Site-specific modification and PEGylation of pharmaceutical proteins mediated by transglutaminase

Zirconium 89 and Copper 64 for ImmunoPET: From Antibody Bioconjugation and Radiolabeling to Molecular Imaging

Immunotherapy has transformed cancer treatment. Nevertheless, given the heterogeneity of clinical efficacy, the multiplicity of treatment options available and the possibility of serious adverse effects, selecting the most effective treatment has become the greatest challenge. Molecular imaging offers an attractive way for this purpose. ImmunoPET provides specific imaging with positron emission tomography (PET) using monoclonal antibodies (mAb) or its fragments as vector. By combining the high targeting specificity of mAb and the sensitivity of PET technique, immunoPET could noninvasively and dynamically reveal tumor antigens expression and provide theranostic tools of several types of malignancies. Because of their slow kinetics, mAbs require radioelements defined by a consistent half-life. Zirconium 89 (89Zr) and Copper 64 (64Cu) are radiometals with half-lives suitable for mAb labeling. Radiolabeling with a radiometal requires the prior use of a bifunctional chelate agent (BFCA) to functionalize mAb for radiometal chelation, in a second step. There are a number of BFCA available and much research is focused on antibody functionalization techniques or on developing the optimum chelating agent depending the selected radiometal. In this manuscript, we present a critical account of radiochemical techniques with radionuclides 89Zr and 64Cu and their applications in preclinical and clinical immuno-PET imaging.

Investigation on Adaptability and Applicability of Polymer-Mediated Cell Surface Engineering by Ligation with Transglutaminase

Polymer-mediated cell surface engineering can be a powerful tool to modify the cell's biological behavior, but a simple ligation strategy must be identified. This manuscript assessed the use of transglutamination as a versatile and adaptable approach for cell surface engineering in various cellular models relevant to biomedical applications. This enzymatic approach was evaluated for its feasibility and potential for conjugating polymers to diverse cell surfaces and its biological effects. Transglutaminase-mediated ligation was successfully performed at temperatures ranging from 4 to 37 °C in as quickly as 30 min, while maintaining biocompatibility and preserving cell viability. This approach was successfully applied to nine different cell surfaces (including adherent cells and suspension cells) by optimizing the enzyme source (guinea pig liver vs microbial), buffer compositions, and incubation conditions. Finally, polymer-mediated cell surface engineering using transglutaminase exhibited immunocamouflage abilities for endothelial cells, T cells, and red blood cells by preventing the recognition of cell surface proteins by antibodies. Employing transglutaminase in polymer-mediated cell surface engineering is a promising approach to maximize its application in cell therapy and other biomedical applications.

Stabilization challenges and aggregation in protein-based therapeutics in the pharmaceutical industry

Protein-based therapeutics have revolutionized the pharmaceutical industry and become vital components in the development of future therapeutics. They offer several advantages over traditional small molecule drugs, including high affinity, potency and specificity, while demonstrating low toxicity and minimal adverse effects. However, the development and manufacturing processes of protein-based therapeutics presents challenges related to protein folding, purification, stability and immunogenicity that should be addressed. These proteins, like other biological molecules, are prone to chemical and physical instabilities. The stability of protein-based drugs throughout the entire manufacturing, storage and delivery process is essential. The occurrence of structural instability resulting from misfolding, unfolding, and modifications, as well as aggregation, poses a significant risk to the efficacy of these drugs, overshadowing their promising attributes. Gaining insight into structural alterations caused by aggregation and their impact on immunogenicity is vital for the advancement and refinement of protein therapeutics. Hence, in this review, we have discussed some features of protein aggregation during production, formulation and storage as well as stabilization strategies in protein engineering and computational methods to prevent aggregation.

Improving the Therapeutic Potential of G-CSF through Compact Circular PEGylation Based on Orthogonal Conjugations

In this study, a circular conjugate of granulocyte colony-stimulating factor (G-CSF) was prepared by conjugating the two end-chains of poly(ethylene glycol) (PEG) to two different sites of the protein. For the orthogonal conjugation, a heterobifunctional PEG chain was designed and synthesized, bearing the dipeptide ZGln-Gly (ZQG) at one end-chain, for transglutaminase (TGase) enzymatic selective conjugation at Lys41 of G-CSF, and an aldehyde group at the opposite end-chain, for N-terminal selective reductive alkylation of the protein. The cPEG-Nter/K41-G-CSF circular conjugate was characterized by physicochemical methods and compared with native G-CSF and the corresponding linear monoconjugates of G-CSF, PEG-Nter-G-CSF, and PEG-K41-G-CSF. The results demonstrated that the circular conjugate had improved physicochemical and thermal stability, prolonged pharmacokinetic interaction, and retained the biological activity of G-CSF. The PEGylation strategy employed in this study has potential applications in the design of novel protein-based therapeutics.

Non-Native Site-Selective Enzyme Catalysis

The ability to site-selectively modify equivalent functional groups in a molecule has the potential to streamline syntheses and increase product yields by lowering step counts. Enzymes catalyze site-selective transformations throughout primary and secondary metabolism, but leveraging this capability for non-native substrates and reactions requires a detailed understanding of the potential and limitations of enzyme catalysis and how these bounds can be extended by protein engineering. In this review, we discuss representative examples of site-selective enzyme catalysis involving functional group manipulation and C-H bond functionalization. We include illustrative examples of native catalysis, but our focus is on cases involving non-native substrates and reactions often using engineered enzymes. We then discuss the use of these enzymes for chemoenzymatic transformations and target-oriented synthesis and conclude with a survey of tools and techniques that could expand the scope of non-native site-selective enzyme catalysis.

Transglutaminase in Foods and Biotechnology

Stabilization and reusability of enzyme transglutaminase (TGM) are important goals for the enzymatic process since immobilizing TGM plays an important role in different technologies and industries. TGM can be used in many applications. In the food industry, it plays a role as a protein-modifying enzyme, while, in biotechnology and pharmaceutical applications, it is used in mediated bioconjugation due to its extraordinary crosslinking ability. TGMs (EC 2.3.2.13) are enzymes that catalyze the formation of a covalent bond between a free amino group of protein-bound or peptide-bound lysine, which acts as an acyl acceptor, and the γ-carboxamide group of protein-bound or peptide-bound glutamine, which acts as an acyl donor. This results in the modification of proteins through either intramolecular or intermolecular crosslinking, which improves the use of the respective proteins significantly.

Polymer selection impacts the pharmaceutical profile of site-specifically conjugated Interferon-α2a

Conjugation of poly(ethylene glycol) (PEG) to biologics is a successful strategy to favorably impact the pharmacokinetics and efficacy of the resulting bioconjugate. We compare bioconjugates synthesized by strain-promoted azide-alkyne cycloaddition (SPAAC) using PEG and linear polyglycerol (LPG) of about 20 kDa or 40 kDa, respectively, with an azido functionalized human Interferon-α2a (IFN-α2a) mutant. Site-specific PEGylation and LPGylation resulted in IFN-α2a bioconjugates with improved in vitro potency compared to commercial Pegasys. LPGylated bioconjugates had faster disposition kinetics despite comparable hydrodynamic radii to their PEGylated analogues. Overall exposure of the PEGylated IFN-α2a with a 40 kDa polymer exceeded Pegasys, which, in return, was similar to the 40 kDa LPGylated conjugates. The study points to an expanded polymer design space through which the selected polymer class may result in a different distribution of the studied bioconjugates.

Rational design of a disulfide bridge increases the thermostability of microbial transglutaminase

Microbial transglutaminase (MTG) has numerous industrial applications in the food and pharmaceutical sectors. Unfortunately, the thermostability of MTG is too low to tolerate the desired conditions used in many of these commercial processes. In a previous study, we used protein engineering to improve the thermostability of MTG. Specifically, we generated a T7C/E58C mutant of MTG from Streptomyces mobaraensis that displayed enhanced resistance to thermal inactivation. In this study, a rational structure-based approach was adopted to introduce a disulfide bridge to further increase the thermostability of MTG. In all, four new mutants, each containing a novel disulfide bond, were engineered. Of these four mutants, D3C/G283C showed the most promising thermostability with a significantly higher ∆T₅₀ (defined as the temperature of incubation at which 50% of the initial activity remains) of + 9 °C by comparison to wild-type MTG. Indeed, D3C/G283C combined enhanced thermostability with a 2.1-fold increased half-life at 65 °C compared with the wild-type enzyme. By structure-based rational design, we were able to create an MTG variant which might be useful for expanding the scope of application in food. KEY POINTS: • Microbial transglutaminase (MTG) is an enzyme used in many food applications • The applicability of MTG to various industrial processes other than the food sector is being investigated • Improvement of thermostability was confirmed for the disulfide bridge mutant D3C/G283C.

Preparation and Characterization of Site-Specific Fatty Chain-Modified Recombinant Human Granulocyte Colony Stimulating Factor

The clinical use of recombinant human granulocyte colony-stimulating factor (rhG-CSF) is limited by its short serum half-life. In this study, a long-acting strategy for site-specific modification of rhG-CSF with 1-pentadecyl-1H-pyrrole-2,5-dione (C15 fatty chain-maleimide, C15-MAL) was studied in mixed DMSO-aqueous solutions. The factors influencing the conjugation reaction were investigated and optimized, and a high yield of the desired product (C15-rhG-CSF) was achieved. Subsequently, C15-rhG-CSF product was efficiently purified using preparative liquid chromatography, and further characterized. Circular dichroism spectroscopy analysis showed that the secondary structure of C15-rhG-CSF had no significant difference from unmodified rhG-CSF. C15-rhG-CSF retained 87.2% of in vitro bioactivity of unmodified rhG-CSF. The pharmacokinetic study showed that the serum half-life of C15-rhG-CSF in mice was 2.08-fold longer than that of unmodified rhG-CSF. Furthermore, C15-rhG-CSF by single-dose subcutaneous administration showed better in vivo efficacy than those of both PEG_10k-rhG-CSF by single-dose administration and rhG-CSF by multiple doses administration. This study demonstrated the potential of C15-rhG-CSF being developed into a novel drug candidate as well as an efficient process for the development of long-acting protein and peptide drugs.

Direct N- or C-Terminal Protein Labeling Via a Sortase-Mediated Swapping Approach

Site-specific protein labeling is important in biomedical research and biotechnology. While many methods allow site-specific protein modification, a straightforward approach for efficient N-terminal protein labeling is not available. We introduce a novel sortase-mediated swapping approach for a one-step site-specific N-terminal labeling with a near-quantitative yield. We show that this method allows rapid and efficient cleavage and simultaneous labeling of the N or C termini of fusion proteins. The method does not require any prior modification beyond the genetic incorporation of the sortase recognition motif. This new approach provides flexibility for protein engineering and site-specific protein modifications.

Absolute Quantification of Phosphorylated ERβ Amino Acids in the Hippocampus of Women and in A Rat Model of Menopause

The rapid decline of circulating 17β-estradiol (E2) at menopause leads to negative neurological consequences, although hormone therapy paradoxically has both harmful and positive effects depending on the age at which it is delivered. The inconsistent response to E2 suggests unappreciated regulatory mechanisms for estrogen receptors (ERs), and we predicted it could be due to age-related differences in ERβ phosphorylation. We assessed ERβ phosphorylation using a sensitive mass spectrometry approach that provides absolute quantification (AQUA-MS) of individually phosphorylated residues. Specifically, we quantified phosphorylated ERβ in the hippocampus of women (aged 21-83 years) and in a rat model of menopause at 4 residues with conserved sequence homology between the 2 species: S105, S176, S200, and Y488. Phosphorylation at these sites, which spanned all domains of ERβ, were remarkably consistent between the 2 species, showing high levels of S105 phosphorylation (80%-100%) and low levels of S200 (20%-40%). Further, S200 phosphorylation decreased with aging in humans and loss of E2 in rats. Surprisingly, Y488 phosphorylation, which has been linked to ERβ ligand-independent actions, exhibited approximately 70% phosphorylation, unaltered by species, age, or E2, suggesting ERβ's primary mode of action may not require E2 binding. We further show phosphorylation at 2 sites directly altered ERβ DNA-binding efficiency, and thus could affect its transcription factor activity. These findings provide the first absolute quantification of ERβ phosphorylation in the human and rat brain, novel insights into ERβ regulation, and a critical foundation for providing more targeted therapeutic options for menopause in the future.

Enzymatic Methods for the Site-Specific Radiolabeling of Targeting Proteins

Site-specific conjugation of proteins is currently required to produce homogenous derivatives for medicine applications. Proteins derivatized at specific positions of the polypeptide chain can actually show higher stability, superior pharmacokinetics, and activity in vivo, as compared with conjugates modified at heterogeneous sites. Moreover, they can be better characterized regarding the composition of the derivatization sites as well as the conformational and activity properties. To this aim, several site-specific derivatization approaches have been developed. Among these, enzymes are powerful tools that efficiently allow the generation of homogenous protein-drug conjugates under physiological conditions, thus preserving their native structure and activity. This review will summarize the progress made over the last decade on the use of enzymatic-based methodologies for the production of site-specific labeled immunoconjugates of interest for nuclear medicine. Enzymes used in this field, including microbial transglutaminase, sortase, galactosyltransferase, and lipoic acid ligase, will be overviewed and their recent applications in the radiopharmaceutical field will be described. Since nuclear medicine can benefit greatly from the production of homogenous derivatives, we hope that this review will aid the use of enzymes for the development of better radio-conjugates for diagnostic and therapeutic purposes.

Effect of introducing a disulfide bridge on the thermostability of microbial transglutaminase from Streptomyces mobaraensis

Microbial transglutaminase (MTG) has been used extensively in academic research and the food industry through cross-linking or posttranslational modification of proteins. In our previous paper, the activity-increased MTG mutants were obtained by means of rational mutagenesis and random mutagenesis coupled with the newly developed screening system. In addition, the improvement of heat resistance of MTG is needed to expand further its industrial applications. Here, a structure-based rational enzyme engineering approach was applied to improve the thermostability of MTG by introducing an artificial disulfide bridge. As a result of narrowing down candidates using a rational approach, we successfully engineered a disulfide bridge into the N-terminal region of MTG by substituting Thr-7 and Glu-58 with cysteine. The T7C/E58C mutant was observed to have a de novo disulfide bridge and showed an increased melting temperature (Tm value) of 4.3 °C with retained enzymatic activity. To address the benefit-gained reason, we focused on the Cβ temperature factor of the amino-acid residues that might form a disulfide bridge in MTG. Introducing the disulfide bridge had no remarkable effect on the mutant aiming to stabilize the high temperature factor. On the other hand, the mutation was effective on the relatively stable region. The introduction of a disulfide bridge may therefore be effective to stabilize further the relatively stable part. This finding is considered to be useful for the rational design of mutants aiming at heat resistance of proteins.Key Points• Microbial transglutaminase (MTG) is used as a binder in the food industry.• MTG has the potential for use in the manufacturing of various commercial materials.• Enhanced thermostability was observed for the disulfide bridge mutant, T7C/G58C.

A Complete and Versatile Protocol: Decoration of Cell-Derived Matrices with Mass-Encoded Peptides for Multiplexed Protease Activity Detection

This article provides guidance toward a platform technology for monitoring enzyme activity within the extracellular matrix (ECM) assessed by quantifying reporters secreted into the cell culture supernatant and analyzed by tandem mass spectrometry. The reporters are enzymatically and covalently bound to the ECM by transglutaminases (TG) using the peptide sequence of human insulin-like growth factor I's (IGF-I) D-domain which is known to be bound to the ECM by transglutaminase. The IGF-I D-domain sequence is followed by a peptide sequence cleaved by the intended target protease. This protease-sensitive peptide sequence (PSS) is cleaved off the ECM and can be used to monitor target-enzyme activity by employing a downstream mass tag designed according to isobaric mass encoding strategies, i.e., the combination of isotopically labeled, heavy amino acids. Thereby, cleavage events are linked to the appearance of encoded mass tags, readily allowing multiplexing. This article presents the design and synthesis of these mass reporters. It further aims at detailing the search for peptide sequences responding to target proteases to facilitate future work on enzyme activity measurement for enzymatic activities of hitherto unknown enzymes. In conclusion, the goal of this article is to arm scientists interested in measurements of local enzymatic activities within the ECM with robust protocols and background knowledge.

The evolution of polymer conjugation and drug targeting for the delivery of proteins and bioactive molecules

Polymer conjugation can be considered one of the leading approaches within the vast field of nanotechnology-based drug delivery systems. In fact, such technology can be exploited for delivering an active molecule, such as a small drug, a protein, or genetic material, or it can be applied to other drug delivery systems as a strategy to improve their in vivo behavior or pharmacokinetic activities such as prolonging the half-life of a drug, conferring stealth properties, providing external stimuli responsiveness, and so on. If on the one hand, polymer conjugation with biotech drug is considered the linchpin of the protein delivery field boasting several products in clinical use, on the other, despite dedicated research, conjugation with low molecular weight drugs has not yet achieved the milestone of the first clinical approval. Some of the primary reasons for this debacle are the difficulties connected to achieving selective targeting to diseased tissue, organs, or cells, which is the main goal not only of polymer conjugation but of all delivery systems of small drugs. In light of the need to achieve better drug targeting, researchers are striving to identify more sophisticated, biocompatible delivery approaches and to open new horizons for drug targeting methodologies leading to successful clinical applications. This article is categorized under: Therapeutic Approaches and Drug Discovery > Emerging Technologies Therapeutic Approaches and Drug Discovery > Nanomedicine for Oncologic Disease Toxicology and Regulatory Issues in Nanomedicine > Regulatory and Policy Issues in Nanomedicine.

Generation and testing of engineered multimeric Fabs of trastuzumab

Recombinant antibodies fragments in several new formats are routinely investigated and used in diagnostic and therapeutic applications as anti-cancers molecules. New antibody formats are generated to compensate the need for multispecificity and site-specific introduction of fluorescent dyes, cytotoxic payloads or for generating semisynthetic multimeric molecules. Fabs of trastuzumab bearing transglutaminase (MTG) reactive sites were generated by periplasmic expression in E. coli and purified. Multimeric Fabs were generated by either disulfide bridge formation or by using MTG-sensitive peptide linkers. Binding to receptor was assessed by ELISA and SPR methods. Internalization and growth inhibition assays were performed on BT-474 and SKBR3 Her2+ cells. Fabs were successfully produced and dimerized or trimerized using MTG and suitably designed peptide linkers. Site-specific derivatizations with fluorophores were similarly achieved. The monomeric, dimeric and trimeric variants bind the receptor with affinities similar or superior to the full antibody. Fab and Fab2 are rapidly internalized in Her2+ cells and exhibit growth inhibition abilities similar to the full antibody. Altogether, the data show that the recombinant Fabs can be produced in E. coli and converted into multimeric variants by MTG-based bioconjugation. Similar approaches are extendable to the introduction of cytotoxic payloads for the generation of novel Antibody Drug Conjugates.

Expanding the Versatility of Microbial Transglutaminase Using α-Effect Nucleophiles as Noncanonical Substrates

The substrate promiscuity of microbial transglutaminase (mTG) has been exploited in various applications in biotechnology, in particular for the attachment of alkyl amines to glutamine-containing peptides and proteins. Here, we expand the substrate repertoire to include hydrazines, hydrazides, and alkoxyamines, resulting in the formation of isopeptide bonds with varied susceptibilities to hydrolysis or exchange by mTG. Furthermore, we demonstrate that simple unsubstituted hydrazine and dihydrazides can be used to install reactive hydrazide handles onto the side chain of internal glutamine residues. The distinct hydrazide handles can be further coupled with carbonyls, including ortho-carbonylphenylboronic acids, to form site-specific and functional bioconjugates with tunable hydrolytic stability. The extension of the substrate scope of mTG beyond canonical amines thus substantially broadens the versatility of the enzyme, providing a new approach to facilitate novel applications.

Site-Specific Antibody Conjugation Strategy to Functionalize Virus-Based Nanoparticles

Amine/thiol-reactive chemistries are commonly used to conjugate antibodies to pharmaceuticals or nanoparticles. Yet, these conjugation strategies often result in unfavorable outcomes such as heterogeneous antibody display with hindered biological activity or aggregation due to multivalent interactions of the antibody and nanoparticles. Here, we report the application of a site-specific and enzymatically driven antibody conjugation strategy to functionalize virus-based nanoparticles (VNPs). Specifically, an azide-handle was introduced into the Fc region of a set of immunoglobulins using a two-step enzymatic reaction: (1) cleavage of N-linked glycan in the Fc region by a glycosidase and (2) conjugation of a chemically reactive linker (containing an azide functional handle) using a microbial transglutaminase. Conjugation of the azide-functional antibodies to several VNPs was achieved by making use of strain-promoted azide-alkyne cycloaddition. We report the conjugation of three immunoglobulin (IgG) isotypes (human IgG from sera, anti-CD47 Rat IgG2a, κ, and Trastuzumab recombinant humanized IgG1, κ) to the plant virus cowpea mosaic virus (CPMV) and the lysine mutant of tobacco mosaic virus (TMVlys) as well as bacteriophage Qβ. Site-specific conjugation resulted in stable and functional antibody-VNP conjugates. In stark contrast, the use of heterobifunctional linkers targeting thiols and amines on the antibodies and VNPs, respectively, led to aggregation due to nonspecific and multivalent coupling between the antibodies and VNPs. We demonstrate that antibody-VNP conjugates were functional, and Trastuzumab-displaying VNPs targeted HER2-positive SKOV-3 human ovarian cancer cells. This bioconjugation strategy adds to the portfolio of methods that can be used for designing functional antibody-VNP conjugates.

A recent update on the use of microbial transglutaminase for the generation of biotherapeutics

The recent scientific progresses on the use of enzyme-mediated reactions in organic, non-aqueous and aqueous media have significantly supported the growing demand of new biotechnological and/or pharmacological products. Today, a plethora of microbial enzymes, used as biocatalysts, are available. Among these, microbial transglutaminases (MTGs) are broadly used for their ability to catalyse the formation of an isopeptide bond between the γ-amide group of glutamines and the ε-amino group of lysine. Due to their promiscuity towards primary amine-containing substrates and the more stringent specificity for glutamine-containing peptide sequences, several combined approaches can be tailored for different settings, making MTGs very attractive catalysts for generating protein-protein and protein small molecule's conjugates. The present review offers a recent update on the modifications attainable by MTG-catalysed bioreactions as reported between 2014 and 2019. In particular, we present a detailed and comparative overview on the MTG-based methods for proteins and antibodies engineering, with a particular outlook on the synthesis of homogeneous antibody-drug conjugates.

Review transglutaminases: part II-industrial applications in food, biotechnology, textiles and leather products

Because of their protein cross-linking properties, transglutaminases are widely used in several industrial processes, including the food and pharmaceutical industries. Transglutaminases obtained from animal tissues and organs, the first sources of this enzyme, are being replaced by microbial sources, which are cheaper and easier to produce and purify. Since the discovery of microbial transglutaminase (mTGase), the enzyme has been produced for industrial applications by traditional fermentation process using the bacterium Streptomyces mobaraensis. Several studies have been carried out in this field to increase the enzyme industrial productivity. Researches on gene expression encoding transglutaminase biosynthesis were performed in Streptomyces lividans, Escherichia coli, Corynebacterium glutamicum, Yarrowia lipolytica, and Pichia pastoris. In the first part of this review, we presented an overview of the literature on the origins, types, mediated reactions, and general characterizations of these important enzymes, as well as the studies on recombinant microbial transglutaminases. In this second part, we focus on the application versatility of mTGase in three broad areas: food, pharmacological, and biotechnological industries. The use of mTGase is presented for several food groups, showing possibilities of applications and challenges to further improve the quality of the end-products. Some applications in the textile and leather industries are also reviewed, as well as special applications in the PEGylation reaction, in the production of antibody drug conjugates, and in regenerative medicine.

Preparation of dendritic polymer-based magnetic carrier for application of bromelain separation and purification

Bromelain has wide applications in different industries, such as food, textile, and medicine. Traditional approaches for bromelain separation and purification from solution still have many problems, including unsatisfactory binding efficiency, time-consuming operation, and costly equipment. In the present study, a new type of dendritic polymer-based magnetic carrier (GO@Fe₃ O₄ @PEI-Cu²⁺ ) was first prepared for bromelain separation and purification in solution. The histidine existing in bromelain could bind to Cu²⁺ cations adsorbed on the surface of the magnetic carrier, and the magnetic carrier showed excellent performance for bromelain separation and purification in solution, with the adsorption capacity up to 357 mg/g. The magnetic carrier also exhibited excellent property in the aspect of recyclability. It was found that the magnetic carrier also presented desirable performance for the separation and purification of bromelain from the crude extract of pineapple peel, and the bromelain structure remained intact before and after elution process. PRACTICAL APPLICATIONS: Considering many advantages of bromelain in the applications of pharmaceutical and food industries, this study is aimed at presenting a novel magnetic carrier with high stability and fabulous performance for bromelain separation and purification in solution and achieving the practical application that the magnetic carrier can efficiently separate bromelain from the crude extract of pineapple peel.

A comparative analysis of catalytic activity and stability of microbial transglutaminase in controlled denaturing conditions

Microbial transglutaminases (MTGs) catalyzes the formation of Gln-Lys isopeptide bonds and are widely used for the cross-linking of proteins and peptides in food and in biotechnological applications for bioconjugation reactions. In view of its practical utility, a comparative study of the catalytic activity and stability of the enzyme in a wide range of denaturing conditions has been performed through Circular Dichroism (CD), fluorescence and activity assays performed with model substrates. In agreement with previous results, we show that MTG has a significant structural and functional tolerance to pH changes, whereas the enzyme stability and activity decrease in presence of increasing amounts of denaturing agents, such as urea and guanidinium chloride (GdnHCl). Noteworthy, the activity of MTG in denaturing conditions differs markedly from that in pseudo-physiological settings, shifting unexpectedly toward higher substrate specificity. Also, the use of controlled amounts of denaturing agents (1.0-1.5 M urea) largely improves yields and purity of the final products of 10-15% and 25-30%, respectively. These findings widen the range of applicability of the MTG-mediated biocatalysis for industrial and biotechnological purposes.

Nanomedicines for the Delivery of Biologics

A special symposium of the Academy of Pharmaceutical Sciences Nanomedicines Focus Group reviewed the current status of the use of nanomedicines for the delivery of biologics drugs. This meeting was particularly timely with the recent approval of the first siRNA-containing product Onpattro™ (patisiran), which is formulated as a lipid nanoparticle for intravenous infusion, and the increasing interest in the use of nanomedicines for the oral delivery of biologics. The challenges in delivering such molecules were discussed with specific emphasis on the delivery both across and into cells. The latest developments in Molecular Envelope Technology® (Nanomerics Ltd, London, UK), liposomal drug delivery (both from an academic and industrial perspective), opportunities offered by the endocytic pathway, delivery using genetically engineered viral vectors (PsiOxus Technologies Ltd, Abingdon, UK), Transint™ technology (Applied Molecular Transport Inc., South San Francisco, CA, USA), which has the potential to deliver a wide range of macromolecules, and AstraZeneca's initiatives in mRNA delivery were covered with a focus on their uses in difficult to treat diseases, including cancers. Preclinical data were presented for each of the technologies and where sufficiently advanced, plans for clinical studies as well as early clinical data. The meeting covered the work in progress in this exciting area and highlighted some key technologies to look out for in the future.

Polymer-drug conjugate therapeutics: advances, insights and prospects

Polymer-drug conjugates have long been a mainstay of the drug delivery field, with several conjugates successfully translated into clinical practice. The conjugation of therapeutic agents to polymeric carriers, such as polyethylene glycol, offers several advantages, including improved drug solubilization, prolonged circulation, reduced immunogenicity, controlled release and enhanced safety. In this Review, we discuss the rational design, physicochemical characteristics and recent advances in the development of different classes of polymer-drug conjugates, including polymer-protein and polymer-small-molecule drug conjugates, dendrimers, polymer nanoparticles and multifunctional systems. Current obstacles hampering the clinical translation of polymer-drug conjugate therapeutics and future prospects are also presented.

Stapling of two PEGylated side chains increases the conformational stability of the WW domain via an entropic effect

Hydrocarbon stapling and PEGylation are distinct strategies for enhancing the conformational stability and/or pharmacokinetic properties of peptide and protein drugs. Here we combine these approaches by incorporating asparagine-linked O-allyl PEG oligomers at two positions within the β-sheet protein WW, followed by stapling of the PEGs via olefin metathesis. The impact of stapling two sites that are close in primary sequence is small relative to the impact of PEGylation alone and depends strongly on PEG length. In contrast, stapling of two PEGs that are far apart in primary sequence but close in tertiary structure provides substantially more stabilization, derived mostly from an entropic effect. Comparison of PEGylation + stapling vs. alkylation + stapling at the same positions in WW reveals that both approaches provide similar overall levels of conformational stability.

Recent progress in enzymatic protein labelling techniques and their applications

Protein-based conjugates are valuable constructs for a variety of applications. Conjugation of proteins to fluorophores is commonly used to study their cellular localization and the protein-protein interactions. Modification of therapeutic proteins with either polymers or cytotoxic moieties greatly enhances their pharmacokinetics or potency. To label a protein of interest, conventional direct chemical reaction with the side-chains of native amino acids often yields heterogeneously modified products. This renders their characterization complicated, requires difficult separation steps and may impact protein function. Although modification can also be achieved via the insertion of unnatural amino acids bearing bioorthogonal functional groups, these methods can have lower protein expression yields, limiting large scale production. As a site-specific modification method, enzymatic protein labelling is highly efficient and robust under mild reaction conditions. Significant progress has been made over the last five years in modifying proteins using enzymatic methods for numerous applications, including the creation of clinically relevant conjugates with polymers, cytotoxins or imaging agents, fluorescent or affinity probes to study complex protein interaction networks, and protein-linked materials for biosensing. This review summarizes developments in enzymatic protein labelling over the last five years for a panel of ten enzymes, including sortase A, subtiligase, microbial transglutaminase, farnesyltransferase, N-myristoyltransferase, phosphopantetheinyl transferases, tubulin tyrosin ligase, lipoic acid ligase, biotin ligase and formylglycine generating enzyme.

Structure of a glutamine donor mimicking inhibitory peptide shaped by the catalytic cleft of microbial transglutaminase

The protein cross-linking enzyme transglutaminase from Streptomyces mobaraensis (MTG) is frequently used to modify therapeutic proteins. In order to reveal the binding mode of glutamine donor substrates, we have now crystallized MTG covalently linked to large inhibitory peptides. A series of peptide structures were examined but DIPIGSKMTG, which was chloroacetylated at serine, was the only inhibitory molecule that resulted in an interpretable density map. We found that, besides the warhead (modified Ser6), Ile4 and Gly5 of the inhibitory peptide occupy the tight but extended hydrophobic bottom of the MTG-binding cleft. Both termini of the peptide protrude along the cleft walls almost perpendicular to the bottom of the extended cleft. This peptide model suggests a zipper-like cross-linking mechanism of self-assembled substrate proteins by MTG.

Site-specific chelator-antibody conjugation for PET and SPECT imaging with radiometals

Antibodies and their derivatives radiolabelled with positron- and gamma-emitting radiometals enable sensitive and quantitative molecular Positron Emission Tomography (PET) and Single Photon Emission Computed Tomography (SPECT) imaging of antibody distribution in vivo. Chelators that are covalently attached to antibodies allow radiolabelling with metallic PET and SPECT radioisotopes. Conventional strategies for chelator-protein conjugation generate heterogeneous mixtures of bioconjugates that can exhibit reduced affinity for their receptor targets, and undesirable biodistribution and pharmacokinetics. Recent advances in bioconjugation technology enable site-specific modification to generate well-defined constructs with superior properties. Herein we survey existing site-specific chelator-protein conjugation methods. These include chelator attachment to cysteines/disulfide bonds or the glycan region of the antibody, enzyme-mediated chelator conjugation, and incorporation of sequences of amino acids that chelate the radiometal. Such technology will allow better use of PET and SPECT imaging in the development of antibody-based therapies.

The development of nanobiocatalysis via the immobilization of cellulase on composite magnetic nanomaterial for enhanced loading capacity and catalytic activity

In this study, graphene oxide (GO) decorated with 4arm‑PEG‑NH₂ (molecular weight (MW) 5 K or 10 K) was constructed on magnetic Fe₃O₄, denoted as GO@Fe₃O₄@4arm‑PEG‑NH₂. The morphology, structure and magnetic property of GO@Fe₃O₄@4arm‑PEG‑NH₂ were characterized by Fourier transform infrared (FTIR), vibrating-sample magnetometer (VSM), thermogravimetric analysis (TGA), transmission electron microscopy (TEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) in details. The saturation loading capacity of GO@Fe₃O₄@4arm‑PEG‑NH₂ (MW 5 K and 1 K) carriers toward cellulase was 429 and 575 mg/g, respectively. Additionally, the immobilized cellulase had exhibited enhanced thermostability, storability and reusability than free enzyme. The two kinds of immobilized cellulose (MW 5 K and 10 K) retained 57% and 60% of its initial activity after 3 h at 70 °C, and retained 47% and 50% of its initial activity after 30 days' storage at room temperature. After eight times reuse, immobilized cellulose (MW 5 K and 10 K) retained 40% and 45% of its initial activity, respectively. In practical application, glucose generated by the saccharification with the immobilized cellulase was much higher than free enzyme (immobilized enzyme is kept at 2.04-2.83 times of the free enzyme), when the loading amount of enzyme was 2-8 mg, indicating the potential of the prepared biocatalyst.

Protein Labeling via a Specific Lysine-Isopeptide Bond Using the Pilin Polymerizing Sortase from Corynebacterium diphtheriae

Proteins that are site-specifically modified with peptides and chemicals can be used as novel therapeutics, imaging tools, diagnostic reagents and materials. However, there are few enzyme-catalyzed methods currently available to selectively conjugate peptides to internal sites within proteins. Here we show that a pilus-specific sortase enzyme from Corynebacterium diphtheriae (CdSrtA) can be used to attach a peptide to a protein via a specific lysine-isopeptide bond. Using rational mutagenesis we created CdSrtA3M, a highly activated cysteine transpeptidase that catalyzes in vitro isopeptide bond formation. CdSrtA3M mediates bioconjugation to a specific lysine residue within a fused domain derived from the corynebacterial SpaA protein. Peptide modification yields greater than >95% can be achieved. We demonstrate that CdSrtA3M can be used in concert with the Staphylococcus aureus SrtA enzyme, enabling dual, orthogonal protein labeling via lysine-isopeptide and backbone-peptide bonds.

Oxidative Modification of Tryptophan-Containing Peptides

We herein present a broadly useful method for the chemoselective modification of a wide range of tryptophan-containing peptides. Exposing a tryptophan-containing peptide to 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) resulted in a selective cyclodehydration between the peptide backbone and the indole side chain of tryptophan to form a fully conjugated indolyl-oxazole moiety. The modified peptides show a characteristic and significant emission maximum at 425 nm, thus making the method a useful strategy for fluorescence labeling.

Site-specific derivatization of human interferon β-1a at lysine residues using microbial transglutaminase

Microbial transglutaminase (TGase) has been successfully used to produce site-specific protein conjugates derivatized at the level of glutamine (Gln) or lysine (Lys) residues with diverse applications. Here, we study the drug human interferon β-1a (IFN) as a substrate of TGase. The derivatization reaction was performed using carbobenzoxy-L-glutaminyl-glycine to modify Lys residues and dansylcadaverine for Gln residues. The 166 amino acids polypeptide chain of IFN β-1a contains 11 Lys and 11 Gln residues potential sites of TGase derivatization. By means of mass spectrometry analyses, we demonstrate the highly selective derivatization of this protein by TGase at the level of Lys115 and as secondary site at the level of Lys33, while no reactive Gln residue was detected. Limited proteolysis experiments were performed on IFN to determine flexible regions of the protein under physiological conditions. Interestingly, primary and secondary sites of limited proteolysis and of TGase derivatization occur at the same regions of the polypeptide chain, indicating that the extraordinary selectivity of the TGase-mediated reaction is dictated by the conformational features of the protein substrate. We envisage that the TGase-mediated derivatization of IFN can be used to produce interesting derivatives of this important therapeutic protein.

Illuminating structure and acyl donor sites of a physiological transglutaminase substrate from Streptomyces mobaraensis

Transglutaminase from Streptomyces mobaraensis (MTG) has become a powerful tool to covalently and highly specifically link functional amines to glutamine donor sites of therapeutic proteins. However, details regarding the mechanism of substrate recognition and interaction of the enzyme with proteinaceous substrates still remain mostly elusive. We have determined the crystal structure of the Streptomyces papain inhibitory protein (SPI_p ), a substrate of MTG, to study the influence of various substrate amino acids on positioning glutamine to the active site of MTG. SPI_p exhibits a rigid, thermo-resistant double-psi-beta-barrel fold that is stabilized by two cysteine bridges. Incorporation of biotin cadaverine identified Gln-6 as the only amine acceptor site on SPI_p accessible for MTG. Substitution of Lys-7 demonstrated that small and hydrophobic residues in close proximity to Gln-6 favor MTG-mediated modification and are likely to facilitate introduction of the substrate into the front vestibule of MTG. Moreover, exchange of various surface residues of SPI_p for arginine and glutamate/aspartate outside the glutamine donor region influences the efficiency of modification by MTG. These results suggest the occurrence of charged contact areas between MTG and the acyl donor substrates beyond the front vestibule, and pave the way for protein engineering approaches to improve the properties of artificial MTG-substrates used in biomedical applications.

Transglutaminase-mediated conjugation and nitride-technetium-99m labelling of a bis(thiosemicarbazone) bifunctional chelator

An assessment study involving the use of the transglutaminase (TGase) conjugation method and the nitride-technetium-99m labelling on a bis(thiosemicarbazone) (BTS) bifunctional chelating agent is presented. The previously described chelator diacetyl-2-(N⁴-methyl-3-thiosemicarbazone)-3-(N⁴-amino-3-thiosemicarbazone), H₂ATSM/A, has been functionalized with 6-aminohexanoic acid (ε-Ahx) to generate the bifunctional chelating agent diacetyl-2-(N⁴-methyl-3-thiosemicarbazone)-3-[N⁴-(amino)-(6-aminohexanoic acid)-3-thiosemicarbazone], H₂ATSM/A-ε-Ahx (1), suitable for conjugation to glutamine (Gln) residues of bioactive molecules via TGase. The feasibility of the TGase reaction in the synthesis of a bioconjugate derivative was investigated using Substance P (SP) as model peptide. Compounds 1 and H₂ATSM/A-ε-Ahx-SP (2) were labelled with nitride-technetium-99m, obtaining the complexes [^99mTc][Tc(N)(ATSM/A-ε-Ahx)] (^99mTc1) and [^99mTc][Tc(N)(ATSM/A-ε-Ahx-SP)] (^99mTc2). The chemical identity of ^99mTc1 and ^99mTc2 was confirmed by radio/UV-RP-HPLC combined with ESI-MS analysis on the respective carrier-added products ^99g/99mTc1 and ^99g/99mTc2. The stability of the radiolabelled complexes after incubation in various environments was investigated. All the results were compared with those obtained for the corresponding ⁶⁴Cu-analogues, ⁶⁴Cu1 and ⁶⁴Cu2. The TGase reaction allows the conjugation of 1 with the peptide, but it is not highly efficient due to instability of the chelator in the required conditions. The SP-conjugated complexes are unstable in mouse and human sera. However, indeed the BTS system can be exploited as nitride-technetium-99m chelator for highly efficient technetium labelling, thus making compound 1 worthy of further investigations for new targeted technetium and copper radiopharmaceuticals encompassing Single Photon Emission Computed Tomography and Positron Emission Tomography imaging.

Transglutaminase and Sialyltransferase Enzymatic Approaches for Polymer Conjugation to Proteins

Proteins hold a central role in medicine and biology, also confirmed by the several therapeutic applications based on biologic drugs. Such therapies are of great relevance thanks to high potency and safety of proteins. Nevertheless, many proteins as therapeutics might present issues like fast kidney clearance, rapid enzymatic degradation, or immunogenicity. Such defects implicate frequent administrations or administrations at high doses of the therapeutics, thus yielding or exacerbating potential side effects. A successful technology for improving the clinical profiles of proteins is the conjugation of polymers to the protein surface. The design of a protein-polymer conjugate presents critical aspects that determine the efficacy and safety of the final product. The control over stoichiometry and conjugation site is a strict criterion on which researchers have been intensively focused during the years, in order to obtain homogeneous and batch-to-batch reproducible products. An innovative site-specific conjugation strategy relies on the use of enzymes as tools to mediate polymer conjugation. Enzymatic approaches are attractive because they allow site-selective polymer conjugation at specific protein amino acids. In these reactions, the polymer is a substrate analog that replaces the native substrate. Furthermore, enzymes can count other advantages such as high yields of conversion and physiological conditions of reaction. This chapter provides a meaningful description of protein-polymer conjugation through transglutaminase-mediated and sialyltransferase-mediated enzymatic strategies, reporting the mechanism of action and some relevant examples.

Transglutaminase mediated PEGylation of nanobodies for targeted nano-drug delivery

Targeted delivery of anticancer drugs that selectively accumulate in malignant cells could enhance drug efficacy and reduce side effects of conventional chemotherapy. In this work, we designed a single domain antibody (nanobody) based drug delivery system for targeted delivery of anticancer drugs. An anti-EGFR nanobody (Nb) was constructed with a C₃-tag and a Q-tag for site specific modifications under physiological conditions. The site specific PEGylation of the nanobody was achieved via a transglutaminase catalyzed reaction through the coupling of the Q-tag with PEG-NH₂. As a proof of concept, the PEGylated nanobody was tethered to HSA coated upconversion nanoparticles (UCNPs) through the C₃-tag, and an anticancer drug, doxorubicin (DOX), was loaded. Results showed that the Nb-conjugated drug delivery system exhibits superior specificity to the EGFR positive tumor cells. The drug delivery system is highly accumulated in the EGFR positive tumor cells (A431), whereas there was no detectable accumulation in the EGFR negative cells (MCF-7). Consequently, the drug loaded particles demonstrated significantly higher anti-proliferation to A431 cells than to MCF-7 cells. This work provides an effective approach for site-specific modification of nanobodies for the construction of targeted drug delivery systems.