Overexpression and Characterization of L-Lactate Dehydrogenase from Lacticaseibacillus casei HY2782 to Control Post-Acidification in Yogurt

Post-acidification is a common phenomenon in fermented milk that can cause adverse effects, in which lactic acid bacteria used as starter cultures continue the lactic acid fermentation during storage. Lactate dehydrogenase (LDH) is a key enzyme for lactic acid fermentation. Therefore, the control of LDH could be one of the solutions to prevent post-acidification. In the present study, four genes (ldhL1, ldhL2, ldhL3, and ldhL4) encoding L-LDH were identified by whole genome sequencing of the Lacticaseibacillus casei HY2782, cloned into expression vector, pET22b (+), transformed into Escherichia coli BL21 (DE3), and expressed by IPTG induction. Four recombinant L-LDHs were purified using Ni-NTA agarose column and characterized. The molecular weights of purified L-LDHs were 30–37 kDa on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Among the four L-LDHs, L-LDH3 exhibited the highest enzyme activity. L-LDH3 exhibited maximal activity of 302,343.16 U/mg at 43℃ and pH 4.0 in the presence of 8 mM fructose 1,6-bisphosphate. In acidic condition, L-LDH3 was activated by 2 mM of Mg2+, Ca2+ and Mn2+ ions, inhibited by Zn2+ and Cu2+ ions. In addition, activity of L-LDH3 was gradually decreased as KH2PO4 concentration increased. Consequently, the L-LDH3 seems to be the major enzyme among the L-LDHs of L. casei HY2782 and may play an important role in lactic acid fermentation of this strain. Therefore, the inhibition of L-LDH3 activity using Zn2+, Cu2+and KH2PO4 could be one of the solutions to mitigate the post-acidification caused by L. casei HY2782.