Herein is reported a method for the enzymatic production of a thioester comprising incubating i) a first polypeptide comprising the amino acid sequence LPXTG (SEQ ID NO: 01, wherein X can be any amino acid residue), ii) a second polypeptide that has at its N-terminus a cysteine amino acid residue or is a cysteinyl compound, and iii) a third polypeptide with sortase A activity.