At least 50% of all human proteins are predicted to experience one or more posttranslational modification, PTM, during their life cycle. These PTMs can result from enzymatic or non- enzymatic reactions. Enzymatic PTMs are well-known for being involved in regulating many cellular events such as gene expression, intracellular and extracellular signal transduction, protein- protein as well as cell-cell interactions. Non-enzymatic posttranslational modifications, nPTMs, are increasingly recognized to affect such events as well, with a special emphasis on age-related, metabolic and neurodegenerative diseases.

Here examples will be discussed that involve different semi-synthesis routes to access site-specific PTMs within proteins such as Hsp27, G-CSF and Tau. For the small heat shock protein Hsp27, which has been shown to carry non-enzymatic argpyrimidine, Apy, modifications, we can demonstrate that such Apy modifications significantly influence chaperon function for a variety of client proteins. Furthermore, new tools to access challenging protein targets via semi-synthesis will be discussed.