Synthesis of O-Sulfated Human Syndecan-1-like Glyco-polypeptides by Incorporating Peptide Ligation and O-Sulfated Glycopeptide Cassette Strategies

Advances in the chemical synthesis of human proteoforms

Access to structurally-defined human proteoforms is essential to the biochemical studies on human health and medicine. Chemical protein synthesis provides a bottom-up and atomic-resolution approach for the preparation of homogeneous proteoforms bearing any number of post-translational modifications of any structure, at any position, and in any combination. In this review, we summarize the development of chemical protein synthesis, focusing on the recent advances in synthetic methods, product characterizations, and biomedical applications. By analyzing the chemical protein synthesis studies on human proteoforms reported to date, this review demonstrates the significant methodological improvements that have taken place in the field of human proteoform synthesis, especially in the last decade. Our analysis shows that although further method development is needed, all the human proteoforms could be within reach in a cost-effective manner through a divide-and-conquer chemical protein synthesis strategy. The synthetic proteoforms have been increasingly used to support biomedical research, including spatial-temporal studies and interaction network analysis, activity quantification and mechanism elucidation, and the development and evaluation of diagnostics and therapeutics.

Oxidation and Phenolysis of Peptide/Protein C-Terminal Hydrazides Afford Salicylaldehyde Ester Surrogates for Chemical Protein Synthesis

With the growing popularity of serine/threonine ligation (STL) and cysteine/penicillamine ligation (CPL) in chemical protein synthesis, facile and general approaches for the preparation of peptide salicylaldehyde (SAL) esters are urgently needed, especially those viable for obtaining expressed protein SAL esters. Herein, we report the access of SAL ester surrogates from peptide hydrazides (obtained either synthetically or recombinantly) via nitrite oxidation and phenolysis by 3-(1,3-dithian-2-yl)-4-hydroxybenzoic acid (SAL(-COOH)^PDT). The resulting peptide SAL(-COOH)^PDT esters can be activated to afford the reactive peptide SAL(-COOH) esters for subsequent STL/CPL. While being operationally simple for both synthetic peptides and expressed proteins, the current strategy facilitates convergent protein synthesis and combined application of STL with NCL. The generality of the strategy is showcased by the N-terminal ubiquitination of the growth arrest and DNA damage-inducible protein (Gadd45a), the efficient synthesis of ubiquitin-like protein 5 (UBL-5) via a combined N-to-C NCL-STL strategy, and the C-to-N semisynthesis of a myoglobin (Mb) variant.

Sulfated Glycoprotein Synthesis

Chemical protein synthesis has achieved tremendous progress in the past decades. With the development of chemical ligation as powerful tools, the scope of synthetic protein is greatly expanded. Proteoglycans are a class of sulfated glycoproteins widely distributed on the cell surface and in the extracellular matrix, which are extensively engaged in cellular communication events. Consisting of protein backbone and glycosaminoglycan(s) side chain, proteoglycans are highly complex and heterogeneous in nature. Chemical synthesis provides facile and reliable approach to these molecules, with defined glycan structure and sulfation pattern. One remaining problem is that the acid-labile sulfates could hardly survive during the typical solid phase peptide synthesis (SPPS) process. In this chapter, strategic design of a "glycopeptide cassette" for the preparation of sulfated glycoprotein is described. In particular, we provide protocols for the chemical synthesis of ectodomain fragment (23-120) of sulfated glycoprotein syndecan-1.

Exploration of human xylosyltransferase for chemoenzymatic synthesis of proteoglycan linkage region

Proteoglycans (PGs) play important roles in many biological processes including tumor progression, cell adhesion, and regulation of growth factor activities. With glycosaminoglycan chains attached to the core proteins in nature, PGs are highly challenging synthetic targets due to the difficulties in integrating the sulfated glycans with the peptide backbone. To expedite the synthesis, herein, the utility of human xylosyltransferase I (XT-I), the enzyme responsible for initiating PG synthesis, has been explored. XT-I was found to be capable of efficiently installing the xylose unit onto a variety of peptide structures on mg scales. Furthermore, an unnatural sugar, i.e., 6-azidoglucose can be transferred by XT-I introducing a reactive handle onto the glycopeptide for selective functionalization. XT-I can be coupled with β-4-galactosyl transferase-7 for one pot synthesis of glycopeptides bearing galactose-xylose disaccharide, paving the way toward efficient chemoenzymatic synthesis of PG glycopeptides and glycoproteins.

Chemoenzymatic Synthesis of Glycopeptides Bearing Galactose-Xylose Disaccharide from the Proteoglycan Linkage Region

Proteoglycans have important biological activities. To improve the overall synthetic efficiency, a new chemoenzymatic route has been established for the proteoglycan linkage region bearing a galactose-xylose disaccharide. The xylosylated glycopeptides were synthesized via solid phase synthesis, which was followed by the addition of the galactose unit by the galactosyl transferase β4GalT7. This work leads to a better understanding of the acceptor preference of β4GalT7 and opens the door for expeditious synthesis of the proteoglycan linkage region.