Method for synthesising a peptide comprising the sequence His-X-Glu-Gly-Thr-Phe-Thr-Ser-Asp-Val-Ser-Ser-Tyr-Leu-Glu-Gly-Gln-Ala-Ala- Y -Glu-Phe-Ile-Ala-Trp-Leu-Val- Z -Gly-Arg-Gly, comprising enzymatically coupling (a) a peptide C-terminal ester or thioester comprising a first peptide fragment comprising the sequence His-X-Glu-Gly-Thr-Phe-Thr-Ser-Asp-Val-Ser-(thio)ester; and (b) a peptide nucleophile having an N-terminally unprotected amine comprising a second peptide fragment comprising the sequence H-Ser-Tyr-Leu-Glu-Gly-Gln-Ala-Ala- Y -Glu-Phe-Ile-Ala-Trp-Leu-Val- Z -Gly-Arg-Gly wherein - X is Ala or an α-amino-isobutyric acid (Aib) residue; - Y is Lys, which Lys has a free side-chain ε-amino group or of which Lys the side-chain ε-amino group is protected with a protective group or of which Lys the side-chain ε-amino group is functionalised with an amino acid or another functional group, in particular a functional group selected from the group consisting of γ-Glu-OH, Pal-γ-Glu-OH, AEEA-AEEA-γ-Glu-OH and AEEA-AEEA-γ-Glu-N-17-carboxyheptadecanoyl-OH, wherein Pal is palmitoyl and AEEA-AEEA is -2-[2-(2-aminoethoxy)ethoxy]acetyl-2-[2-(2-aminoethoxy)ethoxy]acetyl; - Z is Arg or Lys; which enzymatic coupling is catalysed by a ligase, wherein the ligase is a subtilisin BPN' variant or a homologue thereof, which comprises the following mutations compared to subtilisin BPN' represented by SEQ ID NO: 2 or a homologue sequence thereof: - a deletion of the amino acids corresponding to positions 75-83; - a mutation at the amino acid position corresponding to S221, the mutation being S221C or S221 selenocysteine; - preferably a mutation at the amino acid position corresponding to P225; wherein the amino acid positions are defined according to the sequence of subtilisin BPN' represented by SEQ ID NO: 2, and wherein said ligase is a subtilisin BPN' variant, or a homologue thereof having at least 80%, or 85%, or 90%, preferably 95%, sequence identity with SEQ ID NO: 2. (a) a peptide C-terminal ester or thioester comprising a first peptide fragment comprising the sequence His-X-Glu-Gly-Thr-Phe-Thr-Ser-Asp-Val-Ser-(thio)ester; and (b) a peptide nucleophile having an N-terminally unprotected amine comprising a second peptide fragment comprising the sequence H-Ser-Tyr-Leu-Glu-Gly-Gln-Ala-Ala- Y -Glu-Phe-Ile-Ala-Trp-Leu-Val- Z -Gly-Arg-Gly wherein - X is Ala or an α-amino-isobutyric acid (Aib) residue; - Y is Lys, which Lys has a free side-chain ε-amino group or of which Lys the side-chain ε-amino group is protected with a protective group or of which Lys the side-chain ε-amino group is functionalised with an amino acid or another functional group, in particular a functional group selected from the group consisting of γ-Glu-OH, Pal-γ-Glu-OH, AEEA-AEEA-γ-Glu-OH and AEEA-AEEA-γ-Glu-N-17-carboxyheptadecanoyl-OH, wherein Pal is palmitoyl and AEEA-AEEA is -2-[2-(2-aminoethoxy)ethoxy]acetyl-2-[2-(2-aminoethoxy)ethoxy]acetyl; - Z is Arg or Lys; - X is Ala or an α-amino-isobutyric acid (Aib) residue; - Y is Lys, which Lys has a free side-chain ε-amino group or of which Lys the side-chain ε-amino group is protected with a protective group or of which Lys the side-chain ε-amino group is functionalised with an amino acid or another functional group, in particular a functional group selected from the group consisting of γ-Glu-OH, Pal-γ-Glu-OH, AEEA-AEEA-γ-Glu-OH and AEEA-AEEA-γ-Glu-N-17-carboxyheptadecanoyl-OH, wherein Pal is palmitoyl and AEEA-AEEA is -2-[2-(2-aminoethoxy)ethoxy]acetyl-2-[2-(2-aminoethoxy)ethoxy]acetyl; - Z is Arg or Lys; which enzymatic coupling is catalysed by a ligase, wherein the ligase is a subtilisin BPN' variant or a homologue thereof, which comprises the following mutations compared to subtilisin BPN' represented by SEQ ID NO: 2 or a homologue sequence thereof: - a deletion of the amino acids corresponding to positions 75-83; - a mutation at the amino acid position corresponding to S221, the mutation being S221C or S221 selenocysteine; - preferably a mutation at the amino acid position corresponding to P225; - a deletion of the amino acids corresponding to positions 75-83; - a mutation at the amino acid position corresponding to S221, the mutation being S221C or S221 selenocysteine; - preferably a mutation at the amino acid position corresponding to P225; wherein the amino acid positions are defined according to the sequence of subtilisin BPN' represented by SEQ ID NO: 2, and wherein said ligase is a subtilisin BPN' variant, or a homologue thereof having at least 80%, or 85%, or 90%, preferably 95%, sequence identity with SEQ ID NO: 2.