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 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 Escherichiacoli BL21 (DE3), and expressed by IPTG induction. Four recombinant _L-LDHs were purified using Ni-NTA agarose column and characterized. The molecular weight of purified _L-LDHs were 30~37 kDa on SDS-PAGE. Among the four _L-LDHs, _L-LDH3 exhibited highest enzyme activity. The _L-LDH3 exhibited maximal activity at 43℃ and pH 4.0 with 8 mM fructose 1,6-diphosphate (FDP) by 302,343.16 U/mg. 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 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 solutions to mitigate the post-acidification caused by L. casei HY2782.