Relative catalytic efficiencies and transcript levels of three d- and two l-lactate dehydrogenases for optically pure d-lactate production in Sporolactobacillus inulinus

As the optical purity of the lactate monomer is pivotal for polymerization, the production of optically pure d‐lactate is of significant importance. Sporolactobacillus inulinus YBS1‐5 is a superior optically pure d‐lactate‐producing bacterium. However, little is known about the relationship between lactate dehydrogenases in S. inulinus YBS1‐5 and the optical purity of d‐lactate. Three potential d‐lactate dehydrogenase (D‐LDH1‐3)‐ and two putative l‐lactate dehydrogenase (L‐LDH1‐2)‐encoding genes were cloned from the YBS1‐5 strain and expressed in Escherichia coli D‐LDH1 exhibited the highest catalytic efficiency toward pyruvate, whereas two L‐LDHs showed low catalytic efficiency. Different neutralizers significantly affected the optical purity of d‐lactate produced by strain YBS1‐5 as well as the transcription levels of ldhDs and ldhLs. The high catalytic efficiency of D‐LDH1 and elevated ldhD1 mRNA levels suggest that this enzyme is essential for d‐lactate synthesis in S. inulinus YBS1‐5. The correlation between the optical purity of d‐lactate and transcription levels of ldhL1 in the case of different neutralizers indicate that ldhL1 is a key factor affecting the optical purity of d‐lactate in S. inulinus YBS1‐5.

Combined utilization of nutrients and sugar derived from wheat bran for d-Lactate fermentation by Sporolactobacillus inulinus YBS1-5

To decrease d-Lactate production cost, wheat bran, a low-cost waste of milling industry, was selected as the sole feedstock. First, the nutrients were recovered from wheat bran by acid protease hydrolysis. Then, cellulosic hydrolysates were prepared from protease-treated samples after acid pretreatment and enzymatic saccharification. The combined use of nutrients and hydrolysates as nitrogen and carbon sources for fermentation by S. inulinus YB1-5 resulted in d-Lactate levels of 99.5g/L, with an average production efficiency of 1.94g/L/h and a yield of 0.89g/g glucose. Moreover, fed-batch simultaneous saccharification and fermentation process at 40°C, 20% (w/v) solid loading and 20FPU/g solid cellulase concentration was obtained. d-Lactate concentrations, yield, productivity, and optical purity were 87.3g/L, 0.65g/g glucose, 0.81g/L/h and 99.1%, respectively. This study provided a feasible procedure that can help produce cellulosic d-Lactate using agricultural waste without external nutrient supplementation.

Sodium ions activated phosphofructokinase leading to enhanced D-lactic acid production by Sporolactobacillus inulinus using sodium hydroxide as a neutralizing agent

Sporolactobacillus inulinus is a superior D-lactic acid-producing bacterium and proposed species for industrial production. The major pathway for D-lactic acid biosynthesis, glycolysis, is mainly regulated via the two irreversible steps catalyzed by the allosteric enzymes, phosphofructokinase (PFK) and pyruvate kinase. The activity level of PFK was significantly consistent with the cell growth and D-lactic acid production, indicating its vital role in control and regulation of glycolysis. In this study, the ATP-dependent PFK from S. inulinus was expressed in Escherichia coli and purified to homogeneity. The PFK was allosterically activated by both GDP and ADP and inhibited by phosphoenolpyruvate; the addition of activators could partly relieve the inhibition by phosphoenolpyruvate. Furthermore, monovalent cations could enhance the activity, and Na⁺ was the most efficient one. Considering this kind activation, NaOH was investigated as the neutralizer instead of the traditional neutralizer CaCO₃. In the early growth stage, the significant accelerated glucose consumption was achieved in the NaOH case probably for the enhanced activity of Na⁺-activated PFK. Using NaOH as the neutralizer at pH 6.5, the fermentation time was greatly shortened about 22 h; simultaneously, the glucose consumption rate and the D-lactic acid productivity were increased by 34 and 17%, respectively. This probably contributed to the increased pH and Na⁺-promoted activity of PFK. Thus, fermentations by S. inulinus using the NaOH neutralizer provide a green and highly efficient D-lactic acid production with easy subsequent purification.

D-Lactic acid production by Sporolactobacillus inulinus YBS1-5 with simultaneous utilization of cottonseed meal and corncob residue

d-Lactic acid, is an important organic acid produced from agro-industrial wastes by Sporolactobacillus inulinus YBS1-5 was investigated to reduce the raw material cost of fermentation. The YBS1-5 strain could produce d-lactic acid by using cottonseed meal as the sole nitrogen source. For efficient utilization, the cottonseed meal was enzymatically hydrolyzed and simultaneously utilized during d-lactic acid fermentation. Corncob residues are rich in cellulose and can be enzymatically hydrolyzed without pretreatment. The hydrolysate of this lignocellulosic waste could be utilized by strain YBS1-5 as a carbon source for d-lactic acid production. Under optimal conditions, a high d-lactic acid concentration (107.2g/L) was obtained in 7-L fed-batch fermenter, with an average productivity of 1.19g/L/h and a yield of 0.85g/g glucose. The optical purity of d-lactic acid in the broth was 99.2%. This study presented a new approach for low-cost production of d-lactic acid for an industrial application.