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Proteins are involved in and perform a myriad of biological functions. Understanding their structures, functions and interactions with small molecules or other proteins is essential for modern day science. In recent years, proteins as therapeutics have become more and more relevant in the pharmaceutical industry, because of their low toxicity, low risk of side effects as well as their high specificity. Hence, obtaining pure and homogenous samples of proteins has become an important field of research. Although recombinant production allows access to a large number of proteins, it does present limitations: incorporation of unnatural amino acids and the highly sought after posttranslational modifications (PTMs) – that are often critical for the biological activity of the protein of interest – are difficult, if not impossible, to implement. Chemical protein synthesis appears to be a good alternative to the recombinant approach, as it can easily bypass the mentioned limitations. It can deliver homogenous protein samples with almost any kind of amino acid component, without biological contaminants. In 2006, the Bode group developed a method called the -ketoacid–hydroxylamine ligation (KAHA ligation) which allows the chemical synthesis of proteins by ligating two unprotected peptide segments. The KAHA ligation proceeds via the decarboxylative condensation of an -ketoacid with a hydroxylamine to form an amide bond under acidic organic buffering conditions at slightly elevated temperature. Since its discovery, the KAHA ligation has been subject to constant development, allowing the Bode group to synthesize proteins like SUMO2, SUMO3, irisin, AS48, NP4, betatrophin, interleukin-2 proteins and IFITM3. This dissertation describes, in the first chapter, the synthetic improvements made for two main building blocks: the cyanosulfur ylide linker and the (S)-N-Boc-5-oxaproline, used for the preparation of -ketoacid peptide segments and hydroxylamine peptide segments respectively. The new procedures enable the production of these monomers on a multigram scale, allowing our group to scale up the synthesis of synthetic proteins. The second chapter describes the usage of the KAHA ligation for the preparation of multimilligram quantities of the protein hormone betatrophin. This protein had attracted attention after primary results claimed it had direct effects on the proliferation of -cells, giving rise to hope for a new approach to treat diabetes. This synthesis represents the first report of a synthetic protein assembled via five peptide segments by KAHA ligation. Integral membrane proteins are, because of their hydrophobicity, challenging to obtain both by recombinant overexpression and chemical synthesis. In a third chapter, the KAHA ligation was used to synthesize one of these challenging molecules: the antiviral transmembrane protein IFITM3. The unique features of the KAHA ligation – acidic organic solvents and the formation of a depsi-peptide at the ligation site – appeared to be ideally suited for the synthesis of such hydrophobic proteins, and we could successfully synthesize the desired protein in a multimilligram quantity. A phosphorylated, a palmitoylated and a fluorescent variant of IFITM3 could also be synthesized using the KAHA ligation. We showed that the synthetic IFITM3 was incorporating itself into a lipid membrane and displaying an alpha helix fingerprint, as predicted by the calculated 3D structure. Preliminary results using lipid vesicles and fluorescently labeled viruses showed that our synthetic material displays antiviral activity and more experiments are currently being conducted. |
