Expanding the Scope of Native Chemical Ligation in Glycopeptide Synthesis

Chemical Synthesis and Biological Applications of O-GlcNAcylated Peptides and Proteins

All human cells use O-GlcNAc protein modifications (O-linked N-acetylglucosamine) to rapidly adapt to changing nutrient and stress conditions through signaling, epigenetic, and proteostasis mechanisms. A key challenge for biologists in defining precise roles for specific O-GlcNAc sites is synthetic access to homogenous isoforms of O-GlcNAc proteins, a result of the non-genetically templated, transient, and heterogeneous nature of O-GlcNAc modifications. Toward a solution, this review details the state of the art of two strategies for O-GlcNAc protein modification: advances in "bottom-up" O-GlcNAc peptide synthesis and direct "top-down" installation of O-GlcNAc on full proteins. We also describe key applications of synthetic O-GlcNAc peptide and protein tools as therapeutics, biophysical structure-function studies, biomarkers, and as disease mechanistic probes to advance translational O-GlcNAc biology.

Going Native: Synthesis of Glycoproteins and Glycopeptides via Native Linkages To Study Glycan-Specific Roles in the Immune System

Glycosylation plays a myriad of roles in the immune system: Certain glycans can interact with specific immune receptors to kickstart a pro-inflammatory response, whereas other glycans can do precisely the opposite and ameliorate the immune response. Specific glycans and glycoforms can themselves become the targets of the adaptive immune system, leading to potent antiglycan responses that can lead to the killing of altered self- or pathogenic species. This hydra-like set of roles glycans play is of particular importance in cancer immunity, where it influences the anticancer immune response, likely playing pivotal roles in tumor survival or clearance. The complexity of carbohydrate biology requires synthetic access to glycoproteins and glycopeptides that harbor homogeneous glycans allowing the probing of these systems with high precision. One particular complicating factor in this is that these synthetic structures are required to be as close to the native structures as possible, as non-native linkages can themselves elicit immune responses. In this Review, we discuss examples and current strategies for the synthesis of natively linked single glycoforms of peptides and proteins that have enabled researchers to gain new insights into glycoimmunology, with a particular focus on the application of these reagents in cancer immunology.

Rapid total synthesis of DARPin pE59 and barnase

We report the convergent total synthesis of two proteins: DARPin pE59 and Bacillus amyloliquefaciens RNase (Barnase). Leveraging our recently developed fast-flow peptide-synthesis platform, we rapidly explored numerous conditions for the assembly of long polypeptides, and were able to mitigate common side reactions, including deletion and aspartimide products. We report general strategies for improving the synthetic quality of difficult peptide sequences with our system. High-quality protein fragments produced under optimal synthetic conditions were subjected to convergent native chemical ligation, which afforded native full-length proteins after a final desulfurization step. Both DARPin and Barnase were folded and found to be as active as their recombinant analogues.