Semisynthesis of a Homogeneous Glycoprotein Enzyme: Ribonuclease C: Part 2

One-pot ligation of multiple peptide segments via N-terminal thiazolidine deprotection chemistry

Peptide ligation chemistries have revolutionized the synthesis of proteins with site-specific modifications or proteomimetics through assembly of multiple peptide segments. In order to prepare polypeptide chains consisting of 100-150 amino acid residues or larger generally assembled from three or more peptide segments, iterative purification process that decreases the product yield is usually demanded. Accordingly, methodologies for one-pot peptide ligation that omit the purification steps of intermediate peptide segments have been vigorously developed so far to improve the efficiency of chemical protein synthesis. In this chapter, we first outline the concept and recent advances of one-pot peptide ligation strategies. Then, the practical guideline for the preparation of peptide segments for one-pot peptide ligation is described with an emphasis on diketopiperazine thioester synthesis. Finally, we disclose the explicit protocols for one-pot four segment ligation via repetitive deprotection of N-terminal thiazolidine by a 2-aminobenzamide type aldehyde scavenger.

A cell-permeable Ub-Dha probe for profiling E1-E2-E3 enzymes in live cells

Activity-based ubiquitin probes (Ub-ABPs) have recently been developed as effective tools for studying the capabilities of E1-E2-E3 enzymes, but most of them can only be used in cell lysates. Here, we report the first cell-penetrating Ub-Dha probes based on thiazolidine-protected cysteines, which enable successful delivery into cells confirmed by a fluorophore at the N-terminus of Ub and live-cell fluorescence microscopy. A total of 18 E1-E2-E3 enzymes in live cells were labelled and enriched in combination with label-free quantification (LFQ) mass spectrometry. This work provided a new cell-penetrating Ub tool for studying the activity and function of Ub-related enzymes.

Glycoconjugates: Synthesis, Functional Studies, and Therapeutic Developments

Glycoconjugates are major constituents of mammalian cells that are formed via covalent conjugation of carbohydrates to other biomolecules like proteins and lipids and often expressed on the cell surfaces. Among the three major classes of glycoconjugates, proteoglycans and glycoproteins contain glycans linked to the protein backbone via amino acid residues such as Asn for N-linked glycans and Ser/Thr for O-linked glycans. In glycolipids, glycans are linked to a lipid component such as glycerol, polyisoprenyl pyrophosphate, fatty acid ester, or sphingolipid. Recently, glycoconjugates have become better structurally defined and biosynthetically understood, especially those associated with human diseases, and are accessible to new drug, diagnostic, and therapeutic developments. This review describes the status and new advances in the biological study and therapeutic applications of natural and synthetic glycoconjugates, including proteoglycans, glycoproteins, and glycolipids. The scope, limitations, and novel methodologies in the synthesis and clinical development of glycoconjugates including vaccines, glyco-remodeled antibodies, glycan-based adjuvants, glycan-specific receptor-mediated drug delivery platforms, etc., and their future prospectus are discussed.

Synthesis of glycopeptides and glycopeptide conjugates

Protein glycosylation is a key post-translational modification important to many facets of biology. Glycosylation can have critical effects on protein conformation, uptake and intracellular routing. In immunology, glycosylation of antigens has been shown to play a role in self/non-self distinction and the effective uptake of antigens. Improperly glycosylated proteins and peptide fragments, for instance those produced by cancerous cells, are also prime candidates for vaccine design. To study these processes, access to peptides bearing well-defined glycans is of critical importance. In this review, the key approaches towards synthetic, well-defined glycopeptides, are described, with a focus on peptides useful for and used in immunological studies. Special attention is given to the glycoconjugation approaches that have been developed in recent years, as these enable rapid synthesis of various (unnatural) glycopeptides, enabling powerful carbohydrate structure/activity studies. These techniques, combined with more traditional total synthesis and chemoenzymatic methods for the production of glycopeptides, should help unravel some of the complexities of glycobiology in the near future.

Automated Peptide Synthesizers and Glycoprotein Synthesis

The development and application of commercially available automated peptide synthesizers has played an essential role in almost all areas of peptide and protein research. Recent advances in peptide synthesis method and solid-phase chemistry provide new opportunities for optimizing synthetic efficiency of peptide synthesizers. The efforts in this direction have led to the successful preparation of peptides up to more than 150 amino acid residues in length. Such success is particularly useful for addressing the challenges associated with the chemical synthesis of glycoproteins. The purpose of this review is to provide a brief overview of the evolution of peptide synthesizer and glycoprotein synthesis. The discussions in this article include the principles underlying the representative synthesizers, the strengths and weaknesses of different synthesizers in light of their principles, and how to further improve the applicability of peptide synthesizers in glycoprotein synthesis.

Efficient Preparation and Bioactivity Evaluation of Glycan-Defined Glycoproteins

Owing to the generation of heterogeneous glycoproteins in cells, it is highly difficult to study glycoprotein-mediated biological events and to develop biomedical agents. Thus, general and efficient methods to prepare homogeneous glycoproteins are in high demand. Herein, we report a general method for the efficient preparation of homogeneous glycoproteins that utilizes a combination of genetic code expansion and chemoselective ligation techniques. In the protocol to produce glycan-defined glycoproteins, an alkyne tag-containing protein, generated by genetic encoding of an alkynylated unnatural amino acid, was quantitatively coupled via click chemistry to versatile azide-appended glycans. The glycoproteins produced by the present strategy were found to recognize mammalian cell-surface lectins and enter the cells through lectin-mediated internalization. Also, cell studies exhibited that the glycoprotein containing multiple mannose-6-phosphate residues enters diseased cells lacking specific lysosomal glycosidases by binding to the cell-surface M6P receptor, and subsequently migrates to lysosomes for efficient degradation of stored glycosphingolipids.

Harnessing the power of transition metals in solid-phase peptide synthesis and key steps in the (semi)synthesis of proteins

Peptides and proteins can be either synthesized using solid-phase peptide synthesis (SPPS) or by applying a combination of SPPS and ligation approaches to address fundamental questions related to human health and disease, among others. The demand for their production either by chemical or biological methods continues to raise significant interests from the synthetic community. In this context, transition metals such as Pd, Ag, Hg, Tl, Au, Zn, Ni, and Cu have also contributed to the field of peptide and protein synthesis such as in peptide conjugation, extending native chemical ligation (NCL), and for regioselective disulfide bonds formation. In this review, we highlight, summarize, and evaluate the use of various transition metals in the chemical synthesis of peptides and proteins with emphasis on recent developments in this exciting research area.

Cell-Free Synthetic Glycobiology: Designing and Engineering Glycomolecules Outside of Living Cells

Glycans and glycosylated biomolecules are directly involved in almost every biological process as well as the etiology of most major diseases. Hence, glycoscience knowledge is essential to efforts aimed at addressing fundamental challenges in understanding and improving human health, protecting the environment and enhancing energy security, and developing renewable and sustainable resources that can serve as the source of next-generation materials. While much progress has been made, there remains an urgent need for new tools that can overexpress structurally uniform glycans and glycoconjugates in the quantities needed for characterization and that can be used to mechanistically dissect the enzymatic reactions and multi-enzyme assembly lines that promote their construction. To address this technology gap, cell-free synthetic glycobiology has emerged as a simplified and highly modular framework to investigate, prototype, and engineer pathways for glycan biosynthesis and biomolecule glycosylation outside the confines of living cells. From nucleotide sugars to complex glycoproteins, we summarize here recent efforts that harness the power of cell-free approaches to design, build, test, and utilize glyco-enzyme reaction networks that produce desired glycomolecules in a predictable and controllable manner. We also highlight novel cell-free methods for shedding light on poorly understood aspects of diverse glycosylation processes and engineering these processes toward desired outcomes. Taken together, cell-free synthetic glycobiology represents a promising set of tools and techniques for accelerating basic glycoscience research (e.g., deciphering the "glycan code") and its application (e.g., biomanufacturing high-value glycomolecules on demand).

Peptidyl ω-Asp Selenoesters Enable Efficient Synthesis of N-Linked Glycopeptides

Chemical synthesis is an attractive approach allows for the assembly of homogeneous complex N-linked glycopeptides and glycoproteins, but the limited coupling efficiency between glycans and peptides hampered the synthesis and research in the related field. Herein we developed an alternative glycosylation to construct N-linked glycopeptide via efficient selenoester-assisted aminolysis, which employs the peptidyl ω-asparagine selenoester and unprotected glycosylamine to perform rapid amide-bond ligation. This glycosylation strategy is highly compatible with the free carboxylic acids and hydroxyl groups of peptides and carbohydrates, and readily available for the assembly of structure-defined homogeneous N-linked glycopeptides, such as segments derived from glycoprotein EPO and IL-5.

Chemoenzymatic Semisynthesis of Proteins

Protein semisynthesis-defined herein as the assembly of a protein from a combination of synthetic and recombinant fragments-is a burgeoning field of chemical biology that has impacted many areas in the life sciences. In this review, we provide a comprehensive survey of this area. We begin by discussing the various chemical and enzymatic methods now available for the manufacture of custom proteins containing noncoded elements. This section begins with a discussion of methods that are more chemical in origin and ends with those that employ biocatalysts. We also illustrate the commonalities that exist between these seemingly disparate methods and show how this is allowing for the development of integrated chemoenzymatic methods. This methodology discussion provides the technical foundation for the second part of the review where we cover the great many biological problems that have now been addressed using these tools. Finally, we end the piece with a short discussion on the frontiers of the field and the opportunities available for the future.

The Journey for the Total Chemical Synthesis of a 53 kDa Protein

Chemical protein synthesis has been proved as an efficient way to afford medium-sized proteins with high homogeneity in workable quantities for various biochemical, structural, and functional studies. In particular, chemical protein synthesis has enabled access to proteins that are difficult or impossible to prepare by molecular biology approaches, such as those with post-translational modifications and mirror-image proteins. One prominent example is related to ubiquitination, a well-known modification that mediates a variety of cellular processes (e.g., proteasomal degradation). Ubiquitination is considered as a modification that is difficult to introduce into proteins in a test tube to generate ubiquitin (Ub) conjugates with high homogeneity with respect to the chain length and the anchored Lys residue in workable quantities to perform the biochemical and biophysical studies. Chemical protein synthesis has emerged as a powerful approach to prepare Ub conjugates for studies aiming to understand ubiquitination in great detail and decipher its roles in cell processes. Nevertheless, in order to answer more challenging questions in this field, it has been clear that researchers must also prepare Ub conjugates with increased size and complexity. Employing solid-phase peptide synthesis and chemoselective ligation, chemical protein synthesis offers a powerful way to furnish polypeptides composed of 100-200 residues. However, to synthesize larger proteins such as Ub conjugates, longer and more segments are required. This on the other hand leads to difficulties related to solubility, purification, ligation, and late-stage modifications. These challenges have encouraged us to explore more practical synthetic tools to facilitate the synthesis of complex Ub conjugates. In this Account, we summarize the synthetic tools that we have developed to achieve these goals. These include (1) δ-mercaptolysine-mediated isopeptide chemical ligation, (2) chemical synthesis of Ub building blocks, (3) palladium-mediated deprotection of key side chains during protein synthesis, (4) one-pot ligation and desulfurization, and (5) improving the solubility of peptide segments. The developed chemical toolbox has been a key for our successes in the synthesis of diverse and complex Ub conjugates. In this Account, we describe our approaches for generating various Ub conjugates, including (1) the K48 tetra-Ub chain composed of 304 amino acids, (2) the ubiquitinated histones and their analogues made of >200 amino acids, (3) the di-Ub-SUMO-2 hybrid chain composed of 245 amino acids, and (4) the 53 kDa tetra-Ub-α-globin composed of 472 amino acids, which represents the largest protein composed of natural amino acids ever made using chemical protein synthesis. The last target, Flag-Ub-Ub-Ub-Myc-Ub-(HA-α-globin), was prepared in the labeled form where the proximal Ub and distal Ub in the chain were labeled with Myc and Flag tags, respectively, while the α-globin was labeled with the HA tag. Applying the tetra-Ub-α-globin in proteasomal degradation studies assisted us to shed light on the proteolytic signal and the fates of the Ub moieties in the chains. Although these developments have contributed to the synthesis of interesting and challenging targets related to Ub signaling, several other targets may enforce new synthetic challenges. Hence, there is still a need to optimize the current synthetic tools and explore novel synthetic approaches to facilitate this process.

Improved strategy for large scale isolation of sialylglycopeptide (SGP) from egg yolk powder

Chicken egg yolk is an easily available source for the isolation of sialylglycopeptides (SGP) carrying homogenous biantennary N-glycans. This approach has gained much attention in the last decade since these SGPs can easily be used for the semi-synthesis of glycoconjugates circumventing laborious full-synthetic methodologies. Here we report an optimised, significantly shorter (one day instead of five) and environmentally friendly procedure for the mg scale isolation of SGP using commercially available egg yolk powder. A single chromatographic step following chloroform/methanol precipitation of proteins and lipids yielded desired approximately 200 mg SGP from 250 g egg yolk powder within a day.

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Environmentally friendly procedure for isolation of sialylglycopeptide from Egg yolk powder.

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Reduced the protocol from five days down to one.

One-pot multi-segment condensation strategies for chemical protein synthesis

This paper describes recent advances of one-pot multi-segment condensation strategies based on kinetically controlled strategies and/or protecting group-removal strategies in chemical protein synthesis.

Triple Function of 4-Mercaptophenylacetic Acid Promotes One-Pot Multiple Peptide Ligation

One-pot multiple peptide ligation is a key technology to improve the efficiency of chemical protein synthesis. One-pot repetitive peptide ligation requires a cycle of three steps: peptide ligation, removal of a protecting group, and inactivation of the deprotection reagent. However, previous strategies are not sufficient because of harsh deprotection conditions, slow deprotection rates, and difficulty in quenching the deprotection reagent. To address these issues, we developed a rapid, efficient deprotection and subsequent quenching strategy using an allyloxycarbonyl group to protect the N-terminal cysteine residue. 4-Mercaptophenylacetic acid (MPAA), a thiol additive for native chemical ligation, functioned not only as a scavenger for π-allyl palladium complexes, but also as a quencher of palladium(0) complexes. By utilizing the multifunctionality of MPAA, we carried out a one-pot five-segment ligation to afford histone H2AX (142 amino acids), which was isolated in 59 % yield.

Chemoenzymatic Methods for the Synthesis of Glycoproteins

Glycosylation is one of the most prevalent posttranslational modifications that profoundly affects the structure and functions of proteins in a wide variety of biological recognition events. However, the structural complexity and heterogeneity of glycoproteins, usually resulting from the variations of glycan components and/or the sites of glycosylation, often complicates detailed structure-function relationship studies and hampers the therapeutic applications of glycoproteins. To address these challenges, various chemical and biological strategies have been developed for producing glycan-defined homogeneous glycoproteins. This review highlights recent advances in the development of chemoenzymatic methods for synthesizing homogeneous glycoproteins, including the generation of various glycosynthases for synthetic purposes, endoglycosidase-catalyzed glycoprotein synthesis and glycan remodeling, and direct enzymatic glycosylation of polypeptides and proteins. The scope, limitation, and future directions of each method are discussed.

Thirteen decades of peptide synthesis: key developments in solid phase peptide synthesis and amide bond formation utilized in peptide ligation

A historical overview of peptide chemistry from T. Curtius to E. Fischer to M. Bergmann and L. Zervas is first presented. Next, the fundamentals of peptide synthesis with a focus on solid phase peptide synthesis by R. B. Merrifield are described. Immobilization strategies to attach the first amino acid to the resin, coupling strategies in stepwise peptide chain elongation, and approaches to synthesize difficult peptide sequences are also shown. A brief comparison between tert-butyloxycarbonyl (Boc)/benzyl (Bzl) strategy and 9-fluorenylmethoxycarbonyl (Fmoc)/tert-butyl (t -Bu) strategy utilized in solid phase peptide synthesis is given with an emphasis on the latter. Finally, the review focuses on the discovery and development of peptide ligation and the latest advances in this field including native amide bond formation strategies, these include the native chemical ligation, α-ketoacid-hydroxylamine ligation, and serine/threonine ligation which are the most commonly used chemoselective ligation methods that provide amide bond at the ligation site. This review provides an overview of the literature concerning the most important advances in the chemical synthesis of proteins and peptides covering the period from 1882 to 2017.

It is all about the solvent: on the importance of the mobile phase for ZIC-HILIC glycopeptide enrichment

Glycopeptide enrichment is a crucial step in glycoproteomics for which hydrophilic interaction chromatography (HILIC) has extensively been applied due to its low bias towards different glycan types. A systematic evaluation of applicable HILIC mobile phases on glycopeptide enrichment efficiency and selectivity is, to date, however, still lacking. Here, we present a novel, simplified technique for HILIC enrichment termed “Drop-HILIC”, which was applied to systematically evaluate the mobile phase effect on ZIC-HILIC (zwitterionic type of hydrophilic interaction chromatography) glycopeptide enrichment. The four most commonly used MS compatible organic solvents were investigated: (i) acetonitrile, (ii) methanol, (iii) ethanol and (iv) isopropanol. Glycopeptide enrichment efficiencies were evaluated for each solvent system using samples of increasing complexity ranging from well-defined synthetic glycopeptides spiked into different concentrations of tryptic BSA peptides, followed by standard glycoproteins, and a complex sample derived from human (depleted and non-depleted) serum. ZIC-HILIC glycopeptide efficiency largely relied upon the used solvent. Different organic mobile phases enriched distinct glycopeptide subsets in a peptide backbone hydrophilicity-dependant manner. Acetonitrile provided the best compromise for the retention of both hydrophilic and hydrophobic glycopeptides, whereas methanol was confirmed to be unsuitable for this purpose. The enrichment efficiency of ethanol and isopropanol towards highly hydrophobic glycopeptides was compromised as considerable co-enrichment of unmodified peptides occurred, though for some hydrophobic glycopeptides isopropanol showed the best enrichment properties. This study shows that even minor differences in the peptide backbone and solvent do significantly influence HILIC glycopeptide enrichment and need to be carefully considered when employed for glycopeptide enrichment.

Graphical Abstract

Obstacles and solutions for chemical synthesis of syndecan-3 (53-62) glycopeptides with two heparan sulfate chains

Proteoglycans play critical roles in many biological events. Due to their structural complexities, strategies towards synthesis of this class of glycopeptides bearing well-defined glycan chains are urgently needed. In this work, we give the full account of the synthesis of syndecan-3 glycopeptide (53-62) containing two different heparan sulfate chains. For assembly of glycans, a convergent 3+2+3 approach was developed producing two different octasaccharide amino acid cassettes, which were utilized towards syndecan-3 glycopeptides. The glycopeptides presented many obstacles for post-glycosylation manipulation, peptide elongation, and deprotection. Following screening of multiple synthetic sequences, a successful strategy was finally established by constructing partially deprotected single glycan chain containing glycopeptides first, followed by coupling of the glycan-bearing fragments and cleavage of the acyl protecting groups.

Distinguishing N-acetylneuraminic acid linkage isomers on glycopeptides by ion mobility-mass spectrometry

Differentiating the structure of isobaric glycopeptides represents a major challenge for mass spectrometry-based characterisation techniques. Here we show that the regiochemistry of the most common N-acetylneuraminic acid linkages of N-glycans can be identified in a site-specific manner from individual glycopeptides using ion mobility-mass spectrometry analysis of diagnostic fragment ions.

Convergent Synthesis of N-Linked Glycopeptides via Aminolysis of ω-Asp p-Nitrophenyl Thioesters in Solution

An efficient N-linked glycosylation reaction between glycosylamines and p-nitrophenyl thioester peptides has been developed. The reaction conditions are mild and compatible with the C-terminal free carboxylic acid group and the unprotected N-linked sialyloligosaccharide. By means of this convergent strategy, a versatile N-glycopeptide fragment containing an N-terminal Thz and a C-terminal thioester was readily prepared, which is available for the synthesis of long glycopeptides and glycoproteins using the protocol of native chemical ligation.

Chemical Synthesis of Phosphorylated Histone H2A at Tyr57 Reveals Insight into the Inhibition Mode of the SAGA Deubiquitinating Module

Monoubiquitination of histone H2B plays a central role in transcription activation and is required for downstream histone-methylation events. Deubiquitination of H2B by the Spt-Ada-Gcn5 acetyltransferase (SAGA) coactivator complex is regulated by a recently discovered histone mark, phosphorylated H2AY57 (H2AY57p), which inhibits deubiquitination of H2B by the SAGA complex as well as restricting demethylation of H3 and increasing its acetylation. Evidence for the effect of H2AY57p, however, was indirect and was investigated in vivo by monitoring the effects of chemical inhibition of Tyr kinase CK2 or by mutating the phosphorylation site. We applied the total chemical synthesis of proteins to prepare H2AY57p efficiently and study the molecular details of this regulation. This analogue, together with semisynthetically prepared ubiquitinated H2B, enabled us to provide direct evidence for the cross-talk between those two marks and the inhibition of SAGA activity by H2AY57p.

Synthesis of Glc1Man9-Glycoprotein Probes by a Misfolding/Enzymatic Glucosylation/Misfolding Sequence

Glycoproteins in non-native conformations are often toxic to cells and may cause diseases, thus the quality control (QC) system eliminates these unwanted species. Lectin chaperone calreticulin and glucosidase II, both of which recognize the Glc1 Man9 oligosaccharide on glycoproteins, are important components of the glycoprotein QC system. Reported herein is the preparation of Glc1 Man9 -glycoproteins in both native and non-native conformations by using the following sequence: misfolding of chemically synthesized Man9 -glycoprotein, enzymatic glucosylation, and another misfolding step. By using synthetic glycoprotein probes, calreticulin was found to bind preferentially to a hydrophobic non-native glycoprotein whereas glucosidase II activity was not affected by glycoprotein conformation. The results demonstrate the ability of chemical synthesis to deliver homogeneous glycoproteins in several non-native conformations for probing the glycoprotein QC system.

Advances in Fmoc solid-phase peptide synthesis

Today, Fmoc SPPS is the method of choice for peptide synthesis. Very-high-quality Fmoc building blocks are available at low cost because of the economies of scale arising from current multiton production of therapeutic peptides by Fmoc SPPS. Many modified derivatives are commercially available as Fmoc building blocks, making synthetic access to a broad range of peptide derivatives straightforward. The number of synthetic peptides entering clinical trials has grown continuously over the last decade, and recent advances in the Fmoc SPPS technology are a response to the growing demand from medicinal chemistry and pharmacology. Improvements are being continually reported for peptide quality, synthesis time and novel synthetic targets. Topical peptide research has contributed to a continuous improvement and expansion of Fmoc SPPS applications.

A PEGylated photocleavable auxiliary mediates the sequential enzymatic glycosylation and native chemical ligation of peptides

Research aimed at understanding the specific role of glycosylation patterns in protein function would greatly benefit from additional approaches allowing direct access to homogeneous glycoproteins. Herein the development and application of an efficient approach for the synthesis of complex homogenously glycosylated peptides based on a multifunctional photocleavable auxiliary is described. The presence of a PEG polymer within the auxiliary enables sequential enzymatic glycosylation and straightforward isolation in excellent yields. The auxiliary-modified peptides can be directly used in native chemical ligations with peptide thioesters easily obtained by direct hydrazinolysis of the respective glycosylated peptidyl resins and subsequent oxidation. The ligated glycopeptides can be smoothly deprotected by UV irradiation. We apply this approach to the preparation of variants of the epithelial tumor marker MUC1 carrying one or more Tn, T, or sialyl-T antigens.

An efficient one-pot four-segment condensation method for protein chemical synthesis

Successive peptide ligation using a one-pot method can improve the efficiency of protein chemical synthesis. Although one-pot three-segment ligation has enjoyed widespread application, a robust method for one-pot four-segment ligation had to date remained undeveloped. Herein we report a new one-pot multisegment peptide ligation method that can be used to condense up to four segments with operational simplicity and high efficiency. Its practicality is demonstrated by the one-pot four-segment synthesis of a plant protein, crambin, and a human chemokine, hCCL21.

Chemical and chemoenzymatic synthesis of glycoproteins for deciphering functions

Glycoproteins are an important class of biomolecules involved in a number of biological recognition processes. However, natural and recombinant glycoproteins are usually produced as mixtures of glycoforms that differ in the structures of the pendent glycans, which are difficult to separate in pure glycoforms. As a result, synthetic homogeneous glycopeptides and glycoproteins have become indispensable probes for detailed structural and functional studies. A number of elegant chemical and biological strategies have been developed for synthetic construction of tailor-made, full-size glycoproteins to address specific biological problems. In this review, we highlight recent advances in chemical and chemoenzymatic synthesis of homogeneous glycoproteins. Selected examples are given to demonstrate the applications of tailor-made, glycan-defined glycoproteins for deciphering glycosylation functions.

Semisynthesis of biologically active glycoforms of the human cytokine interleukin 6

Human interleukin 6 (IL-6) is a potent cytokine with immunomodulatory properties. As the influence of N-glycosylation on the in vivo activities of IL-6 could not be elucidated so far, a semisynthesis of homogeneous glycoforms of IL-6 was established by sequential native chemical ligation. The four cysteines of IL-6 are convenient for ligations and require only the short synthetic glycopeptide 43-48. The Cys-peptide 49-183 could be obtained recombinantly by cleavage of a SUMO tag. The fragment 1-42 was accessible by the simultaneous cleavage of two inteins, leading to the 1-42 thioester with the native N-terminus. Ligation and refolding studies showed that the inherently labile Asp-Pro bond 139-140 was detrimental for the sequential C- to N-terminal ligation. A reversed ligation sequence using glycopeptide hydrazides gave full-length IL-6 glycoproteins, which showed full bioactivity after efficient refolding and purification.