Active RNase glycoprotein from three pieces: The glycoprotein enzyme ribonuclease C, which contains a complex saccharide N-glycan, was synthesized by sequential native chemical ligation. An optimized ligation and isolation protocol allowed the efficient assembly and refolding of the 124 amino acid enzyme.

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.

Dedicated to Professor Hans Paulsen on the occasion of his 90th birthday N-Glycosylation is an important posttranslational modification of proteins. A carbohydrate is transferred to an asparagine within an Asn-X-Ser/Thr consensus sequence.[1]The study of the biological aspects of N-glycosylation often requires the synthesis of N-glycopeptides, [2] which are accessible by two main approaches. In the sequential mode glycosylamino acid cassettes are used for peptide elongation. After incorporation of larger oligosaccharides the solubility and reactivity of the peptide is affected and side reactions, for example, involving free OH groups [3] complicate further elongation. In the convergent mode (Lansbury aspartylation) [4] the sugar is connected to an aspartate after complete assembly of the peptide (Scheme 1). The main drawback of the convergent mode is the formation of cyclic aspartimides during peptide elongation and sugar coupling; [5] this formation depends on the peptide sequence, [6] and the coupling conditions. [7] We found that a pseudoproline (Ypro) [8] at the consensus-sequence Ser/Thr residue (Asn-X-Ser/Thr(Ypro)) efficiently suppresses the formation of aspartimides in the convergent synthesis of N-glycopeptides on the solid phase.The lack of pure N-glycoproteins for biological studies has stimulated research into their synthesis; these syntheses were carried out mainly by ligation techniques.[9] The required glycopeptides and their thioesters are difficult to obtain as the sugar component interferes with the peptide synthesis. The syntheses of longer glycopeptide thioesters are particularly difficult as they require, for example, additional ligation steps [9f, 10] or segment couplings.[3b] For longer N-glycopeptides the convergent approach provides advantages over the sequential approach, [11] especially as the Lansbury aspartylation [4] has been shown to be efficient also on the solid phase.[12]The undesired aspartimide formation can be avoided by peptide backbone (NH) protection, [13] however, this approach is tedious for residues other than glycine, and causes racemization when coupling backbone-protected dipeptides.[12b] Aspartimide formation during peptide elongation can be reduced by using bulky groups to protect the Asp side chain, [14] for example, the 2-phenylisopropylester (PhiPr); [12a] however, trityl anchors are also cleaved under the reaction conditions for PhiPr removal. Dmab-protected [15] Asp residues are compatible with trityl anchors, but the backbone protection of the neighboring amino acid is required. [13a, 16] To provide complex N-glycopeptide thioesters by solidphase Lansbury aspartylation we compared three Asp-sidechain protecting groups for the Fmoc-SPPS of an interleukin-6 (IL-6) 43-48 hexapeptide. The allyl-protected peptide [17] 6 a showed the highest percentage of aspartimide 7 ai formation (15 %), followed by the Dmab peptide 6 b (8 %), and the PhiPr [12a] peptide 6 c (< 1 %; Scheme 2). As well as the protecting group many factors are known to contribute to aspartim...

Seven in one blow: The efficient formation of mixed disulfides on the thiol-rich fusion protein A followed by subsequent intein cleavage gave the fragment B with all seven cysteines protected against oxidation. The native chemical ligation of B with synthetic glycopeptide thioesters provides glycoproteins.

These are not the final page numbers! Ü Ü Scheme 3. Solid phase coupling of GlcNAc-NH 2 2 to peptides. DIPEA = diisopropylethylamine, DMF = N,N-dimethylformamide, GlcNAc = Nacetylglucosamine, PyBOP = (benzotriazol-1-yloxy)tripyrrolidinophosphonium hexafluorophosphate. Angewandte Chemie 3These are not the final page numbers! Ü Ü Scheme 4. Convergent synthesis of glycoprotein segments: a) RNase 1-39, b) EPO 1-28). DIC = diisopropylcarbodiimide, DMSO = dimethylsulfoxide, Cl-HOBt = 6-chloro-1-hydroxybenzotriazole, 2-Cl-Trt-CM = 2-chlorotrityl ChemMatrix, HATU = 2-(7-Aza-1H-benzotriazole-1-yl)-1,1,3,3-tetramethyluronium hexafluorophosphate, HOAt = 1-hydroxy-7-azabenzotriazole.