L-Lactate oxidase-mediated removal of l-lactic acid derived from fermentation medium for the production of optically pure D-lactic acid.

Abstract

There has been an increasing demand for optically pure d-lactic and l-lactic acid for the production of stereocomplex-type polylactic acid. The d-lactic acid production from lignocellulosic biomass is important owing to its great abundance in nature. Corn steep liquor (CSL) is a cheap nitrogen source used for industrial fermentation, though it contains a significant amount of l-lactic acid, which decreases the optical purity of d-lactic acid produced. To remove l-lactic acid derived from the CSL-based medium, l-lactate oxidase (LoxL) from Enterococcus sp. NBRC 3427 was expressed in an engineered Lactiplantibacillus plantarum (formally called Lactobacillus plantarum) strain KOLP7, which exclusively produces d-lactic acid from both hexose and pentose sugars. When the resulting strain was applied for d-lactic acid fermentation from the mixed sugars consisting of the major constituent sugars of lignocellulose (35g L-1 glucose, 10g L-1 xylose, and 5g L-1 arabinose) using the medium containing 10g L-1 CSL, it completely removed l-lactic acid derived from CSL (0.52g L-1 ) and produced 41.7g L-1 of d-lactic acid. The l-lactic acid concentration was below the detection limit, and improvement in the optical purity of d-lactic acid was observed (from 98.2% to>99.99%)by the overexpression of LoxL. The LoxL-mediated consumption of l-lactic acid would enable the production of optically pure d-lactic acid in any medium contaminated by l-lactic acid.