Reduction of lactate production in Lactococcus lactis, a combined strategy: metabolic engineering by introducing foreign alanine dehydrogenase gene and hemin addition

Abstract

An approach to decreasing the lactate production in Lactococcus lactis by metabolic engineering is presented. The inhibitory effects of a low pH due to the accumulation of lactate on cell growth and nisin production in L. lactis are well known. To avoid such inhibitory effects, a new strategy by rerouting carbon flow was considered. In an effort to suppress lactate production, a new gene was introduced into L. lactis to create a novel pathway for alanine synthesis to reroute the metabolic flow of lactate. Alanine dehydrogenase (E.C.1.4.1.1) encoded by alaD from Bacillus sphaericus was expressed in L. lactis. The enzyme was expressed to a specific activity of nearly 0.39 U/mg protein in the transformant. Hemin addition was also considered to decrease the lactate production in L. lactis. The effect of hemin on the alanine production in the transformant was investigated. This study showed that using the combined strategy, stronger effects on lactate and alanine productions were observed in the transformant.