Effects of l-lysine and d-lysine on ɛ-Poly-l-lysine biosynthesis and their metabolites by Streptomyces ahygroscopicus GIM8

Abstract

ɛ-Poly-l-lysine (ɛ-PL), produced by Streptomyces or Kitasatospora strains, is a homo-poly-amino acid of l-lysine, which is used as a safe food preservative. In this study, the effects of l-lysine and its isomer, d-lysine, on ɛ-PL biosynthesis and their metabolites by the ɛ-PL-producing strain Streptomyces ahygroscopicus GIM8 were determined. The results indicated that l-lysine added into the fermentation medium in the production phase mainly served as a precursor for ɛ-PL biosynthesis during the flask culture phase, leading to greater ɛ-PL production. At an optimum level of 3 mM l-lysine, a ɛ-PL yield of 1.16 g/L was attained, with a 41.4% increment relative to the control of 0.78 g/L. Regarding d-lysine, the production of ɛ-PL increased by increasing its concentrations up to 6 mM in the initial fermentation medium. Interestingly, ɛ-PL production (1.20 g/L) with the addition of 3 mM d-lysine into the initial fermentation medium in flasks was higher than that of the initial addition of 3 mM L-lysine (1.06 g/L). The mechanism by which d-lysine improves ɛ-PL biosynthesis involves its utilization that leads to greater biomass. After S. ahygroscopicus GIM8 was cultivated in the defined medium with L-lysine, several key metabolites, including 5-aminovalerate, pipecolate, and l-2-aminoadipate formed in the cells, whereas only l-2-aminoadipate was observed after d-lysine metabolism. This result indicates that l-lysine and d-lysine undergo different metabolic pathways in the cells. Undoubtedly, the results of this study are expected to aid the understanding of ɛ-PL biosynthesis and serve as reference for the formulation of an alternative approach to improve ɛ-PL productivity using l-lysine as an additional substrate in the fermentation medium.