Chemically Precise Glycoengineering Improves Human Insulin.

Diabetes is a leading cause of death worldwide and results in over 3 million annual deaths. While insulin manages the disease well, many patients fail to comply with injection schedules, and despite significant investment, a more convenient oral formulation of insulin is still unavailable. Studies suggest that glycosylation may stabilize peptides for oral delivery, but the demanding production of homogeneously glycosylated peptides has hampered transition into the clinic. We report here the first total synthesis of homogeneously glycosylated insulin. After characterizing a series of insulin glycoforms with systematically varied O-glycosylation sites and structures, we demonstrate that O-mannosylation of insulin B-chain Thr27 reduces the peptide's susceptibility to proteases and self-association, both critical properties for oral dosing, while maintaining full activity. This work illustrates the promise of glycosylation as a general mechanism for regulating peptide activity and expanding its therapeutic use.

Quantitative Effects of O-Linked Glycans on Protein Folding.

Protein O-glycosylation is a diverse, common, and important post-translational modification of both proteins inside the cell and those that are secreted or membrane-bound. Much work has shown that O-glycosylation can alter the structure, function, and physical properties of the proteins to which it is attached. One gap remaining in our understanding of O-glycoproteins is how O-glycans might affect the folding of proteins. Here, we took advantage of synthetic, homogeneous O-glycopeptides to show that certain glycosylation patterns have an intrinsic effect, independent of any cellular folding machinery, on the folding pathway of a model O-glycoprotein, a carbohydrate binding module (CBM) derived from the Trichoderma reesei cellulase TrCel7A. The strongest effect, a 6-fold increase in overall folding rate, was observed when a single O-mannose was the glycan, and the glycosylation site was near the N-terminus of the peptide sequence. We were also able to show that glycosylation patterns affected the kinetics of each step in unique ways, which may help to explain the observations made here. This work is a first step toward quantitative understanding of how O-glycosylation might control, through intrinsic means, the folding of O-glycoproteins. Such an understanding is expected to facilitate future investigations into the effects of glycosylation on more biological processes related to protein folding.

Structural Insight into the Stabilizing Effect of O-Glycosylation.

Protein glycosylation has been shown to have a variety of site-specific and glycan-specific effects, but so far, the molecular logic that leads to such observations has been elusive. Understanding the structural changes that occur and being able to correlate those with the physical properties of the glycopeptide are valuable steps toward being able to predict how specific glycosylation patterns will affect the stability of glycoproteins. By systematically comparing the structural features of the O-glycosylated carbohydrate-binding module of a Trichoderma reesei-derived Family 7 cellobiohydrolase, we were able to develop a better understanding of the influence of O-glycan structure on the molecule's physical stability. Our results indicate that the previously observed stabilizing effects of O-glycans come from the introduction of new bonding interactions to the structure and increased rigidity, while the decreased stability seemed to result from the impaired interactions and increased conformational flexibility. This type of knowledge provides a powerful and potentially general mechanism for improving the stability of proteins through glycoengineering.