Stabilization of bacterially expressed erythropoietin by single site-specific introduction of short branched PEG chains at naturally occurring glycosylation sites

Microbial Transglutaminase-Mediated Formation of Erythropoietin-Polyester Conjugates

Erythropoietin (EPO) is a glycoprotein hormone that has been used to treat anemia in patients with chronic kidney disease and in cancer patients who are receiving chemotherapy. Here, we investigated the accessibility of the glutamine (Gln, Q) residues of recombinant human erythropoietin (rHuEPO) towards a thermoresistant variant microbial transglutaminase (mTGase), TG¹⁶ with the aim of developing novel rHuEPO conjugates that may potentially enhance its biological efficacy. As a model bioconjugation, we studied the reactivity of rHuEPO towards TG¹⁶ with a low molar mass amine group containing substrate, monodansyl cadaverine (MDC). The reactions were carried out at a T_m of 54.3 °C, the transition temperature of rHuEPO. Characterization by SDS-PAGE and mass spectrometry confirmed the conjugates formation. Then, we examined the conjugation of rHuEPO with a biodegradable and biocompatible polyester, poly(D-sorbitol adipate) (PDSA). To achieve this, PDSA was enzymatically synthesized using lipase B from Candida antartica (CAL-B), chemically modified with side chains having free primary amine (NH₂) groups that can be acyl acceptor substrate of TG¹⁶, thoroughly characterized by ¹H NMR spectroscopy, and then applied for the TG¹⁶-mediated conjugation reaction with rHuEPO. rHuEPO conjugates generated by this approach were identified by SDS-PAGE proving that the amine-grafted PDSA is accepted as a substrate for TG¹⁶. The successful conjugation was further verified by the detection of high molar mass fluorescent bands after labelling of amine-grafted PDSA with rhodamine B-isothiocyanate. Overall, this enzymatic procedure is considered as an effective approach to prepare biodegradable rHuEPO-polymer conjugates even in the presence of N- and O-glycans.

Bioorthogonal Reactions of Triarylphosphines and Related Analogues

Bioorthogonal phosphines were introduced in the context of the Staudinger ligation over 20 years ago. Since that time, phosphine probes have been used in myriad applications to tag azide-functionalized biomolecules. The Staudinger ligation also paved the way for the development of other phosphorus-based chemistries, many of which are widely employed in biological experiments. Several reviews have highlighted early achievements in the design and application of bioorthogonal phosphines. This review summarizes more recent advances in the field. We discuss innovations in classic Staudinger-like transformations that have enabled new biological pursuits. We also highlight relative newcomers to the bioorthogonal stage, including the cyclopropenone-phosphine ligation and the phospha-Michael reaction. The review concludes with chemoselective reactions involving phosphite and phosphonite ligations. For each transformation, we describe the overall mechanism and scope. We also showcase efforts to fine-tune the reagents for specific functions. We further describe recent applications of the chemistries in biological settings. Collectively, these examples underscore the versatility and breadth of bioorthogonal phosphine reagents.

Synthesis of Erythropoietins Site-Specifically Conjugated with Complex-Type N-Glycans

The biological activity of the glycoprotein hormone erythropoietin (EPO) is dependent mainly on the structure of its N-linked glycans. We aimed to readily attach defined N-glycans to EPO through copper-catalyzed azide alkyne cycloaddition. EPO variants with an alkyne-bearing non-natural amino acid (Plk) at the N-glycosylation sites 24, 38, and 83 were obtained by amber suppression followed by protein purification and refolding. Click conjugation of the alkynyl EPOs with biantennary N-glycan azides provided biologically active site-specifically modified EPO glycoconjugates.

Cell-free protein synthesis as a novel tool for directed glycoengineering of active erythropoietin

As one of the most complex post-translational modification, glycosylation is widely involved in cell adhesion, cell proliferation and immune response. Nevertheless glycoproteins with an identical polypeptide backbone mostly differ in their glycosylation patterns. Due to this heterogeneity, the mapping of different glycosylation patterns to their associated function is nearly impossible. In the last years, glycoengineering tools including cell line engineering, chemoenzymatic remodeling and site-specific glycosylation have attracted increasing interest. The therapeutic hormone erythropoietin (EPO) has been investigated in particular by various groups to establish a production process resulting in a defined glycosylation pattern. However commercially available recombinant human EPO shows batch-to-batch variations in its glycoforms. Therefore we present an alternative method for the synthesis of active glycosylated EPO with an engineered O-glycosylation site by combining eukaryotic cell-free protein synthesis and site-directed incorporation of non-canonical amino acids with subsequent chemoselective modifications.

Tub-Tag Labeling; Chemoenzymatic Incorporation of Unnatural Amino Acids

Tub-tag labeling is a chemoenzymatic method that enables the site-specific labeling of proteins. Here, the natural enzyme tubulin tyrosine ligase incorporates noncanonical tyrosine derivatives to the terminal carboxylic acid of proteins containing a 14-amino acid recognition sequence called Tub-tag. The tyrosine derivative carries a unique chemical reporter allowing for a subsequent bioorthogonal modification of proteins with a great variety of probes. Here, we describe the Tub-tag protein modification protocol in detail and explain its utilization to generate labeled proteins for advanced applications in cell biology, imaging, and diagnostics.

Semisynthetic prion protein (PrP) variants carrying glycan mimics at position 181 and 197 do not form fibrils

Semisynthesis and characterization of homogeneously mono- and di-PEGylated full length PrP variants to study the impact of PEGylation (as N-glycan mimics) on protein folding and aggregation.

The prion protein (PrP) is an N-glycosylated protein attached to the outer leaflet of eukaryotic cell membranes via a glycosylphosphatidylinositol (GPI) anchor. Different prion strains have distinct glycosylation patterns and the extent of glycosylation of potentially pathogenic misfolded prion protein (PrP^Sc) has a major impact on several prion-related diseases (transmissible spongiform encephalopathies, TSEs). Based on these findings it is hypothesized that posttranslational modifications (PTMs) of PrP influence conversion of cellular prion protein (PrP^C) into PrP^Sc and, as such, modified PrP variants are critical tools needed to investigate the impact of PTMs on the pathogenesis of TSEs. Here we report a semisynthetic approach to generate PrP variants modified with monodisperse polyethyleneglycol (PEG) units as mimics of N-glycans. Incorporating PEG at glycosylation sites 181 and 197 in PrP induced only small changes to the secondary structure when compared to unmodified, wildtype PrP. More importantly, in vitro aggregation was abrogated for all PEGylated PrP variants under conditions at which wildtype PrP aggregated. Furthermore, the addition of PEGylated PrP as low as 10 mol% to wildtype PrP completely blocked aggregation. A similar effect was observed for synthetic PEGylated PrP segments comprising amino acids 179–231 alone if these were added to wildtype PrP in aggregation assays. This behavior raises the question if large N-glycans interfere with aggregation in vivo and if PEGylated PrP peptides could serve as potential therapeutics.

Site-Specific and Stoichiometric Stealth Polymer Conjugates of Therapeutic Peptides and Proteins

As potent and selective therapeutic agents, peptides and proteins are an important class of drugs, but they typically have suboptimal pharmacokinetic profiles. One approach to solve this problem is their conjugation with "stealth" polymers. Conventional methods for conjugation of this class of polymers to peptides and proteins are typically carried out by reactions that have poor yield and provide limited control over the site of conjugation and the stoichiometry of the conjugate. To address these limitations, new chemical and biological approaches have been developed that provide new molecular tools in the bioconjugation toolbox to create stealth polymer conjugates of peptides and proteins with exquisite control over their properties. This review article highlights these recent advances in the synthesis of therapeutic peptide- and protein-stealth polymer conjugates.

Expansion of bioorthogonal chemistries towards site-specific polymer–protein conjugation

Bioorthogonal chemistries have been used to achieve polymer-protein conjugation with the retained critical properties.

Bis(arylmethyl)-substituted unsymmetrical phosphites for the synthesis of lipidated peptides via Staudinger-phosphite reactions

With this study we introduce new unsymmetrical phosphites to obtain lipidated peptide-conjugates starting from easily accessible azide-modified amino acid or peptide precursors.