Glycopeptide/Glycoprotein Synthesis Ci Xu and Xuechen Li* Department of Chemistry, The University of Hong Kong, Hong Kong, People’ s Republic of China Abstract Protein glycosylation has dramatically enriched both the functional and structural diversity of proteins in mammals. Due to the non-template biosynthesis of glycoproteins in the nature, glyco- proteins always exist as heterogeneous mixtures with different glycan structures, which have complicated the isolation of pure and well-defined glycoforms for detailed mechanistic and functional studies. Over the past years, different strategies including chemical and chemoenzymatic methods have been developed for obtaining the homogenous glycoproteins, which will offer new opportunities to conduct an extensive assessment of relationship between the structure and function of glycoproteins and to reveal the biological role of the individual oligosaccharide on a glycoprotein for the evolution and development of cells. This chapter gives insight into the recent progress in the development of chemical and chemoenzymatic synthesis of homogenous glycoproteins including native chemical ligation, serine/threonine ligation, and chemoenzymatic glycoprotein remodeling. Examples are selected to demonstrate successful applications of synthetic strategies developed so far. Keywords Glycoproteins; Chemical synthesis; Chemoenzymatic; Solid-phase peptide synthesis; Native chemical ligation; Serine/threonine ligation Introduction Most of proteins present in the nature are posttranslationally modified with carbohydrates (glycans), which is known as protein glycosylation. A variety of studies have revealed the important biological roles of protein glycosylation. For example, these oligosaccharides play a critical role in the maintenance of protein solubility and conformation, protection of target protein structures from recognition by proteases or antibodies, and modulation of protein functions, and they also serve as cell surface ligands/receptors for specific binding events to mediate protein targeting, cell-matrix interactions, or cell-cell interactions. When investigating these vital biological events where glycoproteins are involved, it seems a formidable task due to the fact that glycoproteins are naturally produced as heterogeneous mixtures, which is known as glycoforms. Compared with nucleotides and peptides, the biosynthesis of glycan structures of glycoproteins is not template mediated, but determined by the level and the activity of various enzymes such as glycotransferases and glycosidases. As a result, the polypeptide chain of a given glycoprotein in nature is usually decorated by oligosaccharides either at different *Email: xuechenl@hku.hk Glycoscience: Biology and Medicine DOI 10.1007/978-4-431-54836-2_30-1 # Springer Japan 2014 Page 1 of 8