Protein chemical synthesis by serine and threonine ligation

Abstract

Landmark advances in the field of synthetic protein chemistry have enabled the preparation of complex, homogeneous proteins, including those that carry specific posttranslational modifications (PTMs). In addition, chemical synthesis will allow one to incorporate unnatural elements to generate new biologics with altered properties and functions. Native chemical ligation (NCL) is a milestone in the chemical synthesis of proteins (Kent et al., Science, 1994, 266, 776-779), in which a C-terminal peptide thioester and an N-terminal cysteine (Cys)-containing peptide-both in side-chain unprotected forms-are selectively coupled to generate a natural peptidic linkage at the site of ligation. This method requires a cysteine at the optimal convergent ligation site. However, Cys is one of the least abundant amino acids in natural proteins. Therefore, the development of new ligation methods at other amino acids will be necessary and important in this regard.

Along these lines, our laboratory has developed a novel thiol-independent approach-serine/threonine ligation (STL). It uses the N-terminal serine or threonine of a peptide segment to chemoselectively react with another peptide segment with a C-terminal salicylaldehyde ester to form an N,O-benzylidene acetal linked product, followed by acidolysis to afford the final product at the natural Ser/Thr site. To extend the application of STL in chemical protein synthesis, we have developed a robust method for the preparation of peptide salicylaldehyde esters via Fmoc-based solid phase peptide synthesis. Furthermore, we have successfully applied this ligation method in the convergent synthesis of peptide drugs of significant therapeutic importance, including Teriparatide (Forteo), Corticorelin (oCRH), Exenatide (Byetta) and Tesamorelin (hGHRH). Of significance, we have demonstrated the effectiveness of our STL in the assembly of a more complex target of biological interest: human erythrocyte acylphosphatase (~ 11 kDa).

In summary, we have developed a new serine/threonine ligation, which can be effectively used to synthesize peptides and proteins. As there are countless serine and threonine residues in natural proteins, particularly those carrying posttranslational modifications, this method is anticipated to offer new opportunities in synthetic protein chemistry and chemical biology.