Characterization of an l-α,β-diaminopropionic acid polymer with comb-like structure isolated from a poly(ε-l-lysine)-producing Streptomyces sp

Polymers of basic amino acids function as polycationic compounds under physiological conditions and exhibit intriguing biological properties, such as antimicrobial and antiviral activities, immunopotentiating ability, and DNA-binding activity. Poly(ε- l -lysine) (ε-PL) produced by some strains of Streptomyces spp. is a cationic homopolymer of l -lysine linking between ε-amino and α-carboxylic acid functional groups and has been used as a food preservative based on its biocompatibility and biodegradability. An ε-PL-producing strain of Streptomyces sp. USE-33 was found to secrete a novel polycationic substance into its culture broth along with ε-PL. High-performance liquid chromatography analyses and one- and two-dimensional 1 H and 13 C nuclear magnetic resonance (NMR) experiments, accompanied by NMR titration studies, revealed that the secreted substance was poly[β-( l -diaminopropionyl- l -diaminopropionic acid)], PAP, characterized by an isopeptide backbone linking between the β-amino and α-carboxylic acid groups of l -α,β-diaminopropionic acid ( l -Dpr) with pendent l -Dpr residues. PAP had a molecular weight of 500 to 1400, and copolymers composed of the two amino acids l -Dpr and l -lysine were not detected in the producer strain USE-33. The strain coproduced high levels of the two poly(amino acid)s in the presence of glycerol, citrate, and ammonium sulfate at pH 4.0 in a two-stage cultivation procedure. PAP exhibited strong inhibitory activities against several yeasts and weaker activities against bacteria than ε-PL. PAP may share a number of biological functions with ε-PL, and the use of PAP along with ε-PL has potential as a specific and advanced material for technical applications in various fields. Key points • Novel cationic poly(amino acid) was found in an ε-PL-producing Streptomyces species. • The l -α,β-diaminopropionic acid polymer was characterized by a comb-like structure. • The novel poly(amino acid), PAP, exhibited antibacterial and antifungal activities. Graphical abstract