Effects of Polymer Molecular Weight on the Size, Activity, and Stability of PEG-Functionalized Trypsin

Research progress on the PEGylation of therapeutic proteins and peptides (TPPs)

With the rapid advancement of genetic and protein engineering, proteins and peptides have emerged as promising drug molecules for therapeutic applications. Consequently, there has been a growing interest in the field of chemical modification technology to address challenges associated with their clinical use, including rapid clearance from circulation, immunogenicity, physical and chemical instabilities (such as aggregation, adsorption, deamination, clipping, oxidation, etc.), and enzymatic degradation. Polyethylene glycol (PEG) modification offers an effective solution to these issues due to its favorable properties. This review presents recent progress in the development and application of PEGylated therapeutic proteins and peptides (TPPs). For this purpose, firstly, the physical and chemical properties as well as classification of PEG and its derivatives are described. Subsequently, a detailed summary is provided on the main sites of PEGylated TPPs and the factors that influence their PEGylation. Furthermore, notable instances of PEG-modified TPPs (including antimicrobial peptides (AMPs), interferon, asparaginase and antibodies) are highlighted. Finally, we propose the chemical modification of TPPs with PEG, followed by an analysis of the current development status and future prospects of PEGylated TPPs. This work provides a comprehensive literature review in this promising field while facilitating researchers in utilizing PEG polymers to modify TPPs for disease treatment.

Arginase from Priestia megaterium and the Effects of CMCS Conjugation on Its Enzymological Properties

Arginase has shown promising potential in treating cancers by arginine deprivation therapy; however, low enzymatic activity and stability of arginase are impeding its development. This study was aimed to improve the enzymological properties of a marine bacterial arginase by carboxymethyl chitosan (CMCS) conjugation. An arginase producing marine bacterium Priestia megaterium strain P6 was isolated and identified. The novel arginase PMA from the strain was heterologously expressed, purified, and then conjugated to CMCS by ionic gelation with calcium chloride as the crosslinking agent. Enzymological properties of both PMA and CMCS-PMA conjugate were determined. The optimum temperature for PMA and CMCS-PMA at pH 7 were 60 °C and 55 °C, respectively. The optimum pH for PMA and CMCS-PMA at 37 °C were pH 10 and 9, respectively. CMCS-PMA showed higher thermostability than PMA over 55-70 °C and higher pH stability over pH 4-11 with the highest pH stability at pH 7. At 37 °C and pH of 7, i.e., around the human blood temperature and pH, CMCS-PMA was higher than the free PMA in enzymatic activity and stability by 24% and 21%, respectively. CMCS conjugation not only changed the optimum temperature, optimum pH, and enzymatic activity of PMA, but also improved its pH stability and temperature stability, and thus made it more favorable for medical application.

Vascular Endothelial Growth Factor Receptor 1 Targeting Fusion Polypeptides with Stimuli-Responsiveness for Anti-angiogenesis

Genetically engineered fusion polypeptides have been investigated to introduce unique bio-functionality and improve some therapeutic activity for anti-angiogenesis. We report herein that stimuli-responsive, vascular endothelial growth factor receptor 1 (VEGFR1) targeting fusion polypeptides composed of a VEGFR1 (fms-like tyrosine kinase-1 (Flt1)) antagonist, an anti-Flt1 peptide, and a thermally responsive elastin-based polypeptide (EBP) were rationally designed at the genetic level, biosynthesized, and purified by inverse transition cycling to develop potential anti-angiogenic fusion polypeptides to treat neovascular diseases. A series of hydrophilic EBPs with different block lengths were fused with an anti-Flt1 peptide, forming anti-Flt1-EBPs, and the effect of EBP block length on their physicochemical properties was examined. While the anti-Flt1 peptide decreased phase-transition temperatures of anti-Flt1-EBPs, compared with EBP blocks, anti-Flt1-EBPs were soluble under physiological conditions. The anti-Flt1-EBPs dose dependently inhibited the binding of VEGFR1 against vascular endothelial growth factor (VEGF) as well as tube-like network formation of human umbilical vein endothelial cells under VEGF-triggered angiogenesis in vitro because of the specific binding between anti-Flt1-EBPs and VEGFR1. Furthermore, the anti-Flt1-EBPs suppressed laser-induced choroidal neovascularization in a wet age-related macular degeneration mouse model in vivo. Our results indicate that anti-Flt1-EBPs as VEGFR1-targeting fusion polypeptides have great potential for efficacious anti-angiogenesis to treat retinal-, corneal-, and choroidal neovascularization.

Impact of the PEG length and PEGylation site on the structural, thermodynamic, thermal, and proteolytic stability of mono-PEGylated alpha-1 antitrypsin

Conjugation to polyethylene glycol (PEG) is a widely used approach to improve the therapeutic value of proteins essentially by prolonging their body residence time. PEGylation may however induce changes in the structure and/or the stability of proteins and thus on their function(s). The effects of PEGylation on the thermodynamic stability can either be positive (stabilization), negative (destabilization), or neutral (no effect). Moreover, various factors such as the PEG length and PEGylation site can influence the consequences of PEGylation on the structure and stability of proteins. In this study, the effects of PEGylation on the structure, stability, and polymerization of alpha1-antitrypsin (AAT) were investigated, using PEGs with different lengths, different structures (linear or 2-armed) and different linking chemistries (via amine or thiol) at two distinct positions of the sequence. The results show that whatever the size, position, and structure of PEG chains, PEGylation (a) does not induce significant changes in AAT structure (either at the secondary or tertiary level); (b) does not alter the stability of the native protein upon both chemical- and heat-induced denaturation; and (c) does not prevent AAT to fully refold and recover its activity following chemical denaturation. However, the propensity of AAT to aggregate upon heat treatment was significantly decreased by PEGylation, although PEGylation did not prevent the irreversible inactivation of the enzyme. Moreover, conjugation to PEG, especially 2-armed 40 kDa PEG, greatly improved the proteolytic resistance of AAT. PEGylation of AAT could be a promising strategy to prolong its half-life after infusion in AAT-deficient patients and thereby decrease the frequency of infusions.

Efficacy and safety of scleral crosslinking using poly(ethylene glycol)ether tetrasuccinimidyl glutarate for form-deprivation myopia progression in rabbits

Myopia is becoming increasingly prevalent worldwide at an alarming rate. However, no effective treatment is available for inhibiting myopia progression. Materials chemistry advancements have made it possible to regulate mechanical properties and rate of degradation with good compatibility by developing newly crosslinking systems such as the branched polyethylene glycol (PEG) systems. Herein, we presented a PEG molecule with N-hydroxysuccinimide (NHS) ester functional groups at the chain ends as a macromolecular crosslinking agent for the treatment of myopia. We found that the scleral collagen crosslinked with the four-armed star-shaped PEG molecule with NHS ester functional group (4S-PEG) showed better biomechanical properties, increased thermal stability and higher resistance to degradation. 4S-PEG exhibited relatively low cytotoxicity for human fetal scleral fibroblasts. The retrobulbar injection of 4S-PEG at a relatively low concentration (2.5 mM) showed good effective control of the progression of form-deprivation myopia in rabbits. There were no signs of adverse effect or damage by repeated injections with 4S-PEG in rabbits. The results of this work demonstrate that 4S-PEG can serve as a robust macromolecular crosslinking agent and is expected to have promise for application in the treatment of the progression of myopia.

From Synthesis to Characterization of Site-Selective PEGylated Proteins

Covalent attachment of therapeutic proteins to polyethylene glycol (PEG) is widely used for the improvement of its pharmacokinetic and pharmacological properties, as well as the reduction in reactogenicity and related side effects. This technique named PEGylation has been successfully employed in several approved drugs to treat various diseases, even cancer. Some methods have been developed to obtain PEGylated proteins, both in multiple protein sites or in a selected amino acid residue. This review focuses mainly on traditional and novel examples of chemical and enzymatic methods for site-selective PEGylation, emphasizing in N-terminal PEGylation, that make it possible to obtain products with a high degree of homogeneity and preserve bioactivity. In addition, the main assay methods that can be applied for the characterization of PEGylated molecules in complex biological samples are also summarized in this paper.

Temperature-induced aggregation behavior in bovine pancreas trypsin solutions

In this study, we investigate the effect of temperature treatment on Bovine Pancreas Trypsin (BPT) in aqueous solutions using dynamic, static and electrophoretic light scattering, fluorescence spectroscopy and circular dichroism. Static and dynamic light scattering at various solution conditions i.e. different salt content and pH, reveals that BPT aggregation is enhanced as temperature increases in a non-reversible manner. At acidic pH protein monomers are the dominant population over aggregates of globules, nevertheless the two populations co-exist at neutral and basic pH. The surface charge of the aggregates is intensified by aggregation and it is dominated by the negative residues of the protein at all pH conditions. Protein unfolding upon thermal treatment is probed by variation of the fluorescence spectrum which is caused by the exposure of tryptophan to the aqueous environment. The exposure of the hydrophobic interior of BPT upon heating may be considered as the reason of aggregation at the molecular level. Τhis study provides information that can be useful for utilizing thermal treatment protocols of BPT towards manufacturing protein-based nano formulated drugs.

Insight into the molecular mechanism behind PEG-mediated stabilization of biofluid lipases

Bioconjugates established between anionic polyethylene glycol (PEG) based polymers and cationic proteins have proven to be a promising strategy to engineer thermostable biocatalysts. However, the enzyme activity of these bioconjugates is very low and the mechanism of non-covalent PEG-stabilization is yet to be understood. This work presents experimental and molecular dynamics simulation studies, using lipase-polymer surfactant nanoconjugates from mesophile Rhizomucor miehei (RML), performed to evaluate the effect of PEG on enzyme stability and activity. Results demonstrated that the number of hydrogen bonds between the cationized RML and PEG chain correlates with enzyme thermostability. In addition, an increase of both the number of PEG-polymers units and cationization degree of the enzyme leads to a decrease of enzyme activity. Modelling with SAXS data of aqueous solutions of the biofluid lipases agrees with previous hypothesis that these enzymes contain a core constituted of folded protein confined by a shell of surfactants. Together results provide valuable insight into the mechanism of non-covalent PEG mediated protein stabilization relevant for engineering active and thermostable biofluids. Furthermore, the first biofluids RML with activity comparable to their cationized counterpart are presented.

The Locational Impact of Site-Specific PEGylation: Streamlined Screening with Cell-Free Protein Expression and Coarse-Grain Simulation

Although polyethylene glycol (PEG) is commonly used to improve protein stability and therapeutic efficacy, the optimal location for attaching PEG onto proteins is not well understood. Here, we present a cell-free protein synthesis-based screening platform that facilitates site-specific PEGylation and efficient evaluation of PEG attachment efficiency, thermal stability, and activity for different variants of PEGylated T4 lysozyme, including a di-PEGylated variant. We also report developing a computationally efficient coarse-grain simulation model as a potential tool to narrow experimental screening candidates. We use this simulation method as a novel tool to evaluate the locational impact of PEGylation. Using this screen, we also evaluated the predictive impact of PEGylation site solvent accessibility, conjugation site structure, PEG size, and double PEGylation. Our findings indicate that PEGylation efficiency, protein stability, and protein activity varied considerably with PEGylation site, variations that were not well predicted by common PEGylation guidelines. Overall our results suggest current guidelines are insufficiently predictive, highlighting the need for experimental and simulation screening systems such as the one presented here.

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.

Elucidating Surface Ligand-Dependent Kinetic Enhancement of Proteolytic Activity at Surface-Modified Quantum Dots

Combining biomolecules such as enzymes with nanoparticles has much to offer for creating next generation synergistically functional bionanomaterials. However, almost nothing is known about how these two disparate components interact at this critical biomolecular-materials interface to give rise to improved activity and emergent properties. Here we examine how the nanoparticle surface can influence and increase localized enzyme activity using a designer experimental system consisting of trypsin proteolysis acting on peptide-substrates displayed around semiconductor quantum dots (QDs). To minimize the complexity of analyzing this system, only the chemical nature of the QD surface functionalizing ligands were modified. This was accomplished by synthesizing a series of QD ligands that were either positively or negatively charged, zwitterionic, neutral, and with differing lengths. The QDs were then assembled with different ratios of dye-labeled peptide substrates and exposed to trypsin giving rise to progress curves that were monitored by Förster resonance energy transfer (FRET). The resulting trypsin activity profiles were analyzed in the context of detailed molecular dynamics simulations of key interactions occurring at this interface. Overall, we find that a combination of factors can give rise to a localized activity that was 35-fold higher than comparable freely diffusing enzyme-substrate interactions. Contributing factors include the peptide substrate being prominently displayed extending from the QD surface and not sterically hindered by the longer surface ligands in conjunction with the presence of electrostatic and other productive attractive forces between the enzyme and the QD surface. An intimate understanding of such critical interactions at this interface can produce a set of guidelines that will allow the rational design of next generation high-activity bionanocomposites and theranostics.

How PEGylation influences protein conformational stability

PEGylation is an important strategy for enhancing the pharmacokinetic properties of protein therapeutics. The development of chemoselective side-chain modification reactions has enabled researchers to PEGylate proteins with high selectivity at defined locations. However, aside from avoiding active sites and binding interfaces, there are few guidelines for the selection of optimal PEGylation sites. Because conformational stability is intimately related to the ability of a protein to avoid proteolysis, aggregation, and immune responses, it is possible that PEGylating a protein at sites where PEG enhances conformational stability will result in PEG-protein conjugates with enhanced pharmacokinetic properties. However, the impact of PEGylation on protein conformational stability is incompletely understood. This review describes recent advances toward understanding the impact of PEGylation on protein conformational stability, along with the development of structure-based guidelines for selecting stabilizing PEGylation sites.

Fabrication of Sealed Nanostraw Microdevices for Oral Drug Delivery

The oral route is preferred for systemic drug administration and provides direct access to diseased tissue of the gastrointestinal (GI) tract. However, many drugs have poor absorption upon oral administration due to damaging enzymatic and pH conditions, mucus and cellular permeation barriers, and limited time for drug dissolution. To overcome these limitations and enhance oral drug absorption, micron-scale devices with planar, asymmetric geometries, termed microdevices, have been designed to adhere to the lining of the GI tract and release drug at high concentrations directly toward GI epithelium. Here we seal microdevices with nanostraw membranes-porous nanostructured biomolecule delivery substrates-to enhance the properties of these devices. We demonstrate that the nanostraws facilitate facile drug loading and tunable drug release, limit the influx of external molecules into the sealed drug reservoir, and increase the adhesion of devices to epithelial tissue. These findings highlight the potential of nanostraw microdevices to enhance the oral absorption of a wide range of therapeutics by binding to the lining of the GI tract, providing prolonged and proximal drug release, and reducing the exposure of their payload to drug-degrading biomolecules.

Reductively PEGylated carbon nanomaterials and their use to nucleate 3D protein crystals: a comparison of dimensionality

A range of carbon nanomaterials, with varying dimensionality, were dispersed by a non-damaging and versatile chemical reduction route, and subsequently grafted by reaction with methoxy polyethylene glycol (mPEG) monobromides. The use of carbon nanomaterials with different geometries provides both a systematic comparison of surface modification chemistry and the opportunity to study factors affecting specific applications. Multi-walled carbon nanotubes, single-walled carbon nanotubes, graphite nanoplatelets, exfoliated few layer graphite and carbon black were functionalized with mPEG-Br, yielding grafting ratios relative to the nanocarbon framework between ca. 7 and 135 wt%; the products were characterised by Raman spectroscopy, TGA-MS, and electron microscopy. The functionalized materials were tested as nucleants by subjecting them to rigorous protein crystallization studies. Sparsely functionalized flat sheet geometries proved exceptionally effective at inducing crystallization of six proteins. This new class of nucleant, based on PEG grafted graphene-related materials, can be widely applied to promote the growth of 3D crystals suitable for X-ray crystallography. The association of the protein ferritin with functionalized exfoliated few layer graphite was directly visualized by transmission electron microscopy, illustrating the formation of ordered clusters of protein molecules critical to successful nucleation.

Site-Specific Polymer Conjugation Stabilizes Therapeutic Enzymes in the Gastrointestinal Tract

The site-specific conjugation of polymers to multiple engineered cysteine residues of a prolyl endopeptidase leads to its stabilization in the gastrointestinal tract of rats, without compromising the activity relative to the native enzyme. The importance of polymer attachment sites is investigated, as well as the significance of polymer structure.

Exploring Carbon Nanomaterial Diversity for Nucleation of Protein Crystals

Controlling crystal nucleation is a crucial step in obtaining high quality protein crystals for structure determination by X-ray crystallography. Carbon nanomaterials (CNMs) including carbon nanotubes, graphene oxide, and carbon black provide a range of surface topographies, porosities and length scales; functionalisation with two different approaches, gas phase radical grafting and liquid phase reductive grafting, provide routes to a range of oligomer functionalised products. These grafted materials, combined with a range of controls, were used in a large-scale assessment of the effectiveness for protein crystal nucleation of 20 different carbon nanomaterials on five proteins. This study has allowed a direct comparison of the key characteristics of carbon-based nucleants: appropriate surface chemistry, porosity and/or roughness are required. The most effective solid system tested in this study, carbon black nanoparticles functionalised with poly(ethylene glycol) methyl ether of mean molecular weight 5000, provides a novel highly effective nucleant, that was able to induce crystal nucleation of four out of the five proteins tested at metastable conditions.

Correlating enzyme density, conformation and activity on nanoparticle surfaces in highly functional bio-nanocomposites

The biological activity of the immobilized enzyme is crucial for the performance of different nanoparticle mediated enzymatic assays, where enzymatic conversion can be used for label-free analyte detection. In this article we have addressed two significant aspects of enzyme-nanoparticle interactions. First, we have developed copper sulfide (CuS) nanoparticles with an average diameter of 25 nm as a potential enzyme-interface using trypsin protease as a model enzyme. CuS nanoparticles showed high trypsin immobilization capacity of about 14.0 mg m(-2) with the significant retention of native enzymatic activity (75-98%) at room temperature, even beyond the calculated tightly packed monolayer coverage (which is around 4.1 mg m(-2)). Second, we report a quantitative correlation between the structure-functional relationship and the density of immobilized trypsin on a nanoparticle surface. The in situ conformation of immobilized trypsin could be efficiently analyzed by fluorescence, circular dichroism and FT-IR spectroscopic measurements because of the small size of the nanoparticles. Trypsin molecules appear to retain their close-native tertiary and secondary structural features (with a small loss of 1-2% of helical content) in the entire surface density range (2.0-14.0 mg m(-2)) on the CuS nanoparticles. However, interestingly, at a low surface coverage (2.0 mg m(-2)), immobilized trypsin retains almost 98% of its native enzymatic activity, leading to a highly functional bio-nanocomposite. However, at higher surface coverages, the enzyme activity decreases to 77%, indicating the influence of steric crowding. Furthermore, the high functionality of the immobilized trypsin at low surface density on CuS nanoparticle was also confirmed by determining the kinetic parameters of enzymatic activity.

Bioactivity and circulation time of PEGylated NELL-1 in mice and the potential for osteoporosis therapy

Osteoporosis is a progressive bone disease due to low osteoblast activity and/or high osteoclast activity. NELL-1 is a potential therapy for osteoporosis because it specifically increases osteoblast differentiation. However, similar to other protein drugs, the bioavailability of NELL-1 may be limited by its in vivo half-life and rapid clearance from body. The purpose of the present study is to prolong NELL-1 circulation time in vivo by PEGylation with three monomeric PEG sizes (5, 20, 40 kDa). While linear PEG 5k yielded the most efficient PEGylation and the most thermally stable conjugate, linear PEG 20k resulted in the conjugate with the highest Mw and longest in vivo circulation. Compared to non-modified NELL-1, all three PEGylated conjugates showed enhanced thermal stability and each prolonged the in vivo circulation time significantly. Furthermore, PEGylated NELL-1 retained its osteoblastic activity without any appreciable cytotoxicity. These findings motivate further studies to evaluate the efficacy of PEGylated NELL-1 on the prevention and treatment of osteoporosis.

Porous silicon-cell penetrating peptide hybrid nanocarrier for intracellular delivery of oligonucleotides

The largest obstacle to the use of oligonucleotides as therapeutic agents is the delivery of these large and negatively charged biomolecules through cell membranes into intracellular space. Mesoporous silicon (PSi) is widely recognized as a potential material for drug delivery purposes due to its several beneficial features like large surface area and pore volume, high loading capacity, biocompatibility, and biodegradability. In the present study, PSi nanoparticles stabilized by thermal oxidation or thermal carbonization and subsequently modified by grafting aminosilanes on the surface are utilized as an oligonucleotide carrier. Splice correcting oligonucleotides (SCOs), a model oligonucleotide drug, were loaded into the positively charged PSi nanoparticles with a loading degree as high as 14.3% (w/w). Rapid loading was achieved by electrostatic interactions, with the loading efficiencies reaching 100% within 5 min. The nanoparticles were shown to deliver and release SCOs, in its biologically active form, inside cells when formulated together with cell penetrating peptides (CPP). The biological effect was monitored with splice correction assay and confocal microscopy utilizing HeLa pLuc 705 cells. Furthermore, the use of PSi carrier platform in oligonucleotide delivery did not reduce the cell viability. Additionally, the SCO-CPP complexes formed in the pores of the carrier were stabilized against proteolytic digestion. The advantageous properties of protecting and releasing the cargo and the possibility to further functionalize the carrier surface make the hybrid nanoparticles a potential system for oligonucleotide delivery.

Molecular insight into the steric shielding effect of PEG on the conjugated staphylokinase: biochemical characterization and molecular dynamics simulation

PEGylation is a successful approach to improve potency of a therapeutic protein. The improved therapeutic potency is mainly due to the steric shielding effect of PEG. However, the underlying mechanism of this effect on the protein is not well understood, especially on the protein interaction with its high molecular weight substrate or receptor. Here, experimental study and molecular dynamics simulation were used to provide molecular insight into the interaction between the PEGylated protein and its receptor. Staphylokinase (Sak), a therapeutic protein for coronary thrombolysis, was used as a model protein. Four PEGylated Saks were prepared by site-specific conjugation of 5 kDa/20 kDa PEG to N-terminus and C-terminus of Sak, respectively. Experimental study suggests that the native conformation of Sak is essentially not altered by PEGylation. In contrast, the bioactivity, the hydrodynamic volume and the molecular symmetric shape of the PEGylated Sak are altered and dependent on the PEG chain length and the PEGylation site. Molecular modeling of the PEGylated Saks suggests that the PEG chain remains highly flexible and can form a distinctive hydrated layer, thereby resulting in the steric shielding effect of PEG. Docking analyses indicate that the binding affinity of Sak to its receptor is dependent on the PEG chain length and the PEGylation site. Computational simulation results explain experimental data well. Our present study clarifies molecular details of PEG chain on protein surface and may be essential to the rational design, fabrication and clinical application of PEGylated proteins.

Impact of site-specific PEGylation on the conformational stability and folding rate of the Pin WW domain depends strongly on PEG oligomer length

Protein PEGylation is an effective method for reducing the proteolytic susceptibility, aggregation propensity, and immunogenicity of protein drugs. These pharmacokinetic challenges are fundamentally related to protein conformational stability, and become much worse for proteins that populate the unfolded state under ambient conditions. If PEGylation consistently led to increased conformational stability, its beneficial pharmacokinetic effects could be extended and enhanced. However, the impact of PEGylation on protein conformational stability is currently unpredictable. Here we show that appending a short PEG oligomer to a single Asn side chain within a reverse turn in the WW domain of the human protein Pin 1 increases WW conformational stability in a manner that depends strongly on the length of the PEG oligomer: shorter oligomers increase folding rate, whereas longer oligomers increase folding rate and reduce unfolding rate. This strong length dependence is consistent with the possibility that the PEG oligomer stabilizes the transition and folded states of WW relative to the unfolded state by interacting favorably with side-chain or backbone groups on the WW surface.

Mechanistic insights into the stabilization of srcSH3 by PEGylation

Protein PEGylation (attaching PEG chains to proteins) has been widely used in pharmaceuticals and nanotechnology. Although it is widely known that PEGylation can increase the thermodynamic stability of proteins, the underlying mechanism remains elusive. In this Article, we studied the effect of PEGylation on the thermodynamic and kinetic stability of a protein, SH3. We show that the thermodynamic stability of SH3 is enhanced upon PEGylation, mainly due to the slowing of the unfolding rate. Moreover, PEGylation can decrease the solvent-accessible surface area of SH3, leading to an increase of the m-value (the change in free energy with respect to denaturant concentration, which is a measure of the transition cooperativity between corresponding states). Such an effect also causes an enhancement of the thermodynamic stability. We quantitatively measured how the physical properties of PEG, such as the molecular weight and the number of PEGylation sites, affect the stabilization effect. We found that the stabilization effect is largely dependent on the number of PEGylation sites but only has a weak correlation with the molecular weight of the attached PEG. These experimental findings inspire us to derive a physical model based on excluded volume effect, which can satisfactorily describe all experimental observations. This model allows quantitatively calculating the free energy change upon PEGylation based on the change of water excluded zone on the protein surface. Although it is still unknown whether such a mechanism can be extended to other proteins, our work represents a key step toward the understanding of the nature of protein stabilization upon PEGylation.

N-PEGylation of a reverse turn is stabilizing in multiple sequence contexts, unlike N-GlcNAcylation

The intrinsic stabilization of therapeutic proteins by N-glycosylation can endow them with increased shelf and serum half-lives owing to lower populations of misfolded and unfolded states, which are susceptible to aggregation and proteolysis. Conjugation of poly(ethylene glycol) (PEG) oligomers to nucleophilic groups on the surfaces of folded proteins (i.e., PEGylation) is a chemical alternative to N-glycosylation, in that it can also enhance the pharmacologic attributes of therapeutic proteins. However, the energetic consequences of PEGylation are currently not predictable. We find that PEGylation of an Asn residue in reverse turn 1 of the Pin WW domain is intrinsically stabilizing in several sequence contexts, unlike N-glycosylation, which is only stabilizing in a particular sequence context. Our thermodynamic data are consistent with the hypothesis that PEGylation destabilizes the protein denatured state ensemble via an excluded volume effect, whereas N-glycosylation-associated stabilization results primarily from native state interactions between the N-glycan and the protein.

Design, modeling, expression, and chemoselective PEGylation of a new nanosize cysteine analog of erythropoietin

Background:

Recombinant human erythropoietin (rhEPO) is considered to be one of the most pivotal pharmaceutical drugs in the market because of its clinical application in the treatment of anemia-associated disorders worldwide. However, like other therapeutic proteins, it does not have suitable pharmacokinetic properties for it to be administrated at least two to three times per week. Chemoselective cysteine PEGylation, employing molecular dynamics and graphics in in silico studies, can be considered to overcome such a problem.

Methods:

A special kind of EPO analog was elicited based on a literature review, homology modeling, molecular dynamic simulation, and factors affecting the PEGylation reaction. Then, cDNA of the selected analog was generated by site-directed mutagenesis and subsequently cloned into the expression vector. The construct was transfected to Chinese hamster ovary/dhfr⁻ cells, and highly expressed clones were selected via methotrexate amplification. Ion-immobilized affinity and size exclusion (SE) chromatography techniques were used to purify the expressed analog. Thereafter, chemoselective PEGylation was performed and a nanosize PEGylated EPO was obtained through dialysis. The in vitro biologic assay and in vivo pharmacokinetic parameters were studied. Finally, E31C analog Fourier transform infrared, analytical SE-high-performance liquid chromatography, zeta potential, and size before and after PEGylation were characterized.

Results:

The findings indicate that a novel nanosize EPO31-PEG has a five-fold longer terminal half-life in rats with similar biologic activity compared with unmodified rhEPO in proliferation cell assay. The results also show that EPO31-PEG size and charge versus unmodified protein was increased in a nanospectrum, and this may be one criterion of EPO biologic potency enhancement.

Discussion:

This kind of novel engineered nanosize PEGylated EPO has remarkable advantages over rhEPO.