Total Synthesis of Glycosylated Interleukin-25 Via Integrated Chemical Ligations

This dissertation, "Total Synthesis of Glycosylated Interleukin-25 via Integrated Chemical Ligations" by Chi-lung, Lee, 李志龍, was obtained from The University of Hong Kong (Pokfulam, Hong Kong) and is being sold pursuant to Creative Commons: Attribution 3.0 Hong Kong License. The content of this dissertation has not been altered in any way. We have altered the formatting in order to facilitate the ease of printing and reading of the dissertation. All rights not granted by the above license are retained by the author. Abstract: Abstract of thesis entitled TOTAL SYNTHESIS OF GLYCOSYLATED INTERLEUKIN-25 VIA INTEGRATED CHEMICAL LIGATIONS Submitted by Chi Lung LEE for the degree of Doctor of Philosophy at The University of Hong Kong in April 2016 Chemical protein synthesis is a powerful tool as it can exert atomically precise control on the composition of protein and, thus, can overcome the potential limitations of protein expression. It enables the assembly of complex and homogeneous proteins bearing site-specific post-translational modifications, such as glycosylation, for the study of structure-activity-relationship (SAR) of proteins. To maximize the efficiency of protein synthesis, a convergent synthetic approach is adopted as this involves ligations in both the N-to-C and C-to-N direction which are inherently parallel and complementary. To this end, kinetically controlled ligation (KCL) was developed. However, for the synthesis involving multi-fragments ligation using serine/threonine ligation (STL), current strategies are suitable only for the sequential ligations in the C-to-N direction. In this thesis, a novel N-to-C assembly strategy has been developed for STL-based convergent ii synthesis of protein/glycoprotein. The key to the success in STL in the N-to-C direction was the use of semicarbazone as a protecting group to selectively switch on/off on and off the activity of the peptide salicylaldehyde (SAL ) ester, permitting the control of the dual activity of peptide containing N-terminal Ser/Thr and C- terminal SAL ester. By combining this strategy with traditional C-to-N STL, convergent synthesis by STL is now realized. In addition to STL, a combined ligation strategy involving sequential STL and NCL was also revealed in the N- to-C direction. This new strategy could significantly increase the choices for ligation sites and thus allow the assembly of protein targets bearing insufficient cysteines or cysteines next to proline or valine. The feasibility of the N-to-C assembly using an integrated ligation strategy to produce peptide fragments for convergent synthesis was demonstrated by the synthesis of an N-glycosylated protein Interleukin (IL)-25 (also known as IL-17E), where its aggregation-prone nature at its C-terminal end renders its synthesis difficult via traditional C-to-N STL. With this successful illustration, it is anticipated that the compatibility of STL and NCL will provide a versatile approach for the future convergent synthesis of proteins of many kinds. iii Subjects: Interleukins