Abstract
PurposeThe 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.MethodsGlucose 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 0361T.ResultsT. laevilacticus SK5–6 produced D-lactate at higher yield, productivity, and optical purity compared with S. laevolacticus 0361T. 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 0361T. 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.ConclusionThe 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.
Highlights
Lactic acid as a chiral compound provides essential roles in industrial applications in food industries and life sciences
To clearly explain the obtained experimental data, the high inoculum approach typically developed for the catalasepositive lactate producing isolate was applied in D-lactate production by T. laevilacticus SK5–6 (Prasirtsak et al 2017; Thitiprasert et al 2017)
This study evaluated the fermentation performance and glucose bioconversion for D-lactate production in terms of the specific activities of the key enzymes in T. laevilacticus SK5– 6, the potent D-lactate producer
Summary
Lactic acid as a chiral compound provides essential roles in industrial applications in food industries and life sciences. 70% of lactic acid produced is utilized in the food industry (Es et al 2018). Attention was given to the commercially valuable products by microbial biotechnology due to the raised issues in global energy and environmental concerns. This provides the great potential for producing green, biodegradable, and biocompatible polylactic acid (PLA) for replacement of the plastic commodities from petrochemical based feedstocks. Microbial fermentation of L-lactic acid has long been commercially established, there are relatively few studies on D-lactic acid fermentation (Tsuge et al 2014).
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