Influence of modulated structural dynamics on the kinetics of alpha-chymotrypsin catalysis. Insights through chemical glycosylation, molecular dynamics and domain motion analysis

Enhancing Protein Solubility via Glycosylation: From Chemical Synthesis to Machine Learning Predictions

Glycosylation is a valuable tool for modulating protein solubility; however, the lack of reliable research strategies has impeded efficient progress in understanding and applying this modification. This study aimed to bridge this gap by investigating the solubility of a model glycoprotein molecule, the carbohydrate-binding module (CBM), through a two-stage process. In the first stage, an approach involving chemical synthesis, comparative analysis, and molecular dynamics simulations of a library of glycoforms was employed to elucidate the effect of different glycosylation patterns on solubility and the key factors responsible for the effect. In the second stage, a predictive mathematical formula, innovatively harnessing machine learning algorithms, was derived to relate solubility to the identified key factors and accurately predict the solubility of the newly designed glycoforms. Demonstrating feasibility and effectiveness, this two-stage approach offers a valuable strategy for advancing glycosylation research, especially for the discovery of glycoforms with increased solubility.

Glycosylation shapes the efficacy and safety of diverse protein, gene and cell therapies

Over recent decades, therapeutic proteins have had widespread success in treating a myriad of diseases. Glycosylation, a near universal feature of this class of drugs, is a critical quality attribute that significantly influences the physical properties, safety profile and biological activity of therapeutic proteins. Optimizing protein glycosylation, therefore, offers an important avenue to developing more efficacious therapies. In this review, we discuss specific examples of how variations in glycan structure and glycoengineering impacts the stability, safety, and clinical efficacy of protein-based drugs that are already in the market as well as those that are still in preclinical development. We also highlight the impact of glycosylation on next generation biologics such as T cell-based cancer therapy and gene therapy.

In Search of a Dynamical Vocabulary: A Pipeline to Construct a Basis of Shared Traits in Large-Scale Motions of Proteins

The paradigmatic sequence–structure–dynamics–function relation in proteins is currently well established in the scientific community; in particular, a large effort has been made to probe the first connection, indeed providing convincing evidence of its strength and rationalizing it in a quantitative and general framework. In contrast, however, the role of dynamics as a link between structure and function has eluded a similarly clear-cut verification and description. In this work, we propose a pipeline aimed at building a basis for the quantitative characterization of the large-scale dynamics of a set of proteins, starting from the sole knowledge of their native structures. The method hinges on a dynamics-based clusterization, which allows a straightforward comparison with structural and functional protein classifications. The resulting basis set, obtained through the application to a group of related proteins, is shown to reproduce the salient large-scale dynamical features of the dataset. Most interestingly, the basis set is shown to encode the fluctuation patterns of homologous proteins not belonging to the initial dataset, thus highlighting the general applicability of the pipeline used to build it.

Chemical (neo)glycosylation of biological drugs

Biological drugs, specifically proteins and peptides, are a privileged class of medicinal agents and are characterized with high specificity and high potency of therapeutic activity. However, biologics are fragile and require special care during storage, and are often modified to optimize their pharmacokinetics in terms of proteolytic stability and blood residence half-life. In this review, we showcase glycosylation as a method to optimize biologics for storage and application. Specifically, we focus on chemical glycosylation as an approach to modify biological drugs. We present case studies that illustrate the success of this methodology and specifically address the highly important question: does connectivity within the glycoconjugate have to be native or not? We then present the innovative methods of chemical glycosylation of biologics and specifically highlight the emerging and established protecting group-free methodologies of glycosylation. We discuss thermodynamic origins of protein stabilization via glycosylation, and analyze in detail stabilization in terms of proteolytic stability, aggregation upon storage and/or heat treatment. Finally, we present a case study of protein modification using sialic acid-containing glycans to avoid hepatic clearance of biological drugs. This review aims to spur interest in chemical glycosylation as a facile, powerful tool to optimize proteins and peptides as medicinal agents.

Lysosomal protein thermal stability does not correlate with cellular half-life: global observations and a case study of tripeptidyl-peptidase 1

Late-infantile neuronal ceroid lipofuscinosis (LINCL) is a neurodegenerative lysosomal storage disorder caused by mutations in the gene encoding the protease tripeptidyl-peptidase 1 (TPP1). Progression of LINCL can be slowed or halted by enzyme replacement therapy, where recombinant human TPP1 is administered to patients. In this study, we utilized protein engineering techniques to increase the stability of recombinant TPP1 with the rationale that this may lengthen its lysosomal half-life, potentially increasing the potency of the therapeutic protein. Utilizing multiple structure-based methods that have been shown to increase the stability of other proteins, we have generated and evaluated over 70 TPP1 variants. The most effective mutation, R465G, increased the melting temperature of TPP1 from 55.6°C to 64.4°C and increased its enzymatic half-life at 60°C from 5.4 min to 21.9 min. However, the intracellular half-life of R465G and all other variants tested in cultured LINCL patient-derived lymphoblasts was similar to that of WT TPP1. These results provide structure/function insights into TPP1 and indicate that improving in vitro thermal stability alone is insufficient to generate TPP1 variants with improved physiological stability. This conclusion is supported by a proteome-wide analysis that indicates that lysosomal proteins have higher melting temperatures but also higher turnover rates than proteins of other organelles. These results have implications for similar efforts where protein engineering approaches, which are frequently evaluated in vitro, may be considered for improving the physiological properties of proteins, particularly those that function in the lysosomal environment.

Redesigning N-glycosylation sites in a GH3 β-xylosidase improves the enzymatic efficiency

BACKGROUND

β-Xylosidases are glycoside hydrolases (GHs) that cleave xylooligosaccharides and/or xylobiose into shorter oligosaccharides and xylose. Aspergillus nidulans is an established genetic model and good source of carbohydrate-active enzymes (CAZymes). Most fungal enzymes are N-glycosylated, which influences their secretion, stability, activity, signalization, and protease protection. A greater understanding of the N-glycosylation process would contribute to better address the current bottlenecks in obtaining high secretion yields of fungal proteins for industrial applications.

RESULTS

In this study, BxlB-a highly secreted GH3 β-xylosidase from A. nidulans, presenting high activity and several N-glycosylation sites-was selected for N-glycosylation engineering. Several glycomutants were designed to investigate the influence of N-glycans on BxlB secretion and function. The non-glycosylated mutant (BxlB^non-glyc) showed similar levels of enzyme secretion and activity compared to the wild-type (BxlB^wt), while a partially glycosylated mutant (BxlB^N1;5;7) exhibited increased activity. Additionally, there was no enzyme secretion in the mutant in which the N-glycosylation context was changed by the introduction of four new N-glycosylation sites (BxlB^CC), despite the high transcript levels. BxlB^wt, BxlB^non-glyc, and BxlB^N1;5;7 formed similar secondary structures, though the mutants had lower melting temperatures compared to the wild type. Six additional glycomutants were designed based on BxlB^N1;5;7, to better understand its increased activity. Among them, the two glycomutants which maintained only two N-glycosylation sites each (BxlB^N1;5 and BxlB^N5;7) showed improved catalytic efficiency, whereas the other four mutants' catalytic efficiencies were reduced. The N-glycosylation site N5 is important for improved BxlB catalytic efficiency, but needs to be complemented by N1 and/or N7. Molecular dynamics simulations of BxlB^non-glyc and BxlB^N1;5 reveals that the mobility pattern of structural elements in the vicinity of the catalytic pocket changes upon N1 and N5 N-glycosylation sites, enhancing substrate binding properties which may underlie the observed differences in catalytic efficiency between BxlB^non-glyc and BxlB^N1;5.

CONCLUSIONS

This study demonstrates the influence of N-glycosylation on A. nidulans BxlB production and function, reinforcing that protein glycoengineering is a promising tool for enhancing thermal stability, secretion, and enzymatic activity. Our report may also support biotechnological applications for N-glycosylation modification of other CAZymes.

Improving the catalytic, kinetic and thermodynamic properties of Bacillus subtilis KU710517 milk clotting enzyme via conjugation with polyethylene glycol

Milk clotting enzyme (MCE) produced by Bacillus subtilis KU710517 was conjugated to several activated polysaccharides. Among all the conjugates, the enzyme conjugated with polyethylene glycol (PEG) exhibited the highest retained activity (551U/mg protein) with a recovered activity of 95.3%. The activation energy of PEG-conjugated enzyme was calculated as 24.56kJ·mol^-1which was lower than that of the native one (29.27kJ·mol^-1) however, the temperature quotient (Q₁₀) was about 1.08 for the two forms of the enzyme. The calculated half-life times of PEG-conjugated enzyme at 55 and 60°C were 317.78 and 128.6min respectively, whereas at the same temperatures the native enzyme had lower half-life times (53 and 19.6min respectively). The data of thermodynamic analysis for substrate catalysis including the specificity constant (V_max/K_m), turnover number (k_cat), catalytic efficiency (k_cat/K_m), enthalpy of activation (ΔH*), free energy of activation (ΔG*), free energy for transition state formation ΔG*_E-T and free energy of substrate binding ΔG*_E-S were determined for both native and PEG-conjugated enzyme. In addition, the thermodynamic parameters for irreversible inactivation (ΔH, ΔG, ΔS) were evaluated. The calculated results indicated that the catalytic properties after the PEG-conjugation were significantly improved.

Co-solvent effects on reaction rate and reaction equilibrium of an enzymatic peptide hydrolysis

This work presents an approach that expresses the Michaelis constant KaM and the equilibrium constant K_th of an enzymatic peptide hydrolysis based on thermodynamic activities instead of concentrations.

Two-step polymer- and liposome-enzyme prodrug therapies for cancer: PDEPT and PELT concepts and future perspectives

Polymer-directed enzyme prodrug therapy (PDEPT) and polymer enzyme liposome therapy (PELT) are two-step therapies developed to provide anticancer drugs site-selective intratumoral accumulation and release. Nanomedicines, such as polymer-drug conjugates and liposomal drugs, accumulate in the tumor site due to extravasation-dependent mechanism (enhanced permeability and retention - EPR - effect), and further need to cross the cellular membrane and release their payload in the intracellular compartment. The subsequent administration of a polymer-enzyme conjugate able to accumulate in the tumor tissue and to trigger the extracellular release of the active drug showed promising preclinical results. The development of polymer-enzyme, polymer-drug conjugates and liposomal drugs had undergone a vast advancement over the past decades. Several examples of enzyme mimics for in vivo therapy can be found in the literature. Moreover, polymer therapeutics often present an enzyme-sensitive mechanism of drug release. These nanomedicines can thus be optimal substrates for PDEPT and this review aims to provide new insights and stimuli toward the future perspectives of this promising combination.

Functional analysis of TcCTLP-5C2, a chymotrypsin-like serine protease needed for molting in Tribolium castaneum

In a previous study, we have characterized a gene family encoding chymotrypsin-like proteases from the red flour beetle, Tribolium castaneum (TcCTLPs). We identified 14 TcCTLP genes that were predominantly expressed in the midgut, where they presumably function in digestion. Two genes (TcCTLP-6C and TcCTLP-5C₂), however, additionally showed considerable expression in the carcass, and RNAi studies demonstrated that they are required for molting (Broehan et al., 2010; Insect Biochem. Mol. Biol 40, 274-83). Thus, the enzyme has distinct functions in different physiological environments. To study molecular adaptations that facilitate enzyme function in different environments, we performed an in-depth analysis of the molecular and enzymatic properties of TcCTLP-5C_2. We expressed different mutated versions of TcCTLP-5C₂ in form of factor Xa activatable pro-enzymes in insect cells using a baculoviral expression system, and purified the recombinant proteins by affinity chromatography. By measuring and comparing the enzyme activities, we obtained information about the significance of single amino acid residues in motifs that determine substrate specificity and pH tolerance. Further, we showed that TcCTLP-5C₂ is modified by N-glycosylation at amino acid position N137, which lies opposite to the catalytic cleft. Comparison of the enzymatic properties of non-glycosylated and glycosylated TcCTLP-5C₂ versions showed that N-glycosylation decreases V_max (maximum velocity) and k_cat (turnover) while leaving the K_m (specificity) unchanged. Thus, we provide evidence that N-glycosylation alters catalytic behavior by allosteric effects presumably due to altered structural dynamics as observed for chemically glycosylated enzymes.

Catalytic, kinetic and thermodynamic properties of stabilized Bacillus stearothermophilus alkaline protease

Bacillus stearothermophilus alkaline protease was conjugated to several oxidized polysaccharides of different chemical structure. The conjugates were evaluated for the kinetic and thermodynamic stability. The conjugated enzyme with oxidized pectin had the highest retained activity (79.5%) and the highest half-life (T_1/2) at 50°C and pH 9.0. Compared to the native protease, the conjugated preparation exhibited lower activation energy (E_a), lower deactivation constant rate (k_d), higher T_1/2, and higher D values (decimal reduction time) within the temperature range of 50-60°C. The thermodynamic parameters for irreversible inactivation of native and conjugated protease indicated that conjugation significantly decreased entropy (ΔS*) and enthalpy (ΔH*) of deactivation. The calculated value of activation energy for thermal denaturation (E_ad) for the conjugated enzyme was 20.4KJmole^-1 higher over the native one. The results of thermodynamic analysis for substrate hydrolysis indicated that the enthalpy of activation (ΔH*) and free energy of activation (free energy of substrate binding) ΔG*_E-S and (ΔG*), (free energy of transition state) ΔG*_E-T values were lower for the modified protease. Similarly, there was significant improvement of k_cat, k_cat/K_m values. The enzyme proved to be metalloprotease and significantly stimulated by Ca²⁺ and Mg²⁺ whereas Hg²⁺, Fe³⁺ Cu²⁺ and Zn²⁺ inhibited the enzyme activity. There was no pronounced effect on substrate specificity after conjugation.

Effect of protein structure and/or conformation on the dityrosine cross-linking induced by haem-hydrogen peroxide

BACKGROUND

Haem, an essential cofactor in aerobic organisms, can cause oxidative stress and impose toxic effects on tissues and organs. It can induce aggregation of proteins via dityrosine cross-linking and cause neurodegenerative diseases. Although dityrosine cross-linking in many proteins induced by haem has been reported, not all the proteins have the same effect or the efficiency of cross-linking varies, while the reason has not been clarified.

METHODS

The correlation of protein structure/conformation with its aggregation tendency via dityrosine induced by hematin (oxidized form of haem) in the presence of hydrogen peroxide (H2O2) was studied through reducing sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), fluorescence and circular dichroism (CD) measurements, and the mechanism was investigated by performing UV-Vis absorbance, Raman spectroscopy and low-temperature electron spin resonance (ESR) experiments.

RESULTS

It was found that proteins in unstructured state are more readily to be cross-linked via dityrosine formation by hematin-H2O2. The unstructured protein without steric effect can coordinate with hematin to form six-coordinated protein-hematin complex, in which the produced tyrosyl radicals by H2O2 are with high tendency to dimerize to form dityrosine.

CONCLUSIONS

Our results demonstrate that protein structure/conformation can affect its coordination state with haem, and the tendency of reaction of two tyrosyl radicals, further influencing the yield and efficiency of dityrosine cross-linking in the presence of H2O2.

GENERAL SIGNIFICANCE

This research can help to deepen our understanding of the protein aggregation and inactivation mechanisms in varied sophisticated conditions, and especially give us the new insight into the toxic effects under haem stress.

Graphene oxide as a protein matrix: influence on protein biophysical properties

Background

This study provides fundamental information on the influence of graphene oxide (GO) nanosheets and glycans on protein catalytic activity, dynamics, and thermal stability. We provide evidence of protein stabilization by glycans and how this strategy could be implemented when GO nanosheets is used as protein immobilization matrix. A series of bioconjugates was constructed using two different strategies: adsorbing or covalently attaching native and glycosylated bilirubin oxidase (BOD) to GO.

Results

Bioconjugate formation was followed by FT-IR, zeta-potential, and X-ray photoelectron spectroscopy measurements. Enzyme kinetic parameters (k_m and k_cat) revealed that the substrate binding affinity was not affected by glycosylation and immobilization on GO, but the rate of enzyme catalysis was reduced. Structural analysis by circular dichroism showed that glycosylation did not affect the tertiary or the secondary structure of BOD. However, GO produced slight changes in the secondary structure. To shed light into the biophysical consequence of protein glycosylation and protein immobilization on GO nanosheets, we studied structural protein dynamical changes by FT-IR H/D exchange and thermal inactivation.

Conclusions

It was found that glycosylation caused a reduction in structural dynamics that resulted in an increase in thermostability and a decrease in the catalytic activity for both, glycoconjugate and immobilized enzyme. These results establish the usefulness of chemical glycosylation to modulate protein structural dynamics and stability to develop a more stable GO-protein matrix.

Electronic supplementary material

The online version of this article (doi:10.1186/s12951-015-0134-0) contains supplementary material, which is available to authorized users.

Chemically glycosylation improves the stability of an amperometric horseradish peroxidase biosensor

We constructed a biosensor by electrodeposition of gold nano-particles (AuNPs) on glassy carbon (GC) and subsequent formation of a 4-mercaptobenzoic acid self-assembled monolayer (SAM). The enzyme horseradish peroxidase (HRP) was then covalently immobilized onto the SAM. Two forms of HRP were employed: non-modified and chemically glycosylated with lactose. Circular dichroism (CD) spectra showed that chemical glycosylation did neither change the tertiary structure of HRP nor the heme environment. The highest sensitivity of the biosensor to hydroquinone was obtained for the biosensor with HRP-lactose (414 nA μM(-1)) compared to 378 nA μM(-1) for the one employing non-modified HRP. The chemically glycosylated form of the enzyme catalyzed the reduction of hydroquinone more rapidly than the native form of the enzyme. The sensor employing lactose-modified HRP also had a lower limit of detection (74 μM) than the HRP biosensor (83 μM). However, most importantly, chemically glycosylation improved the long-term stability of the biosensor, which retained 60% of its activity over a four-month storage period compared to only 10% for HRP. These results highlight improvements by an innovative stabilization method when compared to previously reported enzyme-based biosensors.

Chemical glycosylation of cytochrome c improves physical and chemical protein stability

Background

Cytochrome c (Cyt c) is an apoptosis-initiating protein when released into the cytoplasm of eukaryotic cells and therefore a possible cancer drug candidate. Although proteins have been increasingly important as pharmaceutical agents, their chemical and physical instability during production, storage, and delivery remains a problem. Chemical glycosylation has been devised as a method to increase protein stability and thus enhance their long-lasting bioavailability.

Results

Three different molecular weight glycans (lactose and two dextrans with 1 kD and 10 kD) were chemically coupled to surface exposed Cyt c lysine (Lys) residues using succinimidyl chemistry via amide bonds. Five neo-glycoconjugates were synthesized, Lac₄-Cyt-c, Lac₉-Cyt-c, Dex₅(10kD)-Cyt-c, Dex₈(10kD)-Cyt-c, and Dex₃(1kD)-Cyt-c. Subsequently, we investigated glycoconjugate structure, activity, and stability. Circular dichroism (CD) spectra demonstrated that Cyt c glycosylation did not cause significant changes to the secondary structure, while high glycosylation levels caused some minor tertiary structure perturbations. Functionality of the Cyt c glycoconjugates was determined by performing cell-free caspase 3 and caspase 9 induction assays and by measuring the peroxidase-like pseudo enzyme activity. The glycoconjugates showed ≥94% residual enzyme activity and 86 ± 3 to 95 ± 1% relative caspase 3 activation compared to non-modified Cyt c. Caspase 9 activation by the glycoconjugates was with 92 ± 7% to 96 ± 4% within the error the same as the caspase 3 activation. There were no major changes in Cyt c activity upon glycosylation. Incubation of Dex₃(1 kD)-Cyt c with mercaptoethanol caused significant loss in the tertiary structure and a drop in caspase 3 and 9 activation to only 24 ± 8% and 26 ± 6%, respectively. This demonstrates that tertiary structure intactness of Cyt c was essential for apoptosis induction. Furthermore, glycosylation protected Cyt c from detrimental effects by some stresses (i.e., elevated temperature and humidity) and from proteolytic degradation. In addition, non-modified Cyt c was more susceptible to denaturation by a water-organic solvent interface than its glycoconjugates, important for the formulation in polymers.

Conclusion

The results demonstrate that chemical glycosylation is a potentially valuable method to increase Cyt c stability during formulation and storage and potentially during its application after administration.

Optimization of physicochemical and pharmacological properties of peptide drugs by glycosylation

Many biological interactions and functions are mediated by glycans, leading to the emerging importance of carbohydrate and glycoconjugate chemistry in the design of novel drug therapeutics. In addition to direct effects on biological activity, sugar addition appears to alter many physicochemical and pharmacological properties of the peptide backbone. Consequently, glycosylation has been often used to improve various less than optimal features of peptide drug leads.In order to study the effects that naturally occurring and/or nonnatural glycans have on peptide drug solubility, conformation, proteolytic resistance, membrane permeability, and toxicity, it is essential to have convenient synthetic access toward synthesis of glycopeptide analogs. The crucial step in the synthesis of glycopeptides is the introduction of the carbohydrate group. The preformed glycosyl amino acid building block is the most commonly employed approach used in glycopeptide synthesis.In this review, we will describe various synthetic approaches to prepare N- and O-glycopeptides bearing simple monosaccharides as a tool to improve peptide therapeutic efficacy by glycosylation.

Structured crowding and its effects on enzyme catalysis

Macromolecular crowding decreases the diffusion rate, shifts the equilibrium of protein-protein and protein-substrate interactions, and changes protein conformational dynamics. Collectively, these effects contribute to enzyme catalysis. Here we describe how crowding may bias the conformational change and dynamics of enzyme populations and in this way affect catalysis. Crowding effects have been studied using artificial crowding agents and in vivo-like environments. These studies revealed a correlation between protein dynamics and function in the crowded environment. We suggest that crowded environments be classified into uniform crowding and structured crowding. Uniform crowding represents random crowding conditions created by synthetic particles with a narrow size distribution. Structured crowding refers to the highly coordinated cellular environment, where proteins and other macromolecules are clustered and organized. In structured crowded environments the perturbation of protein thermal stability may be lower; however, it may still be able to modulate functions effectively and dynamically. Dynamic, allosteric enzymes could be more sensitive to cellular perturbations if their free energy landscape is flatter around the native state; on the other hand, if their free energy landscape is rougher, with high kinetic barriers separating deep minima, they could be more robust. Above all, cells are structured; and this holds both for the cytosol and for the membrane environment. The crowded environment is organized, which limits the search, and the crowders are not necessarily inert. More likely, they too transmit allosteric effects, and as such play important functional roles. Overall, structured cellular crowding may lead to higher enzyme efficiency and specificity.

Glycosylation improves α-chymotrypsin stability upon encapsulation in poly(lactic-co-glycolic)acid microspheres

Enhancing protein stability upon encapsulation and release from polymers is a key issue in sustained release applications. In addition, optimum drug dispersion in the polymer particles is critical for achieving release profiles with low unwanted initial “burst” release. Herein, we address both issues by formulating the model enzyme α-chymotrypsin (α-CT) as nanoparticles to improve drug dispersion and by covalently modifying it with glycans to afford improved stability during encapsulation in poly(lactic-co-glycolic) acid (PLGA) microspheres. α-CT was chemically modified with activated lactose (500 Da) to achieve molar ratios of 4.5 and 7.1 lactose-to-protein. The bioconjugates were co-lyophilized with methyl-β-cyclodextrin followed by suspension in ethyl acetate to afford nanoparticles. Nanoparticle formation did not significantly impact protein stability; less than 5% of the protein was aggregated and the residual activity remained above 90% for all formulations. Using a solid-in-oil-in-water (s/o/w) methodology developed in our laboratory for nanoparticles, we obtained a maximum encapsulation efficiency of 61%. Glycosylation completely prevented otherwise substantial protein aggregation and activity loss during encapsulation of the non-modified enzyme. Moreover, in vitro protein release was improved for glycosylated formulations. These results highlight the potential of chemical glycosylation to improve the stability of pharmaceutical proteins in sustained release applications.

Moisture-induced solid state instabilities in alpha-chymotrypsin and their reduction through chemical glycosylation

Background

Protein instability remains the main factor limiting the development of protein therapeutics. The fragile nature (structurally and chemically) of proteins makes them susceptible to detrimental events during processing, storage, and delivery. To overcome this, proteins are often formulated in the solid-state which combines superior stability properties with reduced operational costs. Nevertheless, solid protein pharmaceuticals can also suffer from instability problems due to moisture sorption. Chemical protein glycosylation has evolved into an important tool to overcome several instability issues associated with proteins. Herein, we employed chemical glycosylation to stabilize a solid-state protein formulation against moisture-induced deterioration in the lyophilized state.

Results

First, we investigated the consequences of moisture sorption on the stability and structural conformation of the model enzyme α-chymotrypsin (α-CT) under controlled humidity conditions. Results showed that α-CT aggregates and inactivates as a function of increased relative humidity (RH). Furthermore, α-CT loses its native secondary and tertiary structure rapidly at increasing RH. In addition, H/D exchange studies revealed that α-CT structural dynamics increased at increasing RH. The magnitude of the structural changes in tendency parallels the solid-state instability data (i.e., formation of buffer-insoluble aggregates, inactivation, and loss of native conformation upon reconstitution). To determine if these moisture-induced instability issues could be ameliorated by chemical glycosylation we proceeded to modify our model protein with chemically activated glycans of differing lengths (lactose and dextran (10 kDa)). The various glycoconjugates showed a marked decrease in aggregation and an increase in residual activity after incubation. These stabilization effects were found to be independent of the glycan size.

Conclusion

Water sorption leads to aggregation, inactivation, and structural changes of α-CT as has been similarly shown to occur for many other proteins. These instabilities correlate with an increase in protein structural dynamics as a result of moisture exposure. In this work, we present a novel methodology to stabilize proteins against structural perturbations in the solid-state since chemical glycosylation was effective in decreasing and/or preventing the traditionally observed moisture-induced aggregation and inactivation. It is suggested that the stabilization provided by these chemically attached glycans comes from the steric hindrance that the sugars conveys on the protein surface therefore preventing the interaction of the protein internal electrostatics with that of the water molecules and thus reducing the protein structural dynamics upon moisture exposure.

Active-site motions and polarity enhance catalytic turnover of hydrated subtilisin dissolved in organic solvents

The enzyme subtilisin Carlsberg was surfactant-solubilized into two organic solvents, isooctane and tetrahydrofuran, and hydrated through stepwise changes in the thermodynamic water activity, a(w). The apparent turnover number k(cat)(app) in these systems ranged from 0.2 to 80 s(-1) and increased 11-fold in isooctane and up to 50-fold in tetrahydrofuran with increasing a(w). (19)F NMR relaxation experiments employing an active-site inhibitor were used to assess the dependence of active-site motions on a(w). The rates of NMR-derived fast (k > 10(7) s(-1)) and slow (k < 10(4) s(-1)) active-site motions increased in both solvents upon hydration, but only the slow motions correlated with k(cat). The (19)F chemical shift was a sensitive probe of the local electronic environment and provided an empirical measure of the active-site dielectric constant epsilon(as), which increased with hydration to epsilon(as) approximately 13 in each solvent. In both solvents, the transition state free energy data and epsilon(as) followed Kirkwood's model for the continuum solvation of a dipole, indicating that water also enhanced catalysis by altering the active-site's electronic environment and increasing its polarity to better stabilize the transition state. These results reveal that favorable dynamic and electrostatic effects both contribute to accelerated catalysis by solubilized subtilisin Carlsberg upon hydration in organic solvents.

On the role of protein structural dynamics in the catalytic activity and thermostability of serine protease subtilisin Carlsberg

The effect of structural dynamics on enzyme activity and thermostability has thus far only been investigated in detail for the serine protease alpha-chymotrypsin (for a recent review see Solá et al., Cell Mol Life Sci 2007, 64(16): 2133-2152). Herein, we extend this type of study to a structurally unrelated serine protease, specifically, subtilisin Carlsberg. The protease was incrementally glycosylated with chemically activated lactose to obtain various subtilisin glycoconjugates which were biophysically characterized. Near UV-CD spectroscopy revealed that the tertiary structure was unaffected by the glycosylation procedure. H/D exchange FT-IR spectroscopy was performed to assess the changes in structural dynamics of the enzyme. It was found that increasing the level of glycosylation caused a linearly dependent reduction in structural dynamics. This led to an increase in thermostability and a decrease in the catalytic turnover rate for both, the enzyme acylation and deacylation steps. These results highlight the possibility that a structural dynamics-activity relationship might be a phenomenon generally found in serine proteases.

Effects of glycosylation on the stability of protein pharmaceuticals

In recent decades, protein-based therapeutics have substantially expanded the field of molecular pharmacology due to their outstanding potential for the treatment of disease. Unfortunately, protein pharmaceuticals display a series of intrinsic physical and chemical instability problems during their production, purification, storage, and delivery that can adversely impact their final therapeutic efficacies. This has prompted an intense search for generalized strategies to engineer the long-term stability of proteins during their pharmaceutical employment. Due to the well known effect that glycans have in increasing the overall stability of glycoproteins, rational manipulation of the glycosylation parameters through glycoengineering could become a promising approach to improve both the in vitro and in vivo stability of protein pharmaceuticals. The intent of this review is therefore to further the field of protein glycoengineering by increasing the general understanding of the mechanisms by which glycosylation improves the molecular stability of protein pharmaceuticals. This is achieved by presenting a survey of the different instabilities displayed by protein pharmaceuticals, by addressing which of these instabilities can be improved by glycosylation, and by discussing the possible mechanisms by which glycans induce these stabilization effects.

Enzymatic activity and thermal stability of PEG-alpha-chymotrypsin conjugates

Alpha-chymotrypsin was chemically modified with methoxypoly(ethylene glycol) (PEG) of different molecular weights (700, 2,000, and 5,000 Da) and the amount of polymer attached to the enzyme was varied systematically from 1 to 9 PEG molecules per enzyme molecule. Upon PEG conjugation, enzyme catalytic turnover (k (cat)) decreased by 50% and substrate affinity was lowered as evidenced by an increase in the K (M) from 0.05 to 0.19 mM. These effects were dependent on the amount of PEG bound to the enzyme but were independent of the PEG size. In contrast, stabilization toward thermal inactivation depended on the PEG molecular weight with conjugates with the larger PEGs being more stable.

Stabilization of alpha-chymotrypsin upon PEGylation correlates with reduced structural dynamics

Protein stability remains one of the main factors limiting the realization of the full potential of protein therapeutics. Poly(ethylene glycol) (PEG) conjugation to proteins has evolved into an important tool to overcome instability issues associated with proteins. The observed increase in thermodynamic stability of several proteins upon PEGylation has been hypothesized to arise from reduced protein structural dynamics, although experimental evidence for this hypothesis is currently missing. To test this hypothesis, the model protein alpha-chymotrypsin (alpha-CT) was covalently modified with PEGs with molecular weights (M(W)) of 700, 2,000 and 5,000 and the degree of modification was systematically varied. The procedure did not cause significant tertiary structure changes. Thermodynamic unfolding experiments revealed that PEGylation increased the thermal transition temperature (T(m)) of alpha-CT by up to 6 degrees C and the free energy of unfolding [DeltaG(U) (25 degrees C)] by up to 5 kcal/mol. The increase in stability was found to be independent of the PEG M(W) and it leveled off after an average of four PEG molecules were bound to alpha-CT. Fourier-transformed infrared (FTIR) H/D exchange experiments were conducted to characterize the conformational dynamics of the PEG-conjugates. It was found that the magnitude of thermodynamic stabilization correlates with a reduction in protein structural dynamics and was independent of the PEG M(W). Thus, the initial hypothesis proved positive. Similar to the thermodynamic stabilization of proteins by covalent modification with glycans, PEG thermodynamically stabilizes alpha-CT by reducing protein structural dynamics. These results provide guidance for the future development of stable protein formulations.