An Investigation into the Effects of Glycosylation On the Properties of L-Proline in Peptides
Owens, Neil Wayne
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The amino acid L-proline plays a critical role in many biological processes. Therefore, efforts have been made to understand and control its influence. Since glycosylation is a common post-translational modification known to affect the characteristics of peptides and proteins, in a series of experiments, the effects of glycosylation on the properties of L-proline in peptides have been investigated. A conformationally constrained C-glucosyl proline hybrid is introduced, which has the capacity to vary the N-terminal amide equilibrium in model peptides through derivatization of the carbohydrate scaffold. For the first time, a comprehensive study of the effects of O-glycosylation on the kinetics and thermodynamics of prolyl amide isomerization is reported. The O-glycosylation of 4-hydroxy-L-proline has different effects on amide isomerization depending on the stereochemistry of the 4-hydroxyl group, which alters the orientation of the glycan with respect to the prolyl side chain. 4S-Galactosylation of 4-hydroxy-L-proline affects both the amide isomer equilibrium and the rate of amide isomerization, whereas 4R-galactosylation does not measurably influence either parameter. However, close contacts between the carbohydrate and prolyl rings lead to changes in the conformation and stability of longer peptides. As an expansion on these initial model studies, the effects of prolyl O-glycosylation on the properties of model peptides of two extremely important structural proteins are investigated. O-Galactosylation of 4R-hydroxy-L-proline residues in collagen model peptides does not preclude formation of the collagen triple helix, where the anomeric linkage of the Hyp O-glycan has slightly different influences on the conformational stability of the peptides. Also, the O-galactosylation of Hyp residues in polyproline model peptides causes a large increase in conformational stability. In both cases, interactions between the glycan and the peptide backbone and changes in hydration are implicated in contributing to the conformational stabilization of the peptides. These studies demonstrate that both natural and unnatural glycosylation of L-proline can be used as a means to control amide isomerization, and can increase the conformational stability of peptides, properties that will likely contribute to the development of new biomaterials. Also, these experiments provide further insight into the broad role glycosylation plays in affecting peptide and protein structure.