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Enzyme‐Catalyzed Formation of β‐Peptides: β‐Peptidyl Aminopeptidases BapA and DmpA Acting as β‐Peptide‐Synthesizing Enzymes

In recent studies, we discovered that the three β‐peptidyl aminopeptidases, BapA from Sphingosinicella xenopeptidilytica 3‐2W4, BapA from S. microcystinivorans Y2, and DmpA from Ochrobactrum anthropi LMG7991, possess the unique feature of cleaving N‐terminal β‐amino acid residues from β‐ and α/β‐peptides. Herein, we investigated the use of the same three enzymes for the reverse reaction catalyzing the oligomerization of β‐amino acids and the synthesis of mixed peptides with N‐terminal β‐amino acid residues. As substrates, we employed the β‐homoamino acid derivatives H‐βhGly‐pNA, H‐β3hAla‐pNA, H‐(R)‐β3hAla‐pNA, H‐β3hPhe‐pNA, H‐(R)‐β3hPhe‐pNA, and H‐β3hLeu‐pNA. All three enzymes were capable of coupling the six β‐amino acids to oligomers with chain lengths of up to eight amino acid residues. With the enzyme DmpA as the catalyst, we observed very high conversion rates, which correspond to dimer yields of up to 76%. The β‐dipeptide H‐β3hAla‐β3hLeu‐OH and the β/α‐dipeptide H‐βhGly‐His‐OH (carnosine) were formed with almost 50% conversion, when a five‐fold excess of β3‐homoleucine or histidine was incubated with H‐β3hAla‐pNA and H‐βhGly‐pNA, respectively, in the presence of the enzyme BapA from S. microcystinivorans Y2. BapA from S. xenopeptidilytica 3‐2W4 turned out to be a versatile catalyst capable of coupling various β‐amino acid residues to the free N‐termini of β‐ and α‐amino acids and even to an α‐tripeptide. Thus, these aminopeptidases might be useful to introduce a β‐amino acid residue as an N‐terminal protecting group into a ‘natural’ α‐peptide, thereby stabilizing the peptide against degradation by other proteolytic enzymes.