights into the binding mechanisms. An important contribution to multimodal ligand-F_C binding in these preferred regions was shown to be electrostatic interactions and π-π stacking of surface-exposed histidines with the ligands. This combined biophysical and simulation approach has provided a deeper molecular-level understanding of multimodal ligand-F_C interactions and sets the stage for future analyses of even more complex biotherapeutics.

Impact of the isoelectric point of model parvoviruses on viral retention in anion-exchange chromatography

Anion-exchange chromatography (AEX) is used in the downstream purification of monoclonal antibodies to remove impurities and potential viral contamination based on electrostatic interactions. Although the isoelectric point (pI) of viruses is considered a key factor predicting the virus adsorption to the resin, the precise molecular mechanisms involved remain unclear. To address this question, we compared structurally homologous parvoviruses that only differ in their surface charge distribution. A single charged amino acid substitution on the capsid surface of minute virus of mice (MVM) provoked an increased apparent pI (pI_app ) 6.2 compared to wild-type MVM (pI_app  = 4.5), as determined by chromatofocusing. Despite their radically different pI_app , both viruses displayed the same interaction profile in Mono Q AEX at different pH conditions. In contrast, the closely related canine parvovirus (pI_app  = 5.3) displayed a significantly different interaction at pH 5. The detailed structural analysis of the intricate three-dimensional structure of the capsids suggests that the charge distribution is critical, and more relevant than the pI, in controlling the interaction of a virus with the chromatographic resin. This study contributes to a better understanding of the molecular mechanisms governing virus clearance by AEX, which is crucial to enable robust process design and maximize safety.

Steering protein adsorption at charged surfaces: electric fields and ionic screening

Protein adsorption at charged surfaces is a common process in the development of functional technological devices.

When the Label Matters: Adsorption of Labeled and Unlabeled Proteins on Charged Surfaces

Fluorescent labels are often attached to proteins to monitor binding and adsorption processes. Docking simulations for native hen egg white lysozyme (HEWL) and HEWL labeled with fluorescein isothiocyanate show that these adsorb differently on charged surfaces. Attachment of even a small label can significantly change the interaction properties of a protein. Thus, the results of experiments with fluorescently labeled proteins should be interpreted by modeling the structures and computing the interaction properties of both labeled and unlabeled species.

Lysozyme adsorption at a silica surface using simulation and experiment: effects of pH on protein layer structure

Hen Egg White Lysozyme (HEWL) is a widely used exemplar to study protein adsorption on surfaces and interfaces. Here we use fully atomistic Molecular Dynamics (MD) simulations, Multi-Parametric Surface Plasmon Resonance (MP-SPR), contact angle and zeta potential measurements to study HEWL adsorption at a silica surface. The simulations provide a detailed description of the adsorption mechanism and indicate that at pH7 the main adsorption driving force is electrostatics, supplemented by weaker hydrophobic forces. Moreover, they reveal the preferred orientation of the adsorbed protein and show that its structure is only slightly altered at the interface with the surface. This provides the basis for interpreting the experimental results, which indicate the surface adsorbs a close-packed monolayer at about pH10 where the surface has a large negative zeta potential and the HEWL is positively charged. At higher pH, the adsorption amount of the protein layer is greatly reduced due to the loss of charge on the protein. At lower pH, the smaller zeta potential of the surface leads to lower HEWL adsorption. These interpretations are complemented by the contact angle measurements that show how the hydrophobicity of the surface is greatest when the surface coverage is highest. The simulations provide details of the hydrophobic residues exposed to solution by the adsorbed HEWL, completing the picture of the protein layer structure.

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.

Mesoscopic coarse-grained simulations of lysozyme adsorption

Coarse-grained simulations are adopted to study the adsorption behavior of lysozyme on different (hydrophobic, neutral hydrophilic, zwitterionic, negatively charged, and positively charged) surfaces at the mesoscopic microsecond time scale (1.2 μs). Simulation results indicate the following: (i) the conformation change of lysozyme on the hydrophobic surface is bigger than any other studied surfaces; (ii) the active sites of lysozyme are faced to the hydrophobic surface with a "top end-on" orientation, while they are exposed to the liquid phase on the hydrophilic surface with a "back-on" orientation; (iii) the neutral hydrophilic surface can induce the adsorption of lysozyme, while the nonspecific protein adsorption can be resisted by the zwitterionic surface; (iv) when the solution ionic strength is low, lysozyme can anchor on the negatively charged surface easily but cannot adsorb on the positively charged surface; (v) when the solution ionic strength is high, the positively charged lysozyme can also adsorb on the like-charged surface; (vi) the major positive potential center of lysozyme, especially the residue ARG128, plays a vital role in leading the adsorption of lysozyme on charged surfaces; (vii) when the ionic strength is high, a counterion layer is formed above the positively charged surface, which is the key factor why lysozyme can adsorb on a like-charged surface. The coarse-grained method based on the MARTINI force field for proteins and the BMW water model could provide an efficient way to understand protein interfacial adsorption behavior at a greater length scale and time scale.

Illuminating protein crystal growth using fluorophore-labelled proteins

Incorporation of trace amounts of fluorophore-labelled proteins is used to study several optical properties and the growth history of protein crystals.

The use of quantitative structure-activity relationship models to develop optimized processes for the removal of tobacco host cell proteins during biopharmaceutical production

The production of recombinant pharmaceutical proteins in plants benefits from the low cost of upstream production and the greater scalability of plants compared to fermenter-based systems. Now that manufacturing processes that comply with current good manufacturing practices have been developed, plants can compete with established platforms on equal terms. However, the costs of downstream processing remain high, in part because of the dedicated process steps required to remove plant-specific process-related impurities. We therefore investigated whether the ideal strategy for the chromatographic removal of tobacco host cell proteins can be predicted by quantitative structure-activity relationship (QSAR) modeling to reduce the process development time and overall costs. We identified more than 100 tobacco proteins by mass spectrometry and their structures were reconstructed from X-ray crystallography, nuclear magnetic resonance spectroscopy and/or homology modeling data. The resulting three-dimensional models were used to calculate protein descriptors, and significant descriptors were selected based on recently-published retention data for model proteins to develop QSAR models for protein retention on anion, cation and mixed-mode resins. The predicted protein retention profiles were compared with experimental results using crude tobacco protein extracts. Because of the generic nature of the method, it can easily be transferred to other expression systems such as mammalian cells. The quality of the models and potential improvements are discussed.

Analytical characterization of complex, biotechnological feedstocks by pH gradient ion exchange chromatography for purification process development

The accelerating growth of the market for proteins and the growing interest in new, more complex molecules are bringing new challenges to the downstream process development of these proteins. This results in a demand for faster, more cost efficient, and highly understood downstream processes. Screening procedures based on high-throughput methods are widely applied nowadays to develop purification processes for proteins. However, screening highly complex biotechnological feedstocks, such as complete cell lysates containing target proteins often expressed with a low titre, is still very challenging. In this work we demonstrate a multidimensional, analytical screening approach based on pH gradient ion exchange chromatography (IEC), gel electrophoresis and protein identification via mass spectrometry to rationally characterize a biotechnological feedstock for the purpose of purification process development. With this very simple characterization strategy a two-step purification based on consecutive IEC operations was rapidly laid out for the purification of a diagnostic protein from a cell lysate reaching a purity of ∼80%. The target protein was recombinantly produced using an insect cell expression system.

Zonal rate model for axial and radial flow membrane chromatography. Part I: knowledge transfer across operating conditions and scales

The zonal rate model (ZRM) has previously been applied for analyzing the performance of axial flow membrane chromatography capsules by independently determining the impacts of flow and binding related non-idealities on measured breakthrough curves. In the present study, the ZRM is extended to radial flow configurations, which are commonly used at larger scales. The axial flow XT5 capsule and the radial flow XT140 capsule from Pall are rigorously analyzed under binding and non-binding conditions with bovine serum albumin (BSA) as test molecule. The binding data of this molecule is much better reproduced by the spreading model, which hypothesizes different binding orientations, than by the well-known Langmuir model. Moreover, a revised cleaning protocol with NaCl instead of NaOH and minimizing the storage time has been identified as most critical for quantitatively reproducing the measured breakthrough curves. The internal geometry of both capsules is visualized by magnetic resonance imaging (MRI). The flow in the external hold-up volumes of the XT140 capsule was found to be more homogeneous as in the previously studied XT5 capsule. An attempt for model-based scale-up was apparently impeded by irregular pleat structures in the used XT140 capsule, which might lead to local variations in the linear velocity through the membrane stack. However, the presented approach is universal and can be applied to different capsules. The ZRM is shown to potentially help save valuable material and time, as the experiments required for model calibration are much cheaper than the predicted large-scale experiment at binding conditions.

Protein diffusion and long-term adsorption states at charged solid surfaces

The diffusion pathways of lysozyme adsorbed to a model charged ionic surface are studied using fully atomistic steered molecular dynamics simulation. The simulations start from existing protein adsorption trajectories, where it has been found that one particular residue, Arg128 at the N,C-terminal face, plays a crucial role in anchoring the lysozyme to the surface [Langmuir 2010 , 26 , 15954 - 15965]. We first investigate the desorption pathway for the protein by pulling the Arg128 side chain away from the surface in the normal direction, and its subsequent readsorption, before studying diffusion pathways by pulling the Arg128 side chain parallel to the surface. We find that the orientation of this side chain plays a decisive role in the diffusion process. Initially, it is oriented normal to the surface, aligning in the electrostatic field of the surface during the adsorption process, but after resorption it lies parallel to the surface, being unable to return to its original orientation due to geometric constraints arising from structured water layers at the surface. Diffusion from this alternative adsorption state has a lower energy barrier of ∼0.9 eV, associated with breaking hydrogen bonds along the pathway, in reasonable agreement with the barrier inferred from previous experimental observation of lysozyme surface clustering. These results show the importance of studying protein diffusion alongside adsorption to gain full insight into the formation of protein clusters and films, essential steps in the future development of functionalized surfaces.

Modelling of lysozyme binding to a cation exchange surface at atomic detail: the role of flexibility

Different approaches were made to predict the adsorbed orientation based on rigid, flexible, or a mixture of both models. To determine the role of flexibility during adsorption, the orientation of lysozyme adsorbed to a negatively charged ligand surface was predicted by a rigid and a flexible model based on two differing protein structures at atomic resolution. For the rigid model, the protein structures were placed at different distances from the ligand surface and the electrostatic interaction energy was calculated for all possible orientations. The results were compared to a flexible model where the binding to the ligand surface was modeled by multiple molecular dynamics simulations starting with 14 initial orientations. Different aspects of the adsorption process were not covered by the rigid model and only detectable by the flexible model. Whereas the results of the rigid model depended sensitively on the protein-surface distance and the protein structure, the preferred orientation obtained by the flexible model was closer to a previous experimental determined orientation, robust toward the initial orientation and independent of the initial protein structure. Additionally, it was possible to obtain insights into the preferred binding process of lysozyme on a negatively charged surface by the flexible model.

Process evaluations and economic analyses of recombinant human lysozyme and hen egg-white lysozyme purifications

Human lysozyme and hen egg-white lysozyme have antibacterial, antiviral, and antifungal properties with numerous potential commercial applications. Currently, hen egg-white lysozyme dominates low cost applications but the recent high-level expression of human lysozyme in rice could provide an economical source of lysozyme. This work compares human lysozyme and hen egg-white lysozyme adsorption to the cation exchange resin, SP-Sepharose FF, and the effect of rice extract components on lysozyme purification. With one exception, the dynamic binding capacities of human lysozyme were lower than those of hen egg-white at pH 4.5, 6, and 7.5 with ionic strengths ranging from 0 to 100 mM (5-20 mS). Ionic strength and pH had a similar effect on the adsorption capacities, but human lysozyme was more sensitive to these two factors than hen egg-white lysozyme. In the presence of rice extract, the dynamic binding capacities of human and hen egg-white lysozymes were reduced by 20-30% and by 32-39% at pH 6. Hen egg-white lysozyme was used as a benchmark to compare the effectiveness of human lysozyme purification from transgenic rice extract. Process simulation and cost analyses for human lysozyme purification from rice and hen egg-white lysozyme purification from egg-white resulted in similar unit production costs at 1 ton per year scale.

Protein-surface interaction maps for ion-exchange chromatography

In this paper, protein-surface interaction maps were generated by performing coarse-grained protein-surface calculations. This approach allowed for the rapid determination of the protein-surface interaction energies at a range of orientations and distances. Interaction maps of lysozyme indicated that there was a contiguous series of orientations corresponding to several adjacent preferred binding regions on the protein surface. Examination of these orientations provided insight into the residues involved in surface interactions, which qualitatively agreed with the retention data for single-site mutants. Interaction maps of lysozyme single-site mutants were also generated and provided significant insight into why these variants exhibited significant differences in their chromatographic behavior. This approach was also employed to study the binding behavior of CspB and related mutants. The results indicated that, in addition to describing general trends in the data, these maps provided significant insight into retention data of the single-site mutants. In particular, subtle retention trends observed with the K12 and K13 mutants were well-described using this interaction map approach. Finally, the number of interaction points with energies stronger than -2 kcal/mol was shown to be able to semi-quantitatively predict the behavior of most of the mutants. This rapid approach for calculating protein-surface interaction maps is expected to facilitate future method development for separating closely related protein variants in ion-exchange systems.

Mechanism of hen egg white lysozyme adsorption on a charged solid surface

The mechanism of hen egg white lysozyme (HEWL) adsorption on a negatively charged, hydrophilic surface has been studied using atomistic molecular dynamics (MD) simulation. Sixteen 90 ns trajectories provide adequate data to allow a detailed description of the adsorption process to be formulated. Two distinct adsorption sites have been identified. The main one is located on the N,C-terminal protein face and comprises Arg128 (the crucial one), supplemented by Arg125, Arg5, and Lys1; the minor one is used accidentally and contains only Arg68. Adsorption of this protein is driven by electrostatics, where the orientation of the protein dipole moment defines the direction of protein movement. The diffusion range on the surface depends on protein side-chain penetration through the surface water layers. This is facilitated by the long-range electric field of the charged surface, which can align polar side chains to be perpendicular to the surface. A simulation of adsorption onto a neutral ionic surface shows no such surface water layer penetration. Therefore, protein flexibility is seen to be an important factor, and to adsorb the HEWL has to adjust its structure. Nevertheless, at a flat surface only a slight loss of α-helical content is required. The adsorbed HEWL molecule is oriented between side-on and end-on ways, where the angle between the protein long axis (which mostly approximates the dipole moment) and the surface varies between 45° and 90°. Simulations with targeted mutations confirm the picture that emerges from these studies. The active site is located on the opposite face to the main adsorption site; hence, the activity of the immobilized HEWL should not be affected by the surface interactions. Our results provide a detailed insight into the adsorption mechanism and protein mobility at the surface. This knowledge will aid the proper interpretation of experimental results and the design of new experiments and functional systems.

Studies of lysozyme binding to histamine as a ligand for hydrophobic charge induction chromatography

Histamine was immobilized on Sepharose CL-6B (Sepharose) for use as a ligand of hydrophobic charge induction chromatography (HCIC) of proteins. Lysozyme adsorption onto Histamine-Sepharose (HA-S) was studied by adsorption equilibrium and calorimetry to uncover the thermodynamic mechanism of the protein binding. In both the experiments, the influence of salt (ammonium sulfate and sodium sulfate) was examined. Adsorption isotherms showed that HA-S exhibited a high salt tolerance in lysozyme adsorption. This property was well explained by the combined contributions of hydrophobic interaction and aromatic stacking. The isotherms were well fitted to the Langmuir equation, and the equilibrium parameters for lysozyme adsorption were obtained. In addition, thermodynamic parameters (DeltaH(ads), DeltaS(ads), and DeltaG(ads)) for the adsorption were obtained by isothermal titration calorimetry by titrating lysozyme solutions into the adsorbent suspension. Furthermore, free histamine was titrated into lysozyme solution in the same salt-buffers. Compared with the binding of lysozyme to free histamine, lysozyme adsorption onto HA-S was characterized by a less favorable DeltaG(ads) and an unfavorable DeltaS(ads) because histamine was covalently attached to Sepharose via a three-carbon-chain spacer. Consequently, the immobilized histamine could only associate with the residues on the protein surface rather than those in the hydrophobic pocket, causing a less favorable orientation between histamine and lysozyme. Further comparison of thermodynamic parameters indicated that the unfavorable DeltaS(ads) was offset by a favorable DeltaH(ads), thus exhibiting typical enthalpy-entropy compensation. Moreover, thermodynamic analyses indicated the importance of the dehydration of lysozyme molecule and HA-S during the adsorption and a substantial conformational change of the protein during adsorption. The results have provided clear insights into the adsorption mechanisms of lysozyme onto the new HCIC material.