A green-by-design bioprocess for l-carnosine production integrating enzymatic synthesis with membrane separation

Abstract

L-Carnosine (L-Car, β-alanyl-L-histidine) is a bioactive dipeptide with important physiological functions. Direct coupling of unprotected β-Ala (β-alanine) with L-His (L-histidine) mediated by an enzyme is a promising method for L-Car synthesis. In this study, a new recombinant dipeptidase (SmPepD) from Serratia marcescens with a high synthetic activity toward L-Car was identified by a genome mining approach and successfully expressed in Escherichia coli. Divalent metal ions strongly promoted the synthetic activity of SmPepD, with up to 21.7-fold increase of activity in the presence of 0.1 mM MnCl₂. Higher temperature, lower pH and increasing substrate loadings facilitated the L-Car synthesis. Pilot biocatalytic syntheses of L-Car were performed comparatively in batch and continuous modes. In the continuous process, an ultra-filtration membrane reactor with a working volume of 5 L was employed for catalyst retention. The dipeptidase, SmPepD, showed excellent operational stability without a significant decrease in space–time yield after 4 days. The specific yield of L-Car achieved was 105 g_Car g_catalyst⁻¹ by the continuous process and 30.1 g_Car g_catalyst⁻¹ by the batch process. A nanofiltration membrane was used to isolate the desired product L-Car from the reaction mixture by selectively removing the excess substrates, β-Ala and L-His. As a result, the final L-Car content was effectively enriched from 2.3% to above 95%, which gave L-Car in 99% purity after ethanol precipitation with a total yield of 60.2%. The recovered substrate mixture of β-Ala and L-His can be easily reused, which will enable the economically attractive and environmentally benign production of the dipeptide L-Car.