D-Lactic acid fermentation performance and the enzyme activity of a novel bacterium Terrilactibacillus laevilacticus SK5–6

Purpose The aim of this study was to prove that Terrilactibacillus laevilacticus SK5-6, a novel D-lactate producer, exhibited a good fermentation performance comparing to the reference D-lactate producer Sporolactobacillus sp. Methods Glucose bioconversion for D-lactate production and the activity of five key enzymes including phosphofructokinase (PFK), pyruvate kinase (PYK), D-lactate dehydrogenase (D-LDH), L-lactate dehydrogenase (L-LDH), and lactate isomerase (LI) were investigated in the cultivation of T. laevilacticus SK5–6 and S. laevolacticus 0361 T . Results T. laevilacticus SK5–6 produced D-lactate at higher yield, productivity, and optical purity compared with S. laevolacticus 0361 T . T. laevilacticus SK5–6, the catalase-positive isolate, simultaneously grew and produced D-lactate without lag phase while delayed growth and D-lactate production were observed in the culture of S. laevolacticus 0361 T . The higher production of D-lactate in T. laevilacticus SK5–6 was due to the higher growth rate and the higher specific activities of the key enzymes observed at the early stage of the fermentation. The low isomerization activity was responsible for the high optical purity of D-lactate in the cultivation of T. laevilacticus SK5–6. Conclusion The lowest specific activity of PFK following by PYK and D/L-LDHs, respectively, indicated that the conversion of fructose-6-phosphate was the rate limiting step. Under the well-optimized conditions, the activation of D/L-LDHs by fructose-1,6-phosphate and ATP regeneration by PYK drove glucose bioconversion toward D-lactate. The optical purity of D-lactate was controlled by D/L-LDHs and the activation of isomerases. High D-LDH with limited isomerase activity was preferable during the fermentation as it assured the high optical purity.