50 Shades of Sugarcoating: Recent Advances in Convergent Peptide Glycosylation for Homogeneous Glycoprotein Synthesis

Abstract

Protein glycosylation is a key post-translational modification that affects protein folding, stability, intracellular trafficking, and protein-protein-interactions. Native glycoprotein biosynthesis is mediated by a diverse set of enzymes that yield a heterogeneous mixture of glycoforms. These glycosylation patterns play a significant role in cancer, Alzheimer’s, and various autoimmune diseases, so elucidating biological functions of specific glycoforms is of great importance. Studying this requires structurally well-defined homogeneous glycoproteins, which can be obtained through enzymatic protein glycosylation, linear assembly of solid-phase peptide synthesis (SPPS)-derived glycopeptides, or convergent chemical attachment of glycans to protein structures. This review describes new convergent protein glycosylation approaches from the past 5 years. Convergent glycosylation is herein defined as the nonenzymatic covalent attachment of a mono- or oligosaccharide to a peptide sequence of at least 5 amino acids. These convergent methods were not only developed as new applications of pre-established ligation principles, but also arose from entirely novel biocompatible reaction mechanisms. Together, the glycosylation approaches enable the preparation of natural N- and O-glycosidic linkages, as well as tailor-made glycoproteins with near- and non-native alternatives. This includes several N-glycosylation methods, direct O-glycosylation of tyrosine residues, cysteine-targeted S-glycosylation and an approach for C-linked glycosylation via dehydroalanine protein tag. Finally, a novel approach is described for non-native tyrosine glycosylation via electrochemical activation of phenyl-urazole. These efforts, combined with traditional total chemical synthesis and chemoenzymatic methods, enhance glycan complexity, compatibility with secondary protein structures, site-selectivity and stereochemical homogeneity. Ultimately, they facilitate access to well-defined glycoproteins and thereby aid in future glycobiology research.