Fully Enzymatic Peptide Synthesis using C-Terminal tert-Butyl Ester Interconversion

Chemoenzymatic peptide synthesis is potentially the most cost-efficient technology for the synthesis of short and medium-sized peptides with some important advantages. For instance, stoichiometric amounts of expensive coupling reagents are not required and racemisation does not occur, thus rendering purification easier compared to chemical peptide synthesis. The economically most attractive synthesis runs in the N -> C terminal direction wherein a cheap C-terminally protected amino acid is employed as the building block for elongation. However, C-terminal deprotection and activation after an elongation step - without cleavage of the side-chain protective groups or the peptide bonds - was hitherto still a challenge. In this paper we describe a novel C-terminal ester interconversion catalysed by the serine endopeptidase Alcalase. C-Terminally protected peptide tert-butyl esters were enzymatically converted into primary alkyl esters in quantitative yield and used directly in the next enzymatic elongation step with another amino acid tert-butyl ester. This fully enzymatic N -> C elongation strategy by C-terminal ester interconversion was applied toward the synthesis of biologically active peptides up to the pentamer level.