Optimization Of The Peptide Hydrazide Ligation And Its Application In Synthesis Of Cyclic Peptides And Glycoproteins

Glycoproteins are a class of natural proteins that are widely found in vivo and play a vital role in various biological processes.These are derived from a kind of important post-translational modification of protein that is glycosylation.In vivo,the precursor proteins are glycosylated to produce a variety of glycoprotein complex which contain heterogeneous glycoforms and play different biological functions.At the same time,the heterogeneous glycoforms induced by glycosylation of protein make obtaining the homogeneous glycoproteins becomes a very challenging task.The heterogeneity also hindered the study of structure-activity relationship of glycoprotein and its development in drug discovery.In this context,the chemical synthesis of proteins has become a popular method for the preparation of homologous glycoproteins,especially for proteins with precise and unnatural tags.Solid-phase synthesis of long peptide beyond 50 amino acids encountered challenges on product purity and purification procedures,therefore peptide ligation is crucial to elongate the polypeptide in protein synthesis.The process of glycoproteins synthesis mainly includes the preparation of glycopeptides and peptide fragments and ligation of corresponding peptide fragments.For peptide ligation,the native chemical ligation is currently the most mature and most widely used method of peptide ligation.The widely recognized mechanism is that first step is the occurrence of thioester exchange between the peptide C-terminal thioester and the N-terminal cysteine peptide to from a new thioester immediate and the next step is the intramolecular acyl shift from S to N to produce the native peptide bond.However,due to the alkaline instability of peptide thioesters and the low abundance of cysteine,the application of native chemical ligation in chemical protein synthesis is limited to a certain extent.Accordingly,a number of peptide ligation methods which are based on the mechanisms of native chemical ligation have been developed in recent decades.The author of this dissertation developed a kind of peptide ligation method which based on peptide hydrazide ligation and auxiliary ligation and applied well themethod to the synthesis of homologous glycoproteins with chemical enzymatic synthesis.In addition to the synthesis of natural glycoproteins,the author optimized the peptide hydrazide ligation and applied the new method to synthesize the branched cyclic peptide.