Chemical Protein Synthesis with the α-Ketoacid–Hydroxylamine Ligation

The coupling of an α-ketoacid and a hydroxylamine (KAHA ligation) affords amide bonds under aqueous, acidic conditions without the need for protecting groups or coupling agents. Translating this finding into a general approach to chemical protein synthesis required the identification of methods to incorporate the key functional groups into unprotected peptide segments-ideally using well-established Fmoc solid-phase peptide synthesis protocols. A decade of effort has now led to robust, convenient methods for preparing peptides bearing free or masked C-terminal α-ketoacids and N-terminal hydroxylamines. The facile synthesis of the segments and the aqueous, acidic conditions of the KAHA ligation make it ideal for the construction of small proteins (up to 200 residues), including SUMO and related modifier proteins, betatrophin and other protein hormones, nitrophorin 4, S100A4, and the cyclic protein AS-48. Key to the successful development of this protein synthesis platform was the identification and gram-scale synthesis of (S)-5-oxaproline. This hydroxylamine monomer is completely stable toward standard methods and practices of solid-phase peptide synthesis while still performing very well in the KAHA ligation. This reaction partner-in contrast to all others examined-affords esters rather than amides as the primary ligation product. The resulting depsipeptides often offer superior solubility and handling and have been key in the chemical synthesis of hydrophobic and ampiphilic proteins. Upon facile O-to-N acyl shift, peptides bearing a noncanonical homoserine residue at the ligation site are formed. With proper choice of the ligation site, the incorporation of this unnatural amino acid does not appear to affect the structure or biological activity of the protein targets. The development of the chemical methods for preparing and masking peptide α-ketoacids and hydroxyalmines, the preparation of several protein targets by convergent ligation strategies, and the synthesis of new hydroxylamine monomers affording either natural or unnatural residues at the ligation site are discussed. By operation under acidic conditions and with a distinct preference for the ligation site, these efforts establish KAHA ligation as a complementary method to the venerable native chemical ligation (NCL) for chemical protein synthesis. This Account documents both the state of the KAHA ligation and the challenges in identifying, inventing, and optimizing new reactions and building blocks needed to interface KAHA ligation with Fmoc solid-phase peptide chemistry. With these challenges largely addressed, peptide segments ready for ligation are formed directly upon resin cleavage, facilitating rapid assembly of four to five segments into proteins. This work sets the stage for applications of the KAHA ligation to chemical biology and protein therapeutics.