Method for producing dipeptide

A protein consisting of an amino acid sequence obtained by substituting one or more amino acid residues selected from the group consisting of amino acid residues corresponding to positions 107, 108, and 110 in an amino acid sequence of an original protein according to the following in [4] or [5] below with amino acid residues of the following [1′] to [3′], respectively, the protein having L-amino acid α ligase activity with improved substrate specificity as compared with the original protein:

• [1′] for the amino acid residue at position 107, an amino acid residue selected from the group consisting of L-serine, L-alanine, L-histidine, L-glutamine, L-aspartic acid, L-glutamic acid, glycine, and L-methionine;
• [2′] for the amino acid residue at position 108, an amino acid residue selected from the group consisting of L-phenylalanine, L-methionine, L-serine, L-proline, L-threonine, L-alanine, L-tyrosine, glycine, and L-leucine;
• and
• [3′] for the amino acid residue at position 110, an amino acid residue selected from the group consisting of L-methionine, L-valine, L-alanine, L-asparagine, L-cysteine, L-tryptophan, and L-phenylalanine;
• [4] a mutant protein consisting of an amino acid sequence obtained by deleting, substituting, inserting, and/or adding 1 to 20 amino acids in an amino acid sequence set forth in SEQ ID NO:
• 2, and having L-amino acid α ligase activity;
• and
• [5] a homologous protein consisting of an amino acid sequence having 80% or more identity to the amino acid sequence set forth in SEQ ID NO:
• 2, and having L-amino acid α ligase activity.